The role of different natural organic acids in postharvest fruit quality management and its mechanism

Abstract

Fresh fruits have good flavor and high nutritional value, but in the postharvest stage they will age and decay rapidly, so we need to find green postharvest fruit preservatives. And natural organic acids are promising natural fruit preservatives due to their safety and effectiveness. The present work reviews recent applications of natural organic acids in postharvest fruit quality management and discussed its potential biochemical mechanisms. This work indicates that numerous natural organic acids are effective postharvest fruit preservatives, such as oxalic acid, salicylic acid, citric acid, ascorbic acid, phenolic acid and terpenic acid. Natural organic acid treatments can improve postharvest fruit quality, as reflected by delaying senescence, alleviating chilling injury, controlling disease, and inhibiting browning. And natural organic acid can have multiple benefits on postharvest fruit. Natural organic acids also play a variety of roles in postharvest fruit disease control. In addition, chemical modification and cotreatment of natural organic acids can be performed to improve the efficiency of postharvest fruit preservation applications. The work provides an important reference for postharvest fruit quality management and the development of natural OAs fruit preservatives.

INTRODUCTION

Organic acids (OAs) are organic compounds that are acidic in nature. The most common OAs is a carboxylic acid (R-COOH), whose acidity originates from the carboxyl group (-COOH). The carboxyl group is the functional group of carboxylic acid. Except for formic acid (H-COOH), carboxylic acid can be regarded as a derivative of a hydrogen atom in a hydrocarbon molecule after being replaced by carboxyl group. It can be expressed by the general formula (Ar) R-COOH (Xin et al., 2021). OAs include natural OAs and synthetic OAs. Natural OAs are mainly OAs with a certain physiological activity obtained from plants or agricultural by-products in nature, while synthetic OAs are OAs obtained through chemical synthesis, enzymatic catalysis and microbial fermentation. In addition to carboxyl groups, natural OAs also contain some other functional groups, including sulfonic acid (RSO₃H), sulfinic acid (RSOOH), sulfuric acid (RCOSH), and phenolic hydroxyl groups. Different chemical groups usually give OAs different characteristics. For example, phenolic acid (such as caffeic acid, chlorogenic acid, gallic acid, protocatechuic acid, and ferulic acid), OAs with the phenol rings, its structure gives phenolic acid significant antioxidant activity. Although amino acids also have carboxyl groups, their characteristic structures do not conform to OAs and therefore do not belong to natural OAs (Xin et al., 2021). Natural OAs are widely distributed in fruit and medicinal plant, and their antioxidant, antibacterial and anti-inflammatory properties in human health have been well-documented (Ożarowski & Karpiński, 2019). OAs commonly found in fruit are mainly divided into aliphatic OAs, including citric acid (CA), oxalic acid, tartaric acid, malic acid, butyric acid and ascorbic acid (ASA); and aromatic OAs, including salicylic acid (SA) and caffeic acid, etc. These OAs are the main flavor nutrients in fruit, and most of them also have positive effects on human health (Liu et al., 2016).

Fresh fruit not only have a unique and delicious flavor, but also have many essential nutrients, including vitamins, polyphenols, dietary fiber and minerals, which are essential components of people's daily diets (Zhang & Jiang, 2019). The consumption of fresh fruit is known to prevent a variety of diseases, including stomach cancer, colon cancer, heart disease and diabetes (Clifton et al., 2014; Lunet et al., 2005). However, postharvest fruit are still metabolically active organisms with rapid senescence, and as senescence proceeds, numerous quality deteriorations occur, including the degradation of nutrients. In addition, fresh fruit and vegetables are highly susceptible to microbial infestation due to their fragility and high water content, resulting in the occurrence of decay, which can cause substantial losses in the postharvest stage (Zhang et al., 2021). Therefore, in recent years scientists have worked to find effective preservation techniques to reduce postharvest fruit losses, such as 1-MCP and SO₂. However, improper application of 1-MCP may cause abnormal ripening of climacteric fruits, which is not conducive to postharvest quality (Song et al., 2020). Considering the safety issues caused by residues of some chemical preservatives, such as SO₂, finding natural postharvest fruit preservatives is the primary goal. The OAs metabolism is an important metabolic pathway for plant growth and development, and it also plays an important regulatory role during fruit ripening and senescence (Liu et al., 2016). It has been reported that intracellular OAs accumulation could enhance plant stress resistance and affect the synthesis of other plant hormones (Lv et al., 2021).

OAs, as natural preservatives, have great potential in improving the quality of postharvest fruit products. In recent years, some exogenous natural OAs could be used to manage the quality of postharvest fruit and extend the shelf life of postharvest fruit (Huang et al., 2013). Common natural OAs that have the ability to preserve postharvest fruit include oxalic acid (Huang et al., 2013), CA (Liu et al., 2016), ASA (Sogvar et al., 2016) and SA (Moosa et al., 2021). In addition, the application of some other natural OAs has also improved the quality of postharvest fruit, including phenolic acid (Su et al., 2019), malic acid (Huang et al., 2016), and ursolic acid (Shu et al., 2019). Among those that have received more attention from scientists in recent years are exogenous salicylic and oxalic acid applications to alleviate chilling injury, increase disease resistance and delay senescence of postharvest fruit (Huang et al., 2013; Moosa et al., 2021). It is worth noting that the application of different exogenous natural OAs manages the quality of postharvest fruit through different biochemical mechanisms. Moreover, in recent years, the application form of natural OAs has changed from a single application to a diversified form, for example in the form of an edible coating, combined with ultrasound and UV-C treatment and other chemical substances such as methyl jasmonate (Chen et al., 2016; Khademi et al., 2019; Siboza et al., 2017; Sogvar et al., 2016).

Although many studies have revealed the beneficial effects of exogenous natural OAs in postharvest fruit quality management, the current relevant information is relatively scattered, and there is no relevant review work. Therefore, this work summarizes the role of common natural OAs in improving the quality of postharvest fruit, and discusses the biochemical mechanisms that may be involved, providing a reference for the application of natural OAs in postharvest fruit.

OXALIC ACID

Oxalic acid, the simplest dibasic acid, is a metabolite of carbon metabolism in living organisms and is widely distributed in plants and animals, performing different functions in different living organisms (Zhang et al., 2021). In recent years, more and more studies have found that exogenous oxalic acid application could not only delay the senescence process of postharvest fruit, but also improve the disease resistance of postharvest fruit, reducing the occurrence of postharvest fruit chilling injury and browning, thus effectively extending the shelf life of postharvest fruit (Figure 1) (Table 1).

FIGURE 1. Different roles of oxalic acid in postharvest fruit quality management.

Delaying the senescence

Postharvest fruit firmness, soluble solids content (SSC), titratable acid (TA) content and respiration rate are important indicators of postharvest fruit senescence (Zhang et al., 2020). It was shown that postharvest peach fruit treated with 1 and 5 mM oxalic acid soaks maintained higher firmness and lower respiration rates compared to untreated peach fruit (Zheng et al., 2007). Oxalic acid treatment delayed postharvest peach fruit senescence, which is mainly related to the enhancement of its antioxidant system. The 5 mM oxalic acid treatment significantly increased the activities of peach fruit antioxidant enzymes, including superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) (Razavi & Hajilou, 2016; Zheng et al., 2007). Oxalic acid enhanced the antioxidant system of postharvest fruit owing to the fact that it is a natural antioxidant in itself and can act as a proton donor and metal ion chelator capable of scavenging free radicals, increasing antioxidant enzyme activity, and directly or indirectly affecting antioxidant metabolism (Kayashima & Katayama, 2002; Malenčić et al., 2004). In addition, oxalic acid treatment can also enhance the nonenzymatic antioxidant system of postharvest fruit. In 2 mM oxalic acid-treated fruit, the activity of antioxidant enzymes (SOD and CAT) and the total phenol content and total antioxidant level in litchi peel and pulp tissue are significantly higher (Shafique et al., 2016). Similarly, oxalic acid-treated plum fruit exhibited a delayed senescence process during the postharvest storage period, as evidenced by lower firmness and TA loss and reduced ethylene production compared to control fruit. The activities of antioxidant compounds (phenolics, anthocyanins, and carotenoids) and antioxidant enzymes were higher in oxalic acid-treated plums than in the control fruit during 50 days of postharvest cold storage (Martínez-Esplá et al., 2019).

