Valorization of Food Waste through Controlled Oxidation: A Case Study on Densification of Onion Waste

Abstract

Around 20% of the fruits and vegetables produced globally are lost between harvest and retail owing to infrastructure and logistics challenges. There are no technologies available to utilize this waste comprehensively, which amounts to billions in economic losses annually. While methods are available for extraction of bio-active compounds, the technologies to harness fibers and sugars, which are the major components of the waste, are not reported. This work reports a new technique for valorization of waste onion, as it consists of carbohydrates and fibers and represents the typical composition of food and vegetable waste. The controlled oxidation of onion waste under ambient conditions led to decomposition of 90% of the sugars, which resulted in crosslinking of the fibers to create a strong and rigid densified matrix of the fibers. The densified matrix had a compression strength of 101.7 MPa and remained stable up to 100 °C. Such densified materials can be used for packaging applications or as one of the components of vegan leather. The onion waste and densified material were characterized using FTIR, XRD, SEM imaging, elemental analysis, and compression testing to identify the structure–property relationship.

Keywords

Food waste - Oxidation - Onion - Packaging material

Significance

Vegetable and fruit waste are generated at every stage of the farm to the fork chain. Onion represents a classic case of vegetable waste containing high moisture content, carbohydrates, and fibers with a short shelf life. The technology in this work demonstrates that carbohydrates in waste onion can be oxidized to generate energy, which can be utilized for crosslinking fibers. The resulting material demonstrates high compression strength and can be used as a packaging material or as a vegan leather component.

Introduction

Food waste is a global challenge that impacts food security, climate change, and the economy. It is imperative to develop technologies that can generate utility and revenue from this waste. Around 1.3 billion tons of edible food is lost or wasted each year.[1] The disposal of such waste in landfill leads to environmental, economic, and social problems. The prevention of food waste necessitates infrastructure for cold and dry storage, which is often lacking in several regions of the world. The alternative is to find ways to utilize this waste by converting it into high-value products. Valorizing food waste facilitates the transition to a circular bio economy, in alignment with the United Nations Sustainable Development Goals.[2]

Onion waste, in particular, is produced in substantial quantities in Asia.[3] It is composed of moisture, carbohydrates, fibers, and several bioactive compounds ([Table 1]). Due to the short shelf life, onions become waste if not stored properly and directly impact the agricultural economy. Over the past two decades, global onion production has grown by at least 25%, making onions the second-most important horticultural crop.[4] The increased demand for processed onions has also led to a rise in waste generation. India produces approximately 250,000 metric tons of onions annually; however, a significant portion is wasted due to inadequate storage facilities.[5] To tackle the high levels of onion waste, it is essential to identify sustainable, eco-friendly processes and develop solutions for recovering key natural products in usable forms or utilize the whole waste. Various efforts have been made to utilize different parts of onion waste. Onion is rich in flavonoids (quercetin and its derivatives), anthocyanins (in red varieties), and phenolic acids. These bioactive compounds can be effectively extracted from onion peels and bulbs using solvents such as acetone, ethanol, and deep eutectic solvents.[6] [7] [8] In some cases, a combined process of extraction of bioactive compounds and isolation of biopolymers has been used to make packaging material and films.[9]

Table 1 Composition of onion and onion waste.

Components of the onion waste	Concentration (%)
Moisture	85–90[10]
Carbohydrates	4–9[10] [11]
Fibers	2–4[12] [13]
Bioactive compounds	0.5–1[14]

Most of the processes for onion waste valorization focus on the extraction of active ingredients or flavonoids from onion. While these components have high value, they have a very low yield and the problem of the disposal of the bulk of onion waste is not addressed. The common disposal methods of such post extraction onion waste are landfilling, incineration, and composting. In recent times, few other valorization techniques include enzymatic degradation and the utilization of sugars,[15] microbial reactions, and thermochemical approaches to produce biochar and bio oils have been demonstrated.[16] However, these methodologies incur significant capital and operational expenditures leading to poor economic viability of the valorization process.

