Physicochemical and Thermal Characteristics of Onion Skin from Fifteen Indian Cultivars for Possible Food Applications

Abstract

Every year tons of onion waste is produced worldwide. The dried outer onion skin contributed up to 70% of this waste. Outer-dried skins of fifteen prominent onion cultivars from India were selected for the study. A comparative study was done for proximate profiling, thermal characteristics, functional grouping, and mineral contents. Skin of cv. “NHRDF Red” contained the highest amount of crude protein (5.97 ± 0.15 g/100 g), ash (12.24 ± 0.59 g/100 g), and fiber (8.28 ± 0.20 g/100 g), whereas cv. “Pusa Red” possessed the highest amount of total fat (0.47 ± 0.02 g/100 g) and the maximum carbohydrates (76.66 ± 0.56 g/100 g) were found in “Pusa Riddhi.” Mineral analysis showed that cv. “NHRDF Red” had the maximum concentration of all 9 minerals along with sulphur content. Fourier transform infrared spectroscopy analysis explored the various metabolites present in each cultivar. The thermal analysis explored cv. “Agrifound Dark Red” as highly thermally stable having 70.98% residual mass. The lowest Tg temperature range was found between 64.4°C and 90.6°C for “Agrifound Dark Red.” Skin of cv. “NHRDF Red” was reported as the best source of protein, fiber, and minerals, which may be utilized for developing a food product.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Onion (Allium cepa L.) is the second most important cultivated horticultural commodity after tomato. Globally, 99,968,016 tons of onion is produced from an area of 5,192,651 hectares [1]. China holds the first position in production with 18,122,435 tons and India is the second producer of onion (11,936,706 tons) in the world [1]. Up to 25% of qualitative and quantitative postharvest losses occurred during storage. Processing of onion has also increased which resulted in huge processing waste. Skin contributes maximum to processing waste. The European Union onion processing industry generates more than 500,000 tons of onion waste annually mainly in the UK, The Netherlands, and Spain. This has become an environmental problem. The maximum amount of onion skin is generated by industries during peeling. This wasted skin is not suitable as fodder in higher concentrations due to its peculiar aroma [2].

Various studies explored onion skin as a rich source of polyphenols, antioxidants, fructooligosaccharides, and dietary fibers [3–5]. Moreover, onion skin extract has also been reported as an anticarcinogenic, hypocholesterolemic, good cardiovascular agent, and having an antiasthmatic effect [4, 5]. Onion flavonoids also show antiproliferative activity that can degrade excess polyamine concentration in the body [6]. Hence, onion skin powder may be incorporated into bakery products and bread which are good sources of energy and consumed widely as well [7]. Wheat bread was fortified with the addition of 3% onion skin and it enhanced the antioxidant property of the bread with sensory acceptance [8]. Onion skin can also be utilized for the production of energy. Agricultural waste is the prime source to generate energy and is used as an important fuel after oil, coal, and natural gas worldwide [9]. The scarcity of oil, gas, and coal and the large availability of biomass provide an alternative to the industries for energy generation from biomass. The proper understanding of biomass characterization is essential for its effective utilization [10]. The proximate composition is an important parameter for developing a functional food or nutraceutical product. In addition to this, the thermal properties of onion skins are equally important to produce energy or biogas.

Overall, onion waste has become a disposable problem for onion processing industries because of its huge quantity that consequently creates environmental pollution. However, onion waste has the potential to be used as an active ingredient of a food or nutraceutical product. In this regard, the onion skin from 15 Indian cultivars was studied for the following objectives: (1) determining physicochemical properties, (2) analyzing mineral content and active functional groups, and (3) investigating thermal characteristics. All the parameters were studied to provide a solution to environmental pollution by exploring possible food applications.

2. Materials and Methods

2.1. Plant Material and Samples Preparation

Fifteen popular cultivars of onion (“Agrifound Dark Red,” “Agrifound Light Red,” “Arka Kirthiman,” “Bhima Kiran,” “Bhima Shakti,” “Bhima Shubhra,” “Hissar-2,” “Hissar-3,” “NHRDF Red,” “Phursungi Local,” “Pusa Madhavi,” “Pusa Red,” “Pusa Riddhi,” “Sukhsagar,” and “Udaipur Local”) were procured from All India Network Research Project on Onion and Garlic, Division of Vegetable Sciences, Indian Agricultural Research Institute, New Delhi, India. The skin of the cultivars was collected (100 g each) after curing at an open field (20 days). Skins were washed with chlorinated water (0.5%) for the complete removal of dust and other impurities. Skins were washed again with distilled water, left open in a porous tray for 10 min to remove excess water, and then kept in deep freeze (Vestfrost Solutions, Denmark) at −40°C for 24 h. Samples were freeze-dried using a lyophilizer (Mini Lyodel, Delvac Pumps, Chennai, India) with keeping the plate temperature at −50°C with the pressure of 0.039 mbar and the drying process was continued up to 48 h. Freeze-dried skin was grounded using a mixer-grinder (3053 colts, Usha International Ltd., India) for powder formation and stored at −30°C in airtight plastic containers for further use.

