Onion Peel: Turning a Food Waste into a Resource

Cobalt molybdate nanoflowers decorated bio-waste derived porous activated carbon nanocomposite: A high performance electrode material for supercapacitors

In this work, we report a supercapacitor electrode material based on nano-flower like cobalt molybdate decorated on porous activated carbon derived from waste onion peels (β-CoMoO4-POAC). The obtained POAC exhibits highly porous structure and after the hydrothermal treatment with salts of cobalt and molybdenum, we observed a uniform distribution of β-cobalt molybdate (β-CoMoO4) as nano-flowers on the surface of POAC. The chemical composition, morphology and porosity of the materials were thoroughly analyzed using field emission scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, infrared spectroscopy and Brunauer-Emmet-Teller surface area measurement. Due to its flower like and highly porous morphology, β-CoMoO4@POAC exhibits a high specific capacitance of 1110.72 F/g at a current density of 1 mA/cm2 with superior cyclic retention of 96.03% after 2000 cycles. The best electrochemical performance exhibited by β-CoMoO4@POAC is mainly due to its high surface area and porous nature of the material which assists in active transport of ions. This study reveals the exceptional electrochemical properties of β-CoMoO4@POAC which could be considered as a potential material for advanced energy storage devices.

Antimicrobial Properties of Newly Developed Silver-Enriched Red Onion-Polymer Composites

Simple low-cost, nontoxic, environmentally friendly plant-extract-based polymer films play an important role in their application in medicine, the food industry, and agriculture. The addition of silver nanoparticles to the composition of these films enhances their antimicrobial capabilities and makes them suitable for the treatment and prevention of infections. In this study, polymer-based gels and films (AgRonPVA) containing silver nanoparticles (AgNPs) were produced at room temperature from fresh red onion peel extract ("Ron"), silver nitrate, and polyvinyl alcohol (PVA). Silver nanoparticles were synthesized directly in a polymer matrix, which was irradiated by UV light. The presence of nanoparticles was approved by analyzing characteristic local surface plasmon resonance peaks occurring in UV-Vis absorbance spectra of irradiated experimental samples. The proof of evidence was supported by the results of XRD and EDX measurements. The diffusion-based method was applied to investigate the antimicrobial activity of several types of microbes located in the environment of the produced samples. Bacteria Staphylococcus aureus ATCC 29213, Acinetobacter baumannii ATCC BAA 747, and Pseudomonas aeruginosa ATCC 15442; yeasts Candida parapsilosis CBS 8836 and Candida albicans ATCC 90028; and microscopic fungi assays Aspergillus flavus BTL G-33 and Aspergillus fumigatus BTL G-38 were used in this investigation. The greatest effect was observed on Staphylococcus aureus, Acinetobacter baumannii, and Pseudomonas aeruginosa bacteria, defining these films as potential candidates for antimicrobial applications. The antimicrobial features of the films were less effective against fungi and the weakest against yeasts.

An ethnopharmacological, phytochemical, and pharmacological overview of onion (Allium cepa L.)

ETHNOPHARMACOLOGICAL RELEVANCE

Onion (Allium cepa L.) is one of the most widely distributed species within the Allium genus of family Amaryllidaceae. Onion has been esteemed for its medicinal properties since antiquity. It has been consumed for centuries in various indigenous cultures for the management of several ailments including microbial infections, respiratory, gastrointestinal, skin and cardio-vascular disorders, diabetes, renal colic, rheumatism, sexual impotence, menstrual pain, and headache. However, so far, there is a scarcity of recent data that compiles the plant chemistry, traditional practices, biological features, and toxicity.

AIM OF THE WORK

The aim of this review is to provide a comprehensive and analytical overview of ethnopharmacological uses, phytochemistry, pharmacology, industrial applications, quality control, and toxicology of onion, to offer new perspectives and broad scopes for future studies.

MATERIALS AND METHODS

The information gathered in this review was obtained from various sources including books, scientific databases such as Science Direct, Wiley, PubMed, Google Scholar, and other domestic and foreign literature.

RESULTS

Onion has a long history of use as a traditional medicine for management of various conditions including infectious, inflammatory, respiratory, cardiovascular diseases, diabetes, and erectile dysfunction. More than 400 compounds have been identified in onion including flavonoids, phenolic acids, amino acids, peptides, saponins and fatty acids. The plant extracts and compounds showed various pharmacological activities such as antimicrobial, antidiabetic, anti-inflammatory, anti-hyperlipidemic, anticancer, aphrodisiac, cardioprotective, and neuroprotective activities. In addition to its predominant medicinal uses, onion has found various applications in the functional food industry.