Mechanistically, controlled oxidation involves the regulated oxidation of polyphenolic and carbohydrate fractions, leading to an increased formation of oxygen-containing functional groups and promoting crosslinking or densification of the fibers. NaOCl being a mild oxidizing agent partially oxidizes the fibers such as cellulose.[17] When NaOCl is combined with other catalysts such as TEMPO-mediated oxidation (NaOCl/NaBr) of fibers comprising cellulose obtained from waste biomass such as banana peels, bamboo, and sugarcane bagasse, it selectively oxidizes primary alcohol groups, forming them to carbonyl or carboxyl groups, which in turn enhances the crosslinking ability of fibres.[18] [19] The enhancement of the mechanical property after oxidation is attributed to the availability of carboxyl and carbonyl groups in the fiber, which interact with other primary alcoholic groups present in the fibers and create hydrogen bonding. Moreover, the collective effect of hydrogen bonding and facilitated entanglement enhances the mechanical properties.[20] Carbohydrates get oxidized by the use of sodium hypochlorite.[21] The oxidized carbohydrates further decompose to simpler carbon skeletons based on Millard reaction[22] when heated up the effect of caramelization. The combined effect of heating and oxidation results in the densification of the material. In the context of onion waste, such reactions can enable the development of densified biocomposite materials. The process of densification implies a process wherein loose, heterogeneous organic residues are structurally compacted through physical (e.g., pelletization, extrusion) or chemical (e.g., oxidation, crosslinking) methods, resulting in materials with improved compression strength, enhanced processability, and value addition. In this work, we have demonstrated the controlled oxidation of onion waste. The carbohydrates and sugar molecules present in the onion waste get oxidized, and fibers present in the onion become accessible for crosslinking. This imparts advanced functionalities to the resulting material, such as improved mechanical strength, and makes the material suitable for downstream applications like sustainable packaging, adsorbents, or biorefinery feedstock.

Experimental Section

Materials

Waste onion along with whole onion, roots, peels, and ingrown leaves were collected from Odisha, India (20°18′21.4″N 85°49′38.6″E). Sodium hypochlorite (NaClO) with 8–10% chlorine content and hydrogen peroxide (H₂O₂) were purchased from Avra Synthesis Pvt. Ltd. Accurex eco pak glucose kit was used for glucose analysis. Sugar standards were purchased from HiMedia Laboratories Private Limited. Sulfuric acid of analytical grade was used. Each chemical component and onion waste was used directly without further purification. The nomenclature used for raw and processed onion samples have been listed in [Table 2].

Table 2 Name abbreviations.

Sample	Sample description	Processing conditions
R1	Raw onion waste	Untreated, wet state (as-received)
R2	Onion powder	Raw waste, dried and pulverized
P1	NaOCl-densified onion powder	R2 treated with sodium hypochlorite at 25 °C
P2	H2O2-densified onion powder	R2 treated with hydrogen peroxide at 25 °C
P1.1	Thermally treated P1	P1 subjected to 100 °C for 1 h
P.1.2	Thermally treated P1	P1 subjected to 200 °C for 1 h

Preparation of Raw Onion Waste and Powder

The moisture content in the waste onion bulb samples was determined as per the ASTM E1756 procedure by keeping the samples at 105 °C for 4 h. The waste onion bulb samples had a moisture content of 87% (±3%). The waste onion bulb samples were used without any washing or chemical pretreatment for the densification process and referred to as R1. The onion bulb sample was cut into smaller prices and dried at 70 °C such that the sample had a moisture content of 5–8%, which is suitable for pulverization. This dried onion waste sample was then pulverized using a food-grade grinder, ensuring that there was no increase in temperature during the pulverization and it was stored in a moisture-proof container to avoid further moisture ingress and referred to as R2 (ES Fig. 17 A–C).

Densification of Waste Onion

About 3.84 cm³ of sodium hypochlorite with 8–10% chlorine content was added dropwise at the rate of 130 μL/min to 5 g of the pulverized onion sample R2 at 25 °C and atmospheric pressure under continuous mixing using an overhead stirrer equipped with a mixing paddle. The procedure for densification of onion waste is shown in [Fig. 1] with the mass balance. The processed sample was designated as P1 ([Fig. 2A]). The preparation procedure for P2 closely resembles that of P1. To begin, 5 g of dry onion waste powder (R2) was placed in the mixing paddle, and 3.84 cm³ of H₂O₂ was added dropwise at room temperature until the mixture achieved a uniform consistency and formed a lumpy material. The processed sample was designated as P2 ([Fig. 2B]). Both of the lumpy materials formed were then casted into a mold and dried at 70 °C for 4 h, resulting in the final product. The processed onion samples (P1, P2) were dried at 70 °C for 4 h in an oven.

Fig. 1 Process flow and mass balance (g) for the conversion of raw onion waste (R1) to NaOCl-densified onion powder (P1).

