Onion (Allium cepa L.) bioactives: Chemistry, pharmacotherapeutic functions, and industrial applications

Abstract

Bioactive compounds possess different health benefits. Onion contains various bioactive compounds, such as organosulfur compounds, flavonols, ascorbic acids, and carbohydrate prebiotics, and its by‐products have more content of flavonoids than the bulb. Diallyl monosulfide, diallyl disulfide, diallyl trisulfide, and diallyl tetrasulfide are the major organosulfur compounds, whereas quercetin, kaempferol, anthocyanin, and luteolin are considered as main flavonoids. Ascorbic acid and fructooligosaccharides are also regarded as bioactive compounds. Onion bioactive compounds have the strong antioxidant potential for neutralizing oxidative stress of the cells. These bioactive components are beneficial as anticarcinogenic, antibiotic, anti‐inflammatory, antiplatelet, antidiabetic, and cardioprotective agents along with other nutritional benefits. However, various postharvest practices have an impact on these bioactive compounds, for example, curing mostly enhances the bioactive level and processing temperature generally decreases the concentration of many of them, whereas storage studies suggest an increase of others under optimized conditions. Additionally, conventional extraction techniques showed a negative impact on bioactive compounds of onion, whereas innovative methods yielded a higher amount of bioactive components. There is a need for innovative and integrated procedures in the postharvest sector to maintain or enhance the level of bioactive compounds without compromising the quality of onions. The present review comprehensively describes different bioactive compounds of onion, their chemistry, and their pharmacotherapeutic roles. Moreover, it also explores the effects of various postharvest factors, such as temperature, storage duration, and extraction conditions on the level of the bioactive components. It also suggests industrial applications of onion waste and its bioactive compounds in the food sector.

INTRODUCTION

Onion (Allium cepa L.) has been used as a food and medicinal crop since ancient times, and it is grown worldwide in 170 countries (FAO, 2018; Petropoulos et al., 2017). Because of its peculiar taste, unique flavor, highly valued aroma, and various health benefits, it is sometimes referred to as the “Queen of the kitchen” (Griffiths et al., 2002). There are three types of onion based on color, that is, red, yellow, and white, and all have different flavors and pungency from mild to highly strong according to color (Bahram-Parvar & Lim, 2018; Khandagale & Gawande, 2019). Fresh onion is supplied to the market or processed to manufacture various products such as fresh or fried rings, flakes, and powder. Onion is utilized in different cuisines and recipes as a spice, condiment, salad, or in combination with other vegetables, and as an ingredient in processed foods such as paste and pickles (Edith et al., 2018; Khan et al., 2016; Piechowiak et al., 2020). Commercially, three products are processed from the onion: fresh-cut or fresh rings (minimally processed), onion oil, and dehydrated onion (flakes or powder) for utilization in food products (Wiczkowski, 2011). Onion processing consists of peeling, slicing, and dicing and these operations generate a large amount of waste including skin that is considered as the main part (up to 60%) of processing waste (Gawlik-Dziki et al., 2015). However, this wasted skin is not recycled, for example, as a fertilizer or fodder due to the pungent aroma or the potential contamination by white rot (Sclerotium cepivorum) during growth (Roldán-Marín et al., 2009). Regarding flavor or pungency, the sulfur compounds “allyl propyl disulfides” are responsible for the peculiar smell of onion. Thus, the onion pungency level is chosen as per the end product, for example, moderate pungency bulbs are used for seasoning and cooking, whereas higher pungent types are used for the production of sauces and canned soup extracts (Loredana et al., 2019).

The onion bulb and skin contain various bioactive compounds, such as organosulfur compounds (OSCs), thiosulfinates, polyphenols, including flavonoids, and fructooligosaccharides (FOS) (Benkeblia & Lanzotti, 2007a; Downes et al., 2009; Putnik et al., 2019; Sagar et al., 2018, 2021) and among them, flavonoids are the most effective bioactive compounds. Two principal subgroups of flavonoids are anthocyanins, quercetin, and quercetin derivatives, which impart different colors to onion skins from yellow to purple (Benítez et al., 2011). Quercetin aglycone, quercetin diglucoside, quercetin 4′-glucoside, and kaempferol are the primary flavonoids of onion (Rodríguez et al., 2008; Sagar et al., 2020). The environment, cultivar type, agronomic practices, maturation stage, and storage duration have significant effects on the bioactive compounds of onion (Galdón et al., 2009). Waste onion skin also contains a higher level of flavonoids than the edible part (Duan et al., 2015) due to the oxidation of quercetin flavonol into 3,4- hydroxybenzoic acid and 2,4,6- trihydroxyphenylglycosilic acid and concentrated in dry onion skin to protect the bulb from soil microbes (Takahama & Hirota, 2000).

According to numerous animal research and clinical studies, onion has been used for the treatment of various ailments such as asthmas, cancer, diabetes, hypocholestremic, and osteoporosis (Fukushima et al., 2001; Marrelli et al., 2019). Onion skin also possesses various health benefits for example, anti-asthmatic effect, anticarcinogenic, hypocholesterolemic, and good cardiovascular agent (Hassan et al., 2014; Moreno et al., 2006). Indeed, quercetin is thought to be involved in all these health benefits due to its strong antioxidant activity (Bonaccorsi et al., 2008). Therefore, onion and its by-products have the potential to be used as an ingredient of functional food, such as natural antioxidants, preservatives, and additives.

Nevertheless, postharvest operations, such as processing and storage, can alter the concentration of bioactive compounds of onion, which consequently affect their bioavailability and efficacy (Hithamani et al., 2017; Sans et al., 2019; Zudaire et al., 2017). Various investigations reported the effects of storage time, as well as temperature, packaging, and freeze-drying on the bioactive compounds (Majid & Nanda, 2017; Petropoulos et al., 2017; Zudaire et al., 2017). On the other hand, with the growing demand for onion products, such as canned onion and onion powder, more efficient dehydrating techniques were developed to reduce losses during storage and minimize these effects (Arslan & Özcan, 2010; Edith et al., 2018). Additionally, minimally processing methods such as peeling, shredding, chopping, slicing, dicing, as well as freezing and cooking, were also investigated to determine their effects on the bioactive compounds of onion (Berno et al., 2014; Islek et al., 2015; Pérez-Gregorio, García-Falcón, et al., 2011a). It has been observed that during processing, ketones are released by the Maillard reaction that is responsible for the aroma of onion (Liu et al., 2020). Likewise, extraction is an important step to obtain bioactive components, and it is very important to choose an efficient and suitable method to extract these compounds without significant losses. It has been observed that higher loss is often associated with conventional methods, whereas innovative (green) technologies enhance extraction yield and save energy (Ren et al., 2020). Among these innovative technologies supercritical fluid extraction (SFE) or ultrasound-assisted extraction (UAE) showed promising results and technical efficiency, whereas the ultrasound has the benefit to be an ecofriendly technique and of lower cost.

This review aims to comprehensively explore the main bioactive compounds of onion, their biological roles, fate during the postharvest stage, and their utilization status (Figure 1). The aims will also describe the effects of extraction methods, postharvest processing techniques, and storage on these compounds.

FIGURE 1. Fate and utilization of onion and its waste in different sectors

TYPES OF BIOACTIVE COMPOUNDS

Onion is a perfect blend of valuable bioactive compounds such as FOS, flavonoids, ascorbic acid, and OSCs, and these compounds have shown various health benefits to humans (Griffiths et al., 2002; Corzo-Martínez & Villamiel, 2012). Onion is a prime source of OSCs and flavonoids known for their antioxidant properties, whereas onion by-products possess a significantly higher amount of total phenols, flavonoids, and minerals compared to the edible bulb (Benítez, 2011). However, the concentration and distribution of bioactive compounds may vary in onion types and cultivars. In Table 1, the bioactive compounds of onion are summarized, and the major bioactive compounds found in onion are described in the following sections.

TABLE 1 Types of bioactive compounds in onion and its by-product

Bioactive compounds	Part used	Detection technique	Concentration	References
Organosulfur compounds
Diallylmonosulfide (DMS), diallyldisulphide (DDS), diallyltrisulfide (DTS), diallyltetrasulfide (DTTS), PRENCSO, MCSO, alliin, asallicin, and thiopropanal S-oxide	Bulb	GC	1–5 g/100 g DW	Bystrická et al. (2013)
Diallylmonosulfide (DMS), diallyldisulphide (DDS), diallyltrisulfide (DTS), diallyltetrasulfide (DTTS), PRENCSO, MCSO, alliin, asallicin, and thiopropanal S-oxide	Bulb	HPLC	8.42–0.18 mg/g DW	Kim et al. (2018)
Diallylmonosulfide (DMS), diallyldisulphide (DDS), diallyltrisulfide (DTS), diallyltetrasulfide (DTTS), PRENCSO, MCSO, alliin, asallicin, and thiopropanal S-oxide	Bulb	GC-MS	963.90–1393 mg/kg DW	Loredana et al. (2019)
Sulfur content	Skin	Elemental analyzer	15.6 μmoles/g DW	Benítez et al. (2011)
Phenolic acids
Protocatechuic, vanillic acids, ferulic acid, p-coumaric, cyaniding, Peonidin, pelargonidin, Isoeugenol, Hydroxytyrosol, Sylvopinol, p-Octopamine, 4-Methylcatechol	Bulb	HPLC	26,445–108,300 mg/kg DW	Lachman et al. (2003)
Protocatechuic, vanillic acids, ferulic acid, p-coumaric, cyaniding, Peonidin, pelargonidin, Isoeugenol, Hydroxytyrosol, Sylvopinol, p-Octopamine, 4-Methylcatechol	Bulb	HPLC	519- 2600 mg/kg DW	Ignat et al. (2011)
Protocatechuic, vanillic acids, ferulic acid, p-coumaric, cyaniding, Peonidin, pelargonidin, Isoeugenol, Hydroxytyrosol, Sylvopinol, p-Octopamine, 4-Methylcatechol	Bulb	UPLC-ESI-MS/MS	72.12–551.74 μg/g DW	Zhou et al. (2020)
Flavonoids
Quercetin, kaempferol, isorhamnetin, gallic acid, luteolin, Rhamnazin, Dalbergin,Xanthomicrol, Petunidin 3-galactoside	Bulb	HPLC	300 mg/kg DW	Hollman and Arts (2000)
Quercetin, kaempferol, isorhamnetin, gallic acid, luteolin, Rhamnazin, Dalbergin, Xanthomicrol, Petunidin 3-galactoside	Bulb	HPLC	391.0–1497.5 mg/kg DW	Miean and Mohamed (2001)
Quercetin, kaempferol, isorhamnetin, gallic acid, luteolin, Rhamnazin, Dalbergin, Xanthomicrol, Petunidin 3-galactoside	Bulb	HPLC-MS	125–7475 mg/kg DW	Gennaro et al. (2002)
Quercetin, kaempferol, isorhamnetin, gallic acid, luteolin, Rhamnazin, Dalbergin, Xanthomicrol, Petunidin 3-galactoside	Bulb	HPLC	285.5–580.9 mg/kg FW	Lombard et al. (2005)
Quercetin, kaempferol, isorhamnetin, gallic acid, luteolin, Rhamnazin, Dalbergin, Xanthomicrol, Petunidin 3-galactoside	Bulb	HPLC	0.84–64.90 mg/g DW	Liguori et al. (2017)
Quercetin, kaempferol, isorhamnetin, gallic acid, luteolin, Rhamnazin, Dalbergin, Xanthomicrol, Petunidin 3-galactoside	Bulb	UPLC-ESI-MS/MS	0.46–48.27 μg/g DW	Zhou et al. (2020)
Quercetin, kaempferol, luteolin	Skin or peel	HPLC	34,430–83,477 mg/kg DW	Nuutila et al. (2003)
Quercetin, kaempferol, luteolin	Skin or peel	HPLC	481–5110 mg/kg DW	Prakash et al. (2007)
Quercetin, kaempferol, luteolin	Skin or peel	HPLC	880–17,630 mg/kg DW	Ko et al. (2011)
Quercetin, kaempferol, luteolin	Skin or peel	HPLC	32,810 mg/kg DW	Kwak et al. (2017)
Quercetin, kaempferol, luteolin	Skin or peel	HPLC	56.17–11,885.02 mg/kg DW	Sagar et al. (2020)
Quercetin	Bulb	Spectrophotometric method	22.0–890.5 mg/kg FW	Kaur et al. (2010)
Myricetin	Bulb	HPLC	2.77–4.13 mg/100 g FW	Sellappan and Akoh (2002)
Myricetin	Bulb	HPLC	8.02 mg/100 g DW	Fredotović et al. (2017)
Anthocyanin (cyanidin, peonidin, pelargonidin, delphinidin, and petunidin)	Bulb	HPLC-MS	0.21–0.45 mg/100 g DW	Wu and Prior (2005)
Anthocyanin (cyanidin, peonidin, pelargonidin, delphinidin, and petunidin)	Bulb	HPLC	0.3–0.19 mg/100 g DW	Fredotović et al. (2017)
Anthocyanin (cyanidin, peonidin, pelargonidin, delphinidin, and petunidin)	Bulb	HPLC	1555–250 mg/kg DW?	Ren et al. (2020)
Ascorbic acid	Bulb	Spectrophotometric method	60–2703 mg/kg FW	Lachman et al. (1999)
Ascorbic acid	Bulb	Spectrophotometric method	54–57 mg/kg FW	Iqbal et al. (2006)
Ascorbic acid	Bulb	Spectrophotometric method	35 mg/kg DW	Sun-Waterhouse et al. (2008)
Ascorbic acid	Leaves	Spectrophotometric method	390–5000 mg/kg FW	Lachman et al. (1999)
Organic acids
Folic acid	Bulb	UPLC-ESI-MS/MS	0.10–0.99 μg/g DW	Zhou et al. (2020)
Ferulic acid	Bulb	UPLC-ESI-MS/MS	2.79–24.40 μg/g DW	Zhou et al. (2020)
Triethyl citrate	Bulb	UPLC-ESI-MS/MS	0.15–8.90 μg/g DW	Zhou et al. (2020)
Pivmecillinam	Bulb	UPLC-ESI-MS/MS	0.41–11.56 μg/g DW	Zhou et al. (2020)
Pyruvic acid	Bulb	UPLC-ESI-MS/MS	1.45–2.96 μg/g DW	Zhou et al. (2020)
Ureidoisobutyricacis	Bulb	UPLC-ESI-MS/MS	0.51–0.70 μg/g DW	Zhou et al. (2020)
Valaciclovir acid	Bulb	UPLC-ESI-MS/MS	1.68–15.01 μg/g DW	Zhou et al. (2020)
Alkaloids
Ergoloid	Bulb	UPLC-ESI-MS/MS	4.17–14.91 μg/g DW	Zhou et al. (2020)
Galantamine	Bulb	UPLC-ESI-MS/MS	0.15–1.73 μg/g DW	Zhou et al. (2020)
Harman	Bulb	UPLC-ESI-MS/MS	3.08–7.59 μg/g DW	Zhou et al. (2020)
3-Hydroxyquinine	Bulb	UPLC-ESI-MS/MS	0.39–4.18 μg/g DW	Zhou et al. (2020)
Morphine-6-Glucuronide	Bulb	UPLC-ESI-MS/MS	0.46–58.31 μg/g DW	Zhou et al. (2020)
Fructooligosaccharides	Bulb	HPAEC	27.1 mg/g	Benkeblia et al. (2002)
Fructooligosaccharides	Bulb	HPLC	24.2–26.3 mg/g	Benkeblia et al. (2004)
Fructooligosaccharides	Bulb	HPLC	1.1–3.1 mg/g	Campbell et al. (1997)
Fructooligosaccharides	Bulb	HPLC	1.20–1.27 mg/g FW	Benkeblia and Shiomi (2006b)
Fructooligosaccharides	Bulb	HPLC	0.20–0.46 mg/g FW	Shiomi et al. (2005)
Saponins
TropeosideA1	Bulb (red)	HPLC	26.4 mg/kg FW	Corea et al. (2005)
TropeosideA2	Bulb (red)	HPLC	38.0 mg/kg FW	Corea et al. (2005)
TropeosideB1	Bulb (red)	HPLC	46.1 mg/kg FW	Corea et al. (2005)
TropeosideB2	Bulb (red)	HPLC	60.3 mg/kg FW	Corea, Fattorusso, Lanzotti, Capasso, and Izzo (2005)
Ceposide A	Bulb (white)	TLC followed by HPLC	9.8 mg/kg FW	Lanzotti et al. (2012)
Ceposide B	Bulb (white)	TLC followed by HPLC	3.0 mg/kg FW	Lanzotti et al. (2012)
Ceposide C	Bulb (white)	TLC followed by HPLC	2.1 mg/kg FW	Lanzotti et al. (2012)
Ascalonicoside A	Bulb (white)	TLC followed by HPLC	1.4 mg/kg FW	Lanzotti et al. (2012)

