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

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.


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. e European Union onion processing industry generates more than 500,000 tons of onion waste annually mainly in the UK, e Netherlands, and Spain. is has become an environmental problem. e maximum amount of onion skin is generated by industries during peeling. is 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][4][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]. e scarcity of oil, gas, and coal and the large availability of biomass provide an alternative to the industries for energy generation from biomass. e proper understanding of biomass characterization is essential for its effective utilization [10]. e 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.

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. e 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.

Physicochemical Analysis.
Water activity (a w ) of skin powder was analyzed by a water activity meter (Aqua LabTM-Dewpoint, US). e 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: Carbohydrate % � 100 − moisture content + total fiber + crude protein + total fat + ash content.
(1) e total sulphur content was also analyzed by a CHNS analyzer (EURO Elemental Analyzer, Wegberg, Germany). e investigation was subjected to the total oxidation of the sample and complete combustion which changes the sample into SO 2 (combustion product). e 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.

Analysis of Mineral Content.
e onion skin powder was taken (0.5 g) into digestion tubes from each sample. Briefly, 2 mL of concentrated nitric acid (HNO 3 ) 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]. e 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].

Assessment of Functional Groups.
e 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]. e graphs were evaluated by MicroLab FTIR software.

ermal 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]. e sample mass used for each sample was 4.0 ± 0.1 mg. e 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. e 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.
is 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.

Journal of Food Quality 3
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. e 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.

Mineral Analysis.
e 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  e 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). e variation in mineral content of cultivars might be due to soil composition, harvesting time, and varietal differences.

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. e 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)). e 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. is 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.
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. e sodium, potassium, and phosphorus content exhibited a negative correlation with the iron, potassium, copper, and zinc content in the cultivars. is 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.

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]. e 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 . e 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 CO 2 . e other captured peaks between 900 and 800 cm −1 are assigned to the C-H stretch of aromatic (aryl) ring frequency [31].
e 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].

ermal Characterization.
e thermal decomposition analysis was carried out by TGA for the detection of mass change/loss of samples. TGA curves are shown in Figure 3.
ermograms 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." e 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%. e maximum mass loss and total residual mass of each cultivar are presented in Table 3. A significant (p ≤ 0.05) difference was obtained between cultivars. Mass loss at the first step was probably due to the evaporation of available water.
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. e 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. e Tg temperature for cv. "Arka Kirthiman" was recorded between the temperature ranges of 111.1°C-132.7°C. e Tg of cv. "NHRDF Red" was found between 120.2°C and 124.5°C and  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].

Conclusions
In the present study, the skins of fifteen onion cultivars were investigated for various important parameters. e 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][6][7]10].

Data Availability
All the data pertaining to this work have been provided within the text.

Conflicts of Interest
e authors declare no conflicts of interest with respect to this work.

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.
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. e financial support provided by UGC to the first author vide letter no. F.15-6(DEC.2018)/2019(NET) is highly acknowledged.