2.2. Physicochemical Analysis

Water activity (a_w) of skin powder was analyzed by a water activity meter (Aqua LabTM–Dewpoint, US). The moisture content of the powder was analyzed by a moisture analyzer (Citizen®, India). Crude protein and total fat were estimated by Kjeldahl’s method (protein-nitrogen conversion factor 6.25) and the Soxhlet apparatus, respectively [10]. Ash content was obtained by placing 3 g sample in a muffle furnace at 550°C for 3 h; after cooling down, the sample was transferred in a dryer and calculated [11]. Total fiber concentration was estimated by calculating the difference in weights before calcination and after calcination. Each sample was digested with the solutions of sulphuric acid (1.25%) and sodium hydroxide (1.25%) and then the residue was calcined. Carbohydrate content was calculated by the following formula:$$\begin{array}{}\text{(1)}& \text{Carbohydrate\hspace{0.17em}\%}=\text{100}-\text{moisture\hspace{0.17em}content}+\text{total\hspace{0.17em}fiber}+\text{crude\hspace{0.17em}protein}+\text{total\hspace{0.17em}fat}+\text{ash\hspace{0.17em}content}\text{.}\end{array}$$

The total sulphur content was also analyzed by a CHNS analyzer (EURO Elemental Analyzer, Wegberg, Germany). The investigation was subjected to the total oxidation of the sample and complete combustion which changes the sample into SO₂ (combustion product). The analysis was carried out according to Benitez et al. [12] with slight modifications. 1 mg sample of onion skin of each cultivar was taken for the analysis and sulfanilamide was used as a reference standard.

2.3. Analysis of Mineral Content

The onion skin powder was taken (0.5 g) into digestion tubes from each sample. Briefly, 2 mL of concentrated nitric acid (HNO₃) was poured into each tube and placed into the digestion block at 150°C. After digestion, clear solutions were obtained which were used for mineral detection. Mineral concentration was detected by ICP-AES (inductive coupled plasma-atomic emission spectroscopy, Fujitsu Quality Lab Ltd., Japan). Results were expressed in mg/kg using external standards of K, Ca, P, Mn, Na, Mg, Fe, Zn, and Cu [13]. The calculation of adequate intake (AI) and population reference index (PRI) was done using the latest recommendations of the European Food Safety Authority (EFSA) [14]. Additionally, the recommended dietary allowances (RDA) given by the Food Safety and Standards Authority of India (FSSAI) for the minerals were also compared with the results [15].

2.4. Assessment of Functional Groups

The presence of various functional groups was assessed by FTIR (Fourier transform infrared spectroscopy, Agilent Cary 630, US) in transmission mode from 4000 to 600 cm⁻¹ as described by Kumar et al. [16]. The graphs were evaluated by MicroLab FTIR software.

2.5. Thermal Characterization

Mass loss of samples was estimated using TGA Libra (Netzsch, Germany) under nitrogen atmosphere (60 mL/min) at the heating rate of 5°C/min in the temperature range from 20°C to 300°C [17]. The sample mass used for each sample was 4.0 ± 0.1 mg. The curves were analyzed by Netzsch Proteus software (version 6.1.10). Phase change with glass transition temperature was obtained by DSC 200 F3Maia (Netzsch, Germany). Sample (4.0 ± 0.1 mg) mass was packed in a hermetically sealed aluminum pan in a nitrogen atmosphere (60 mL/min) at the heating rate of 5°C/min. The temperature range was taken from 30°C to 300°C and data analysis was carried out using Netzsch Proteus software (version 6.1.10).

2.6. Statistical Analysis

One-way analysis of variance (ANOVA) was applied for data analysis using IBM® SPSS statistics (version 20). All the samples were carried out in triplicate and the results were presented as mean ± standard deviation on a dry weight basis. Moreover, the Principal Component Analysis (PCA) was applied to the results of proximate composition and mineral analysis to explore a comprehensive view of data.

3. Results and Discussion

3.1. Physicochemical Analysis

Water activity is an essential parameter, which is related to food safety, quality, and shelf life. Skin powder of cv. “Pusa Madhavi” had the highest water activity (0.40 ± 0.01), while “Agrifound Dark Red,” “Arka Kirthiman,” “Bhima Shakti,” and “Pusa Red” cultivars exhibited the least water activity (0.09 ± 0.01). This indicates that cv. “Pusa Madhavi” contained excessive water for biological reactions in comparison to other cultivars, while cultivars with the least water activity had the lowest quantity of excess water. There was a significant difference (p ≤ 0.05) among cv. “Agrifound Dark Red,” “Agrifound Light Red,” “Bhima Shubhra,” “Hissar-2,” “Phursungi Local,” “Pusa Madhavi,” and “Udaipur Local” but no difference was observed among cv. “Bhima Kiran,” “NHRDF Red,” “Pusa Riddhi,” and “Sukhsagar” (Table 1). Maximum moisture content was found in cv. “Pusa Madhavi” (13.23 ± 0.90% d.b.), while cv. “Pusa Riddhi” possessed the lowest moisture content with 6.27 ± 0.56% d.b. Moisture content showed a significant difference ($p\le 0.05$) among “Agrifound Light Red,” “Bhima Kiran,” “Phursungi Local,” “Pusa Red,” “Pusa Madhavi,” and “Pusa Riddhi,” whereas cv. “Agrifound Dark Red,” “Bhima Shakti,” “Hissar-3,” “NHRDF Red,” “Phursungi Local,” and “Sikh Sagar” were found to be at par with each other (Table 1). Compared to the onion bulb slices, Rapusas and Driscoll [18] reported 15.7% moisture content (wet basis) at 2.23 ± 0.09 J/kg specific heat. In addition to this, Ismail et al. [19] analyzed Egyptian onion skins and found 3% moisture content on a dry basis. In addition to this, Pereira et al. [20] obtained 9.41 ± 0.09% moisture content in dried onion waste. The differences in the fiber content of skin and cultivar types were the main reasons behind the variation in the moisture content. In addition, moisture content also depends upon the water available in the sample and the vapor pressure in the atmosphere [21].