DW, dry weight basis; FW, fresh weight basis; GC-MS, gas chromatography- Mass spectroscopy; HPAEC, high-performance anion exchange chromatography; HPLC, high-performance liquid chromatography; TLC, thin-layer chromatography.

Thiosulfinates and organosulfur compounds

Like other Allium species onion is one of the major sources of flavoring OSCs, including thiosulfinates. Enzyme alliinase forms thiosulfinates by catalytic action, which produces lachrymatory compounds and onion flavor (Benkeblia & Lanzotti, 2007a). Total thiosulfinates may differ according to the color of onion cultivars (Benkeblia & Lanzotti, 2007a). As per a high-performance liquid chromatography (HPLC) analysis, the red, yellow, and white onions had 0.20, 0.35, and 0.14 μmol/g FW of thiosulfinates, respectively (Block et al., 1992). Depending on the reaction conditions, thiosulfinates can convert into several sulfur compounds, such as diallyl, vinyldithiins, diethyl mono, di, tri, tetra, penta, hexa sulfides, methyl allyl, and (E)- and (Z)-ajoene.

From the chemical point of view, the OSCs of onion include diallyl monosulfide (DMS), diallyl disulfide (DADS), diallyl trisulfide (DATS), and diallyl tetrasulfide (DTTS) (Pareek et al., 2017). Onion also contains other sulfur-containing compounds, such as S-propyl-l-cysteine sulfoxide, S-methyl-l-cysteine sulfoxide, and S-propenyl-l-cysteine sulfoxide. In addition, sulfoxides, such as (+)-S-(1-propenyl)-L-cysteine sulfoxide (PRENCSO) and (+) S-methyl-L-cysteine sulfoxide (MCSO) are also known as sulfur-containing compounds (Bystrická et al., 2013; Mateljan, 2015). Indeed, S-alk(en)yl-l-cysteine sulfoxides (ACSOs) are formed from (+)-S-(trans-1-propenyl)-L-cysteine and S-alk(en)yl-cysteines, such as (+)-S-allyl-L-cysteine (SAC), which are parts of γ-glutamyl peptides. All these molecules are responsible for the peculiar flavor (pungency) of onion and formed on the disruption of onion tissues triggering the conversion reaction (Velisek et al., 2006). Proelyn-L-cysteine sulfoxide is a precursor of lachrymatory and flavor attributes of onion, which irritate few animals. In their study, Kim et al. (2018) investigated onion for ACSOs and found isoallin concentration ranging from 8.42 to 0.18 mg/g dry weight (DW). Loredana et al. (2019) examined four onion varieties, namely, Alife Onion, Montoro Onion, tapered shape Vatolla onion, and spinning top Vatolla onion from the Italian Mediterranean area. They recorded 963.90 ± 69.88, 1322.79 ± 253.22, 1368.63 ± 127.60, and 1393.15 ± 156.48 mg/kg concentration of OSCs (DW basis), respectively. Similarly, Zhou et al. (2020) investigated three types of onions, that is, “onion,” “Storey onion’,” and “Welsh onion” for OSCs and they obtained the highest amount in “Storey onion” (962.20 μg/g FW) followed by “onion” (634.65 μg/g) and “Welsh onion” (606.48 μg/g). Nevertheless, the variation in the content of different OSCs is related to growing conditions, cultivar types, and geographical locations. In addition, onion color also affects the level of OSCs (41). Zamri and Hamid (2019) carried out an experiment using ultrahigh-performance liquid chromatography- quadrupole time-of-flight mass spectrometry (UPLC-QTOF/MS) to detect OSCs in onion and reported organosulfur ions in the range of 302–16,435. S-allylcysteine (SAC) and S-allylmercaptocysteine (SAMC) were reported as stable and primary OSCs with higher organosulfur ions, that is, 5263 and 16,435, respectively, by UPLC-QTOF/MS. The solubility, polarity, and chemical reactivity of SAC and SAMC with solvent were the key factors behind the differences between their concentrations. Additionally, these two (SAMC and SAC) were reported as predominant OSCs in onion (Lanzotti et al., 2014). On the other hand, numerous studies showed a positive relationship between antioxidant activity and OSCs in Allium species. Chemically, OSCs act by inhibiting the production system of reactive oxygen species (ROS), namely, nicotinamide adenine dinucleotide phosphate (NADPH) oxide and prevent the deterioration of antioxidant enzymes like glutathione S-transferase, which lower the level of ROS (Kim et al., 2018; Yin et al., 2002).

Phenolics and flavonoids

Onion is a source of phenolics that derive through cinnamic or benzoic acid. Phenolics are found either in free or bound form and are responsible for the color, taste, bitterness, and aroma of plants. Hydroxycinnamic acids (HCs), monocyclic phenylpropanoids having a C6–C3 skeleton, are found in plants and exemplified by ferulic, sinapic acids, p-coumaric, and caffeic acid. In onion, HCs are found in the form of protocatechuic, ferulic acid, 1-O-ß-D-glucoside, p-coumaric, and vanillic acids either free or conjugated (Lachman et al., 2003). In this regard, purple, red, yellow, and green cultivars of onion were evaluated for total phenols and it was noted that the purple cultivar contained the highest content of phenols (47.3 mg/100 g FW) followed by red (44 mg/100 g FW) and yellow (34.7 mg/100 g FW), whereas green cultivar had the lowest phenolics content (30 mg/100 g FW) (Benkeblia, 2005). Kaur et al. (2010) investigated 34 Indian genotypes of white-, pink-, and red-colored onions and recorded the highest phenolics in red cultivars (867.8 mg/kg FW) followed by pink (702.0 mg/kg FW) and white (165.0 mg/kg FW). Similarly, Sagar et al. (2020) analyzed the skin of fifteen Indian cultivars and found that total phenolics content values in dark red (289.04 mg GAE/g DW), pink (231.73 mg GAE/g DW), and white (19.74 mg GAE/g DW) cultivars were in agreement with other studies, and these results confirm that the concentration of phenols is positively correlated with the color of onion cultivars. The extraction solvents and experimental conditions such as time, temperature, and pressure may also affect the level of phenolic compounds.

Flavonoids, a more complex class of phenolics, are categorized into catechins (flavan-3-ols), flavonols, flavanones, flavanonols, leucoanthocyanidins (flavan-3,4-diols), and anthocyanidins (VELÍŠEK, 2002). In onion, two types of flavonoids, such as anthocyanins and flavonols, are found. Various flavonols have been reported in onions such as quercetin, isorhamnetin, and kaempferol, and quercetin is considered to be the main flavonol (Lanzotti, 2006; Panche et al., 2016; Slimestad et al., 2007). Quercetin is found in onion under both and conjugated forms, and monoglycosides and diglycosides of quercetin comprise 93% of the total flavonols (Lombard et al., 2005). Indeed, about 25 different types of flavonols as derivatives of quercetin have been reported in onion cultivars (Slimestad et al., 2007). Sellappan and Akoh (2002) agreed that quercetin is the main flavonoid and is found under conjugated forms such as quercetin 4′ glucoside and quercetin 3,4′ diglucoside. Two other glycosides of quercetin, quercetin 3,4-O-ß-diglucoside and quercetin 4-O-ß-glucoside, were identified with bioactive effects during oxidative stress (Gülşen et al., 2007; Ioku et al., 2001). From the nutritional point of view, quercetin is an important diet ingredient for humans and is present in most fruits and vegetables. However, onion contains 5–10-folds concentration (300 mg/kg) compared to many other horticultural crops such as broccoli (100 mg/kg), apples (50 mg/kg), and blueberries (40 mg/kg) (Hollman & Arts, 2000; Liguori et al., 2017; Mojzer et al., 2016).

Kaempferol and luteolin are also found in onion as flavonols (Lanzotti, 2006), and red cultivars contain a higher concentration of flavonols and anthocyanin glycosides in the form of peonidin, cyanidin, and pelargonidin (Bystrická et al., 2013; Wu & Prior, 2005a. Anthocyanins are another class of flavonoids found in onions but in a lesser amount compare to flavonols, whereas red onion contains mainly anthocyanins (250 mg/kg) (Ren et al., 2020). In their study, Fossen and Andersen (2003) identified 25 different anthocyanins, including two novel derivatives of 5-carboxypyranocyanidin in red onion cultivars, corresponding to c.a. 10% of total flavonoids. In another study, Fredotović et al. (2017) analyzed methanolic extract of onion and quantified myricetin and isorhamnetin and found 8.02 mg/100 g DW and 4.74 mg/100 g DW concentration, respectively. However, the type and concentration of solvent, cultivar, and experimental and conditions time may impact the flavonoids content of onion. Recently, Zhou et al. (2020) evaluated onion for various flavonoids such as dalbergin, isorhamnetin, rhamnazin, xanthomicrol, kuwanon E, and leucopelargonidin and recorded 7.59, 48.56, 0.25, 4.48, 48.27, and 0.46 μg/g (DW) concentration for the same, respectively. Biologically, polyphenols and flavonoids are known to possess antioxidant activity due to hydroxylated aromatic rings (Bystrická et al., 2013; Nuutila et al., 2003). Main flavonoids of onions have been described below.

Quercetin

Quercetin is made up to five hydroxyl groups, which is responsible for possible number of derivatives and biological activity of the compound (Figure 2a). Major groups of quercetin derivatives are ethers and glycosides, while prenyl and sulfate substituents are present in fewer amounts.

FIGURE 2. Main flavonoids of onion: (a) quercetin, (b) kaempferol, (c) luteolin, and (d) anthocyanin

Kaempferol

It is a tetrahydroxyflavone having four hydroxyl groups present on 3, 5, 7, and 4′ positions (Figure 2b). Being a tetrahydroxyflavoneand 7-hydroxyflavonol, it falls under flavonols family. It comes from kaempferol oxoanion as conjugate acid (Jia & Liu, 2013).

Luteolin

It is a tetrahydroxyflavone same as kaempferol but it contains four hydroxyl groups located at 3′, 4′ 5, and 7 (Figure 2c). According to the structure, it is also known as 3′-hydroxyflavonoid.