CONCLUSION

Extensive literature analysis reveals that onion extracts and bioactive constituents possess diverse pharmacological activities that can be beneficial for treating various diseases. However, the current research primarily revolves around the documentation of ethnic pharmacology and predominantly consists of in vitro studies, with relatively limited in vivo and clinical studies. Consequently, it is imperative for future investigations to prioritize and expand the scope of in vivo and clinical research. Additionally, it is strongly recommended to direct further research efforts towards toxicity studies and quality control of the plant. These studies will help bridge the current knowledge gaps and establish a solid basis for exploring the plant's potential uses in a clinical setting.

Quantification of the Anti-diabetic Effect of Allium cepa

BACKGROUND

Allium cepa, or onion, boosts numerous health benefits, including anti-diabetic effects. Its rich array of antioxidants and sulfur compounds not only aids heart health by lowering cholesterol and blood pressure but also exhibits anti-inflammatory properties. Onion's antibacterial and antiviral properties help combat infections, while its compounds like quercetin show promise in cancer prevention. Additionally, Allium cepa supports respiratory health by relieving coughs and colds and aids digestion with its prebiotic properties. Incorporating onions into a balanced diet can enhance overall well-being, including managing blood sugar levels in individuals with diabetes.

AIM AND OBJECTIVE

This study aims to determine if the ethanolic extract from the dried peel of Allium cepa holds potential as an anti-diabetic agent, with a focus on its ability to manage diabetes and reduce blood sugar levels.

METHODOLOGY

To prepare the ethanolic extract from dried onion peel, the peel was finely ground and soaked in ethanol. The mixture was then agitated and filtered to separate the liquid extract. Finally, the filtrate was concentrated using methods such as rotary evaporation or vacuum distillation to obtain a concentrated extract for further analysis like alpha-amylase inhibition assay and alpha-glucosidase inhibition assay.

RESULTS

The ethanolic extracts derived from dried onion peel demonstrate inhibition of alpha-glucosidase, leading to reduced blood glucose levels. Additionally, this inhibition prompts an increase in insulin production.

CONCLUSION

The study underscores that the efficacy of the ethanolic extract of dried onion peel increases with concentration. It highlights the presence of beneficial compounds like total phenolics, flavonoids, quercetin, and its derivatives in onion peel, known for their therapeutic roles in cardiovascular health, weight management, diabetes control, cancer prevention, and antimicrobial activity. These findings affirm the hypoglycemic and anti-diabetic properties of Allium cepa's ethanolic leaf extract.

Gastroprotective potential of red onion (Allium cepa L.) peel in ethanol-induced gastric injury in rats: Involvement of Nrf2/HO-1 and HMGB-1/NF-κB trajectories

ETHNOPHARMACOLOGICAL RELEVANCE

The utilization of plants with therapeutic properties in traditional medicine has a longstanding practice. Among them, the well-known Allium cepa L. commonly known as onion has been valued for its anti-inflammatory and antioxidant potential in the treatment of various ailments, including gastric ulcers.

AIM OF THE STUDY

This study investigated the gastroprotective potential of red onion peel extract and its fractions in a rat model of ethanol-induced gastric ulcer. Moreover, their phytochemical profiles were compared to identify the active metabolites.

MATERIALS AND METHODS

Mass spectrometry-based metabolomics and chemometrics were performed for phytochemical analysis. Ethanol-induced gastric ulcer model was used to assess the gastroprotective activity. Nine groups of rats were allocated as follows: Group 1 was the normal control; Group 2 rats were used as a positive control/model and received 1 mL of absolute ethanol; and Group 3 rats were treated with famotidine at a dose of 20 mg/kg orally. Group 4 and 5 rats were treated with total acidified ethanolic extract (T1, T2). Group 6 and 7 rats were treated with anthocyanins-rich fractions (P1, P2). Groups 8 and 9 were the flavonoids-rich fraction (S1, S2) treatment. Prior to scarification, the ulcer index in mm was obtained from gastric tissues photographed beside a ruler with further analysis using ImageJ software.

RESULTS

Seventy key major and discriminatory metabolites were identified including flavonoids, anthocyanins, phenolic acids, and miscellaneous compounds. The examined extract and its fractions significantly reduced the ulcer index and inflammatory cytokines via downregulating HMGB-1/NF-κB. Also, they augmented the expression of Nrf2/HO-1 and reduced NOX1/4 mRNA expression. Moreover, there was a significant reduction in the oxidative stress and apoptotic biomarkers as well as a noticeable enhancement in histopathological changes of the stomach tissues.