In another study, mango fruit (Mangifera indica L. cv. Zill) were immersed in 5 mM oxalic acid solution for 10 min at 25°C to study the effect on ripening and decay incidence during storage at room temperature (25°C). The results showed that oxalic acid treatment delayed fruit senescence, including higher firmness and TA content, and reduced the incidence of fruit decay compared to the control fruit. In addition, oxalic acid treatment significantly inhibited postharvest ethylene production in mango fruit and delayed the appearance of peak ethylene release, which was an important reason why oxalic acid delayed postharvest climacteric fruit senescence (Zheng et al., 2007). In oxalic acid-treated jujube fruit, it was also observed that ethylene production was significantly inhibited, and the activity of 1-aminocyclopropane-1-carboxylic acid synthase (ACC), an enzyme related to ethylene biosynthesis, was significantly inhibited (Wang et al., 2009). The mechanism of oxalic acid antagonism against ethylene may be mainly realized with its regulation of Ca²⁺ concentration; oxalic acid can form calcium oxalate with Ca²⁺ of plant tissue cells to control the free state Ca²⁺ level between cell wall and cytoplasm. Ca²⁺, as an important “second messenger” in plant cells, directly influences and regulates intracellular ethylene metabolism (Ping et al., 2002). In addition, it is known that interactions between Ca²⁺ and cell walls play a key role in plant physiology. Ca²⁺ is involved in many mechanisms: for example, stabilization of cell wall structures, acidic growth, ion exchange properties, control of the activities of wall enzymes (Aghdam et al., 2012). All these properties originate from the tight binding of Ca²⁺ to the pectin present in the cell walls. Therefore, oxalic acid could maintain the structural integrity of the cell wall directly by adjusting the Ca²⁺ concentration of the cells of the postharvest fruit, thereby maintaining the firmness and texture of the postharvest fruit. Moreover, oxalic acid can also inhibit the activity of a series of cell wall degrading enzymes by antagonizing ethylene, including polygalacturonase (PG), pectin esterase and pectin methylesterase (PME). Therefore, oxalic acid can directly or indirectly regulate the softening process of postharvest fruit cell walls. The application of exogenous oxalic acid in postharvest banana and pear fruit verified the above view; the application of exogenous oxalic acid maintained postharvest banana fruit firmness, inhibited ethylene production and respiration rate, and enhanced antioxidant enzyme activity; exogenous oxalic acid treatment also maintained postharvest pear fruit firmness and reduced the activity of cell wall degrading enzyme PME (Huang et al., 2013; Kaur et al., 2017).

Inhibiting postharvest disease

As we all know, postharvest fruit disease control has always relied mainly on chemical synthetic fungicides, however, the long-term use of chemical synthetic fungicides can lead to the development of resistance to pathogenic bacteria, and chemical fungicide residues caused by human health is a concern (Zhang et al., 2019). In recent years, studies have found that some natural resistance inducers can control diseases by inducing systemic resistance in postharvest fruit. According to reports, oxalic acid can induce plant systemic resistance to fungi, bacteria, and viruses, and oxalic acid can induce increased plant POD activity and produce new POD isoenzymes and other physiological effects (Zheng et al., 2007; Zhu et al., 2016). Tian et al. (2006) found that oxalic acid treatment induced systemic resistance to Alternaria alternata in postharvest pear fruit, and this resistance was closely related to oxalic acid-induced activities of disease-resistance-related enzymes including polyphenol oxidase (PPO), POD and phenylalanine ammonium lyase (PAL). Wang et al. (2009) suggested that oxalic acid treatment increased the resistance of jujube fruit to Penicillium and was associated with the induction of expression of stress and defense-related proteins such as major allergen, Cu/Zn-SOD and heat shock protein (HSP) 70 in fruit. Similarly, oxalic acid treatment significantly induced resistance to Penicillium expansum during postharvest storage of kiwifruit fruit, as reflected by that oxalic acid treatment increased the activity of a range of defense-related enzymes, including chitinase (CHI), β−1,3-glucanase (GLU), PAL, 4-coumarate CoA ligase, PPO and POD, as well as the content of a number of disease-resistant related active substances, including phenolics, flavonoids and hydroxyproline-rich glycoprotein (Zhu et al., 2016). In addition, besides inducing resistance, the application of oxalic acid in postharvest disease control has a direct inhibitory effect on certain fungal spore germination and mycelial growth. EI Ganaienyrma et al. (2002) found that both 2, 4, 6, 8, and 10 mM concentrations of oxalic acid treatment inhibited the growth of onion wilt mycelium, while 8 mM greatly reduced the germination of spores. Although the direct bacteriostatic effect of oxalic acid is related to its acidity, another study found that when the oxalic acid content was 5 mM or higher, the spore germination and mycelial growth of mango anthracnose were all significantly inhibited regardless of whether the environmental pH was increased or not, indicating that oxalic acid directly inhibits pathogenic bacteria, not only depending on its acidity, but also related to other characteristics (Zheng et al., 2007).

Alleviating chilling injury and browning

Although most of the postharvest fruit are stored at low temperatures, there are some fruits sensitive to low temperatures that would suffer from chilling injury during cold storage, directly causing deterioration in quality. Therefore, improving chilling resistance for low-temperature fruit is essential to extend the shelf life of low-temperature-sensitive fruit (Zhang et al., 2021). Numerous studies have shown that oxalic acid treatment contributes to enhanced cold resistance in postharvest fruit. It was reported that 10 mM oxalic acid treatment significantly alleviated chilling injury symptoms in postharvest green ripe tomatoes stored at 4 ± 0.5°C for 20 days; Compared to control fruit, oxalic acid treatment maintained intracellular energy supply and increased the activity of enzymes related to energy metabolism, including succinic dehydrogenase (SDH), Ca²⁺-ATPase and H⁺-ATPase. This may be part of the reason for oxalic acid alleviating chilling injury in postharvest tomato fruit (Li et al., 2016). In another study, 15 mM oxalic acid treatment significantly alleviated chilling injury development in Hami melons fruit during postharvest cold storage, accompanied by enhanced enzymatic and nonenzymatic antioxidant systems, mainly involving enhanced ASA-GSH cycle (Wang et al., 2018). In addition, it was reported that exogenous oxalic acid application could significantly alleviate chilling injury in postharvest sweet persimmon, apricot, pomegranate and guava fruit (Ehteshami et al., 2021; El-Gawad, 2021; Li et al., 2018; Wang et al., 2016). Therefore, as a green chemical treatment, oxalic acid treatment seems to be a good choice for alleviating chilling injury in postharvest fruit.