Anthocyanins

Anthocyanins are generally found in epidermal layer of plant cell. It is composed of a molecular region with three aromatic rings that has one or more attached sugar moiety. The basic structure of anthocyanins has flavylium cation (2-phenylbenzopyrilium) links with methoxyl (-OCH₃) or hydroxyl (-OH) and one or more sugar units (Figure 2d). It comprises mainly 3-glucosides that is categorized into the sugar free anthocyanin glycosides and anthocyanidin aglycones.

Ascorbic acid

Onion has also been reported to contain ascorbic acid (vitamin C). Red and yellow Korean onions were assessed for ascorbic acid content and red cultivar showed higher concentrations (28.34 mg/100 g FW) than yellow ones (19.20 mg/100 g FW), and it was shown that color variation in cultivars affects ascorbic acid levels (Jeong et al., 2006). Lachman et al. (1999) measured ascorbic acid content and values ranged from 60–2703 mg/kg to 390–5000 mg/kg DW for onion bulb and leaves, respectively. Physical treatment also affects ascorbic acid content. For example, raw onion bulbs had a higher quantity of ascorbic acid (35 mg/kg DW) in comparison with microwaved onion (1 min at 1000 W), which contained 29 mg/kg DW (Sun-Waterhouse et al., 2008).

Fructooligosaccharides

FOS are the main nonstructural carbohydrate storage in onion. Among 60 different vegetables, onion was reported to contain the highest amount of FOS, which has been shown to increase the health-promoting gut microflora (Roberfroid, 2007). In onion, FOSs are composed of several fructofuranosyl sucrose subunits, that is, 1-kestose, neokestose, nystose, other linear (inulin), and branched (inulin neo-series) polymers (Shiomi et al., 2005). The analysis of different varieties showed that the intense color of onion can be correlated with higher content of FOS (Benkeblia & Shiomi, 2006a). Different studies reported a variation in FOS content of different onion varieties. In red onion cultivars, total FOSs averaged 27.1 mg/g FW (Benkeblia et al., 2002) and in yellow cultivars, it was found between 24.2 and 26.3 mg/g FW (Benkeblia et al., 2004; Benkeblia et al., 2005b), whereas white cultivar and welsh onion had 3.1 and 1.1 mg/g FW of FOS, respectively (Campbell et al., 1997). Jaime et al. (2001) investigated FOS in five Spanish onion cultivars and reported a content ranging from 23.3 to 141.9 g/kg DW. Similarly, Galdón et al. (2009) analyzed six Spanish onion cultivars and they reported FOS content from 0.84% to 3.04% g/100 FW. Pöhnl et al. (2017) performed a comparative study between HPAEC-PAD and UHPLC-ELSD techniques for better recovery of FOS in onion and obtained 41 saccharides, including FOS (up to 20 DP) by both techniques. Although, UHPLC had a shorter run time, with HPAEC-PAD superior DP of FOS with lower quantification limits (0.12–2.3 mg/L) was detected compared to UHPLC-ELSD (34–68 mg/L). Furthermore, the baseline separation of FOS was also reported better in HPAEC-PAD; however, the type of instruments, extraction solvents, and detector may affect the recovery of FOS in onions.

However, it is important to note that the quantification of total fructans in plants has been problematic and some discrepancies have been noted between the different studies published. Although FOS content of the different layers of the bulb varies (Jaime et al., 2001), geographical area, cultivation conditions, and cultivars are among the important factors of the variation of FOS concentration in onion (Galdón et al., 2009; Rodríguez Galdón et al., 2008). Other factors impacting FOS concentration are extraction method and the recovery yield (Downes & Terry, 2010), the analytical technique (Benkeblia, 2013), and the storage period (Benkeblia et al., 2005b; Ohanenye et al., 2019).

BIOLOGICAL FUNCTIONS OF BIOACTIVE COMPONENTS OF ONIONS

Bioactive compounds of onion have been shown to possess numerous attributes and actions that impart health benefits to humans and provide protection from various health ailments, including cancer. OSCs possess anti-allergic, anti-inflammatory, antimicrobial, and antithrombotic activity (Souza et al., 2011; Yoshinari et al., 2012). Onions’ bioactive compounds were reported to have many biological functions, such as antioxidant, antimicrobial, antiviral, anti-inflammatory, anticancer activity, antimutagenic activity, and defense of brain, liver, and heart (Ansari et al., 2009; Assefa et al., 2018; Beretta et al., 2017; Harwood et al., 2007; Jia & Liu, 2013; Quecan et al., 2019; Singh et al., 2009; Utesch et al., 2008). The important biological functions are categorized into in vitro and in vivo, and they have been described separately.

In vitro biological activities Antioxidant activity

Extensive literature reported the antioxidant property of onion flavonoids (Calderon-Montaño et al., 2011; Marrelli et al., 2019; Škerget et al., 2009; Santas et al., 2010). ROSs exogenously damage the cells of various organs (Harwood, 2007; Kumar & Pandey, 2013), and flavonoids stabilize the free electrons evolving from ROS in vitro (Pietta, 2000). The hydroxyl configuration of flavonoids plays a crucial role to stabilize ROS by antioxidant activity. This activity occurs when conjugation takes place between the aromatic rings and a free 3-OH by heterocycle of flavonoids initiating antioxidant activity (Ren et al., 2017). Additionally, studies reported that the occurrence, number, and position of sugar residues have an important role in antioxidant activity (Ye et al., 2013). Flavonoids also have a metal-chelating quality that inhibits the formation of free radicals (Mishra et al., 2013, 2013). It is proven that quercetin flavonol plays a better role as an antioxidant compared to selenium and sulfur-containing amino acids of onion (Rodrigue et al., 2017). Kaempferol is also regarded as a potent antioxidant because it induces the production of many antioxidant enzymes, such as superoxide, catalase, and dismutase. Kaempferol has also been shown to prevent low-density lipid protein (LDLP) oxidation for giving protection in atherosclerosis condition (Calderon-Montaño et al., 2011). Two white and one yellow skinned Spanish onion cultivars were evaluated for antioxidant potential, and this activity ranged from 4.55 to 74.86 μmol trolox/g DW) (Santas et al., 2010). Similarly, three different colored Indian onion cultivars were assessed for their antioxidant activities and results showed that the maximum activity was recorded in red-colored onion (Nile & Park, 2013). In another study, Benmalek et al. (2013) experimented with Allium cepa varieties for radical scavenging potential and obtained 2.91 × 10⁻⁵ mg/mL (IC₅₀) for the outer layer of red onion. Recently, Zhou et al. (2020) revealed that aqueous extract of onion exhibited better DPPH (2,2-diphenyl-1-picrylhydrazyl) (IC₅₀ = 1.24 ± 0.52 mg/mL) and antioxidant activity (IC₅₀ = 1.64 ± 0.64 mg/mL) compared to Welsh onion (4.0 ± 0.56 mg/mL). However, these biological activities might vary with some factors, such as the color, cultivar type, solvents, and extraction conditions (Benítez et al., 2011). The incubation period of mixed solution might also play a significant role in the reaction of DPPH molecules, responsible for higher antioxidant potential.

Onion skin contains a significantly higher level of phenols and flavonoids, which consequently imparts better antioxidant activity than the edible portion (Duan et al., 2015). Škerget et al. (2009) investigated the ethanol and acetone extracts of skin and edible portions of red onion and found the highest antioxidant activity in skins are extracted with 60% ethanol and using β-carotene bleaching test.

Indeed, organic solvents such as ethanol and methanol gave better solubility of flavonoids than water, which leads to higher phenolic extraction and antioxidant activity as well (Tram Ngoc et al., 2005). In a study, water and 95% EtOH extracts of onion skin and bulb were prepared for antioxidant capacity using oxygen radical absorbance capacity (ORAC) test, and a higher ORAC value was detected for onion skin extracted with 95% ethanol (Kim et al., 2010). Lee et al. (2014) compared hot water (60℃), hot ethanol (60℃), and two subcritical water extracts (110 and 165℃) extracts of onion peels, and they showed that ethanolic peel extract exhibited the best DPPH activity (72.25%) compared to the other samples. These studies highlight well the importance of solvent in extracting the phenolic compounds particularly from the dry outer onion layers of onion, which showed higher antioxidant activity due to the high level of quercetin and its derivatives, and this extraction is improved by combining temperature and ethanol that disrupt onion skin cells much easier increasing their solubility in comparison with water. Hence, these numerous studies show well that either the bulb or the outer skins of onion can be a valuable and good source of phenolic compounds as functional food and ethnomedicine (Ly et al., 2005).

Antimicrobial activity

Extensive literature reported the antimicrobial activities of phenolics and flavonoids (Kumar & Pandey, 2013; Pandey et al., 2010; Santas et al., 2010) (Table 2). Different mechanisms have been suggested to decipher the inhibition activities of onion compounds. The first inhibition mechanism involves the catalysis prevention of microbial enzymes, adhesins, and transport of proteins during growth (Mishra et al., 2009). Allium bioactive compounds have also been reported to decrease oxygen supply causing the breaking of the fungal cell wall that results in retarding the cell growth (Gupta & Porter, 2001).

TABLE 2 Antimicrobial activities of onion bioactive compounds against different pathogenic organisms