Fig. 2 Visual appearance of densified onion powder treated at 25 °C using (A) NaOCl(P1) and (B) H2O2 (P2).

Analysis of Glucose Content using the GOD POD Method

The glucose content of various samples was analyzed by using the glucose oxidase peroxidase (GOD-POD) method.[23] [24] Accurex eco pak glucose kit was used, consisting of glucose peroxidase enzyme, standard solution, and reagent. In case of solid samples, 1% w/v solution was prepared, mixed with 2 mL of working solution, and incubated at 37 °C for 15 min and then the absorbance was recorded using a UV–vis spectrophotometer with absorbance recorded at 505 nm. The absorbance of the working solution and the standard solution were 0.004 and 0.344, respectively. The absorbance of the actual standard solution was measured by subtracting from the working solution and was 0.340. The concentration of glucose in the sample was determined using the following equation:

Glucose (mg/mL) = Abs of sample/Abs of standard

Dinitrosalicylic Acid (DNSA) Test

The analysis of reducing sugars can be conducted using the DNSA method, which stands for dinitrosalicylic acid.[25] [26] This biochemical technique is used to identify the presence of reducing sugars in both onion powder (R2) and densified material made from dry onion waste powder using NaOCl (P1). To facilitate the analysis of reducing sugars, several calibration curves can be constructed using different reducing sugar standards. In the current protocol, glucose was chosen for analysis, and a calibration curve was created by varying the concentrations of this standard and measuring the corresponding absorbance values. Afterward, the calibration curve was fitted accordingly to determine the unknown concentration of reducing sugars present in the tested samples. The reagent was prepared using a standard protocol where dinitrosalicylic acid was mixed with NaOH solution, and then Rochelle’s salt (KNaC₄H₄O₆·4H₂0) was added to it. Further to the mixture, phenol was added and the mixture was stored. In summary, 1 g of each sample was dissolved in 10 cm³ of water, sonicated for 30 min, filtered using a 0.2 μm filter, and then stored for study. The prepared sample was diluted to 1:40, and 1 cm³ of the diluted sample was taken and 3 cm³ of DNSA solution was added to it. Then, the samples were incubated at 100 °C for 10 min and allowed to cool. The absorbance was then measured at 540 nm. Two sets of samples R2 and P1 were analyzed using this test to identify the reducing sugars present before (R2) and after oxidation process using NaOCl as oxidizing agent (P1).

Thermal Stability Test

Three sets of samples were considered for the thermal stability test and were analyzed using compression strength analysis to assess their structural integrity both before and after thermal treatment. A control sample of NaOCl-densified onion powder (P1) was utilized, along with two additional samples of P1 with the same dimensions. One sample was subjected to heating in an oven at 100 °C for 1 h (P.1.1), while the other was heated at 200 °C for the same duration (P.1.2). Following the heat treatment, the samples were evaluated for compression strength.

Characterization of the Raw and Processed Onion Samples

Fourier transform infrared spectroscopy (FTIR) was performed using the attenuated total reflectance (ATR) technique for the raw and processed onion samples in solid state (Perkin Elmer, Spectrum Two) in the wavenumber range of 400 to 4000 cm⁻¹. A UV visible spectrophotometer from Shimadzu (UV-2600i Plus/UV-2700i Plus) was used for analyzing the glucose content. The surface morphology of the raw and processed onion samples was studied using field emission scanning electron microscopy (FESEM, Quanta FEG 250) with energy dispersive X-ray spectroscopy. Thin slices of raw and processed onion samples (R2 and P2) were loaded on the sample stub and sputter coated with gold using a Lecia EM ACE200 sputter coater. The compression strength of the raw and processed onion samples was measured using an AIC Labs-1000 KN compression testing machine. The crystallinity of the samples was studied using Brucker D8 advance and the X-ray diffraction pattern was studied between 2-theta range of 10° and 90°.

Sample Preparation for Characterization

R2, after undergoing pulverization and ensuring complete moisture evaporation, was taken directly for analysis. P1 and P2 were collected after oxidation and drying, scraped from the edges to obtain the powdered samples and stored for further analysis. For X-ray diffraction (XRD), all powder samples were analyzed within a range of 10–90° with a step size of 0.02. A similar procedure was followed for obtaining samples for Fourier-transform infrared spectroscopy (FTIR), except that for R1, a waste onion sample fraction was analyzed directly without moisture evaporation. For scanning electron microscopy (SEM), both the oxidized and nonoxidized powder samples were thoroughly dried and placed on a carbon tape prior to analysis. After coating the samples, SEM analysis was conducted. Additionally, the same analytical procedure used for SEM was applied for energy-dispersive X-ray spectroscopy (EDX), with the inclusion of a new sample P2, added to the analysis list.