In addition, exogenous oxalic acid treatment can be used as an effective antibrowning agent to control the occurrence of postharvest browning. The browning index of litchi fruit was significantly reduced by 2 and 4 mM oxalic acid treatment. Oxalic acid treatment to reduce browning in litchi fruit may be associated with biochemical events such as improving the integrity of litchi fruit cell membranes, preventing the degradation of anthocyanins, and reducing POD activity and the degree of membrane lipid peroxidation (Zheng & Tian, 2006). Oxalic acid as an antibrowning agent also showed promising effects in postharvest lotus roots, banana fruit, and abiu fruit (Ali et al., 2020; Arif et al., 2023; Huang et al., 2013). This is mainly due to the fact that oxalic acid can directly inhibit the activity of POD to inhibit the occurrence of browning as an acidulant (Zhang et al., 2021). And recent studies have revealed that combining oxalic acid with other postharvest treatments can optimize the efficiency of oxalic acid in postharvest fruit antibrowning and preservation. In recent years, edible coated films and biodegradable food packaging films are effective methods for postharvest fruit preservation (Zhang & Rhim, 2022; Zhang et al., 2023). The combination of oxalic acid and chitosan with polyvinyl alcohol coating significantly inhibited the development of postharvest banana fruit peel browning and reduced the activity of cell degradation enzymes, including cellulase and pectinase (Lo'ay & Dawood, 2017). The effect of oxalic acid on browning of litchi fruit was investigated under 5% CO₂ + 1% O₂ controlled atmosphere and compared with air at 5 ± 1°C for 28 days. The oxalic acid treatment reduced oxidative stress and oxidative enzyme activities, delayed browning and maintained markedly higher total anthocyanins and inhibited browning of litchi compared with control (Ali et al., 2021).

SA

SA is a small molecule aromatic OA that widely exists in prokaryotes and plants. And SA is a derivative of cinnamic acid, and its biosynthetic pathway is mainly through the mangiferous acid pathway, in which its intermediate product phenylalanine is synthesized trans-cinnamic acid through PAL, which is transformed into coumaric acid or benzoic acid and finally forms SA. It is well known that SA, as an important endogenous plant phenolic acid, could regulate many physiological and biochemical processes in plants, such as seed germination, stomatal movements, pigment accumulation, photosynthesis, ethylene biosynthesis, heat production, enzyme activities, abscission reversal, nutrient uptake, flower induction, membrane functions, legume nodulation and overall plant growth and development (Ali, 2021). In recent years, SA has been widely used as a plant growth regulator for postharvest fruit quality management (Table 2).

TABLE 1 Summary of some effects of oxalic acid on some postharvest fruits reported by recent studies.

Fruit	Concentration	Results	References
Peach	1 and 5 mM	Inhibition of ripening. Inhibition of relative leakage rate, flesh firmness, respiration rate, superoxide, H2O2 content and LOX activity. Increase in SOD, POD, CAT, APX, and PPO activities.	Zheng et al. (2007)
Plums	2 mM	Ripening delay and chilling injury reduction. Inhibition of ethylene production. Increase in phenolic, anthocyanin and carotenoid contents and SOD, CAT, APX, and POD activities.	Martínez-Esplá et al. (2019)
Mango	5 mM	Inhibition of decay incidence and ripening. Maintaining higher firmness and TA content. Inhibition of ethylene production.	Zheng et al. (2007)
jujube	5 mM	Senescence delay and blue mold resistance enhancement. Inhibition of ethylene production and ACC activity. Increase in major allergen, Cu/Zn-SOD and HSP70 proteins expression.	Wang et al. (2009)
Banana	20 mM	Inhibition of ripening and ethylene production. Retention of firmness. Increase in SOD activity and antioxidant capability.	Huang et al. (2013)
Pear	2, 4, and 6 mM	Senescence delay, chilling injury and weight loss reduction. Increase in firmness, TSS and TA content, and PME activity.	Kaur et al. (2017)
	1 mM	Decrease in disease incidence. Increase in PPO, POD, PAL and β−1,3-glucanase activities.	Tian et al. (2006)
Kiwifruit	5 mM	Decrease in disease incidence, lesion diameter, and patulin accumulation. Increase in CHI, GLU, PAL, 4CL, POD, and PPO activities, as well as the content of phenolics, flavonoids and hydroxyproline-rich glycoprotein.	Zhu et al. (2016)
Tomato	10 mM	Chilling injury, electrolyte leakage and malondialdehyde content reduction. Retention of ATP and ADP content. Increase in SDH, CCO, Ca2+-ATPase, and H+-ATPase activities.	Li et al. (2016)
Hami melon	15 mM	Chilling injury, electrolyte leakage and malondialdehyde content reduction. Retention of ASA and GSH content. Increase in GR, APX, and POD activities.	Wang et al. (2018)
Sweet persimmon	5 mM	Chilling injury reduction. Inhibition of ethylene production, electrolyte leakage, malondialdehyde content, PPO and POD activities. Increase in water-soluble pectin content and PG activity.	Li et al. (2018)
Apricot	5 mM	Chilling injury, electrolyte leakage, and malondialdehyde content reduction. Increase in glucose and fructose content.	Wang et al. (2016)
Litchi	2 and 4 mM	Pericarp browning and electrolyte leakage reduction. Inhibition of anthocyanin degradation and POD activity.	Zheng and Tian (2006)
Lotus root slices	10 mM	Browning, electrolyte leakage, ROS, and malondialdehyde content reduction. Retention of ASA and total phenolic content. Inhibition of PPO and POD activities.	Shi et al. (2018)
Banana	8 and 20 mM	Browning reduction. Inhibition of PPO and POD activities.	Khademi et al. (2019)
Abiu	10 mM	Browning and malondialdehyde reduction. Increase in TA, ASA, total phenolic contents, and firmness. Inhibition of ethylene production and ACC and PPO activities.	Arif et al. (2023)
Guava	3 and 6 mM	Inhibition of chilling injury, and weight loss. Increase in fruit firmness, SSC and ASA content. Inhibition of TA.	El-Gawad (2021)
Pomegranate	5 mM	Chilling injury, H2O2 and malondialdehyde content reduction. Increase in total phenolic, anthocyanin, ASA and TA content. Inhibition of PPO and POD activities.	Ehteshami et al. (2021)

TABLE 2 Summary of some effects of salicylic acid on some postharvest fruits reported by recent studies.