Antimicrobial activity	Effective bioactive compound	Methoda	Inhibition	Inhibited organism	References
Antibacterial	Organosulfur compounds	Disk-diffusion assay: 50, 100, 200, 300, and 500 mL/L of onion essential oil extract	7.1 ± 0.2–11.1 ± 0.4 mm	Salmonella enteritidis	Benkeblia (2004)
Antibacterial	Organosulfur compounds	Disk-diffusion assay: 50, 100, 200, 300, and 500 mL/L of onion essential oil extract	5.6 ± 0.2–8.8 ± 0.7 mm	Staphylococcus aureus	Benkeblia (2004)
Antibacterial	Quercetin	Disk-diffusion method: 10 mg/mL yellow onion quercetin in DMSO (dimethyl sulfoxide)	11 mm	Helicobacter pylori	Ramos et al. (2006)
Antibacterial	Quercetin	Disk-diffusion method: 10 mg/mL yellow onion quercetin in DMSO (dimethyl sulfoxide)	13 mm	Staphylococcus aureus	Ramos et al. (2006)
Antibacterial	Phenols	Disk-diffusion test: 50, 100, 200, 300, and 500 mL/L onion extract	6.5 ± 0.7–10.4 ± 0.4 mm	Salmonella enteritidis	Benkeblia (2005)
Antibacterial	Phenols	Disk-diffusion test: 50, 100, 200, 300, and 500 mL/L onion extract	6.5 ± 0.7–10.1 ± 0.7 mm	Staphylococcus aureus	Benkeblia (2005)
Antibacterial	Flavonoids, phenols	Disk-diffusion assay: 5 mg/mL onion powder mixed in DMSO	9.5 ± 0.7 mm	Bacillus cereus	Santas et al. (2010)
Antibacterial	Flavonoids, phenols	Disk-diffusion assay: 5 mg/mL onion powder mixed in DMSO	10.5 ± 0.5 mm	Staphylococcus aureus	Santas et al. (2010)
Antibacterial	Flavonoids, phenols	Disk-diffusion assay: 5 mg/mL onion powder mixed in DMSO	12.0 ± 0.0 mm	Listeria monocytogenes	Santas et al. (2010)
Antibacterial	Flavonoids	Agar-well diffusion assay: 1, 10, 50, 100, and 1000 μL onion extract	13 ± 0.7–29 ± 1.0 mm	Staphylococcus aureus	Eltaweel (2013)
Antibacterial	Flavonoids	Disk-diffusion method: 20 μL onion extract of six varieties	5.6 ± 0.4–11.18 ± 0.7 mm	E. coli	Sharma et al. (2018)
Antibacterial	Flavonoids	Disk-diffusion method: 20 μL onion extract of six varieties	8.0 ± 0.5–11.5 ± 0.7 mm	Pseudomonas aeruginosa	Sharma et al. (2018)
Antibacterial	Flavonoids	Disk-diffusion method: 20 μL onion extract of six varieties	8.0 ± 0.6–13.5 ± 0.09 mm	Staphylococcus aureus	Sharma et al. (2018)
Antibacterial	Flavonoids	Disk-diffusion method: 20 μL onion extract of six varieties	8.0 ± 0.6–11.2 ± 0.4 mm	Bacillus cereus	Sharma et al. (2018)
Antibacterial	Flavonoids	Disk-diffusion assay: 2–8 μg onion bulb extract	4.66 ± 0.57–16.66 ± 0.57 mm	Pseudomonas aeruginosa	Loredana et al. (2019)
Antibacterial	Flavonoids	Disk-diffusion assay: 2–8 μg onion bulb extract	4.33 ± 0.57–6.33 ± 0.57 mm	Staphylococcus aureus	Loredana et al. (2019)
Antibacterial	Flavonoids	Disk-diffusion assay: 2–8 μg onion bulb extract	4.66 ± 0.57–14.33 ± 0.57 mm	Bacillus cereus	Loredana et al. (2019)
Antibacterial	Polysaccharides	Agar disk-diffusion method: 0.25%, 0.50%, 0.75%, 1%, 1.25%, 1.50%, 1.75%, and 2% concentration of extracted onion polysaccharide	6.25–13.0 mm	E. coli	Ma et al. (2018)
Antibacterial	Polysaccharides	Agar disk-diffusion method: 0.25%, 0.50%, 0.75%, 1%, 1.25%, 1.50%, 1.75%, and 2% concentration of extracted onion polysaccharide	6.12–14.22 mm	B. subtilis	Ma et al. (2018)
Antibacterial	Polysaccharides	Agar disk-diffusion method: 0.25%, 0.50%, 0.75%, 1%, 1.25%, 1.50%, 1.75%, and 2% concentration of extracted onion polysaccharide	6.21–13.42 mm	S. aureus	Ma et al. (2018)
Antibacterial	Polysaccharides	Agar disk-diffusion method: 0.25%, 0.50%, 0.75%, 1%, 1.25%, 1.50%, 1.75%, and 2% concentration of extracted onion polysaccharide	6.11–13.91 mm	S. typhimurium	Ma et al. (2018)
Antibacterial	Polyphenols	Agar well-diffusion assay: 100 μL onion juice	13 mm	Salmonella typhimurium	Sharma (2015)
Antibacterial	Polyphenols	Agar well-diffusion assay: 100 μL onion juice	23 mm	Klebsiella pnuemoniae	Sharma (2015)
Antibacterial	Polyphenols	Agar well-diffusion assay: 100 μL onion juice	14 mm	Staphylococcus aureus	Sharma (2015)
Antibacterial	Polyphenols	Agar well-diffusion assay: 100 μL onion juice	18 mm	E. coli	Sharma (2015)
Antibacterial	Polyphenols	Agar well-diffusion assay: 100 μL onion juice	14 mm	Bacillus cereus	Sharma (2015)
Antibacterial	Flavonoids	Agar well-diffusion assay: 25 μL onion skin extract	15 mm	Salmonella typhimurium	Sagar and Pareek (2020)
Antibacterial	Flavonoids	Agar well-diffusion assay: 25 μL onion skin extract	11 mm	Bacillus cereus	Sagar and Pareek (2020)
Antiviral	Polyphenols	Chiang's MTT method: both adenoviruses were seeded in 24-well microplate and 80 – 3500 μg/mL onion crude extract	0–13.64%	ADV3	Chen et al. (2011)
Antiviral	Polyphenols	Chiang's MTT method: both adenoviruses were seeded in 24-well microplate and 80–3500 μg/mL onion crude extract	1.2–15.7%	ADV41	Chen et al. (2011)
Antiviral	Flavonols	Plaque assay: onion extract (10 and 50 μL/mL) was mixed with 6.5 log10 PFU of HAV mixture was added to fetal rhesus monkey kidney (FRhK-4) cells	HAV reduced up to 2 log10 PFU/mL and 0.4 log10 PFU/mL for 10 and 50 μL/mL extract	Hepatitis A virus (HAV)	Seo et al. (2017)
Antifungal	Flavonoids, organosulfur compounds	Agar-well assay: 2, 5, and 10 mg/mL quantity of onion extract	3.2, 4.5, and 4 mm for 2, 5, and 10 mg/mL concentration, respectively	Fusarium oxysporum	Cornago et al. (2011)
Antifungal	Flavonoids, organosulfur compounds	Agar-well assay: 2, 5, and 10 mg/mL quantity of onion extract	1, 3, and 4.3 mm for 2, 5, and 10 mg/mL concentration, respectively	Colletotrichum sp.	Cornago et al. (2011)
Antifungal	Quercetin	Agar-well diffusion assay: 0.25%, 0.5%, and 1% concentration of fresh extract	13–26.3 mm	C. albicans	Mahmoudabadi and Nasery (2009)
Antifungal	Quercetin	Agar-well diffusion assay: 0.25%, 0.5%, and 1% concentration of fresh extract	29.3–39.3 mm	Alternaria sp.	Mahmoudabadi and Nasery (2009)
Antifungal	Quercetin	Agar-well diffusion assay: 0.25%, 0.5%, and 1% concentration of fresh extract	29–38 mm	Drechslera sp.	Mahmoudabadi and Nasery (2009)
Antifungal	Quercetin	Agar-well diffusion assay: 0.25%, 0.5%, and 1% concentration of fresh extract	16.3–28.7 mm	Penicillium sp.	Mahmoudabadi and Nasery (2009)
Antifungal	Quercetin	Agar-well diffusion assay: 0.25%, 0.5%, and 1% concentration of fresh extract	12.7–29 mm	Cladosporium sp.	Mahmoudabadi and Nasery (2009)
Antifungal	Quercetin	Agar-well diffusion assay: 0.25%, 0.5%, and 1% concentration of fresh extract	18.7–22 mm	Aspergillus niger	Mahmoudabadi and Nasery (2009)
Antifungal	Quercetin	Agar-well diffusion assay: 0.25%, 0.5%, and 1% concentration of fresh extract	11–29 mm	M. gypseum	Mahmoudabadi and Nasery (2009)
Antifungal	Quercetin	Agar-well diffusion assay: 0.25%, 0.5%, and 1% concentration of fresh extract	15.3–26 mm	E. floccosum	Mahmoudabadi and Nasery (2009)
Antifungal	Quercetin	Agar disk-diffusion method: 0.5–2 mg/mL onion quercetin chitosan gel	5–14 mm	Trichophyton rubrum	George et al. (2019)
Antiparasitic	Polyphenols	96-microtiter plate method: 10–0.07 mg/mL serial dilution of onion extract into wells and 105 parasites (promastigotes stage) added with 100 μL culture medium followed by 72 h incubation at 25℃	Concentration of 1.25 mg/mL showed IC100 (100%) mortality	Leishmania sp., Trichomonas vaginalis	Saleheen et al. (2004)

a

Conditions for antibacterial (nutrient agar medium and incubation at 30 ± 5℃ for 24 h) and antifungal activity (Potato dextrose agar [PDA] medium and incubation at 20 ± 5℃ for 48–72 h).

Onion extracts were reported to inhibit the growth of both Gram-positive and Gram-negative bacteria, that is, Bacillus cereus, Staphylococcus aureus, Pseudomonas aeruginosa, Salmonella typhimurium, and Klebsiella pnuemoniae (Eltaweel, 2013; Induja & Geetha, 2018; Loredana et al., 2019; Santas et al., 2010; Sharma et al., 2015; Sharma et al., 2018). Comparatively, quercetin has been a most extensively studied flavonoid, which retarded the growth of numerous bacteria (Wu et al., 2008), although kaempferol was also showed inhibition against Helicobacter pylori (Calderon-Montaño et al., 2011). Along with quercetin, sulfur compounds were also reported as antimicrobial agents against (multidrug resistant) S. aureus (MRSA) (Škerget et al., 2009), whereas syringaresinol, 4-O-methylquercetin, and phloroglucinol-3,4-dihydroxybenzoate were marked as mild antibacterial components against H. pylori (Ramos et al., 2006). Sulfur compounds also showed antimicrobial activity; however, their use is limited due to their pungent property, strong flavor, and volatile nature (Rose et al., 2005). A combination of aqueous onion extract and bacteriocin (divergicin M 35) was used to inhibit Listeria monocytogenes and this combination inhibited the bacterium completely due to the antagonist activity (Zouhir et al., 2008). Other onion metabolites, such as saponins and proteins, also exhibited antimicrobial activity (Griffiths et al., 2002).

The antifungal activity and the growth inhibition of various fungal pathogens were also reported by the actions of onion bioactive compounds. Diallyl sulfide, DADS, and DATS were reported as active antifungal components in onion extract (Corzo-Martínez et al., 2007). Irkin and Korukluoglu (2007, 2009) prepared acetone and ethanolic extracts from onion bulb and tested on the growth of Aspergillus niger, Candida albicans (ATCC 10231), Fusarium oxysporum, and Metschnikowia fructicola. Total phenols of onion and fungicidal potential against Rhyzopus oryzae were highly correlated (De Souza et al., 2010); however, the antifungal activity depends on the extracting solvent used, because ethyl acetate and methanolic extracts showed higher inhibition compared to aqueous acetate. Interestingly, purified Ace-AMP1 onion protein expressed in Escherichia coli was tested in-vitro against various pathogens such as F. oxysporum f. sp. vasinfectum, Alternaria solani, and Verticilium dahlia, and a significant decrease was observed in the growth (Wu et al., 2011).

The antiviral activity of flavonoids was also subjected to many studies (Arshad et al., 2017; Corzo-Martínez et al., 2007; Goren et al., 2002), and its active mechanisms consist of destroying or blocking viral proteins synthesis (Zandi et al., 2011). Quercetin and kaempferol were reported to have a prominent inhibitory role against various viruses, such as rabies virus, polio virus, megno virus, herpes simplex type I virus, sindbis virus, parainfluenza type 3 virus, pseudorabies, and various respiratory viruses (Calderon-Montaño et al., 2011; Chen et al., 2006; Chen et al., 2011; Chiang et al., 2003; Cushnie & Lamb, 2005; Goren et al., 2002; Kumar & Pandey, 2013). Onion, shallot, garlic, and green onion extracts were evaluated in vitro for their antiviral activities against adenoviruses (ADV41 and ADV3) and significant antiviral results were observed. As onion contains the highest content of quercetin, this compound was also found to be responsible for this antiviral action (Chen et al., 2011), and a cell culture study revealed that quercetin may have an inhibition role against various respiratory viruses (Chen, 2006; Chiang et al., 2003).

Adipogenesis inhibition

Adipogenesis is the formation of subcutaneous adipose tissue resulting from the accumulation of fats, which leads to obesity and many associated diseases. In an experiment, Yoshinari et al. (2012) tested concentrated onion extract on preadipocyte cells incubated for 7 days, and interestingly, lipid accumulation was inhibited. Similarly, quercetin showed a similar effect and the application of rich peel extract of onion on 3T3-L1 preadipocytes cells inhibited both adipogenesis and conversion to adipocytes (Moon et al., 2013). Adipogenesis suppression activity of quercetin-rich onion peel was also reported by Bae et al. (2014), and their results showed that powdered onion skins not only significantly reduced the accumulation of lipids in treated cells, but also suppressed other effectors (C/EBPα and PPAR-√) responsible for adipogenesis.

In vivo effects Antiproliferative activity

Malignant tumors are the main health problem globally (Sawadogo et al., 2012), and it is well admitted that DNA damage caused by oxidative stress is one of the main causes of various cancer types (Waris & Ahsan, 2006). Among various animal studies, the putative effects of onion bioactive compounds against different carcinogenic factors have been well recognized (Table 3). Several studies reported the association between onion consumption and decreased risk of colorectal (Taché et al., 2007), liver (Fukushima et al., 2001), skin (Byun et al., 2010), breast (Kumar & Pandey, 2013), lung (Le Marchand et al., 2000), and prostate cancers (Hsing et al., 2002). More specifically, studies showed the effective role of flavonoids in preventing the risk of cancer (Bianchini & Vainio, 2001; Kamaraj et al., 2009; Linsalata et al., 2010) by inhibiting the tyrosine kinase, regulating p53 protein, blocking the expression of Ras protein, and heat shock protein inhibition (Duthie et al., 2000). In a meta-analysis conducted by Zhou et al. (2011), consumption of Allium vegetables (20 g/day) decreased stomach cancer, whereas onion consumption up to 12 months lowered by 95% the risk of colorectal adenoma (Millen et al., 2007), and by 60% the risk of esophageal cancer (Chen et al., 2009). Quercetin was reported as the key flavonoid for the anticarcinogenic effect. Moreover, quercetin and its derivatives were also found as potential carcinogenic agents against skin, lung, prostate, and liver cancers (Arung et al., 2011; Le Marchand et al., 2000; Vijayababu et al., 2006). Besides quercetin, many animal studies reported the antiangiogenic, antiproliferative, and antimetastatic of luteolin (Bagli et al., 2004; Lee et al., 2006; Manju et al., 2005). For instance, Byun et al. (2010) found that luteolin suppressed skin cancer by inhibiting two protein kinases (c-Src and PKCε) activity in rats. Kaempferol as another Allium phenolic compound has also been shown to act as a preventive agent against lung cancer, ovarian cancer, prostate, breast, leukemia, pancreatic, and bladder cancers (Calderon-Montaño et al., 2011; Chen et al., 2012; Jaganathan & Mandal, 2009; Kim et al., 2013; Nöthlings et al., 2007). In their study, Bang and Kim (2010) fed rats with onion and results showed that preneoplastic liver lesions were less and hepatocellular carcinogenesis was inhibited through suppression of ROS.