Table 1

Physicochemical composition of onion skin powder of fifteen cultivars.

Cultivar	Water activity (aw)	Moisture content (%)	Crude protein (%)	Total fat (%)	Crude fiber (%)	Ash content (%)	Carbohydrate (%)	Sulphur content (%)
Agrifound Dark Red	0.09 ± 0.00a	8.77 ± 0.49bc	5.73 ± 0.31gh	0.35 ± 0.05ab	6.53 ± 0.42ef	9.53 ± 0.35cd	69.08 ± 1.16cd	4.50 ± 0.17g
Agrifound Light Red	0.35 ± 0.01g	11.24 ± 1.07e	5.83 ± 0.15gh	0.32 ± 0.02ab	5.35 ± 0.13bc	7.89 ± 0.51b	69.37 ± 1.02de	3.48 ± 0.24d
Arka Kirthiman	0.09 ± 0.01a	10.88 ± 0.35de	5.27 ± 0.31e	0.35 ± 0.03ab	6.96 ± 0.62f	9.11 ± 0.14c	67.44 ± 0.81b	4.09 ± 0.03f
Bhima Kiran	0.13 ± 0.01ab	7.89 ± 0.31b	4.27 ± 0.15bc	0.34 ± 0.04ab	5.67 ± 0.32cd	11.60 ± 0.46gh	70.20 ± 0.57de	2.87 ± 0.08c
Bhima Shakti	0.09 ± 0.00a	8.79 ± 0.44bc	4.37 ± 0.15bc	0.41 ± 0.02d	5.05 ± 0.23abc	6.90 ± 0.20a	74.48 ± 0.79f	5.08 ± 0.08h
Bhima Shubhra	0.14 ± 0.01bc	6.46 ± 0.47a	3.93 ± 0.15a	0.30 ± 0.02a	5.49 ± 0.49bcd	7.83 ± 0.25b	75.89 ± 0.20g	n.d.
Hissar-2	0.30 ± 0.01f	10.76 ± 0.67de	5.40 ± 0.20ef	0.45 ± 0.05de	5.07 ± 0.15abc	10.93 ± 0.15fg	67.39 ± 0.79b	6.72 ± 0.17l
Hissar-3	0.21 ± 0.02de	8.88 ± 0.29bc	4.43 ± 0.21c	0.41 ± 0.02cd	7.03 ± 0.43f	10.07 ± 0.15d	69.81 ± 0.34cd	5.68 ± 0.21j
NHRDF Red	0.17 ± 0.05bc	8.76 ± 1.24bc	5.97 ± 0.15h	0.43 ± 0.03de	8.28 ± 0.20g	12.24 ± 0.59h	64.32 ± 0.99a	8.63 ± 0.17m
Phursungi Local	0.24 ± 0.04e	9.87 ± 0.50cd	4.30 ± 0.10bc	0.43 ± 0.02de	4.80 ± 0.26ab	9.84 ± 0.31d	70.77 ± 0.89e	5.30 ± 0.17i
Pusa Madhavi	0.40 ± 0.01h	13.23 ± 0.90f	4.10 ± 0.10abc	0.36 ± 0.02b	4.45 ± 0.44a	10.85 ± 0.71ef	67.02 ± 0.73b	6.11 ± 0.04k
Pusa red	0.09 ± 0.02a	10.33 ± 0.95de	4.03 ± 0.15ab	0.47 ± 0.02e	5.67 ± 0.42cd	8.88 ± 0.69c	70.62 ± 1.63de	2.61 ± 0.14b
Pusa Riddhi	0.16 ± 0.04bc	6.27 ± 0.56a	4.80 ± 0.10d	0.36 ± 0.02bc	5.04 ± 0.81abc	6.88 ± 0.39a	76.66 ± 0.56g	3.82 ± 0.06e
Sukhsagar	0.13 ± 0.02b	9.98 ± 0.11cd	5.60 ± 0.20fg	0.33 ± 0.03ab	6.17 ± 0.31de	10.18 ± 0.31de	67.74 ± 0.53bc	4.15 ± 0.04f
Udaipur Local	0.18 ± 0.01cd	6.76 ± 0.43a	3.90 ± 0.10a	0.31 ± 0.02a	4.57 ± 0.35a	8.03 ± 0.15b	76.43 ± 0.69g	n.d.

Values are mean ± standard deviation (n = 3). Values with different superscripts ^ah in the same column are significantly different ($p\le 0.05$) by Duncan’s multiple range test.

Various studies revealed the proximate composition of onion bulbs [18, 19] but there are very few studies available that explore the proximate components of onion skins. Present research for proximate analysis showed that skin of “NHRDF Red” cultivar contained the highest amount of crude protein with 5.97 ± 0.15 g/100 g and the lowest concentration was reported in “Udaipur Local” (3.90 ± 0.10 g/100 g). “Pusa Red” exhibited the highest content of total fat (0.47 ± 0.02 g/100 g) among all the cultivars, while the least concentration was found in “Bhima Shubhra” (0.30 ± 0.02 g/100 g). The harvesting conditions and the types of cultivars may impart the difference in protein content. A significant difference (p ≤ 0.05) was found among “Bhima Shubhra,” “Hissar-3,” “Pusa Red,” and “Pusa Riddhi” cultivars, whereas “Agrifound Dark Red,” “Agrifound Light Red,” “Arka Kirthiman,” “Bhima Kiran,” “Pusa Madhavi,” “Pusa Riddhi,” and “Sukhsagar” were found to be at par with each other for total fat content (Table 1).