CONCLUSION

Red onion peels have a promising dose dependent gastroprotective potential in alcohol-induced ulcers via modulating Nrf2/HO-1 and HMGB-1/NF-κB trajectories. This highlights the potential of red onion peels in treating gastric ulcers.

Red Onion Peel Powder as a Functional Ingredient for Manufacturing Ricotta Cheese

Onion (Allium cepa L.) is a vegetable widely cultivated and consumed due to its rich content in bioactive compounds. Red onion peel (ROP) powder, which is a by-product derived from the onion industry, has been attracting significant interest as a potential functional ingredient for improving the overall quality of foods. The present study explores the potential of ROP powder as a functional ingredient to improve the quality and nutritional value of whey cheese. Despite being frequently viewed as a food processing waste byproduct, ROP is a rich source of bioactive substances, including antioxidants, flavonoids, and dietary fiber, having antioxidant and antibacterial effects. The ROP extract exhibited high amounts of total polyphenols (119.69 ± 2.71 mg GAE/g dw) and antioxidant activity (82.35 ± 1.05%). Different quantities (1 and 3%) of ROP powder were added to cheese formulations, and the subsequent impact on the texture characteristics, sensory attributes, and phytochemical composition of the value-added cheeses was evaluated. The findings show that the addition of ROP powder improved the texture and the color of the cheeses, providing a visually appealing product. Additionally, adding the ROP powder significantly raised the amount of phytochemicals and antioxidant activity (17.08 ± 0.78 µmol TE/g dw for RCROP1, 24.55 ± 0.67 µmol TE/g dw for RCROP2) in the final product's formulation. Moreover, adding powder to cheese is an effective way to increase the value of onion by-products and produce polyphenol-enriched cheese.

Valorization of onion peel waste: From trash to treasure

Globally, fruits and vegetables are consumed as raw, processed, or as an additive, accounting for approximately 50% of total food wastage. Among the fruits and vegetables, onion is well known for its potential bioactive components; however, peels of onion are a major concern for the environmental health and food industries. Effective utilization methods for valorizing the onion peel should be needed to develop value-added products, which are more eco-friendly, cost-effective, and sustainable. Therefore, this review attempts to emphasize the conventional and emerging valorization techniques for onion peel waste to generate value-added products. Several vital applications including anticancerous, antiobesity, antimicrobial, and anti-inflammatory activities are thoroughly discussed. The findings showed that the use of advanced technologies like ultrasound-assisted extraction, microwave-assisted extraction, and enzymatic extraction, demonstrated improved extraction efficiency and higher yield of bioactive compounds, which showed the anticancerous, antiobesity, antimicrobial, and anti-inflammatory properties. However, in-depth studies are recommended to elucidate the mechanisms of action and potential synergistic effects of the bioactive compounds derived from onion peel waste, and to promote the sustainable utilization of onion peel waste in the long-term.

Supplementation with grape seed extract, onion peel extract, or rosemary extract in the diet alleviates growth inhibition, liver damage, and oxidative stress induced by diquat in Lohmann chicks

The present study aimed to assess the impact of grape seed extract (GSE), onion peel extract (OPE), and rosemary extract (ROE) on Diquat-induced growth restriction and oxidative stress in Lohmann chicks. A total of 200 chicks were randomly assigned to 5 diets: the positive control (PC) group, the negative control (NC) group, GSE group, OPE group, and ROE group. During the first 7 d of trial, compared with NC and PC groups, the GSE group enhanced average daily feed intake (ADFI). From day 8-21, diquat injection resulted in reduced growth performance, increased platelet volume distribution width (PWD), malondialdehyde (MDA) concentration, and activities of alanine aminotransferase (ALT) in chick serum; it also decreased total protein (TP), albumin (ALB), globulin (GLB) concentration, activities of superoxide dismutase (SOD) and glutathione S-transferase (GST) in chick serum; furthermore, it increased MDA concentration while decreasing GST activities in liver. The NC group exhibited lower average daily gain (ADG) than other groups. Compared with NC group, GSE group reduced ALT activities, MDA levels, and red cell distribution width (RDW), and PDW concentration; it also increased SOD, GST activities. The ROE group lowered ALT activities and MDA concentration. The OPE group decreased ALT activities, and MDA levels, RDW, and PDW concentration, and increased SOD activities of chicks. These results suggest that supplementing antioxidants in diets alleviated oxidative stress in chicks challenged by improving antioxidant capacity and liver function.