Fruit	Concentration	Results	References
Peach	1 mM	Chilling injury reduction. Increase in Put, Spm, and Spd content and SOD, CAT, GR, and APX activities. Inhibition of LOX activity.	Cao et al. (2010)
Pomegranates	0.5 and 1 mM	Chilling injury reduction. Inhibition of respiration rate and ion leakage. Retention of firmness, sugars, organic acids, total phenolics, and anthocyanins content.	Sayyari et al. (2011)
Plums	1.5 mM	Chilling injury and disease incidence reduction. Inhibition of electrolyte leakage, respiration rate, malondialdehyde and ethylene production, POD and PPO activities. Increase in Put, Spm and Spd contents.	Luo et al. (2011)
Tomato	2 mM	Chilling injury reduction. Inhibition of LOX activity, electrolyte leakage, and malondialdehyde content. Increase in proline content.	Aghdam et al. (2014)
Sweet cherries	1 mM	Retention of firmness, total phenolics and total anthocyanins content and antioxidant activity.	Giménez et al. (2014)
Strawberry	2 mM	Increase in CAT and total antioxidant activities.	Asghari and Hasanlooe (2015)
Apricot	1 mM	Inhibition of respiration rate, ethylene production, and H2O2 content. Increase in PAL activity, total phenols, total flavonoids, SOD, and activity. Increase in hydrophilic total antioxidant activity.	Wang et al. (2015)
Citrus	2 mM	Decrease in disease incidence. Retention of firmness. Increase in and SOD activities, defense-related metabolites and polymethoxylated flavones contents.	Zhu et al. (2016)
Plums	2 mM	Retention of firmness. Inhibition of decay rate, respiration rate, ethylene production, malondialdehyde content, and PME activity.	Sharma and Sharma (2016)
Pomegranate	2 mM	Chilling injury, electrolyte leakage and weight loss reduction. Inhibition of respiration rate, ethylene production. Retention of total phenols and anthocyanins content.	Sayyari et al. (2016)
Agaricus bisporus	0.25 mM	Browning and electrolyte leakage reduction. Retention of total phenols content and PAL, CAT, APX, and SOD activities and DPPH· radical scavenging activity.	Dokhanieh and Aghdam (2016)
Lemon	2 mM	Chilling injury reduction. Increase in HSP70 accumulation, CAT, APX, and GR activities.	Siboza et al. (2017)
Mangosteen	1 mM	Decrease in water loss. Increase in soluble phenolics content and antioxidant activity.	Mustafa et al. (2018a)
Dragon fruit	1 mM	Retention of citric acid content and soluble phenolics content.	Mustafa et al. (2018b)
Grapefruit	2 mM	Inhibition of disease incidence and lesion diameter. Increase in ASA total phenolic and TSS contents, CHI, GLU, and PAL activities.	Shi et al. (2018)
Banana	1 mM	Chilling injury, electrolyte leakage and weight loss reduction. Retention of firmness, TSS, starch index phenol content, and antioxidant activity. Inhibition of PPO activities.	Khademi et al. (2019)
Longan	2 mM	Inhibition of disease incidence. Retention of energy charge level, NADP and NADPH contents, G6PDH, 6PGDH activities	Chen et al. (2020)
Bell pepper	0.2 mM	Chilling injury reduction. Inhibition of electrolyte leakage, malondialdehyde and LOX activity. Retention of water, ASA, and GSH content.	Ge et al. (2020)
Mandarin	4 mM	Inhibition of disease incidence and weight loss. Retention of ASA and total phenolics content. Increase in CAT and SOD activities.	Haider et al. (2020)
Goji berry	2 mM	Inhibition of disease incidence. Increase in CAT, SOD and APX activities. Increase in PAL, C4H, 4CL, CHS, CHI, and CAD activities.	Zhang et al. (2021)
Sweet cherries	0.5 mM	Increase in firmness, TSS, total phenolics, flavonoids, and total anthocyanins contents, SOD, CAT, APX, GR, GPX, PAL, 4CL, and C4H activities, repairing ability on mechanical damage.	Gu et al. (2022)

TABLE 3 Summary of some effects of phenolic acid on some postharvest fruits reported by recent studies.

Fruit	Phenolic acid	Concentration	Results	References
Apple	Chlorogenic acid	0.5 mM	Peel browning reduction. Retention of total flavonoids content, PAL and C4H activities. Inhibition of lignin content, PPO and POD activities.	Wang et al. (2014)
Nectarine	Chlorogenic acid	25 and 50 mg L−1	Inhibition of softening, ethylene production, decay rate and malondialdehyde content. Retention of soluble solids and total phenolics content, ABTS, DPPH, FRAP, and CUPRAC radical scavenging activity.	Xi et al. (2017)
Nectarine	Chlorogenic acid	50 mg L−1	Inhibition of ROS production and POD activity. Retention of SOD, CAT, APX, GR, GST, and MDHAR activities.	Xi et al. (2017)
Apple	Caffeic acid	300 mg L−1	Inhibition of incidence and lesion diameters. Retention of total phenols, flavonoids and lignin content and PAL, C4H, and 4CL activities.	Zhang et al. (2020)
Grape	Cinnamic acid	5 and 10 mM	Inhibition of incidence and disease severity. Retention of PPO and POD activities.	Zhang et al. (2015)
Mandarin	Cinnamic acid	4000 mg L−1	Inhibition of incidence and disease severity	Cheng et al. (2022)
Apple	Ferulic acid	0.80 g L−1	Inhibition of incidence and lesion diameters. Retention of total phenols, flavonoids, and lignin content and PAL, CAD, C4H, laccase, PPO, POD, CCR, and 4CL activities.	He et al. (2019)
Tomato	Ferulic acid	100 μM	Inhibition of incidence and lesion diameters. Retention of PAL, GLU, CHI and PPO activities. Upregulation of PR1, NPR1, MYC2, LoxD expressions.	Shu et al. (2021)
Tomato	Ferulic acid	100 μM	Inhibition of chilling injury. Upregulation of SlMAPK3, SlCBF1, and SlICE1 expressions	Shu et al. (2022)
Jujube	p-Coumaric acid	100 mg L−1	Inhibition of incidence and lesion diameters. Retention of SOD, CAT, APX, PAL, C4H, 4CL, CHI, and POD activities.	Yuan et al. (2019)
Sweet cherry	p-Coumaric acid	100 and 200 mg L−1	Weight loss, SSC and decay rate reduction. Retention of total flavonoids, total phenolics, anthocyanins content, PAL, C4H, 4CL activities and DPPH and ABTS+ radical scavenging capacity.	Liu et al. (2020)

Delaying the senescence

To improve the postharvest nutritional and functional quality of apricot fruit, Wang et al. (2015) treated the fruit with SA by vacuum infiltration. Apricot fruit treated with SA showed a lower level of ethylene production and the respiration rate, maintained the higher firmness and titratable acid content than control fruit. In addition, the results showed that significantly higher hydrophilic total antioxidant activity (H-TAA) and content of phenolic acids, flavonoids were found in fruit treated with SA. Thus, this work suggested that postharvest SA treatment not only delayed postharvest apricot fruit senescence, but also induced the accumulation of nutrients, such as hydrophilic antioxidant substances (Wang et al., 2015). A similar effect of SA on increasing hydrophilic antioxidant substances also has been reported in sweet cherries. Moreover, SA treatment also delayed the senescence of postharvest sweet cherries and improved their quality during storage including color (chroma index), fruit firmness and total acidity (Giménez et al., 2014). Strawberries are highly susceptible to postharvest losses, and SA treatment significantly increased the total antioxidant activity and the activities of a range of antioxidant enzymes, including CAT and POD of postharvest strawberries fruit (Asghari & Hasanlooe, 2015). In postharvest plum fruit, SA treatment reduced ethylene production and the respiration rate, and also maintained fruit firmness by decreasing the activity of wall degrading enzymes (Sharma & Sharma, 2016). In addition, SA treatment also retarded the ripening and senescence process in postharvest mangosteen (Mustafa et al., 2018), dragon (Mustafa et al., 2018), sugar apple (Mo et al., 2008), kiwifruit (Wang et al., 2022; Zhang et al., 2003), pear (Shi et al., 2021), and mango (Ding et al., 2007; Reddy et al., 2016). Therefore, SA, as a promising postharvest preservative, not only could delay postharvest fruit senescence, but also promote the production of other beneficial secondary metabolites in the fruit and increase the nutritional value of the postharvest fruit.