The highest amount of crude fiber was obtained in “NHRDF Red” (8.28 ± 0.20 g/100 g), while the lowest crude fiber content (4.45 ± 0.44 g/100 g) was recorded in “Pusa Madhavi.” There was a significant difference ($p\le 0.05$) among “Agrifound Dark Red,” “Agrifound Light Red,” “Bhima Kiran,” “NHRDF Red,” and “Pusa Madhavi” cultivars for crude fiber content. The highest ash content (12.24 ± 0.59 g/100 g) was recorded in cv. “NHRDF Red” skin and the lowest concentration of ash was reported in cv. “Pusa Riddhi” (6.88 ± 0.39 g/100 g). Near approximate concentration (5.7 ± 0.3 g/100 g) of ash was reported in dry weight of onion skin waste [22]. “Agrifound Dark Red,” “Hissar-3,” “Phursungi Local,” and “Sukhsagar” were found at par with regard to ash content and a significant difference was found among “Bhima Kiran,” “Bhima Shubhra,” “Phursungi Local,” “Pusa Madhavi,” “Pusa Riddhi,” and “Pusa Red.” The concentration of biomolecules may vary with the cultivar types, soil composition, and harvesting time. Maximum carbohydrate content (76.66 ± 0.56 g/100 g) was obtained in cv. “Pusa Riddhi,” which was much higher than onion slices (14.77 ± 0.04 g/100 g) and the extract of onion bulb (6.91 ± 0.02 g/100 g) [18]. The lowest amount of carbohydrate was recorded in “NHRDF Red” cultivar (64.32 ± 0.99 g/100 g). A significant difference ($p\le 0.05$) was found among cv. “Agrifound Dark Red,” “Agrifound Light Red,” “Bhima Shakti,” “Bhima Shubhra,” “NHRDF Red,” “Pusa Madhavi,” and “Udaipur Local” (Table 1). Majid et al. [23] have analyzed onion bulb powder of sprouted and unsprouted cultivars and obtained 3.087–6.08% total fibers, 70.74–79.54% carbohydrate, and 8.57–14.16% proteins. The proximate analysis showed that the color of skin, that is, whited, light red, red, and dark red, imparted the variations in the proximate composition of cultivars. This study showed that onion skin and bulb powder are comparable for total fibers, total carbohydrates, and proteins. Also, the skin is as important for fiber, carbohydrates, and proteins as bulbs.

The highest sulphur concentration was obtained in the skin of cv. “NHRDF Red” (8.63 ± 0.17%) and lowest in cv. “Agrifound Light Red” (3.48 ± 0.24%), while sulphur was not detected in cv. “Bhima Shubhra” and “Udaipur Local.” All the cultivars were significantly different ($p\le 0.05$) from each other except “Arka Kirthiman” and “Sukhsagar” which were at par. Previous studies revealed that the outer skin of onion contains the lowest amount (15.6 ± 0.6 μmoles/g dw) of sulphur compared to its other sections such as inner scale (153.1 ± 5 μmoles/g dw), top-bottom (143.8 ± 3.3 μmoles/g dw), and onion bulb (121.9 ± 3.2 μmoles/g dw) [12]. However, sulphur is an integrated part of ACSOs (S-alk(en)yl-L-cysteine sulfoxides), which are onion flavor precursors, but in various studies, no correlation was reported between the concentration of flavor precursors and total sulphur content, which revealed that there was a poor correlation between pungency and sulphur accumulation. The percentage of S-ACSOs in total sulphur content showed an increasing pattern from the outer section to the inner section of the skin; however, brown outer skin contained almost 30% of the total sulphur content in flavor precursors [12]. Benitez et al. [12] found the lowest flavor precursors in brown skin compared to onion bulb. On the other hand, Sarkar et al. [24] reported 0.44 ± 0.01% of sulphur concentration in onion bulb produced by different mulching practices. It is already proven that mulching and other preharvest factors may affect sulphur content in onion bulb [25]. Concentration and ratio of flavor precursors differ from cultivar to cultivar and its ratio imparts peculiar aroma and taste to the onion.

3.2. Mineral Analysis

The mineral composition of onion skin powder of fifteen cultivars was determined on a dry weight basis. K, P, and Ca were recorded as principal abundant macrominerals in the outermost peel, ranging from 877.82 ± 38.79 to 3474.01 ± 17.16 mg/kg, 796.22 ± 13.01 to 2873.10 ± 46.76 mg/kg, and 762.42 ± 38.21 to 3007.81 ± 16.42 mg/kg, respectively. The study revealed that cv. “NHRDF Red” contained the highest amount of macrominerals, such as K (3474.01 ± 17.16 mg/kg), Ca (3007.81 ± 16.42 mg/kg), P (2873.10 ± 46.76 mg/kg), and Mg (1034.54 ± 49.98 mg/kg) among all the cultivars. The least amount of macronutrients was obtained in cv. “Bhima Shubhra” and “Udaipur Local” (Table 2). However, all cultivars were found to be a valuable source of important minerals according to the RDA of EFSA and FSSAI [14, 15]. According to FSSAI, RDA for women-men should be 3225–3750, 600, 600, and 310–340 mg/d for K, P, Ca, and Mg, respectively (Table 2). The variation in mineral content of cultivars might be due to soil composition, harvesting time, and varietal differences.

Table 2

Mineral composition of onion skins of fifteen cultivars.