A Comparative Study between Onion Peel Extracts, Free and Complexed with β-Cyclodextrin, as a Natural UV Filter to Cosmetic Formulations

The growing concern regarding the adverse effects of synthetic UV filters found in sunscreens has spurred significant attention due to their potential harm to aquatic ecosystems and human health. To address this, the present study aimed to extract and microencapsulate sensitive bioactive compounds derived from by-product onion peel (OP) by molecular inclusion using β-cyclodextrin as the wall material. Identification and quantification of bioactive compounds within the extract were conducted through high-performance liquid chromatography (HPLC-DAD) analysis, revealing quercetin and resveratrol as the primary constituents. The photoprotection capacity, evaluated by the sun protection factor (SPF), revealed a protection factor comparable to the value for a synthetic UV filter. The produced microparticles presented high antioxidant capacity, significant photoprotection capacity, encapsulation efficiency of 91.8%, mean diameter of 31 μm, and polydispersity of 2.09. Furthermore, to comprehensively evaluate the performance of OP extract and its potential as a natural UV filter, five O/W emulsions were produced. Results demonstrated that microparticles displayed superior ability in maintaining SPF values over a five-week period. Photoprotection evaluation-skin reactivity tests revealed that both extract and microparticles absorb UV radiation in other regions of UV radiation, revealing their potential to be used as a natural UV filter to produce a sustainable and eco-friendly value-added sunscreen.

Unlocking the Power of Onion Peel Extracts: Antimicrobial and Anti-Inflammatory Effects Improve Wound Healing through Repressing Notch-1/NLRP3/Caspase-1 Signaling

Onion peels are often discarded, representing an unlimited amount of food by-products; however, they are a valuable source of bioactive phenolics. Thus, we utilized UPLC-MS/MS to analyze the metabolomic profiles of red (RO) and yellow (YO) onion peel extracts. The cytotoxic (SRB assay), anti-inflammatory (Griess assay), and antimicrobial (sensitivity test, MIC, antibiofilm, and SP-SDS tests) properties were assessed in vitro. Additionally, histological analysis, immunohistochemistry, and ELISA tests were conducted to investigate the healing potential in excisional skin wound injury and Candida albicans infection in vivo. RO extract demonstrated antibacterial activity, limited skin infection with C. albicans, and improved the skin's appearance due to the abundance of quercetin and anthocyanin derivatives. Both extracts reduced lipopolysaccharide-induced nitric oxide release in vitro and showed a negligible cytotoxic effect on MCF-7 and HT29 cells. When extracts were tested in vivo for their ability to promote tissue regeneration, it was found that YO peel extract had the greatest impact. Further biochemical analysis revealed that YO extract suppressed NLRP3/caspase-1 signaling and decreased inflammatory cytokines. Furthermore, YO extract decreased Notch-1 levels and boosted VEGF-mediated angiogenesis. Our findings imply that onion peel extract can effectively treat wounds by reducing microbial infection, reducing inflammation, and promoting tissue regeneration.

From Waste to Green: Water-Based Extraction of Polyphenols from Onion Peel and Their Adsorption on Biochar from Grapevine Pruning Residues

Onion peels (OP) are rich in bioactive compounds with a plethora of benefits for human health, but this valuable material is often wasted and underutilized due to its inedibility. Likewise, grapevine pruning residues are commonly treated as agricultural waste, but biochar (BC) obtained from this material has favorable characteristics as an adsorbent. This study investigated the potential of BC in removal of targeted polyphenolic compounds from OP extracts. The OP extracts were obtained adhering to green chemistry principles using deionized water amplified by three methods: maceration (MAC), ultrasound-assisted extraction (UAE), and microwave-assisted extraction (MAE). The extraction efficiency on the polyphenolic profile and antioxidant capacity was investigated with different extraction temperatures and solid-to-liquid (s/l) ratios. For further analysis, UAE at 90 °C with an s/l ratio of 1:100 was used due to higher polyphenolic compound yield. The BC adsorption capacity of individual polyphenols was fitted with the Langmuir and Freundlich isotherm models. Quercetin-3,4'-diglucoside obtained the highest R2 coefficient in both models, and the highest qmax value. The optimum conditions in the dosage experiment suggested an amount of 0.5 g of BC using 3 g/L extracts. The studied BC showed a high affinity for targeted phytochemicals from OP extracts, indicating its potential to be applied for the green adsorption of valuable polyphenolic compounds.