TABLE 3 Biological effects of onion in different models

Activity	Animal/Model	Experimental setupa	Results	References
Hypolipidimic	Alloxan diabetic rats	Isolated S-methyl cysteine sulfoxide (SMCS) was fed orally to Sprague–Dawley rats (200 mg/kg body weight) up to 45 days on 1% cholesterol diet and compared with control group	Tissue lipid level was decreased from 3000 mg/100 g wet tissue (control group) to 2800 mg/100 g wet tissue (treated group); total cholesterol was also decreased: control (130 mg/100 mL) and treated (110 mg/100 mL)	Kumari and Augusti (2007)
Antidiabetic	Mice	Mice were divided into two groups: control (fed soluble starch—1 g/kg) and treated (fed-soluble starch + 0.5% root extract of Welsh onion); blood samples were analyzed at 60, 120, 180, or 240 min	Lower level of plasma glucose was found intreated group up to 240 min and lowest level (50 mg/dL) was obtained at 60 min in compared to control (100 mg/dL)	Kang et al. (2010)
Antidiabetic	Wistar rats	Three groups of diabetic rats: control (standard diet), group-1 (standard diet + 125 mg/kg/d onion extract), and group-2 (standard diet + 250 mg/kg/d onion extract) were divided and diets were fed orally for 4 weeks	Blood glucose level (BGL) was gradually decreased in the order: control (374.5 mg/dL) > group-1 (217.87 mg/dL) > group-2 (209.5 mg/dL)	Tamtaji et al. (2017)
Antidiabetic	Wistar rats	Extract of red onion scales (ROS) was investigated for its antidiabetic activity Three groups were categorized: group-1 or control (diabetic rats with 50 mg/kg streptozotocin [STZ]), group-2 (50 mg/kg/d STZ + 150 mg/kg/d ROS), and group-3 (50 mg/kg STZ + 300 mg/kg/d ROS). A standard diet was fed to each group up to 4 weeks	Lowest BGL was obtained in group-3 (310.2 ± 5.50 mg/dL) followed by group-2 (352.2 ± 13.11 mg/dL) and control (545.4 ± 24.07 mg/dL); Serum insulin level was increased from 2.86 ± 0.30 (control) to 5.98 ± 0.28 (group-2) and 7.60 ± 0.45 (group-3) in μU/mL	Abouzed et al. (2018)
Hypoglycemic	Alloxan diabetic rabbits	Diabetic rabbits were divided into three groups:group-1/control (Peanut oil 5 mL/kg/d), group-2 (100 mg/kg/d onion extract), and group-3 (300 mg/kg/d onion extract) and oral administration was carried out for 30 days	Control group showed highest BGL (300 mg/dL) whereas treated groups, that is, group-2 and group-3 had lower 140 and 90 mg/dL BGL, respectively	Ogunmodede et al. (2012)
Antihyperglycemic	Rats	Diabetic rats were classified into three groups: control/group-1, group-2, and group-3. Natural food, ripe onion juice (15 mL/kg/d), and tolbutamide (250 mL/kg/d) were orally fed to these groups, respectively, for 7 days.	Group-2 exhibited lower blood glucose concentration (315 ± 8 mg/dL) than control (338 ± 13 mg/dL), whereas group-3 had lowest (276 ± 17 mg/dL) BGL	Lee et al. (2013)
Antiobesity	Zucker diabetic fatty rats	Three groups of rats were made: group-1/control- basal diet (casein, cornstarch, sucrose, corn oil, cellulose, vitamin, and mineral mixture), group-2 (basal diet + 3% onion extract), and group-3 (basal diet + 5% onion extract). Proposed diet was given orally for 28 days at 1 g/kg/d concentration	Total cholesterol (mg/dL) was decreased in the order: group-1 (267 ± 6) > group-2 (258 ± 11) > group-3 (257 ± 6); lowest serum triglyceride (113.3 ± 6.4 mg/dL) was reported in group-3 (5% onion extract), whereas maximum in group-1 (205.5 ± 22 mg/dL); serum-free fatty acids were found highest in group-1 (1.264 ± 0.11 mEq/L) whereas least in group-3 (0.942 ± 0.05 mEq/L)	Yoshinari et al. (2012)
Cardiovascular protection	Wistar rats	Three groups of rats were prepared: group-1/control (100% commercial rodent diet), group-2 (control diet+ 2% cholesterol+0.5% cholic acid), and group-3 (control diet+2% cholesterol+10% onion powder) and fed their respective diets orally for 7 weeks	Lowest triglyceride level was obtained in group-3 (0.369 ± 0.02 mmol/L) in compared to group-2 (0.458 ± 0.03 mmol/L) and control (0.953 ± 0.07 mmol/L)	Colina-Coca et al. (2017)
Lowering serum cholesterol	Human	17 females and 10 males (age- 45.9 ± 10.0 years) were fed 150 mL/L pack onion extract (30 mg quercetin) for 10 weeks	The total cholesterol was decreased from 226.7 ± 4.6 to 206.8 ± 3.6 mg/dL and LDL-cholesterol level (mg/dl) was minimized up to 127.1 ± 4.1 from 151.6 ± 5.0	Lee et al. (2010)
Antiproliferative	Human adenocarcinoma cells	Caco-2 strain of adenocarcinoma cells was treated with onion extract and purified flavonoids (quercetin, kaempferol, and myricetin). Cells with onion extract (100 μL) and growth medium were culture on 96-well plate and compared with flavonoids (100 μM). Untreated cells were used as control	Onion extract worked similarly compared flavonoids and showed strong antiproliferative activity (69–80% decrease in absorbance than untreated cells)	Murayyan et al. (2017)
Antiproliferative	Human cancer cells	Five cancer cell lines (HepG2, SPC-A-1, A549, CNE-1, and MGC80-3) were treated with spirostanolsaponins isolated from Chinese onion and compared with control MRC (macrophage RAW264.7 cells) for antiproliferative activity	Saponins showed significant inhibition in proliferation of cancer cells (IC50 2.22–15.58 μM) than control cells (IC50 > 100 μM)	Wang et al. (2016)
Antiproliferative	Human cell lines	Four cancer cell lines: HeLa (cervix), HT-29 (colon), MCF7 (breast), and MRC-5 (fetal lung) were treated with yellow onion extract (20 μl/well) in 180 μL of culture medium and incubated for 48 h at 37℃ and 5% CO2	Extract treatment showed strong inhibition with low IC50 values in MCF7 (25.44 mg/mL), MRC-5 (30.37 mg/mL), and HeLa (36.39 mg/mL) cell lines that indicated enhancement in cytotoxicity	Simin et al. (2013)
Antiproliferative	Cancer cell lines	Five cancer cell lines: ACHN (human renal carcinoma), Calu 1 (human nonsmall cell lung carcinoma), HCT 116 (human colon carcinoma), H460 (human noncell lung carcinoma), and Panc 1 (human pancreatic carcinoma) were treated with 50 μg of Q, QDG, and QMG, and 5 mg of OSW extracts followed by the incubation at 37℃ and 5% CO2 for 48 h	OSW extracts and Q, QDG, and QMG doses effectively inhibited cancer cell lines up to 60–85% than flavopiridol standard (70–86%)	Nile et al. (2018)
Antipeptic ulcer	Rats	Rats with gastric ulcer were divided into four groups: group 1 (1% sodium carboxy-methylcellulose, 2 mL/kg), group-2 (crushed famotidine tablet 12 mg/kg), group-3 (famotidine alginate bead 12 mg/kg), and group-4 (famotidine aginate bead + quercetin 12 mg/kg) and given diet was fed for 15 days	Maximum gastric ulcer index (GUI) was observed in group-1 (42%), whereas other groups showed lower GUI, that is, 30% (group-2), 18% (group-3), and 8%(group-4)	Abourehab et al. (2015)
Immuno-protective	Wistar rats	Cyclophosphamide (CP) immunosuppressed rats were divided into four groups: control/group-1 (100 mg/kg CP), group-2 (100 mg/kg CP + 1 μg onion lection/rat), group-3 (100 mg/kg CP + 10 μg onion lection/rat), and group-4 (100 mg/kg CP + 100 μg onion lection/rat) and oral administration was carried out for 28 days	Onion lectin restored higher lymphoid cell counts (∼ sixfold) in treated groups than control; onion lectin promoted immune response (1.5–2-fold higher) in group-3 and group-4 than control group.	Kumar and Venkatesh (2016)
Immunomodulatory	Wistar rats	Isolated thymocytes and splenocytes were cultured with onion fructo-oligosaccharides (FOS)at 0.5, 5, 50, or 250 μg concentration at CO2 incubator (37℃, 80% humidity, 5% CO2) for 72 h. The MTT# reagent dose (20 μL of 5 mg/mL) was also given before study	Significant increase (threefold) was observed in proliferation of cells with 250 μg FOS; FOS also elevated the production (∼2.5-fold) of nitric oxide and phagocytic activity of peritoneal exudates cells	Kumar et al. (2015)
Anticancerous	Human cancer cells (colon)	Adenocarcinoma cells (2×105 cells/5 mL density) were incubated on tissue culture dish (60 mm) containing 10% fetal bovine serum for 24 h. Then, medium was replaced with quercetin at 0.1, 0.5, 1, 10, 20, 50, 70, or 100 μM and cells were incubated for 48 h and compared with control cells	Putrescine, spermidine level, and ornithine decarboxylase activity compared to control and 0.1 μM concentration; Quercetin dose (70 μM) significantly reduced (two times) the conversion of MTT salt (cell proliferation activity) and induced apoptosis in treated cell than control	Linsalata et al. (2010)
Anticancerous	Human carcinoma cells (breast)	Breast cancer cell lines (MDA-MB-453 and SK-Br3) and normal cells (MCF-10A) were seeded on 100 mm tissue culture plate. Cells were treated up to 96 h with 1, 5, or10 μM quercetin doses in every 24 h. Comparison of cell proliferation was done by hemocytometer	Quercetin treatment (10 μM) showed least cell proliferation in both cancer cell lines: SK-Br3 that is, 100 (×104) than control (400×104) and MDA-MB-453 480 (×104) than control (1400 ×104), whereas no effect was observed in MCF-10A	Jeong et al. (2009)
Anti-inflammatory	Mouse macrophage cells (J774A.1)	J774A.1 cells were treated with onion skin extract at 5, 20, or 50 μg/mL concentration in every 24 h up to 48 h then 10 ng/mL proinflammatory mediator (lipopolysaccharide) was given to treated cells and compared with untreated cells	All the concentrations significantly reduced mRNA level of COX-2 in treated cells, that is, 5 μg/mL (46.82%), 20 μg/mL (67.02%), and 50 μg/mL (65.22%) than control (4.24%) that showed a significant inhibitory effect against inflammation	Albishi et al. (2013)
Anti-inflammatory	Human cancer cells	Five cancer cell lines (HepG2, SPC-A-1, A549, CNE-1, and MGC80-3) were treated with spiro stanol saponins isolated from Chinese onion and compared with control MRC (macrophage cells)	Onion saponins exhibited higher inhibitory range (IC5034.33–88.12 μM) than control (IC50 > 100μM) against inflammation	Wang et al. (2016)
Anti-inflammatory	Wistar rats	Rats were divided into three groups: group-1/control (100% commercial rodent diet), group-2 (control diet+ 2% cholesterol+0.5% cholic acid), and group-3 (control diet+2% cholesterol+10% onion powder) and fed their respective diets orally for 7 weeks	Plasma inflammatory parameter were reduced (12.05−399.47 pg/mL) by onion powder in group-3, whereas group-2 had highest level (15.10 – 545.97 pg/mL) of inflammatory components	Colina-Coca et al. (2017)
Antiaging	Wistar albino rats	Two groups of rats (n = 20): control (standard commercial feed) and treated (standard commercial feed with 2 mL/rat onion husk extract) orally given for 187 days	Higher antioxidant activity for brain was obtained in treated group (52.73) than the control (35.70) on μmol equivalent quercetin/g protein at the end of experiment	Chernukha et al. (2021)

a

All the experiments were carried out in ambient condition (22 ± 3℃, 40–70% humidity, 12:12 h light/dark cycle) and sample size (n = 6–12) for animal studies; #3-(4,5 dimethylthiazol-2yl)−2,5-diphenyl-tetrazolium bromide (MTT).

Sulfur compounds were well admitted to have putative anticancer effects. Dipropyl disulfide, dipropyl sulfide, S-allyl cysteine (SAC), S-methylcysteine (SMC), and N- acetylcysteine were reported to retard the growth of early and late-stage carcinogenesis of the oesophagus, forestomach, lung, mammary gland, kidney, liver, and colon (Bora & Sharma, 2009; Fukushima et al., 2001; Guyonnet et al., 2001), whereas ajoene, methiin, SAMC, DATS, and DADS induced apoptosis and inhibited proliferation of human leukemic cells in vitro (Corzo-Martínez & Villamiel, 2012). Organoselenium compounds of onion were also reported to possess similar anticancer property and onion rich in selenium expresses higher anticancerous activity than other crops (El-Bayoumy et al., 2006; Matsuura, 2004). For example, two main selenium-containing compounds of onion, that is, γ-glutamyl-Se-methyl selenocysteine and Se-methyl selenocysteine with Se-allyl selenocysteine, were reported to prevent a large number of cancers (Block et al., 2001; Hurst et al., 2010). In addition, the selenite, selenocysteine, and selenomethionine exhibited similar anticancerous activity (Kotrebai et al., 2000).

Protection from inflammation

Inflammation is a complex biological process occurring when injuries, infections, or chemical irritations take place in cells or tissues. The roles of quercetin and kaempferol have been investigated in various animal models against inflammation (Bahram-Parvar & Lim, 2018; Jachak, 2001). A diet containing 500 mg of quercetin decreased by 42% dendritic cells (DCs) in peripheral blood of treated rat males corresponding to an anti-inflammatory effect (Nickel et al., 2011). Similar action of quercetin was observed as an inhibitory bioactive compound against isotypes of various mitogen stimulating immunoglobulins like IgA, IgG, and IgM (Pareek et al., 2017). Likewise, flavonoids control inflammation by regulating inflammatory signaling pathways such as cyclooxygenase −2, mechanistic target of rapamycin, and tissue inhibitors of metalloproteinases (Navarro et al., 2001). The saponins have also shown an anti-inflammatory activity against chronic skin inflammation, 12-O-tetradecanoylphorbol-13-acetate (TPA), and induced ear edema (Adão et al., 2011; Navarro et al., 2001; Sparg et al., 2004). OSCs showed similar protection against inflammation (Bahram-Parvar & Lim, 2018; Rose et al., 2005; Wilson & Demmig-Adams, 2007). For instance, lipoxygenase (LOX) and cyclooxygenase (COX) can be converted into arachidonic acid (AA), a proinflammatory enzyme, which is responsible for various disorders in the body, including inflammation (Wilson & Demmig-Adams, 2007). The capaene and thiosulfinate compounds of onion have the ability to neutralize the conversion and production of this AA. Additionally, capaene inhibited the activity of LOX and COX enzymes along with platelet aggregation (Ali et al., 2000). This anti-inflammatory activity of onion was confirmed by many other animal studies via feeding rats with onion bulbs extracts (100–200 mg/kg body weight) (Khajah et al., 2019) and Welsh onion leaf extracts (0.25–1 g/kg body weight) (Wang et al., 2013).