Cultivar	K	Ca	P	Mn	Na	Mg	Fe	Zn	Cu
Agrifound Dark Red	3366.17 ± 28.29k	1197.18 ± 7.82b	2418.49 ± 66.34ef	11.67 ± 0.41d	508.34 ± 17.46de	762.18 ± 25.33h	170.87 ± 0.29j	34.54 ± 0.39j	25.52 ± 0.39i
Agrifound Light Red	3050.16 ± 42.48h	1381.58 ± 23.57d	2551.78 ± 41.45f	12.55 ± 0.51e	610.89 ± 20.85g	335.27 ± 9.42c	150.90 ± 0.13g	29.65 ± 0.28g	21.78 ± 0.29f
Arka Kirthiman	2137.99 ± 56.22c	1222.92 ± 29.35b	2056.44 ± 43.67b	11.67 ± 0.41cd	575.46 ± 42.74g	207.63 ± 6.06a	135.86 ± 0.17d	21.71 ± 0.23c	16.52 ± 0.34c
Bhima Kiran	2614.45 ± 78.93e	1325.89 ± 28.02c	2632.97 ± 55.48g	13.44 ± 0.45fg	674.82 ± 28.11h	297.96 ± 7.29bc	130.74 ± 0.17c	22.62 ± 0.19d	18.66 ± 0.12d
Bhima Shakti	3140.76 ± 56.26i	1492.81 ± 18.16e	2458.65 ± 52.66f	14.03 ± 0.17gh	488.47 ± 20.31d	361.48 ± 32.46de	140.82 ± 0.24e	25.56 ± 0.29e	23.49 ± 0.27g
Bhima Shubhra	959.05 ± 27.72b	770.48 ± 30.61a	796.22 ± 13.01a	10.04 ± 0.17b	127.97 ± 1.52a	168.57 ± 11.23a	110.60 ± 0.37a	17.95 ± 0.06a	12.45 ± 0.43a
Hissar-2	3250.05 ± 36.78j	2892.29 ± 23.28j	2789.47 ± 20.34h	17.37 ± 0.57i	794.25 ± 10.17i	888.78 ± 16.61i	197.01 ± 0.11n	38.39 ± 0.34m	27.95 ± 0.14k
Hissar-3	2667.44 ± 27.70e	2496.18 ± 72.00h	2430.87 ± 50.50de	14.09 ± 0.17gh	599.86 ± 10.8g	641.32 ± 41.48g	181.07 ± 0.84l	31.48 ± 0.23h	24.29 ± 0.25h
NHRDF Red	3474.01 ± 17.16l	3007.81 ± 16.42k	2873.10 ± 46.76i	18.68 ± 0.42j	879.97 ± 26.15j	1034.54 ± 49.98j	211.96 ± 0.08o	39.27 ± 0.34n	29.49 ± 0.32l
Phursungi Local	2233.70 ± 18.23d	2691.22 ± 32.91i	2395.82 ± 55.92de	13.33 ± 0.48fg	431.88 ± 32.34c	443.80 ± 39.69f	160.53 ± 0.33h	33.61 ± 0.40i	21.36 ± 0.29f
Pusa Madhavi	2767.56 ± 30.22f	2702.44 ± 26.78i	2362.49 ± 17.24cd	14.37 ± 0.47h	540.69 ± 45.12ef	743.15 ± 27.33h	175.85 ± 0.18k	35.41 ± 0.42k	24.41 ± 0.20h
Pusa Red	2855.08 ± 21.63g	2699.48 ± 11.41i	2466.60 ± 55.19e	13.26 ± 0.42f	496.23 ± 7.62d	856.01 ± 26.17i	181.72 ± 0.25m	37.46 ± 0.43l	26.10 ± 0.04j
Pusa Riddhi	2662.61 ± 22.77e	2381.53 ± 23.86g	2300.07 ± 11.92c	12.27 ± 0.34e	404.02 ± 5.94c	397.96 ± 8.69e	161.35 ± 0.16i	27.55 ± 0.39f	21.42 ± 0.18f
Sukhsagar	2188.48 ± 10.82cd	2107.15 ± 23.17f	2125.58 ± 81.74b	11.20 ± 0.61c	298.38 ± 8.39b	281.13 ± 19.20b	143.55 ± 0.32f	21.39 ± 0.30c	19.93 ± 0.19e
Udaipur Local	877.82 ± 38.79a	762.42 ± 38.21a	870.27 ± 32.49a	9.11 ± 0.41a	121.07 ± 0.56a	188.16 ± 11.85a	125.80 ± 0.34b	19.55 ± 0.29b	13.42 ± 0.41b
%AI/PRI${}^{\ast }$	124	12	—	126	—	80	191	60	114
RDA (mg/d)#	3225–3750	600	600	4	1900–2100	310–340	17–21	10–12	1.7

Data are mean value ± standard deviation (n = 3) in mg/kg. ${}^{\ast }$As per the recommendations of EFSA [14]. ^#As per the recommendations of FSSAI [15]. —: not available. Values in the same column with different letters are significantly different (P ≤ 0.05) by Duncan’s multiple range test.