Inhibiting postharvest disease

The important role of SA in postharvest fruit quality management is in the defense response to pathogenic bacteria. The resistance response of plants to pathogen infestation is generally divided into two categories, namely systemic acquired resistance (SAR) and induced systemic resistance (ISR). SAR requires the signal molecule SA and is associated with accumulation of pathogenesis-related (PR) proteins, which are believed to contribute to resistance, which leads to early acquisition of resistance to the pathogen (Romanazzi et al., 2016). In recent years, the induction of plant resistance by biological, chemical or physical treatments has been considered as a sustainable strategy for managing postharvest fruit and vegetable decay, and SA is a promising chemical inducer. Postharvest fruit black mold caused by Alternaria alternata caused considerable economic loss, and it was reported that SA treatment significantly suppressed the disease incidence and lesion area of postharvest jujube fruit black mold, mainly due to the induced defense system of the fruit by SA, which mainly includes a number of defense enzymes, including PAL, CHI, and GLU (Cao et al., 2013). Furthermore, in vitro inhibition experiments demonstrated that the low concentrations of SA used for fruit treatment did not exhibit direct inhibition against Alternaria alternata in vitro, suggesting that the beneficial effect of SA on fruit protection is due to its ability to activate several highly coordinated defense systems, rather than its direct bactericidal activity (Cao et al., 2013). In another study, the effect of SA on the defense response of Colletotrichum gloeosporioides in postharvest mango fruit and its mechanism were investigated by in vitro and in vivo tests. The SA treatment significantly decreased the disease incidence and lesion area. SA treatment stimulated the activities of CHI, GLU, PAL and PPO, as well as the content of total phenolic compounds and lignin in mango fruit (He et al., 2017). In addition, SA treatment effectively maintained the firmness of mango fruit by inhibiting the conversion of insoluble protopectin to water-soluble pectin. Correlation analysis showed that fruit firmness was negatively correlated with higher incidence (He et al., 2017). This is mainly related to the fact that the increase in some secondary metabolites due to SA treatment can maintain cell wall modification and also related to the decrease in ethylene production due to SA treatment leads to a decrease in cell wall degrading enzyme activity. And chemical modification of the fruit cell wall could directly lead to an increase in physical barriers to fungal colonization (Zhang & Jiang, 2019). An earlier study showed that SA enhanced the resistance of sweet cherry fruit to Penicillium expansum, resulting in lower disease incidence and lesion diameter. Based on proteomic analysis, 13 and 28 proteins were identified in SA-treated cherry fruit at early (A) and late (B) maturity, respectively. Seven antioxidant proteins and three PR proteins were identified in stage A, while five HSPs and four dehydrogenases were identified only in stage B (Chen et al., 2008). This indicates that the SA treatment induced postharvest cherry fruit defense system mainly at earlier maturity stages. In addition, recent studies have also shown that SA treatment was effective in controlling postharvest decay of mandarin (Haider et al., 2020) and goji berry fruit (Zhang et al., 2021).

Alleviating chilling injury

In addition, the chilling injury in postharvest fruit due to low temperature is also cold stress, so SA as an effective plant activator could also alleviate chilling injury in postharvest fruit by inducing adverse resistance (Table 1). Cao et al. (2009) observed that the SA treatment effectively alleviated CI in postharvest peach fruit. Alleviating of CI was associated with the increase in antioxidant enzymes including SOD, CAT, APX and GLU. In addition, the SA treatment increased the levels of polyamines including putrescine, spermidine, and spermine in peach fruit. This work suggested that the SA treatment may be a useful technique to alleviate chilling injury in cold-stored peach fruit and the reduction in chilling injury by the SA treatment may be due to the induction of antioxidant enzymes and increase in polyamine levels (Cao et al., 2009). Similarly, a reduction in chilling injury associated with an increase in polyamines was observed in SA-treated plum fruit, and the alleviate of chilling injury by SA was reflected in reduced leakage, malonaldehyde content, delayed activities of PPO and POD (Luo et al., 2011). Polyamines are ubiquitous biogenic amines that are associated with a wide range of cellular functions in widely distributed organisms. Polyamines have been reported to bind to negatively charged molecules such as phospholipids, proteins and nucleic acids, which contribute to their maintenance of cell membrane stability in plant stresses (Hussain et al., 2011). Therefore, the biochemical mechanism of the reduction of postharvest fruit chilling injury due to SA treatment is related to the increase of polyamines. In addition, as mentioned earlier, SA was able to induce an enhancement of the postharvest fruit antioxidant system, mainly including an increase in the activity of some antioxidant enzymes and the accumulation of antioxidant substances, which also helped to resist the redox homeostasis of fruit cells caused by chilling injury. For instance, SA treatment reduced the development of chilling injury in postharvest lemon fruit and inhibited the accumulation of intracellular reactive oxygen species (ROS) during cold storage. In addition, biochemical analysis of the fruit showed that SA treatment caused an increase in the activity of antioxidant enzymes, including CAT, APX and glutathione reductase (GR), and that SA also caused the expression of HSP, which also contributed to the reduction of chilling injury in lemon fruit (Siboza et al., 2017). HSPs constitute a family of stress response proteins with molecular weights ranging between 15 and 115 kDa. At the molecular level, the expression of heat shock genes encoding HSPs is regulated by heat shock transcription factors, which have the ability to sense heat and/or cold stress and then respond to cold stress by mediating downstream related genes (Aghdam & Bodbodak, 2013). In addition to antioxidant enzyme system, SA treatment could also promote nonenzymatic antioxidant system in postharvest fruit, mainly including accumulation of some antioxidants. Sayyari et al. (2016) reported that SA treatment reduced chilling injury by increasing the content of total phenols, anthocyanins and ASA in postharvest pomegranate fruit. Similarly, a reduction in chilling injury accompanied by an increase in antioxidants was observed in SA-treated postharvest banana fruit (Khademi et al., 2019) and bell peppers (Ge et al., 2020). Therefore, SA treatment could alleviate chilling injury by affecting multiple physiological metabolic pathways in postharvest fruit.

ASA

ASA, also known as vitamin C, is widely found in fresh fruit and vegetables, and is an essential nutrient in people's daily life. ASA is known to be an effective antioxidant and free radical scavenger, and its participation in the important ASA-GSH cycle in plant cells maintains the homeostasis of ROS in plant cells. In addition, ASA is also involved in some other physiological metabolic pathways, such as metal ion transport, in addition to its role as an antioxidant in plant cells (Smirnoff, 2018). As a natural antioxidant with nutritional value, the application of ASA in postharvest fruit preservation is mainly reflected in the way it acts as an antibrowning agent to reduce browning of fresh-cut fruit (Gonzalez-Aguilar et al., 2008). However, since ASA is easily oxidized in the ambient state, it does play its role as an antibrowning agent only for a short period of time, and it has also been reported that ASA as an antibrowning agent exacerbates the growth of microorganisms and softens the fruit tissue. In addition, when the ASA has been completely oxidized to dehydro-ASA, the reaction resulted in convert back phenolic compounds to quinones, quinones again can be accumulated and undergo browning (Yan et al., 2017). Therefore, in order to increase the efficiency of the application of ASA in the antibrowning of fresh-cut fruit, studies in recent years have focused on the application of ASA together with edible coatings for the quality management of fresh-cut fruit. Robles-Sánchez et al. (2013) developed an alginate-ASA composite edible coating and applied it to fresh-cut mango browning, showing that the edible alginate-ASA composite coating treatment significantly inhibited browning and maintained the content of bioactive substances, including p-hydroxybenzoic acid and ellagic acid. And it is important to note that the antioxidant of fresh-cut fruit containing ASA composite coating treatment was significantly increased, which was not entirely due to the accumulation of antioxidants in the fruit, but rather the contribution of exogenous ASA. Similarly, in another study, an edible coating containing ASA were developed using carboxymethyl cellulose as a polymer for improving the quality of fresh-cut apples. The results showed that the composite coating treatment inhibited browning of fresh-cut apples and maintained firmness as well as bioactive substances content. In addition, the composite coating treatment inhibited the activity of PPO and POD, which could be attributed to the improvement of redox status in fruit cells due to ASA and the barrier effect of edible coating on oxygen (Saba & Sogvar, 2016). In addition, in addition to combining with edible coatings to reduce ASA's own oxidation to improve its efficiency in antibrowning of fresh-cut fruit. Recent studies have shown that there are other ways to use ASA in combination to improve its efficiency as an antibrowning agent. An earlier study showed that combined ASA and ultrasound treatment was more effective in reducing POD and PPO activities in fresh-cut apples (Jang & Moon, 2011). Considering the optimal efficiency of the combined treatment, an idea is to use ASA in combination with other antimicrobial agents to achieve the dual management of browning and microbial growth of fresh-cut fruit (Yan et al., 2017).