Results and Discussion

The oxidation of carbohydrates is an exothermic reaction, and the crosslinking of fibers is an endothermic reaction. It was hypothesized that the exothermic oxidation of carbohydrates resulted in the endothermic crosslinking of the fibers and made densified material. The characterization of the material was focused on validating this hypothesis. The process block diagram in [Fig. 1] explains the entire process in a nutshell. About 38.46 g of raw onion waste (R1) was collected, chopped, and dried at 70 °C until 80% of the moisture was removed. The material was then pulverized to obtain 5 g of onion powder (R2). About 3.84 cm³ of NaClO was mixed with 5 g R2 to form a lumpy material. The lumpy material was casted in a mold and dried to yield 5.64 g of the NaOCl-densified onion powder (P1). The volatile sulfur-containing compounds such as thiosulfonates, dipropyl disulfides, hydrogen sulfide, and methanethiol present in the raw onion waste (R1) are lost during the drying process. [27] [28] [29]

Furthermore, some of the simple sugars present in the onion undergo nonenzymatic browning reactions, such as the Maillard reaction and caramelization.[30] This results in the reduction of the total amount of free glucose in the raw onion samples R2. The reduction in the glucose content between R1, R2, and P1 samples was verified using the GOD POD method ([Table 3]). Glucose oxidase (GOD) converts the glucose present in the sample into gluconic acid and hydrogen peroxide. The hydrogen peroxide formed during the first reaction couples with 4-aminoantipyridine and phenol to produce red quinoeimine dye in the presence of peroxidase (POD). [Scheme 1] explains the reaction pathways during the GOD-POD analytical technique. The red quinoeimine dye has an absorbance maximum at 505 nm, which is detected by UV visible spectroscopy.[23] [24] The concentration of the red quinoeimine dye is directly proportional to the concentration of glucose present in the sample. Sodium hypochlorite (NaOCl) was added to the prepared R2 dropwise during the oxidation process. The exothermic nature of the reaction was observed during the addition of the oxidant. The crosslinking of decomposed sugars and pectin with fibers present in onion is an endothermic reaction, which gets initiated just after the exothermic reaction during the oxidation, as the material starts forming lumps. At the end of the oxidation, the lumpy material was molded in the shape of circular discs (P1) ([Fig. 2A]).

Table 3 Glucose concentrations in raw onion waste (R1), onion powder (R2), and NaOCl-densified onion powder (P1).

Sample	Absorbance (at 505 nm)	Glucose (mg/mL)	Dilution factor	Actual glucose in sample (mg/mL)
R1	1.021	3.002	5	15.01
R2	0.269	0.7911	1	0.79
P1	0.096	0.2823	1	0.28

Scheme 1 Glucose oxidase-peroxidase(GOD-POD) reaction for glucose content estimation.

The raw onion waste (R1) had a glucose concentration of 15.0145 mg/mL as the onion pulp retained both natural sugars and moisture in the cell pockets (ES Fig. 16).[31] The glucose concentration dropped to 0.7911 mg/mL in onion powder (R2) and to 0.28 mg/mL in the NaOCl-densified onion powder (P1). The decrease in the glucose concentration in the R2 and P1 samples with respect to R1 sample can be attributed to decomposition of sugars to acid saccharides, oxidative conversion of sugars to organic acids, and the decomposition of pyruvic acids.[32] [33]

The decrease in the sugar concentration was also validated using the DNSA method. It involves the reduction of DNSA by reducing sugars under alkaline conditions to yield 3-amino-5-nitrosalicylic acid ([Scheme 2]). The initial color of the reaction mixture is yellow due to the presence of the nitro groups. The reaction mixture turns red after the formation of the amino groups. The resulting color intensity is directly proportional to the concentration of reducing sugars in the sample. It was measured spectrophotometrically at 540–580 nm. The unknown sugar concentration in the samples R2 and P1, respectively, was determined using the calibration curve ([Fig. 3]) for the pure glucose sample. The glucose concentration of onion powder (R2) and NaOCl-densified onion powder (P1) was found to be 0.646 and 0.306 mg/mL, respectively.