Among the trace elements, Fe, Zn, and Cu were found in the highest concentration. They were quantified in the range of 110.60 ± 0.37–211.96 ± 0.08 mg/kg, 17.95 ± 0.06–39.27 ± 0.34 mg/kg, and 12.45 ± 0.43–29.49 ± 0.32 mg/kg, respectively. Cultivar “NHRDF Red” was reported to have the highest concentration of trace elements among all the cultivars. The lowest concentration of trace elements was obtained in cv. “Bhima Shubhra.” Cultivars “Agrifound Dark Red,” “Bhima Shakti,” “Hissar-2,” “Hissar-3,” “NHRDF Red,” “Pusa Madhavi,” and “Pusa Red” were found significantly different (P ≤ 0.05) from each other, whereas no significant ($p\le 0.05$) difference was obtained between cultivars “Agrifound Light Red,” “Arka Kirthiman,” “Bhima Kiran,” “Bhima Shubhra,” “Phursungi Local,” and “Sukhsagar.” A previous study showed a similar amount of macrominerals such as K (7297.88 mg/kg), P (2491.04 mg/kg), Ca (1824.29 mg/kg), and Mg (1990.59 mg/kg) and a lower range for trace elements, that is, Fe (160.69 mg/kg), Al (41.46 mg/kg), Zn (38.44 mg/kg), and Cu (27.23 mg/kg) in onion bulb [26]. Chope and Terry [27] analyzed different cultivars of onion and obtained a greater range of macroelements, that is, K (149.00–270.85 mg/100 g), P (15.92–29.25 mg/100 g), Na (3.84–14.57 mg/100 g), and Mg (6.39–10.81 mg/100 g), while Mn (0.11–0.23 mg/100 g) and Zn (0.27–0.45 mg/100 g) were recorded in a lower concentration. Bello et al. [28] analyzed onion bulbs and found a good concentration of K (2.98 mg/100 g), Ca (1.22 mg/100 g), Mn (0.05 mg/100 g), Fe (0.04 mg/100 g), and Cu (0.13 mg/100 g) but lower than skin because onion skin contains a concentrated amount of minerals compared to bulb due to the absence of water. Likewise, Sarkar et al. [24] reported 1.10, 0.29, and 0.26 mg/100 g concentrations of K, P, and Ca in onion bulb cultivated by different mulching techniques, respectively. It showed that intrinsic and extrinsic factors influence the level of minerals. In Japan, onion bulbs from five major growing areas were investigated and a higher range of Na (107 ± 45–343 ± 221 µg/g), P (2530 ± 423–3120 ± 498 µg/g), and Mg (798 ± 131–948 ± 209 µg/g) was reported. Moreover, Ariyama et al. [29] analyzed Japanese onions for Zn and quantified them in the range of 12.8 ± 3.0–17.9 ± 7.2 µg/g. Geographical locations, cultivar types, and growing conditions play an important role in the mineral composition of the onion bulb and skin [26, 27].

3.3. Principal Component Analysis

PCA was performed to identify and visualize the association of the proximate composition and mineral composition in the fifteen onion cultivars that were considered for this study. The results of the PCA analysis of proximate composition showed that two PCs (Dim1 and Dim2) could explain 68.6% of the variability in data (Figure 1(a)). The fat, ash, moisture content, and crude protein showed a positive correlation, while carbohydrate showed a negative correlation with all the other parameters. Moreover, an inverse correlation was also observed between fiber content and water activity. This result was expected since fibers can bind the water molecules within their complex matrix and reduce the overall vapor pressure, thereby preventing it from being detected by the water activity meter. It can also be noted that Udaipur Local, Bhima Shakti, Pusa Riddhi, and Bhima Shubhra have considerably higher carbohydrate content, while Hissar-3, Sukhsagar, Arka Kirthiman, and Agrifound Dark Red had higher fiber and crude protein.

[figures omitted; refer to PDF]

Based on the biplot of the mineral composition (Figure 1(b)), it can be stated that the first principal component (Dim1) explained 100% of the variability in mineral data. The sodium, potassium, and phosphorus content exhibited a negative correlation with the iron, potassium, copper, and zinc content in the cultivars. This shows that the former set of mineral parameters could be a good indicator of the latter set and vice versa. However, the variation in the manganese content for the cultivars did not show any correlation to the other two mineral correlation sets and showed less variability (Dim2 = 0%). It should also be noted that the onion varieties that are clustered together show a similar mineral composition. For instance, the varieties Phursungi Local, Udaipur Local, Pusa Riddhi, and Sukhsagar would show homogeneous properties. Similarly, Pusa Madhavi, Bhima Shakti, NHRDF Red, and Hissar-3 would have a similar mineral composition. Another cluster that can be recognized consists of the Pusa Red, Arka Kirthiman, Bhima Kiran, and Bhima Shubhra varieties; however, the correlation between these varieties would be relatively less as compared to the other two identified clusters due to a more scattered cluster.

3.4. Functional Group Assessment

FTIR spectroscopy is the best-suited technique to investigate the probable chemical compounds or metabolites in samples. It is a nondestructive method of analysis, which saves the sample contents from mechanical or thermal energy [30]. In the present study, FTIR analysis of onion skin powder of fifteen cultivars confirmed the presence of various functional groups. Spectra of different cultivars are shown in Figure 2. Spectral bands between fingerprint regions from 1800 to 750 cm⁻¹ reflect the primary biomolecules like protein, carbohydrate, lipid, nucleic acid, and polyphenols [31]. The skin powder of all the cultivars showed a strong and common absorbance between the wavelength of 1012–1009 cm⁻¹ and 1607–1597 cm⁻¹, while medium absorbance was obtained between the wave number of 1418–1317 cm⁻¹, 2116–2113 cm⁻¹, and 2917–2900 cm⁻¹. The weak wavelength ranged from 1147 to 1100 cm⁻¹, 1245 to 1237 cm⁻¹, 1878 to 1849 cm⁻¹, and 2342 to 2320 cm⁻¹ in all the cultivars. Apart from this, other absorbance ranges were also observed in some cultivars between 840 and 804 cm⁻¹ wavelengths. Both strong absorbances that fall between the wavelength of 1150–1000 cm⁻¹ and 1615–1495 cm⁻¹ are assigned to C-F stretch aliphatic organic halogen compounds and aromatic nitro compounds, respectively [32]. Medium peak frequencies fall between the range of 1420–1300 cm⁻¹, 2140–2100 cm⁻¹, and 2923–2915 cm^-1, which showed the presence of carboxylate (carboxylic acid salt), C≡C stretch terminal alkyne, and methylene in the saturated aliphatic group, respectively [33]. Weak frequencies between 1200 and 1100 cm⁻¹, 1850 and 1650 cm⁻¹, and 2363 and 2313 cm⁻¹, respectively, represent sulfonates (sulphur oxy compounds), carbonyl compounds, and atmospheric CO₂. The other captured peaks between 900 and 800 cm⁻¹ are assigned to the C-H stretch of aromatic (aryl) ring frequency [31]. The study showed that all the cultivars exhibited a similar pattern of peaks; however, the peak sharpness (strong and medium) showed a difference in biomolecules level of onion due to varietal difference. Larrosa et al. [34] investigated vegetable paste and reported the presence of phenolic, aromatic, and carbonic groups. Characterization of red onion skin tannin represented almost the same peaks absorbance at 1650 cm⁻¹, 1437 cm⁻¹, and 1115 cm⁻¹ [35]. Moreover, the FTIR study of onion powder showed the presence of starch and other important compounds between the wavenumbers of 4000–650 cm⁻¹, which supports the present study [36].