In addition to its application as an antibrowning agent for quality management of fresh-cut fruit, ASA is also used as a preservative for postharvest quality management of intact fruit. A recent study found that ASA soaking treatment maintained the quality of postharvest longan fruit during storage, including reduction in weight loss and peel browning index as well as accumulation of nutrient bioactive substances. This was mainly due to the fact that ASA, as an important antioxidant, improved the redox homeostasis of postharvest longan fruit cells (Liu et al., 2021). In addition to single treatments, ASA was also applied to delay postharvest fruit senescence in combination with edible coatings treatment. A chitosan coating containing ASA was developed and applied to postharvest strawberry fruit by Saleem et al. (2021). The results showed that the composite coating treatment maintained the quality, inhibited cell wall degrading enzyme PG, cellulase and pectin methyl esterase activities, and maintained firmness of postharvest strawberry fruit. The combined application of ASA and controlled atmosphere also suppressed browning index, soluble quinones, and activities of PPO and POD of postharvest litchi fruit. The combination of controlled atmosphere along with ASA reduced weight loss and maintained higher anthocyanins, total phenolics, membrane integrity, APX, CAT, GR, and SOD activities compared with control fruit (Ali et al., 2021). Although ASA itself does not have significant antimicrobial activity, recent studies have shown that ASA could enhance the efficiency and capacity of postharvest fruit biocontrol as an enhancer compound to supplement with yeast (Yang et al., 2020). Yang et al. (2017) indicated the possible mechanisms involved in enhancing the biocontrol efficiency of P. caribbica by ASA under oxidative stress, including the cellular oxidative damage, the antioxidative enzymes activity and the antioxidative substances activity of P. caribbica. In addition, ASA has shown promising potential as an excellent antioxidant against postharvest fruit cold stress. ASA treatment improved chilling tolerance in postharvest banana fruit by promoting antioxidant enzyme activity and limiting active oxygen species production to protect cell membrane integrity (Lo'Ay & El-Khateeb, 2018). Also, the combination of ASA and active coating also inhibited the occurrence of chilling injury of postharvest citrus fruit during cold storage by promoting the antioxidant system (Lo'ay & Dawood, 2019). Therefore, ASA could play a different role as an excellent antioxidant not only to inhibit browning of fresh-cut fruit, but also to control postharvest fruit diseases and chilling injury.

CA, MALIC ACID, AND ACETIC ACID

CA, an important natural OA, is easily soluble in water and is a natural preservative and food additive. Since CA contains three carboxyl groups, CA is highly acidic and could inhibit the growth of most microorganisms in food. And as an acidifier, CA can be used to reduce browning and microbial population control in fresh-cut fruit. Treatment with 0.1 M CA significantly suppressed surface browning and decay rate, and reduced the loss in eating quality associated with the contents of ASA, total soluble solid and titratable acidity of fresh-cut Chinese water chestnut. Notably, the low concentration of CA treatment promoted the activity of PPO and accelerated the browning of fresh-cut Chinese water chestnut (Jiang et al., 2004). Similarly, in another study, an increased browning index was observed in fresh-cut apples treated with CA alone, whereas the combined treatment with UV-C and CA significantly alleviated browning and quality deterioration in fresh-cut apples (Chen et al., 2016.) Therefore, the key concentration of CA should be paid attention to when it is applied as an antibrowning agent to control browning of fresh-cut fruit. To investigate the mechanism of the effect of CA on browning of fresh-cut fruit, the sulfuric acid solution with the same pH as CA was used to treat fresh-cut potatoes and found that CA treatment inhibited browning, while sulfuric acid treatment did not inhibit browning, which suggesting that the inhibition of browning in fresh-cut fruit by CA is not due to acidification (Tsouvaltzis & Brecht, 2017). This suggests that CA treatment, not only as an acidifying agent, but can improve fruit quality by affecting other physiological metabolism of fruit cells. A previous study has shown that CA treatment reduced the respiration rate of fresh-cut mango fruit (Chiumarelli et al., 2011). Although CA treatment did not inhibit ethylene production of postharvest peach fruit, CA was significantly effective in maintaining firmness, inhibiting decay, and preventing the decline of titratable acid. Furthermore, CA treatment significantly affected carbon metabolism, including sugar metabolism and OA metabolism, in postharvest peach fruit cells (Yang et al., 2019). Thus, CA, as an important substrate of the tricarboxylic acid cycle in biological cells, could be involved in managing quality of postharvest fruit by affecting the basic carbon metabolism-related pathways. Malic acid, a naturally occurring OA similar to CA that is widely present in fresh fruit, has also been shown to play an important role in the carbon metabolism of postharvest tomato fruit (Centeno et al., 2011). In addition, a recent study has shown that exogenous malic acid treatment reduced cold damage by promoting the antioxidant system in postharvest banana fruit (Huang et al., 2016).

Whereas acetic acid, although also present in fruit, is mostly present in the form of esters, the acetyl group in acetic acid, is the basis of all life in biochemistry. When it is combined with coenzyme A, it becomes the center of carbohydrate and fat metabolism. The application of acetic acid in postharvest fruit quality management is mainly in disease control and could be applied in the form of both solution immersion and steam fumigation. Fumigation with acetic acid vapor has proven to be very effective in disinfecting postharvest pear and grapes fruit (Sholberg et al., 2004; Venditti et al., 2017). A subsequent study also showed that acetic acid vapor treatment was effective in suppressing postharvest citrus decay caused by Penicillium digitatum infection during the shelf life (Venditti et al., 2009). Acetic acid vapor treatment for postharvest fruit disease control was mainly due to its direct antibacterial activity. Hassenberg et al. (2010) showed that acetic acid vapor treatment significantly reduced the number of microorganisms on the surface of postharvest strawberries and reduced the incidence of gray mold. Moreover, acetic acid solution treatment is also effective in controlling postharvest fruit diseases. For example, the germination of P. expansum spores more than 6 Log cycles in vitro was inhibited in 1% acetic acid solution, and it also inhibited the incidence of postharvest apple infection with P. expansum. In addition, acetic acid treatment had no significant application on postharvest apples for quality during storage, including changes in acidity, total soluble solids, total solids, texture and color (Radi et al., 2010). Similarly, treatments of tomato fruit with different concentrations of glacial acetic acid wither liquid or as vapor significantly reduced the growth of Alternaria alternata and Botrytis cinerea in both (in vitro and in vivo) (Alawlaqi & Alharbi, 2014). And a recent study showed that ultrasonic atomization of acetic acid treatment reduced postharvest red bayberry fruit decay, without significant effects on its soluble solids and titratable acids (Perkins et al., 2017). Therefore, acetic acid is a promising antimicrobial agent that can be used in combination with other treatments for postharvest fruit quality management. However, since acetic acid is corrosive, the risk of applying acetic acid to improve postharvest fruit quality is to control the concentration of acetic acid to avoid damage to the fruit, and for acetic acid fumigation applications attention should be paid to damage and corrosion of equipment.