Antidiabetic property

During the past three decades, the prevalence of type 2 diabetes has risen dramatically in all countries regardless of their income levels even though the majority lives in low- and middle-income countries. The death of 1.6 million persons is directly attributed to diabetes each year. Interestingly, onion bioactive compounds like flavonoids were shown to possess an antidiabetic property by inducing higher insulin production and controlling pancreatic cells, therefore, suppressing the symptoms of diabetes (Vessal et al., 2003) (Table 3), whereas quercetin consumption helped in the controlling of type-2 diabetes (Kobori et al., 2009). A study showed that onion flavonoids and SMC increased insulin production via reduction of lipid peroxidation, blood glucose, serum lipids, and oxidative stress (Akash et al., 2014). Onion extract was also found effective to control diabetes via phosphatidylinositol-4,5-bisphosphate 3-kinase/Akt pathway (Gautam et al., 2015). Ojieh et al. (2015) evaluated the hypoglycemic effect of 0.4 and 0.6 g per 100 g body weight of onion juice on diabetic and normoglycemic male rats, and surprisingly, 0.4 g/100 g body weight dose of onion juice decreased fasting blood glucose level by 50% in diabetic rats, and other results showed that onion juice remarkably reduced the activity of α-amylase and α-glucosidase in the blood (Ojieh et al., 2015).

Onion skin extracts showed similar antihyperlipidemic effects by decreasing the triacylglycerol level of plasma without gaining body weight induced by a high-fat diet (Kim 2007) and reducing total cholesterol and plasma cholesterol (Baragob et al., 2015). Indeed, a longer absorption rate of natural flavonoids (quercetin) was suggested to prevent collagens glycation responsible for the cardiovascular problem in diabetic patients (Urios et al., 2007) and the Allium bioactive compounds such as methiin and SAC sulfoxide were demonstrated to play an important role in the stimulation and secretion of insulin from pancreas (Srinivasan, 2005). For example, N-acetylcysteine was analyzed for its antidiabetic activity and the effects on diet-induced obesity (high sucrose), LDL oxidation, and lipid profile in male rats, and this study revealed the capacity of N-acetylcysteine to improve diet-induced obesity (high sucrose) (Souza et al., 2011).

Protection from cardiovascular diseases

Cardiovascular diseases (CVDs) are the first cause of death globally and more people die annually from CVDs than from any other cause or disease. An unhealthy diet is one of the most important behavioral risk factors for heart disease and stroke. Indeed, many factors are linked to CVDs, such as elevated platelet activity, increased blood cholesterol, increased blood homocysteine, obesity, diabetes, and hypertension. Extensive nutritional studies demonstrated that diet is one of the potential solutions to prevent and reduce CVDs, and onion is one element of this diet (Corzo-Martínez et al., 2007). Ischemic heart injury was evaluated by feeding rats with onion bulb extract and 10 g/kg effectively decreased apoptotic cell death and infarct size (Park et al., 2009). Similarly, powdered peel extract containing quercetin 100 mg/g quercetin given to individuals during 12 weeks reduced body mass index (BMI) and body weight confirming the putative role of onion in reducing CVDs (Choi et al., 2015). As a result, the extract-fed rats showed a higher level of cholesterol in their fecal matter, which exhibited a cholesterol-decreasing effect through fecal excretion.

Hepatoprotective activity

Besides the various positive effects of Allium reported above, onion flavonoids have shown significant hepatoprotective effects by protecting the liver from different ailments (Ogunmodede et al., 2012). Quercetin was reported to protect mouse liver from injury due to overloaded iron in hepatic cells (Zhang et al., 2006). Anthocyanins have also been reported to protect the liver because anthocyanin cyanidin-3-O-β- glucoside (C3G) was found to protect hepatic Gclc expression for the activation of protein kinase by increasing cAMP level resulting in phosphorylated element binding protein for Gclc transcription (Zhu et al., 2012). Bioactive compounds are also found to have beneficial antiaging, antihypertension, and neuroprotective actions (Bast et al., 2007; Brüll et al., 2015; Chernukha et al., 2021; Hwang & Yen, 2008; Marrelli et al., 2019).

Based on the above-discussed studies, it can be concluded that almost all bioactive components of onion had many health benefits and active biological roles. However, flavonoids mainly quercetin and its derivatives were reported as the strongest bioactive compounds for a greater health benefit against different types of pathogens and cancer. Moreover, flavonoids were the potent agents to suppress ROS to a higher extent. It can also be suggested that the OSCs showed relatively better anti-inflammatory, anti-allergic, and antithrombotic activity.

EFFECT OF POSTHARVEST FACTORS ON BIOACTIVE COMPOUNDS OF ONIONS

The numerous postharvest practices and processing parameters, such as light, minimal processing, packaging, cooking, frying, boiling, freezing, and storage conditions, have a significant impact on bioactive compounds of onion (Table 4). Most of the studies showed reduction in the level of phenols and flavonoids by heating operations (Juániz et al., 2016; Lee et al., 2008; Ren et al., 2018), whereas few others explored an elevation in TPC and total flavonoids (Siddiq et al., 2013; Teng et al., 2019). The important postharvest factors are discussed in the following sections.

TABLE 4 Effect of postharvest practices and minimally processing and conditions on bioactive compounds of onion

Postharvest/processing practices	Parameters/conditions	Effect on bioactive compounds	References
Curing	Curing for 3 days at 24℃	↑ Quercetin content up to 40%	Rodrigues et al. (2009)
	Curing under fluorescent light for 3 days	↑ Quercetin level	Ko et al. (2015)
	Curing of red onions at 20℃ for 6 weeks	↑ Quercetin, quercetin glucosides and anthocyanin	Downes et al. (2009)
	Curing at 28℃ for 6 days	↓ Individual flavonols content	Cools et al. (2010)
	Curing at 24℃ for 10–14 days	↑ Quercetin glucoside between 100% and 300%	Mogren et al. (2006)
Minimal processing	Onion maceration for 5 h	↓ Flavonols between 10.7% and 17.7%	Makris and Rossiter (2001)
	Peeling	↓ Quercetin glucosides up to 21%	Gennaro et al. (2002)
	Chopping	No significant impact on quercetin concentration	Rodrigues et al. (2009)
	Trimming	↓ Flavonoids (39%)	Ren et al. (2020)
	Cutting	Flavonols induction	Ioannou et al. (2012); Bernaert et al. (2013)
Freezing	Diced onion frozen at −18℃ and stored for 3, 4, and 5 months	↑ Total flavonols and total anthocyanins	Pinho et al. (2015)
Frying	In olive oil for 4–8 min at 180℃	No effect on bioactive compounds	Rodrigues et al. (2009)
	In oil for 4–8 min at 180℃	No effect on flavonoids	Ahmed and Eun (2018)
	In oil for 5–15 min at 180℃	↓ 23% quercetin content	Lee et al. (2008)
	Sautéing for 5 min	↓ 21% flavonoids	Lee et al. (2008)
Boiling	10 min at 100℃	↓ 20% flavonoids	Gorinstein et al. (2009)
	60 min at 90℃	↓ 50–60% bioactive compounds	Németh et al. (2003)
	30 min 100℃	↓ 59% quercetin derivatives	Nemeth et al. (2004)
	3-60 min at 100℃	↓ 18–75% quercetin content	Makris and Rossiter (2001); Lombard et al. (2005)
Heating	Blanching onion for 60 or 70℃ for 3 or 1 min, respectively	↑ Quercetin and its derivatives	Ren et al. (2018)
	Onion processed on 400 MPa pressure at 5℃ temperature for 5 min	↑ Quercetin up to 33%	Roldán-Marín et al. (2009)
	Sterilization of onion by-products followed by pasteurization at 100℃ for 11-17 min	↑ Quercetin and total phenols	Roldán-Marín et al. (2008)
	Microwave heating for 4 min	↓ 18% quercetin glucosides	Lombard et al. (2005)
	Microwave heating for 4 min	No effect on flavonoids	Ahmed and Eun (2018)
	Heating at 70℃	↑ Total phenols from 44.92 to 52.32 mg GAE/100 g	Siddiq et al. (2013)
	Heating at 120℃ for 30 min	↑ Total phenols and total flavonoids	Sharma et al. (2014)
	Heating at 36℃ for 96 h	↓ Total flavonoids content	Olsson et al. (2010)
	Griddled at 110℃ for 5 min	↑ 57.35% flavonoids	Juániz et al. (2016)
Roasting	Roasting for 18 min at 270℃	↓ Flavonoids (15%)	Sans et al. (2019)
	Roasting for 15–30 min at 180℃	No effect on total quercetin	Rodrigues et al. (2009)
Drying	Freez-drying onion slices at −70℃ and 4.2 Pa pressure for 24 h	↑ Flavonoids (32%) and anthocyanins (25%)	Pérez-Gregorio, García-Falcón, et al. (2011)
	Onion dried at 70℃	↓ Ascorbic acid from 1889 ± 69 to 89.0 ± 2 mg/100 g	Mota et al. (2010)
Irradiation	Peeled onion with UV-irradiation at low (1.2 KJ/m2) and medium (6.0 KJ/m2) doses	↑ Flavonols content	Rodov et al. (2010)
	Onion slice under UV-radiation	↑ Quercetin content	Higashio et al. (2005)
	Gamma irradiation at 1.5 kGy	↑ Total phenols (2.4%) and flavonoids (6.4%)	Hussain et al. (2018)
	Gamma irradiation at 1.5 kGy	↓ Total ascorbic acid (8.5 mg/100 g)	Hussain et al. (2018)
	Fresh cut green onion under 1.0 kGy gamma irradiation and 0.1% sodium benzoate treatment and stored for 16 days	↑ Polyphenols	Memon et al. (2020)

“↑“ = Increase; “↓“ = Decrease.

Effect of curing and light

The goal of curing is to remove excess water that consequently strengthens by drying the skin (outer layers) and reduces the infection in onion (Shivakumar & Chandrashekar, 2014). Field curing increased the quercetin content (Rodrigues et al., 2009) as light stimulates the synthesis of flavonoids mainly in epidermal cells to protect the crop from UV radiation, and phenyl ammonia lyase (PAL) is the key enzyme for the stimulation of flavonols (Rodrigue et al., 2017). Similarly, Mogren et al. (2006) revealed that the quercetin level increased after curing in light. Ko et al. (2015) studied the effect of different light exposure, that is, blue, red, UV light, and fluorescent light on onion bioactive compounds for up to 3 days. They found that fluorescent light increased the level of quercetin in peeled onion, whereas the blue light increased quercetin glucosides in onion pulp after 8 h exposure. UV light increased the quercetin level in onion better in comparison with visible light at 24℃ storage (Yoo et al., 2013). The authors also obtained higher total quercetin measured in dry skin (outer layer) but decreased gradually toward the inner layers of the onion (Yoo et al., 2013). It was observed that a higher level of PAL enzyme triggered the production of flavonoids, mainly in the outer layers of the onion, and Higashio et al. (2005) observed an increase of 50–70% in quercetin content when onion slices were kept under UV light for a longer time.

Effect of temperature

Temperature plays an important role in different postharvest processing techniques such as cooking, boiling, drying, frying, freezing, and so on. Heat is responsible for thermal degradation, oxidation, and loss of bioactive components in horticultural crops. The heating treatment (temperature and time) has diverse effects on onion bioactive compounds (Ahmed & Eun, 2018; Tiwari & Cummins, 2013). Many studies have recorded either an increase or a decrease in bioactive compounds of the onion by heating (Juániz et al., 2016; Islek et al., 2015; Makris & Rossiter, 2001; Ren et al., 2018). Generally, cooking decreases the total flavonoids of onion but the reduction level depends on the heating conditions and the duration of the thermal treatment, which can decrease significantly the amount of bioactive compounds in onion (Juániz et al., 2016; Ren et al., 2018). Oven roasting of chopped onion at 180–200℃ for 15 min and frying in olive oil for 4-8 min at 180℃ did not affect flavonol (quercetin) content but intense microwave application decreased flavonols content by 17% (Rodrigues et al., 2009). Nevertheless, a little loss of flavonols was observed when onion was subjected to the frying process (Ioku et al., 2001). Lee et al. (2008) noted a decrease in quercetin level (25−33%) of onion when fried at 180℃. Frying of onion in sunflower oil, olive oil, and griddled enhanced flavonoids content, and this increase might be probably due to disruption of subcellular section and cell wall by oil heat. As griddle needs a higher temperature compare to frying; therefore, it exhibited an increment in the flavonoids concentration up to 57.35% than raw onion (Juániz et al., 2016).