[figure omitted; refer to PDF]

3.5. Thermal Characterization

The thermal decomposition analysis was carried out by TGA for the detection of mass change/loss of samples. TGA curves are shown in Figure 3. Thermograms of TGA showed that the highest mass loss (25.31%) was reported in cv. “Phursungi Local” (Figure 3). A total 1.91% residual mass was obtained for “Phursungi Local.” The least mass decomposition (0.64%) was observed in cv. “Agrifound Dark Red” between the temperature ranges of 220°C–270°C with the residual mass of 70.98%. The maximum mass loss and total residual mass of each cultivar are presented in Table 3. A significant ($p\le 0.05$) difference was obtained between cultivars. Mass loss at the first step was probably due to the evaporation of available water.

[figure omitted; refer to PDF]

Table 3

Maximum mass loss (with respect to the temperature) and residual mass of onion skin powder of fifteen cultivars.

Cultivar	Maximum mass loss (%)	Temperature range (°C)	Residual mass (%) at 299°C
Agrifound Dark Red	12.80 ± 0.05b	220–270	70.98 ± 0.04m
Agrifound Light Red	12.25 ± 0.08b	240–290	50.90 ± 0.06f
Arka Kirthiman	17.16 ± 0.07ef	220–270	31.19 ± 0.02c
Bhima Kiran	13.14 ± 0.05c	220–270	63.75 ± 0.05j
Bhima Shakti	12.55 ± 0.09b	220–270	65.22 ± 0.05k
Bhima Shubhra	16.47 ± 0.03e	220–270	40.68 ± 0.04d
Hissar-2	11.07 ± 0.02a	220–270	67.73 ± 0.07l
Hissar-3	22.16 ± 0.05g	220–270	41.70 ± 0.02d
NHRDF Red	15.27 ± 0.04d	220–270	57.24 ± 0.02h
Phursungi Local	25.31 ± 0.10h	220–270	1.91 ± 0.08a
Pusa Madhavi	11.32 ± 0.06ab	250–299	59.56 ± 0.03i
Pusa Red	22.72 ± 0.04g	220–270	28.70 ± 0.04b
Pusa Riddhi	13.08 ± 0.05c	220–270	48.28 ± 0.01e
Sukhsagar	17.02 ± 0.02ef	220–270	52.04 ± 0.03fg
Udaipur Local	10.67 ± 0.06a	220–270	63.95 ± 0.04j

Data are mean value ± standard deviation (n = 3). Values in the same column with different letters are significantly different (P ≤ 0.05) by Duncan’s multiple range test.

At the other decomposition steps, the mass loss occurred probably because of different components such as secondary metabolites and carbohydrates [37]. At the end of the experiment (299.9°C), the lowest residual mass with 1.91% was found in cv. “Phursungi Local,” while the highest residual mass was obtained in cv. “Agrifound Dark Red” with 70.98% Table 3. To the best of our knowledge, previous studies are not available on the thermal attributes of onion skin powder. However, a TGA study of the powder of guava, sapota, and papaya showed the maximum mass loss from 30% to 41.45% between combined temperature ranges of 132°C–332°C [36], which supports the findings of the present investigation. Moreover, the thermal analysis of the extract of Syzygium cumini L. leaves also revealed a significant mass loss (28.16%) between the temperature range of 209°C–260°C [38]. Different concentrations of crude fiber, ash, and protein are the main reasons for differences among residual masses of different cultivars. TGA revealed “Phursungi Local” as the least thermally stable cultivar and “Agrifound Dark Red” as the best thermally stable cultivar which explores that the onion skin powder of cv. “Agrifound Dark Red” can be utilized for higher temperature processes.

DSC curves explored the phase change and glass transition temperature (Tg) of onion skin powder of fifteen cultivars. DSC curves are given in Figure 4 and Tg for all the cultivars is given in Table 4. The cv. “Agrifound Dark Red” had a Tg temperature between 64.4°C and 90.6°C. It also showed a single endothermic peak at 102.1°C (Figure 4). “Agrifound Light Red” showed the Tg between 135.9°C and 140.9°C. It also showed two endothermic peaks: the first huge at 160.4°C and the second small peak at 244°C. The Tg temperature for cv. “Arka Kirthiman” was recorded between the temperature ranges of 111.1°C–132.7°C. The Tg of cv. “NHRDF Red” was found between 120.2°C and 124.5°C and one huge and second small endothermic peaks occurred at 138.7 and 236.4°C, respectively.