PHENOLIC ACID

It is well known that the main nutrients in fresh fruit are phenolic substances, which include phenolic acids, flavonoids and nonflavonoids, according to the molecular structure and number of phenolic rings (Li et al., 2022). As the structure of phenolic substances is aromatic molecules with hydroxyl groups, phenolic substances generally have high antioxidant properties, while phenolic acids are a class of phenolic substances with carboxyl groups, the common ones are chlorogenic acid, ferulic acid, coumaric acid, gallic acid, caffeic acid, gallic acid, and sinapic acid, etc. (De Souza et al., 2019). In addition to their antioxidant properties that promote health, phenolic substances also have broad-spectrum antimicrobial properties, which are mainly derived from the chemical properties of their hydroxyl groups (Lima et al., 2019). Moreover, most natural phenolic acids are products in the phenylalanine pathway in plant cells and are important molecules involved in the regulation of plant adversity stress and physiological metabolism. Therefore, the search for promising postharvest fruit preservatives from natural phenolic acids is a viable strategy (Table 3).

Chlorogenic acid

Chlorogenic acid (3-O-caffeoylquinic acid), is an important phenolic compound belonging to the hydroxycinnamic acid. The compound's chemical structure consists of a caffeic acid moiety and a quinic acid moiety (Santana-Gálvez et al., 2017). Chlorogenic acid has been reported to be one of the most abundant free phenolic acids in a variety of fresh fruit and has been strongly associated with plant defense responses. In recent years, attention has been paid to the potential of chlorogenic acid in postharvest fruit quality management. It was found that apple varieties sensitive to peel browning had lower levels of free total flavonoids and chlorogenic acid in the peel, and therefore treatment of apple fruit with exogenous chlorogenic acid was found to inhibit the occurrence of peel browning. This was mainly due to the fact that chlorogenic acid treatment increased the PAL and CHI enzyme activities and related gene expressions as well as decreased the PPO and POD enzyme activities and related gene expressions (Wang et al., 2014). The treatment of postharvest nectarine fruit with chlorogenic acid could delay senescence and maintain better quality, including the reduction in ethylene production, softening, decay rate and malondialdehyde accumulation. Chlorogenic acid treatment also promoted the antioxidant system of postharvest nectarine fruit, mainly including an increase in the content of some antioxidants (Xi et al., 2017). And the proteomic analysis of chlorogenic acid-treated nectarine fruit also showed that chlorogenic acid alleviated oxidative stress of postharvest nectarine fruit during senescence mainly by affecting the expression of antioxidant metabolism and defense-related proteins (Xi et al., 2017). The chlorogenic acid treatment reduced P. expansum infection by ISR of postharvest peach fruit, and the increase in resistance to P. expansum caused by the chlorogenic acid treatment was mainly due to the expression of genes related to the SA signaling pathway. In addition, in vitro inhibition assays demonstrated that applied concentrations of chlorogenic acid had no inhibitory effect on P. expansum in vitro. Therefore, chlorogenic acid treatment was used to control the disease by inducing an increase in postharvest peach fruit resistance (Jiao et al., 2018). Chlorogenic acid itself has antifungal activity and can inhibit spore germination and mycelial growth of several pathogenic fungi associated with horticultural crops by disrupting the cell walls of the fungi (Martínez et al., 2017). Zhang et al. (2020) found that chlorogenic acid inhibited spore germination and mycelial growth of Diaporthe sp in a concentration dependent manner, which was mainly due to the impairment of mitochondria caused by the ROS burst caused by chlorogenic acid. Moreover, exogenous chlorogenic acid treatment also inhibited disease development in postharvest kiwifruit due to Diaporthe sp infection, which suggesting that chlorogenic acid can control postharvest fruit disease not only by inducing resistance but also through direct fungal inhibition activity in vitro. This is mainly due to the difference between the concentration of chlorogenic acid and the species of the pathogenic fungi.

In addition, the application of chlorogenic acid in postharvest fruit quality management is also reflected in the application of its modified compounds. And the grafting of chitosan and chlorogenic acid was shown to be synthesized by a green radical-mediated method, and the grafted chitosan-chlorogenic acid complex showed increased antimicrobial activity and antioxidant activity. Importantly, the edible coating treatment prepared from chitosan-chlorogenic acid effectively retarded postharvest peach fruit senescence, mainly in terms of better maintained firmness, soluble solids contents, titratable acidity, and ASA contents (Jiao et al., 2019). Similarly, in a previous study, a complex of chitosan and SA was prepared and an edible coating was prepared for application with postharvest cucumber preservation. The results showed that the chitosan-SA complex coating treatment increased the content of endogenous SA, promoted the activities of antioxidant enzymes SOD, CAT, APX and GR, resulting in reduction in the chilling injury of postharvest cucumber. Thus, natural OAs can be applied not only as additives together with edible coating treatment for postharvest fruit preservation, but also grafted onto biomolecules by chemical modification to form complexes with enhanced functional properties.

Other phenolic acid

In addition to chlorogenic acid, other phenolic acids have shown potential to be used as natural fruit preservatives. Caffeic acid is one of the common phenolic acids found in apples and other fruit. As an important synthetic precursor of lignin, exogenous caffeic acid can enter the phenylpropane pathway, leading to an increase in lignin monomers (Bubna et al., 2011). Exogenous caffeic acid treatment was reported to suppress gray mold by promoting the phenylalanine pathway in postharvest apple fruit. Specifically, exogenous caffeic acid treatment activated POD, PPO, PAL, cinnamic acid-4-hydroxylase and 4CL and also increased the content of total phenols, flavonoids and lignin of postharvest apple fruit (Zhang et al., 2020). In addition, in another study, caffeic acid-chitosan complexes were synthesized enzymatically by using laccase from Pleurotus ostreatus as a catalyst. Better quality was observed in postharvest mulberry fruit treated with caffeic acid-chitosan complex coating treatment, including a reduction in weight loss and accumulation of bioactive substances (Yang et al., 2016).

Cinnamic acid is widely used for the preservation of agricultural products due to its antiseptic and bactericidal effect. It is a natural phenolic compound and aromatic carboxylic acid, which has been approved by FDA for the use in perfumes, pharmaceuticals, and food industries. Recent studies have revealed that cinnamic acid can be used as a postharvest fruit disease preservative through three main roles: first, cinnamic acid can act as an antifungal agent to directly inhibit the growth of fungi; second, cinnamic acid can act as an inducer to enhance the biocontrol efficacy of microbial antagonists; and finally, cinnamic acid can also act as a natural exciton to induce resistance in fruit. For example, cinnamic acid treatment inhibited Botrytis cinerea growth by damaging the cell membrane and resulting in elevated levels of ROS. At the same time, cinnamic acid treatment significantly stimulated the activities of PPO and POD, which are closely related to plant resistance. This suggests that cinnamic acid is effective in controlling postharvest gray mold of fresh grapes by inhibiting the growth of pathogenic bacteria and inducing host resistance (Zhang et al., 2015). Similarly, in another study, cinnamic acid treatment was found to be effective in controlling postharvest citrus sour rot by disrupting the integrity of the plasma membrane, resulting in the intracellular contents exosmosis and finally leading to the death of the Geotrichum citri-aurantii (Cheng et al., 2022). In addition, the combined application of cinnamic acid and Cryptococcus laurentii enhanced the biocontrol efficacy of Cryptococcus laurentii against Penicillium italicum and reduced the incidence of blue mold caused by Penicillium italicum in postharvest mandarin fruit (Li et al., 2019).