Scheme 2 DNS reaction scheme reaction for glucose content estimation.

Fig. 3 Glucose calibration curve compared against onion powder (R2) and NaOCl-densified onion powder (P1) using the DNSA method.

The DNSA method results were in-line with the GOD-POD analysis and, therefore, validate the hypothesis of sugar degradation. Furthermore, high performance liquid chromatography (HPLC) was performed to estimate the exact concentration of different types of sugars present in the raw and processed onion samples. The HPLC analysis also corroborated the findings of the GOD-POD and DNSA tests. The details of the HPLC analysis for the sugar concentration determination in the raw and processed samples are elaborated in the supplementary information (ES Figs. 13 and 14).

In case of oxidation using hydrogen peroxide, significant brightening of the material was observed ([Fig. 2B]) due to the lower activation energy of the bleaching reaction when hydrogen peroxide is used as primary oxidant compared to sodium hypochlorite.[34] When hydrogen peroxide reacts with the wet onion sample, the catalase enzyme present in onion breaks the hydrogen peroxide into water and oxygen.[35] In the case of onion powder (R2), as the enzymes are denatured during the drying process, the oxidation reaction proceeds to decompose the residual sugars in the onion samples and crosslink the fibers. However, it has been reported that the rate of oxidation using hydrogen peroxide is significant only in the presence of enzymes.[36] [37] [38] This might have resulted in lower oxidation rates and lesser crosslinking of the fibers. The H₂O₂-densified onion waste (P2) exhibited lower compression strength than the NaOCl-densified onion waste (P1) ([Table 4]).

Table 4 Compression strength and comparison of densified onion powder treated with NaOCl (P1) and H2O2 (P2) with other literature values.

Sample	Area of cross section (cm2)	Average load (KN)	Compression strength (MPa)
P1	4.52	46	101.7
P2	4.52	36	79.59
Bamboo	NA	NA	70[39]
Douglas fir	NA	NA	45[40] [41]
Aluminum 7178-T6	NA	NA	530[42]

The compression strength of the processed onion samples was also evaluated to ascertain the application of such densified materials from food waste. The NaOCl-densified onion waste (P1) demonstrated a higher compression strength of 101.7 MPa compared to the compression strength of H₂O₂-densified onion waste (P2) of 79.59 MPa. The rate of oxidation and the oxidation levels achieved in the fibers post oxidation using sodium hypochlorite and hydrogen peroxide were different leading to a variable composition of the processed samples. However, the order of magnitude of compression strength for both the processed samples was similar to wood or bamboo. The compression strength values justify the use of this technology for making bio-based packaging material from onion waste.

The onion waste is composed of moisture, carbohydrates in the form of fructan and dimeric sugars, pectin, and fibers in the form of cellulose. Pectin is composed of long galacturonic acid chains linked by alpha 1,4-glycosodic bonds. The FTIR spectra of raw onion waste (R1), onion powder (R2), and NaOCl-densified onion powder (P1), respectively, are represented in [Fig. 4]. The broad absorption between 3250 and 3450 cm⁻¹ is seen due to the stretching vibrations of O–H from carbohydrates,[43] which decreases after drying and oxidation of the raw onion waste, indicating degradation of carbohydrates due to the heat and oxidation, respectively. The presence of galacturonic acid causes a significant absorption at a wavenumber of 1625 cm⁻¹ in raw onion waste samples corresponding to the C=O (carbonyl) stretching vibration.[44] [45] However, this absorption peak shifts slightly due to lower wavenumber after the oxidation of the raw onion powder due to conjugation of carbonyl group and formation of hydrogen bonding post oxidation. The strong absorption at 1010 cm⁻¹ in raw and oxidized onion samples can be attributed to the presence of alpha 1,4-glycosodic bonds.[46] The strong absorption at a wavenumber of 500 cm⁻¹ confirms the presence of polysaccharides in the form of cellulose. The scanning electron microscope images in [Fig. 5] of onion powder (R2) and NaOCl-densified onion powder (P1) corroborate the hypothesis that cellulose fibers crosslink only after the degradation of sugars and pectin. The image of the raw onion powder sample (R2) demonstrates an orderly pattern in the fibers with a gelatinous matrix filled between the fibers. This gelatinous matrix cannot be seen in the processed sample (P1), with an intense crosslinking between the fibers replacing the earlier orderly pattern.

Fig. 4 FTIR spectra of raw onion waste (R1), onion powder (R2), and NaOCl-densified onion powder (P1).