Investigation of the phytochemical composition, antioxidant, antibacterial, anti-osteoarthritis, and wound healing activities of selected vegetable waste

Agri-food wastes, produced following industrial food processing, are mostly discarded, leading to environmental hazards and losing the nutritional and medicinal values associated with their bioactive constituents. In this study, we performed a comprehensive analytical and biological evaluation of selected vegetable by-products (potato, onion, and garlic peels). The phytochemical analysis included UHPLC-ESI-qTOF-MS/MS in combination with molecular networking and determination of the total flavonoid and phenolic contents. Further, the antimicrobial, anti-osteoarthritis and wound healing potentials were also evaluated. In total, 47 compounds were identified, belonging to phenolic acids, flavonoids, saponins, and alkaloids as representative chemical classes. Onion peel extract (OPE) showed the higher polyphenolic contents, the promising antioxidant activity, the potential anti-osteoarthritis activity, and promising antimicrobial activity, especially against methicillin-resistant Staphylococcus aureus (MRSA). Furthermore, OPE revealed to have promising in vivo wound healing activity, restoring tissue physiology and integrity, mainly through the activation of AP-1 signaling pathway. Lastly, when OPE was loaded with nanocapsule based hydrogel, the nano-formulation revealed enhanced cellular viability. The affinities of the OPE major metabolites were evaluated against both p65 and ATF-2 targets using two different molecular docking processes revealing quercetin-3,4'-O-diglucoside, alliospiroside C, and alliospiroside D as the most promising entities with superior binding scores. These results demonstrate that vegetable by-products, particularly, those derived from onion peels can be incorporated as natural by-product for future evaluation against wounds and osteoarthritis.

Liquid submerged fermentation by selected microbial strains for onion skins valorization and its effects on polyphenols

Onion skins, actually recycled as organic fertilizers, could be used as a substrate in environmental-friendly bioprocesses to recover high-value bioactive compounds and food ingredients.In this work, a bioprospecting method was carried out including 94 bacterial and 45 yeast strains from several agri-food and environmental niches to verify their ability to grow on onion skins as unique nutrients source.Red and yellow onion skins were assessed by newly selected starter-driven liquid submerged fermentation assays mainly aimed at the release and modification of polyphenols through microbial activities. Fermented onion skins were also investigated as a inexpensive favourable source of microbial enzymes (amylases, proteases, lipases, esterases, cellulases, xylanases).In red onion skins, the treatment with Lactiplantibacillus plantarum TB 11-32 produced a slight increase of bioactive compounds in terms of total phenolics, whereas with the yeast strain Zygosaccharomyces mrakii CL 30 - 29 the quercetin aglycone content increased of about 25% of the initial raw material.In yellow onion skins inoculated, the highest content of phenolic compounds was detected with the yeast strain Saccharomyces cerevisiae En SC, while quercetin aglycone increased of about 60% of the initial raw material in presence of the bacterial strain L. plantarum C 180 - 34.In conclusion, the proposed microbial pre-treatment method can be a potential strategy to re-use onion skins as a fermentation substrate, and as a first step in the development of a biorefinery process to produce value-added products from onion by-products.

Quercetin extraction from small onion skin (Allium cepa L. var. aggregatum Don.) and its antioxidant activity

In the present study, the maximum yield of quercetin was optimized for the ethanol extraction of small onions (Allium cepa L. var. aggregatum Don.), and the antioxidant activity was investigated in vitro. The extraction of quercetin from the small onion skin was carried out through ethanol solvent extraction with different ratios of ethanol and water. Ethanol: water ratio produced the highest quercetin from the onion skin. RP-HPLC analysis of the extracted material showed 2, 122 mg/g of quercetin and 0.34 mg/g of isorhamnetin. A total of 301.03 mg GAE/g dry weight and 156 mg/g quercetin equivalents were found in the onion skin extract. DPPH and ABTS free radical scavenging potentials of the tested extract (90:10 v/v) were dose-dependent, with IC₅₀ values of 62.27 μg/mL and 53.65 μg/mL, respectively. Therefore, the present study reports that small onion skin extract rich in quercetin may serve as a promising antioxidant and anticancer agent.