Boiling has also a negative impact on the bioactive compounds of onion, such as quercetin and kaempferol (Aoyama & Yamamoto, 2007). Rodrigues et al. (2009) observed that mild boiling resulted in 37% loss of quercetin, whereas intense boiling caused the loss of 64% quercetin flavonol. A decrease of 50–60% in flavonoids was recorded in red and yellow onion compared to other raw vegetables when subjected to 60 min boiling (Németh et al., 2003). Boiling excretes out all the bioactive compounds in the water via cell disruption and consequently decreases the level of flavonoids in onion. Flavonoids lost due to higher temperature might also be a reason for the reduction. Similarly, 15% reduction was observed in total flavonoids during boiling of green Welsh onion for 15 min (Aoyama et al., 2007). In addition, when yellow onion was boiled for 30 min, 59% of quercetin derivatives were lost (Nemeth et al., 2004). Likewise, several studies reported that 3–60 min boiling time imparts 18–75% quercetin loss in onion (Lombard et al., 2005; Makris & Rossiter, 2001).

Cooking of onion solubilizes bioactive compounds such as quercetin and kaempferol in water, thus reducing the flavonoid content in onion but producing a healthy solution (broth) because up to 50% of flavonoids dissolve in water by the disruption of onion cells (Nemeth et al., 2003). Microwave cooking decreased quercetin glucosides of the onion by up to 18% when treated for 4 min (Lombard et al., 2005). Four percent decrease in quercetin was reported when onion was cooked by microwave regardless of the power used (Lee et al., 2008). On the contrary, Ahmed and Eun (2018) observed that microwave cooking without water more quercetins was retained because flavonoids remained intact inside onion cells due to the absence of water. In an experiment, fresh-cut onion was treated with mild heat for 1 min at different temperatures that is, 50, 60, and 70℃ in water, and stored at 4℃ for 21 days. Among all treatments, total phenolics in onion increased slightly from 44.92 to 52.32 mg GAE/100 g FW when heated at 60℃ (Siddiq et al., 2013). Another temperature-dependent study was carried out on red, yellow, and white cultivars. All the cultivars were heated at 80, 100, 120, and 150℃ temperature for 30 min. Though all the cultivars showed a higher level of total phenolic contents, red onion showed a maximum increase in total phenolics from 6631.33 ± 661.21 to 13,712.67 ± 1034.85 μg/g GAE and total flavonoids from 2835.86 ± 121.12 to 3456 ± 185.82 μg Q/g at 120℃ than yellow onion (total phenols from 5381.54 ± 542.24 to 13,611.83 ± 341.61 μg/g GAE and flavonoids from 2665.12 ± 243.12 to 3482.87 ± 117.17 μg Q/g) DW (Sharma et al., 2015). The red onion had the higher level of flavonoids, hence, contained the maximum flavonoids even after heating. Another study revealed that phenolics content of onion powder, increased either by the liberation of glycosylated or esterified bond through cleavage (Sharma et al., 2015). Although quercetin conjugates are found resistant to a lower temperature (Makris & Rossiter, 2001), higher temperatures negatively affect the flavonoid level due to flavonoids degradation. The heating of onion (three varieties) for 24 h at 36℃ did not show any change in total flavonoids, whereas longer heating time (96 h) decreased total flavonoid content (Olsson et al., 2010). Similarly, Woo et al. (2007) did not report the effect of heating up to a certain magnitude of time and temperature, but by increasing temperature to 150℃ for 3 h, total flavonoids started to decrease. Deep thermal penetration led to chemical breakdown and thermal degradation of flavonoids at higher temperature.

As far as the OSCs are known, they are thermolabile and reactive. Various studies explored thermal treatments such as steaming, drying, cooking, and frying in food industries or at home, and these treatments led to the reduction of OSC and as well as their bioavailability (Barba & Orlien, 2017; Putnik et al., 2019). However, a study recorded that steaming, microwaving, and frying increase the ACSOs amount, but blanching showed negative effects because of oxidase catalysis and γ-glutamyl peptidase activity (Kim et al., 2016). Moreover, steaming of onion decreased OSC level and antiplatelet activity. The decrease of OSC content by heat is due to the damage of allinase activity (Hansen et al., 2012), and Nishimura et al. (2000) evaluated Japanese Allium through baking at 150℃ and found a change between disulfides and trisulfides due to vinyldithiins formation.

Effect of minimal processing

Minimal processing creates wounding stress on onion and unit operations, such as peeling, slicing, dicing, and chopping have been studied for their effect on bioactive compounds of onion (Ioannou et al., 2012; Berno et al., 2014). Peroxidase, polyphenol oxidase, and phenylalanine ammonia lyase are the three important enzymes of phenolics metabolism (Rodrigue et al., 2017), and it was observed that wounding stress elevates the level of phenolics in sliced onions (Pérez-Gregorio, Regueiro, et al., 2011). Peeling decreased quercetin concentration by 40% because onion skin contains up to 90% of quercetin, which is removed by peeling, whereas peeling and trimming operations decreased flavonoids by up to 39% (Bahram-Parvar & Lim, 2018). Ren et al. (2020) reported that 39% of flavonoids content was lost during preprocessing operations like trimming, whereas peeling resulted in 21% loss of 17 quercetin glucoside. Likewise, peeling reduced by 70% anthocyanins and 20% quercetin-4-glucoside initially found in red onion (Martínez et al., 2005; Wiczkowski, 2011). On the other hand, cutting enhanced the flavonols level in onion, for example, chopped onion showed a higher level of flavonols (Pérez-Gregorio, Regueiro, et al., 2011), and slicing increased flavonols (Chen et al., 2016). Dicing and slicing enhance the level of flavonoids in onions by disrupting their cells. However, another experiment showed that chopping did not show any change in quercetin content compared to whole onion bulb during 11 storage days at 4℃, but the concentration increased up to 22% after 30 days of storage, whereas Makris and Rossiter (2001) did not find any change in quercetin 3,4′-diglucoside (Q_DG) and quercetin 4′-glucoside (Q_MG) induced by chopping. In contrast, Rodrigues et al. (2009) observed a decrease in Q_MG and Q_DG of onion after chopping followed by storage for 5 h in light; afterward, an increase of quercetin glucosides was observed after 18 h storage, and a reduction in flavonols was noted during 48 h of storage. Conclusively, it seems that some minimal processes like slicing and chopping are better operations because they increase flavonols, while some others, such as maceration and trimming, decrease flavonols content.

Packaging plays an important role for minimally processed onions because it protects products from microbial contamination and extends the shelf-life of the products during storage. An experiment was carried out on minimally processed purple onion stored at 0, 5, 10, and 15℃ and 85–90% relative humidity for 15 days in packages. Results showed that the onion stored at 15℃ contained the highest phenolics level, whereas anthocyanin concentration in onion stored at 0℃ increased by 48% on day 3. Results also showed that flavonoids were not affected by temperature, whereas storage time, that is, 15 days, decreased quercetin level by 26% (Berno et al., 2014). Packing material was also found to affect the quercetin level in onion and chopped products showed a decline in quercetin and its derivatives when stored in refrigerating and packed conditions (Martínez et al., 2005).

When sliced onion was packed in transparent polystyrene cups and stored (1–2℃) under visible light for 16 days, an increase in flavonols (up to 58%) and anthocyanins (up to 39%) was noted (Pérez-Gregorio, Regueiro, et al., 2011). The action of the released ethylene could be the reason behind the significant elevation of onion flavonoids during storage. Ethylene activates PAL enzyme involved in the biosynthesis and accumulation of bioactive compounds (Leja et al., 2003). A positive relationship has also been reported between total phenolics and PAL activity in onion stored for a longer period (Benkeblia, 2000). The effect of atmospheric conditions on flavonoids level was evaluated in fried and sliced onions during storage. Results revealed that vacuum-packed fried onion had the highest flavonoid retention at 5℃ for 7 days in the dark, whereas sliced onion (vacuum-packed in dark) exhibited better flavonoid concentration at 5℃ and −18℃ temperature for 21 days (Islek et al., 2015). Marta et al. (2013) used normal atmosphere (NA) and controlled atmosphere (CA) packaging system in four different combinations, that is, (a) 2% CO₂ + 2% O₂, (b) 2% CO₂ + 5% O₂, (c) 5% CO₂ + 5% O₂, and (d) 5% CO₂ + 2% O₂ to enhance quality of stored onion. CA packaging at 5% CO₂ + 5% O₂ showed maximum flavonoids content in stored onion. Although CA storage enhances the bioactive compound, it is a costly technique; therefore, further research in this area is needed to determine the optimal atmosphere to store minimally processed onion for longer periods. Low temperature also preserves bioactive compounds for a longer duration but there is a need for further investigation on sustainable and efficient storage practices.

Various changes occur in the bioactive compounds of onion during other processing operations. For example, OSCs (ACSOs) such as isoalliin, alliin, and γ-glutamylcysteines are converted into different volatiles like sulfenic acid and 1-propenesulfenic acid when the onion is crushed, chopped, or cut. These acids are chemically converted to thiosulfinates and further into DMS and DADS through self-condensation, dimerization, and hydrolysis (Figure 3), and these compounds are responsible for the peculiar flavor, odor, and biological activities of onion (Pareek et al., 2017).

FIGURE 3. Chemistry of organosulfur compounds during processing of onion. Adapted from Pareek et al. (2017)

Temperature, humidity, genetic material, and sulfate composition of the growing area can also impact the content of precursors releasing the OSCs (Montano et al., 2011; Putnik et al., 2019). Moreover, different flavoring compounds are liberated during the processing (cutting or crushing) of onion (Figure 4).

FIGURE 4. Development of different flavor groups during metabolic pathways in processed onions. Adapted from Griffiths et al. (2002)

Effect of storage conditions

Storage is an important practice since ancient times and is used to ensure food security, food availability, and consumption for a longer time. Onion is stored in whole or minimally processed (peeled, diced, fresh-cut, etc.) forms in storage houses, and storage time and temperature play important role in the stability of onion bioactive compounds (Bahram-Parvar & Lim, 2018).

Storage conditions affect the nutritional and biochemical properties of onion. Several studies have been conducted on the changes in bioactive compounds of onion during storage (Gennaro et al., 2002; Kevers et al., 2007). Sharma and Lee (2016) stored onion at 4, 10, and 25℃ for 9 months with 65–74% RH and they observed that the total flavonoids increased during the first 5 months at 25℃ but gradually decreased during the months 6 and 7. Initially, quercetin glucosides increased during storage at 4 and 10℃ but a reduction was noted during the later months. Likewise, red and white onions were stored under traditional field storage and refrigerated (2℃, 65% RH) conditions. A significant (64%) increase was noted in both onions after months 6 and 7. Red onion of field storage condition had highest flavonoids increase (64%) than refrigerated onions (40%), whereas white onion was found to have 44–60% increase in flavonols. The increased level of flavonoids is directly related to the activity of the PAL enzyme as research showed that storage temperature and time trigger the reaction of PAL (Rodrigues et al., 2010), but under the same conditions, no effect was observed on total anthocyanins content after 6 months in red onion but total anthocyanins were reduced between 40% and 60% after 7 months of storage (Rodrigues et al., 2010). The internal factors, such as the genotype of the cultivar, also impact the level of flavonoids. During 36 weeks of onion storage, an increase in the total flavonoids was observed in red (102.898 to 108.300 mg/kg) and yellow (59.245 to 65.210 mg/kg) cultivars when stored at a higher temperature (22℃) (Lachman et al., 2003). Zudaire et al. (2017) stored onion at 1℃ for 30 days and recorded an increase in flavonoids up to 0.58 ± 0.10 g/kg FW. Similarly, onion was stored at 25 ± 1℃ with 60–70 ± 5% RH and higher flavonoids content was reported (Petropoulos et al., 2016). Polish onion showed no effect on quercetin glucoside during storage at 1℃ (Grzelak et al., 2009), whereas quercetin glucoside level decreased in Swedish onion at the same temperature (1℃) (Mogren et al., 2007). Ren et al. (2020) reported an increased level (30–51%) of quercetin 3,4′ diglucoside, whereas no effect on quercetin 4′ glucoside content when onion was stored at 2℃ for 8 months. The possible reason behind the same level or increase in flavonoids content might be likely due to the change in the chemical composition of onion due to low rate of respiration resulting in an enhanced level of quercetin and its glycoside or water losses causing an increase in dry matter (Majid et al., 2016; Sharma et al., 2014). In another experiment, bioactive contents of red onion were analyzed during storage at 2℃, and decreased level of total anthocyanins was noted but there was no effect on flavonoids indicating that flavonoids are more stable at a lower temperature in comparison with anthocyanins (Gennaro et al., 2002); however, few studies reported an increase in flavonoid concentration of frozen onion (Pinho et al., 2015; Rodrigues et al., 2009).