Fig. 5 SEM images illustrating morphological changes in (A) onion powder (R2) and (B) NaOCl-densified onion powder (P1).

[Fig. 6] represents the XRD spectra for raw onion powder (R2) and processed onion powder synthesized using sodium hypochlorite and hydrogen peroxide as oxidants, respectively, (P1 and P2). The XRD spectrum of onion powder (R2) demonstrates a broad peak at 20° with a shoulder at 22°, which is characteristic of the presence of pectin along with cellulose fibers.[44] [45] [47] However, the spectrum of the processed sample demonstrates only a single broad peak at 22° indicating that pectin has been oxidized and only the cellulosic fibers are left behind. The sharp peaks at 32°, 45°, and 55° are characteristics of the sodium chloride salt (ICDD 05-0628) that is formed after the oxidation, as sodium hypochlorite was used as an oxidant.[48] [49] [50] These characteristic peaks of sodium chloride are absent in the XRD spectrum of the sample that was oxidized using hydrogen peroxide as an oxidant (P2).

Fig. 6 XRD patterns of onion powder (R2) compared to densified onion powder treated with NaOCl (P1) and H2O2 (P2).

The presence of sodium chloride only in the oxidized samples is also verified by the elemental analysis done by energy dispersive X-ray spectroscopy (ES Table 1). When matched with mass balance data ([Fig. 1]), 5.64 g of densified onion powder was obtained from 5 g of onion powder (R2). It suggested the incorporation of sodium chloride post oxidation in the densified material, which is validated from the elemental analysis using energy dispersive X-ray spectroscopy and X-ray diffraction pattern of P1 (ES Table 1 and [Fig. 6]).

From the point of view of using the densified material for packaging applications, the temperature stability of the material was studied by heating the densified material at 100 and 200 °C, respectively, for 1 h and measuring the compression strength ([Fig. 7]).

Fig. 7 Visual changes in NaOCl-densified onion powder (P1) following 1-hour thermal treatment at 100°C (P1.1) and 200°C (P1.2).

The compression strength of the densified material increased by 20% after heating at 100 °C for 1 h ([Table 5]). This increase in the compression strength after heating the material can be attributed to the caramelization of the leftover sugars as the temperature provides sufficient activation energy for the Millard reaction. The sugars harden after caramelization and also increase the crystallinity of the material. However, when the densified material is heated to 200 °C, the residual sugars are charred and make the material brittle. Cracks are induced under stress in the material, and the compression strength decreases when the material is heated beyond 100 °C.

Table 5 Effect of thermal treatment on the compressive strength of NaOCl-densified onion powder at 25 °C (P1), 100 °C (P1.1), and 200 °C (P1.2).

Sample	Temperature (°C)	Time (min)	Area of cross section (cm)	Average load (kN)	Compression strength (MPa)
P1	70	240	4.52	46	101.7
P1.1	100	60	4.98	60	120.4
P1.2	200	60	4.68	40	85

Conclusion

Onion waste consisting of carbohydrates and fibers can be densified by controlled oxidation process at room temperature and atmospheric pressure using sodium hypochlorite. The process does not require any specialized equipment. The carbohydrates in the waste oxidize and generate energy, which is harnessed for the crosslinking of the fibers. The material obtained after oxidation demonstrated reasonable compression strength of 101.7 MPa, which is similar to the compression strength of bamboo or wood. The compression strength of the densified material remained stable even when the material was heated up to 100 °C for 1 h. It justifies the usage of the densified material for packaging applications replacing single-use plastics. This technology can be used to treat other forms of vegetable and fruit waste containing carbohydrates and fibers for generating revenue from otherwise high-tonnage waste material incurring high disposal costs.

Contributorsʼ Statement

Sudam Sankar Padhi: Investigation, Methodology, Writing - original draft. Saurabh Chandrakant Patankar: Conceptualization, Funding acquisition, Methodology, Project administration, Supervision, Validation, Writing - review & editing.

Conflict of Interest

The authors declare that they have no conflict of interest.

Acknowledgement

The authors acknowledge the support from the Central Sophisticated Instrumentation Facility (CSIF), Chemical Engineering Sophisticated Instrumentation Facility (CESIF), BITS Pilani, KK Birla Goa Campus for characterization of the raw and processed onion samples.

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Publication History

Received: 04 November 2025

Accepted after revision: 20 January 2026

Accepted Manuscript online:
20 January 2026

Article published online:
06 February 2026