Ferulic acid, the most common phenolic acid in plants and one of the derivatives of cinnamic acid, has excellent antioxidant and antibacterial activities (Zhang et al., 2023). Therefore, the main application of ferulic acid in postharvest fruit preservation is disease control. First, ferulic acid has direct antimicrobial activity. Hernández et al. (2021) extracted ferulic acid from orange fruit peels and found that ferulic acid had a significant inhibitory effect against the Monilinia fructicola, Botrytis cinerea and Alternaria alternata. Moreover, exogenous ferulic acid as an endogenous fruit polyphenol can effectively induce disease resistance in postharvest fruit. It was reported that postharvest ferulic acid treatment effectively reduced the disease incidence and lesion diameters of gray mold in postharvest apple, which was mainly due to the stimulation of phenylalanine metabolic pathway and phenolic accumulation by ferulic acid (He et al., 2019). Increased disease resistance to Botrytis cinerea was also observed in postharvest tomato fruit treated with ferulic acid, accompanied by upregulation of a number of genes in pathways related to SA and jasmonic acid signaling pathway, including PR1, NPR1, MYC2, LoxD (Shu et al., 2021). And ferulic acid can also play an important role in inducing resistance in cold stress of fruit. It has been reported that exogenous ferulic acid treatment enhanced chilling tolerance in tomato fruit by upregulating the gene expression of CBF transcriptional pathway in MAPK3-dependent manner (Shu et al., 2022). The p-coumaric acid is also an antibacterial and antioxidant phenolic acid in plants (Zhang et al., 2023). Studies in recent years have shown that p-coumaric acid is also effective in controlling postharvest fruit diseases. For example, exogenous p-coumaric treatment inhibited fungal growth in vitro and significantly reduced black spot rot caused by Alternaria alternata in postharvest jujube fruit. p-Coumaric acid regulated the expression of some genes encoding antioxidant enzymes and related enzymatic activities, enhanced the metabolism of the phenylpropane pathway, and activated the expression of genes encoding PR proteins (Yuan et al., 2019). Similarly, exogenous p-coumaric acid treatment inhibited the mycelial growth of Botrytis cinerea and Penicillium expansum. And p-coumaric acid treatment reduced the incidence of disease in postharvest cherry fruit, which was mainly attributed to the activation of the phenylpropane pathway and cell wall modifications due to phenolic accumulation. In addition, p-coumaric acid reduced the production of bacitracin in P. expansum (Liu et al., 2020). Therefore, some natural phenolic acids can be involved in postharvest fruit quality management from multiple perspectives.

TERPENOIDS ACID

A number of terpenoids are widely distributed in the plant and have antibacterial, antioxidant and antiviral properties. And terpenoids are usually found in the waxy layer on the surface of fruit, thus being the first line of defense against pathogenic microbial infections. Therefore, recent studies have focused on the isolation of natural compounds with antimicrobial activity from the waxy layer of plants for food preservative applications. Among them, some terpenoids acid have shown promising potential, such as ursolic acid and oleanolic acid. Ursolic acid (3β-hydroxy-urs-12-en-28-oic-acid) is a pentacyclic triterpenoid carboxylic compound (C₃₀H₄₈O₃), which may occur in the free acid form or as aglycones for triterpenoid saponins. Its isomer, oleanolic acid (3β-hydroxy-olea-12-en-28-oic-acid), presents different substitution of the methyl group, but they have similar molecular structures and pharmacological activity (López-Hortas et al., 2018). Most of the studies on terpenoids acid have been focused on their pharmacological activity and antibacterial effects, while in recent years they have been found to have inhibitory effects on some postharvest fruit fungi as well. It was reported that ursolic acid treatment led to disturbance of membrane permeability and integrity, induced accumulation of intracellular ROS in Alternaria alternata, and resulted in disruption and lysis of the pathogen morphology, leading to direct antifungal activity. In addition, exogenous ursolic acid application significantly inhibited the development of black spot rot of apple fruit, which was mainly attributed to the activity of fruit-related defense enzymes induced by ursolic acid. These results suggest that, in addition to direct antifungal activity, ursolic acid induces a defense response of apple fruit to postharvest pathogen invasion (Shu et al., 2019). However, there is limited information on the study of terpenoids acid in postharvest fruit regulation. As natural substances with high content in fruit wax layer, terpenoids acid are excellent candidates for antifungal preservatives and natural plant inducers, so future research should develop some natural terpenoids acid with postharvest fruit preservation ability.

CONCLUSION AND OUTLOOK

In conclusion, this work indicates that numerous natural OAs could be used not only as food preservatives, but also for postharvest fruit quality management. Numerous studies in recent years have shown that natural OAs can be used as postharvest fruit preservatives to improve postharvest fruit quality, including delaying senescence, controlling disease, alleviating chilling injury and inhibiting browning. Different natural OAs have different roles in postharvest fruit quality regulation; for example, oxalic acid can be used in postharvest fruit as an ethylene inhibitor to delay postharvest fruit senescence and as a plant elicitor to enhance postharvest fruit resistance to diseases and low temperatures to inhibit the occurrence of diseases and chilling injury. In addition, oxalic acid can also be used as an antibrowning agent to inhibit the occurrence of postharvest fruit browning. SA and other phenolic acids are mainly used as plant elicitor to activate the postharvest fruit defense system and reduce the occurrence of postharvest fruit diseases and enrichment of active nutrient substances. It is worth noting that some natural OAs are simultaneously involved in postharvest fruit quality management in multiple roles; for example, some OAs with antifungal activity can be involved in postharvest fruit disease control through their direct antifungal action, induction of postharvest fruit resistance and as compensators to enhance the efficiency of antagonistic microorganisms. Therefore, numerous small molecule natural OAs are promising preservatives. And the key about the application of OAs in postharvest fruit should be noted in addition to the dose effect, the chemical modification of natural organic counts may make them better for postharvest fruit preservation efficiency. However, the current limitations of various natural OAs for large-scale industrial applications are mainly the processing methods and effectiveness.

Future research should focus on the development of some new natural OAs, such as those found in terpenoids. In addition, there is limited information on the contribution of natural OAs to postharvest fruit flavor, and some natural OAs, such as CA, are key substrates in the metabolism of postharvest fruit flavor substances and therefore are likely to affect postharvest fruit flavor formation. Since the lack of postharvest fruit flavor is currently a major concern for consumers, there seems to be potential to use OA treatments to improve postharvest fruit flavor. Although some natural OAs are effective for postharvest fruit quality improvement, they still do not achieve the desired effect with alone application, so future research should focus on the combined application of other postharvest treatments such as ultrasound, cold plasma and pulsed light, among other emerging technologies.

ACKNOWLEDGMENTS

The authors have nothing to report.

CONFLICT OF INTEREST STATEMENT

The authors confirm that they have no conflict of interest to declare for this publication.