Apart from this, few studies are focusing on the postharvest effect of storage on FOS and their enzymatic conversion. In onion, inulin neoseries are produced and considered as a special kind of FOS. The pattern of these 1-SST, 1-FFT, and 6G-FFT enzymes was reported in onion bulbs during storage at 10 and 20℃; however, a slight increase of FOS was observed at 10℃ during the last 3 months of storage time. This increase was stimulated by substrate availability followed by low temperature (Benkeblia et al., 2003). In addition to this, Benkeblia et al. (2005a) determined the activities of these enzymes during 6 months storage at 15℃ and found that 1- FFT increased after 4 months and then reduced to the initial level during storage. Contrary to this, 6G- FFT activity did not exhibit significant change during four months storage time but it reduced afterward during the last storage period. More interesting, fructose monomers produced two new oligosaccharides such as saccharide 1 (inulobiose [β-D-fructofuranosyl- β-fructopyranose]) and saccharide 2 (inulotriose [β-D-fructofuranosyl-β-D-fructofuranosyl -β-fructopyranose]) in onion bulb when stored at 10℃ for four weeks. Similar enzymes, namely, 6G- FFT and 1- FFT were responsible for the synthesis; however, a study revealed that the activities of the 1-FFT and the 6G-FFT in onion tissue are catalyzed by a unique enzyme having the two activities of elongation and branching (Fujishima et al., 2009). Recently, a study revealed that when fully matured onion bulbs treated with 1-methyl cyclopropene (MCP, 1 μL/L) and stored under air or ethylene atmosphere (10 μL/L) for 5 months at 1℃, total FOS concentration increased (128–141.7 g/kg) from preharvest to 6 weeks storage followed by a continuous decline thereafter (Ohanenye et al., 2019). Indeed, hydrolysis of FOS takes place during long term storage consequence of the resprouting of the bulb and the remobilization of the carbohydrate reserve. This hydrolysis occurs by shortening the FOS chain and the production of fructose, sucrose, and glucose. A proper kinetic formula was developed for the estimation of FOS isomers hydrolysis during storage (Benkeblia et al., 2005b). The hydrolysis estimation of polymerized FOS is given by the following formula: $$\begin{equation*}{\rm{Hydrolysis}}\,\left( \% \right) = \left| {\frac{{{C_f}}}{{{C_i}}} - 1} \right| \times 100,\end{equation*}$$where C_i = Initial concentration at time 0 and C_f = Final concentration at time i.

Three onion cultivars were stored at three temperature ranges and analyzed for hydrolysis of polymerized FOS. The range of hydrolysis was reported between 73% and 81%, whereas no effect of temperature was noted (Benkeblia & Shiomi, 2006a). Similarly, Benkeblia et al. (2007) studied the hydrolysis of FOS in “Red Amposta,” “Tenshin,” and “Yellow Spanish” cultivars of onion during 6 months of storage at 10, 20, and 20℃. They observed the highest percentage of FOS hydrolysis at 20℃ (74–83%) followed by 15℃ (63–68%) and 10℃ (47–58%).

EFFECT OF EXTRACTION TECHNIQUES ON BIOACTIVE COMPOUNDS OF ONION AND ITS BY-PRODUCTS

Solid-liquid extraction (SLE) has been used as a conventional method for decades to extract flavonoids from plant samples. SLE utilizes organic solvents such as methanol, ethanol, hexane, acetone, and ethyl acetate in pure or mixed forms for achieving bioactive rich extract (Makris & Kefalas, 2015). However, the utilization of organic solvents for extraction is harmful to the working person and a source of pollution for the environment. Large-scale production of FOS and quercetin from onion waste requires a huge quantity of solvents and consequently creating a significant burden on the environment; therefore, alternative and green extraction techniques must be adopted (Santiago et al., 2019). Indeed, novel extraction techniques have achieved much attention because of the limitations associated with conventional techniques. UAE, microwave-assisted extraction (MAE), pressurized liquid extraction (PLE), subcritical water extraction (SWE), and pressurized hot water extraction (PHWE) are some of the innovative techniques utilized for the extraction of onion bioactive components (Campone et al., 2018; Khan et al., 2018; Kumar et al., 2014). In comparison with conventional methods, these techniques have several advantages like short extraction time, energy efficient, less use of organic solvents, and better recovery of compounds (Liu et al., 2018; Manohar et al., 2017)

PLE method was applied in few studies to extract onion flavonoids and results revealed that PLE is less time-consuming, highly automated, more efficient, parallel extraction, and high purity extract than the conventional method (Søltoft et al., 2009). SWE was also suggested as a good method for the extraction of natural compounds from plants, herbs, and other food materials (Kim et al., 2019; Lee et al., 2014; Munir et al., 2018). It was employed for quercetin extraction and more than eightfold yield was achieved with ethanol in comparison with water and methanol, and this method showed a positive effect on the extraction of flavonoids from onion skin (Ko et al., 2011). Conclusively, novel methods are far better than conventional techniques for higher recovery of bioactive compounds, but they need further work on parameters optimization for better reproducibility and efficiency (Table 5).

TABLE 5 Effect of different extraction methods on bioactive compounds of onion

Extraction method	Onion part	Solvent	Targeted compound	Remarks/Effects	References
Microwave-assisted extraction (MAE)	Yellow onion bulb	None	Flavonoids	No solvent needed, energy efficient, less time-consuming, higher flavonoid yield	Zill-e-Huma et al. (2011); Kumar et al. (2014); Pal and Jadeja (2019)
Pressurized liquid extraction (PLE)	Bulb	Methanol	Quercetin	Automated, best for oxygen-sensitive flavonol, higher yield of quercetin	Søltoft et al. (2009)
Pressurized hot water extraction-enzymatic hydrolysis (PHWE-EH)	Yellow onion bulb	Ethanol and water	Quercetin glycosides	More accurate, environmentally friendly, less time, higher quercetin yield	Lindahl et al. (2013)
Pressurized hot water extraction (PHWE)	Bulb and waste	Ethanol and water	Anthocyanins	Short extraction time, less effect on environment, elevate flavonoids yield	Lindahl et al. (2010); Petersson et al. (2010); Liu et al. (2014)
Ultrasound-assisted extraction (UAE)	Waste	Water	Flavonol	Higher flavonol recovery, easy handling, less energy but high-solvent quantity needed	Jang et al. (2013); Katsampa et al. (2015)
Supercritical fluid extraction (SFE)	Red and yellow onion skin	Ethanol	Quercetin aglycone	Rapid use, controlled temperature and pressure, low-solvent need, higher quercetin recovery but longer time	Martino and Guyer (2004); Campone et al. (2018)
Subcritical water extraction (SWE)	Waste of red and yellow onions	Water	Quercetin Isorhamnetin	Environmentally friendly, less recovery of compounds due to high temperature	; Ko et al. (2011); Lee et al. (2014); Kim et al. (2019) ; Munir et al. (2018)
Pulse electric field (PEF)	Bulb	Water	Flavonoids	Best for water soluble flavonoids, environmentally friendly	Liu et al. (2018)
Pressurized low polarity water (PLPW)	Waste	Water	Flavonoids	No solvent toxicity, ecofriendly, high-quality compound yield	Manohar et al. (2017)

UTILIZATION OF ONION AND ITS BY-PRODUCTS AS INGREDIENTS OF FUNCTIONAL FOODS

Onion has been utilized directly in different food products from ancient times. It is used to produce various products such as onion flakes, dehydrated onions, powder, chutney, pickles, and paste (Wiczkowski, 2011). Though any food product that imparts health benefits is known as a functional food. Onion and its by-products have the maximum concentration of flavonoids and other bioactive compounds in comparison to other crops; therefore, it can be utilized in various value-added and functional products (Prokopov et al., 2018; Putnik et al., 2019; Sagar et al., 2018). In addition, having a high amount of flavonoids, it can be used as a natural antioxidant rather instead of artificial ones (Arshad et al., 2017). For example, onion juice of three different cultivars was mixed with sweet whey in 1:1 ratio to produce a symbiotic product. Four lactic acid bacteria (10⁶ cells/mL) were inoculated into the mixture and incubated at 37℃ for 48 h. Higher growth of Streptococcus macedonicus, Lactobacillus fabifermentans, and L. plantarum was observed and this mixture was reported as a potential and pleasant health drink (Tinello et al., 2017). Various baked and extruded functional foods were also produced using red, white, and yellow onion powder, which was mixed with wheat flour at 2.5%, 5%, 10%, and 20% concentration for making baked rolls. Among them, 5% concentration showed higher (3.81 mg GAE/g) total phenolics content, antioxidant activity, good texture, and good consumer acceptability of the product (Michalak-Majewska et al., 2017). Tonyali and Sensoy (2016) made an extruded product by mixing 3%, 6%, and 9% of onion skin powder with wheat flour and investigated the functional properties of the product. They observed that the 9% mix had the highest antioxidant activity and total phenolics (39.41 ± 1.34 GAE/g) along with a smaller pore size compared to control. Additionally, Świeca et al. (2013) enriched bread with onion skin and examined protein–phenols interaction. They noted a reduction (55% for 4% supplemented bread) in protein digestibility than control bread (78.4%). Indigestible protein–flavonoid complexes were also reported in chromatographic and electrophoretic studies with 25 and 14.5 kDa molecular weight that affected prohealth attributes of enriched bread. A human gastrointestinal tract model (in vitro) was utilized for checking the bioavailability and bioaccessibility of onion skin-enriched bread. Quercetin was found highly bioaccessible during the assay and bread containing 3% onion skin was evaluated as a good antioxidant product with satisfactory acceptability of the consumers (Gawlik-Dziki et al., 2013). Similarly, two functional products, that is, pizza base (Sagar & Pareek, 2020) and multigrain bread (Sagar & Pareek, 2021), were also fortified with the onion skin of “NHRDF Red” cultivar at different concentrations (1–5%) and observed a higher antioxidant activity, total phenols, and flavonoids in the fortified products than the control. Moreover, the shelf-life of the fortified products was extended up to 13 days during the storage at 5℃. In addition to onion and its by-products, extracted quercetin has also been utilized as an ingredient of functional food. An antiglycative functional food (bread) was made by incorporating quercetin at 0.5%, 0.1%, and 0.2% concentrations. The inhibition of advanced glycation end products (AGEs) and total phenolics was investigated in the bread. Enriched bread (0.2%) inhibited 46.52% AGEs and had higher total phenolic content (259.2 ± 12.6 mg GAE/100 g) than the control bread (76.3 ± 5.5) (Lin & Zhou, 2018). In another study, Lin et al. (2019) utilized plant extracted quercetin in western baked bread and oriental steamed bread. They assayed starch digestion, glycemic potential, and bioaccessibility of the products. Approximately 18.3% reduction was observed in starch digestion for 1.5%, 3%, and 6% quercetin enriched bread, and the authors also noted a higher quercetin bio-accessibility in quercetin fortified breads.

On the other hand, various other food ingredients have also been extracted and showed proven benefits as bioactive compounds. Roldán et al. (2008) analyzed onion by-products (juice, paste, and bagasse) as ingredients of functional food, such as antioxidant and polyphenol oxidase enzyme inhibition capacity (PPO). They found that paste is the best source of antioxidant and antibrowning compounds. Helkar et al. (2016) have also suggested that onion extract having anti-PPO activity can also be used as a natural ingredient of functional food. Pectic oligosaccharides (POSs) are used as prebiotics in various food products. The enzymatic production of POS from onion skin was carried out, and extracted pectin from onion skin was treated with viscozyme, EPG-M2, and pectinase. Results revealed that EPG-M2 (5. 2 IU/mL) was the best enzyme for higher POS production (5.2–5.5% w/w) (Babbar et al., 2016). In another study, Baldassarre et al. (2018) valorized onion skin into POS using viscozyme I with membrane enzyme bioreactor. They achieved 22.0 g/L/h volume productivity of POS. Nile et al. (2017) utilized onion solid waste (OSW) for flavonol glycosides and antioxidant ingredients. The authors reported that OSW had 499 mg/100 g of cumulative flavonol glycosides content and cannot only be used in foods but also in cosmetics as antigout and antioxidant agents.

CONCLUSIONS

Onions are one of the richest sources of bioactive compounds, including FOS, anthocyanins, flavonoids, and other antioxidants, ascorbic acid, and organosulphur components. Besides this, onion waste (specifically skin) possesses higher flavonoids compared to the bulb, and these bioactive compounds have potential biological roles in both in vitro and in vivo conditions. However, postharvest factors such as curing, processing, and storage have significant impacts on these bioactive compounds. Broadly, curing has a positive impact on bioactive compounds, whereas processing techniques decrease the content of bioactive components. Various enzymes and intermediate compounds occur during the processing and storage of onion, and the change in biochemical processes alters the concentration of flavonoids, ascorbic acid, organosulphur compounds, and FOS. In addition, novel extraction methods increase the recovery rate of bioactive compounds than conventional methods. Conclusively, good fiber and higher flavonoids in bulbs and its by-products are interesting ingredients for food application as antioxidants, preservatives, and antimicrobial agents for the development of various functional foods.

FUTURE SCOPE

It is well proven that bioactive compounds of onion have various health benefits and must be consumed in an adequate amount. However, well-established toxicological and clinical studies must be performed to analyze the bioabsorption rate of bioactive compounds in the human gut and their impacts on the different organs. The effects of functional foods enriched with onion and its bioactive compounds must also be thoroughly investigated in in vivo conditions.

Although the postharvest factors alter the concentration of bioactive compounds of onion, the entire mechanism of change is not fully understood and needs to be studied further. Moreover, conventional extraction methods that utilize organic solvents for better recovery of biomolecules are polluting the environment. Therefore, there is a need for sustainable, cost-effective, green, and novel methods, which could recover a higher yield of bioactive compounds without compromising health and the environment.

CONFLICTS OF INTEREST

The authors declare no conflicts of interest.

ACKNOWLEDGMENTS

The authors are thankful to the National Institute of Food Technology Entrepreneurship and Management (NIFTEM) and its library, that is, NKC (NIFTEM KNOWLEDGE CENTRE) for providing digital support and facilities for this work. Moreover, the first author deeply acknowledges the financial support given by UGC (University Grant Commission) vide letter no. F.15-6(DEC.2018)/2019(NET).