[figure omitted; refer to PDF]

Table 4

Glass transition temperature (Tg) of onion skin powder of fifteen cultivars.

Cultivar	Glass transition (Tg) temperature (onset °C-end °C)	Temperature of first endothermic peak (°C)	Temperature of second endothermic peak (°C)
Agrifound Dark Red	64.4–90.6	102.1 ± 0.02a	—
Agrifound Light Red	135.9–140.9	160.4 ± 0.04j	244.0 ± 0.09h
Arka Kirthiman	111.1–132.7	146.5 ± 0.03h	234.5 ± 0.10c
Bhima Kiran	65.7–106.9	106.7 ± 0.05b	—
Bhima Shakti	143.3–144.5	148.4 ± 0.05i	238.5 ± 0.08e
Bhima Shubhra	107.5–127.3	131.0 ± 0.03e	223.7 ± 0.11a
Hissar-2	107.0–110.2	127.0 ± 0.06d	228.8 ± 0.13b
Hissar-3	103.6–114.4	123.3 ± 0.04c	228.1 ± 0.09b
NHRDF Red	120.2–124.5	138.7 ± 0.06f	236.4 ± 0.14d
Phursungi Local	94.2–118.9	124.0 ± 0.02c	—
Pusa Madhavi	127.2–128.6	144.7 ± 0.04g	240.7 ± 0.08f
Pusa Red	114.6–131.9	144.3 ± 0.03g	236.0 ± 0.11d
Pusa Riddhi	146.4–146.8	146.6 ± 0.02h	238.8 ± 0.16e
Sukhsagar	158.2–159.8	169.0 ± 0.03k	243.7 ± 0.12g
Udaipur Local	101.7–128.1	130.5 ± 0.06e	224.1 ± 0.09a

“—” no peak found. Data are mean value ± standard deviation (n = 3). Values in the same column with different letters are significantly different ($p$ ≤ 0.05) by Duncan’s multiple range test.

The study of cv. “Pusa Madhavi” revealed that the Tg was taken place between the temperature range of 127.2°C–128.6°C. Two endothermic peaks were also obtained at different temperatures, one huge peak at 144.7°C and the second tiny peak at 240.7°C. Moderate ranges of Tg (phase changing), that is, 111.1°C–132.7°C, 107.5°C–127.3°C, 107.0°C–110.2°C, 103.6°C–114.4°C, and 101.7°C–128.1°C, were obtained, respectively, for “Arka Kirthiman,” “Bhima Shubhra,” “Hissar-2,” “Hissar-3,” and “Udaipur Local” cultivars and a significant ($p$ ≤ 0.05) difference was observed between cultivars (Table 4). Possible occurred reactions were dehydroxylation, decarboxylation, and demethoxylation [39, 40]. As per the study, the Tg temperature revealed that the earliest phase change was taken place in cv. “Agrifound Dark Red” between 64.4°C and 90.6°C, while late phase change was confirmed in cv. “Sukhsagar” between 158.2°C and 159.8°C. Likewise, de Oliveira Cartaxo-Furtado et al. [38] studied the extract of Syzygium cumini L. leaves and found the largest endothermic peak between the temperature range of 114.2°C–150.69°C. Similarly, Da Silva et al. [41] observed a thermal event at 120°C during the analysis of freeze-dried camu camu fruit pulp (Myrciaria dubia). Apart from this, many endothermic peaks were obtained between 126.14°C and 325.50°C in the analysis of the dried extract of Schinopsis brasiliensis Engl. [30].

4. Conclusions

In the present study, the skins of fifteen onion cultivars were investigated for various important parameters. The physicochemical analysis explored that “NHRDF Red” has the highest protein, fiber, and ash content among the cultivars studied, whereas “Pusa Riddhi” and “Pusa Red” were revealed as the best source of carbohydrates and fat among all the cultivars. Additionally, sulphur and mineral analysis showed OSP as a rich source of essential minerals. TGA studies confirmed that cv. “Pusa Madhavi” was the most thermally stable, while “Pusa Red” was reported as the least thermally stable compare to others and DSC confirmed the earliest phase change in cv. “Agrifound Dark Red” but late phase change in cv. “Sukhsagar.” FTIR analysis suggested that all the cultivars were rich in various functional groups, such as methylene in the saturated aliphatic group, aromatic ring, carbohydrate, and sulphur oxy compounds. Onion skin waste in processing industries may be utilized for various purposes like functional/nutraceutical food development, energy, and biogas production [5–7, 10].

Authors’ Contributions

Narashans Alok Sagar and Sunil Pareek conceived the experiment(s); Narashans Alok Sagar contributed to investigation; Vikas contributed to data analysis and PCA application; Narashans Alok Sagar and Ayon Tarafdar were responsible for writing of the original draft; Sunil Pareek contributed to reviewing and editing, supervision, and project administration; Anil Khar contributed to resources and draft editing.

Acknowledgments

The authors are highly grateful to All India Coordinated Research Project on Onion and Garlic, Division of Vegetable Science, Indian Agricultural Research Institute, New Delhi, for providing the onion bulbs for the study. Dr. Khalid Bashir, Department of Basic and Applied Sciences, and Mr. Mayank Nigam, Mr. Naveen Roja, and Mr. Ankur Chaudhary, Laboratory In-Charge, National Institute of Food Technology Entrepreneurship and Management (NIFTEM), are also acknowledged for their extended help. The financial support provided by UGC to the first author vide letter no. F.15–6(DEC.2018)/2019(NET) is highly acknowledged.

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