The ‘Edge Effect’ Phenomenon in Plants: Morphological, Biochemical and Mineral Characteristics of Border Tissues

The ‘edge’ effect is considered one of the fundamental ecological phenomena essential for maintaining ecosystem integrity. The properties of plant outer tissues (root, tuber, bulb and fruit peel, tree and shrub bark, leaf and stem trichomes) mimic to a great extent the ‘edge’ effect properties of different ecosystems, which suggests the possibility of the ‘edge’ effect being applicable to individual plant organisms. The most important characteristics of plant border tissues are intensive oxidant stress, high variability and biodiversity of protection mechanisms and high adsorption capacity. Wide variations in morphological, biochemical and mineral components of border tissues play an important role in the characteristics of plant adaptability values, storage duration of roots, fruit, tubers and bulbs, and the diversity of outer tissue practical application. The significance of outer tissue antioxidant status and the accumulation of polyphenols, essential oil, lipids and minerals, and the artificial improvement of such accumulation is described in connection with plant tolerance to unfavorable environmental conditions. Methods of plant ‘edge’ effect utilization in agricultural crop breeding, production of specific preparations with powerful antioxidant value and green nanoparticle synthesis of different elements have been developed. Extending the ‘edge’ effect phenomenon from ecosystems to individual organisms is of fundamental importance in agriculture, pharmacology, food industry and wastewater treatment processes.

Novel use of kitchen waste: protection of boiler quality steel from corrosion in acidic media using onion waste

An eco-friendly approach to inhibit the corrosion of boiler quality (BQ) steel by onion waste in acidic media was investigated. The extract from onion peel was characterized using the conventional extraction method and was characterized using HPLC. The efficacy of the onion peel extract (OPE) as a green corrosion inhibitor was studied using the weight loss method and a variety of electrochemical techniques, including open-circuit potential (OCP), potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS). The Tafel polarization revealed that at 200 mg L^-1 of onion peel extract (OPE), corrosion current density was reduced maximum in both 1 (M) HCl and 0.5 (M) H₂SO₄ media. From the electrochemical impedance spectroscopy studies, the maximum inhibition efficiencies of 91.30% and 90.71% were found at 200 mg L^-1 in 1 (M) HCl and 0.5 (M) H₂SO₄, respectively. The Langmuir isotherm was determined to be the best-fitting model, and the thermodynamic parameter, such as free energy Δ G ads ∘ , was computed, which indicated the physisorption of OPE onto the BQ surface. In theoretical investigations, density functional theory DFT was used to determine the adsorption efficiency and reactive sites of the OPE molecule by exploring various quantum chemical parameters.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11696-022-02549-7.

Value-added salad dressing enriched with red onion skin anthocyanins entrapped in different biopolymers

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The microencapsulation of ROS extract was performed by gelation and freeze-drying.

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The powders were rich in valuable antioxidants, with over 80% EE for anthocyanins.

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Good stability of phytochemicals over storage and digestion was noticed.

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V2 powder’s functionality was tested by its addition to salad dressing.

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Our results are promising in developing multifunctional ingredients for foods.

In this study, red onion skin extract was used to obtain food ingredients. Complex biopolymeric matrices were dissolved in the anthocyanin-rich aqueous extract, followed by gelation and freeze-drying. Powders were characterized regarding encapsulation efficiency (EE), phytochemical content, color, antioxidant activity, and microstructure. Storage and simulated digestion stability were also assessed. Two powders with high contents of bioactives and antioxidant activity were obtained. The highest EE was acquired for the powder with a higher polysaccharides concentration (V2). In addition, V2 exhibited the best storage stability. The in vitro studies demonstrated that increased carbohydrate concentration delivers the best anthocyanins protection. To prove its functionality, V2 was added to a salad dressing. The addition of powder has improved the concentration of biologically active compounds and the antioxidant activity of the salad dressings. These results support the assumption that microencapsulation may deliver bioactives from red onion skin as functional ingredients for value-added foods.

Comparative Studies on a Standardized Subfraction of Red Onion Peel Ethanolic Extract (Plant Substance), Quercetin (Pure Compound), and Their Cell Mechanism and Metabolism on MDA-MB-231

This study indicates the presence of quercetin in subfraction F1 and the standardized value of F1 derived from research using ultra-performance liquid chromatography (UPLC) and AlCl₃ colorimetric assays, which further proved that both F1 and quercetin are potential growth inhibitors in MDA-MB-231 cells by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. In the process, staining of F1-treated cells with annexin/propidium iodide (PI) reduced cell proliferation and induced only S and G2 phases of cell cycle arrest in the treated cells by flow cytometry. Quercetin reduced cell proliferation by inducing apoptosis and S phase arrest. The 5′-bromo-2′-deoxyuridine (BrdU) incorporation of DNA synthesis in MDA-MB-231 cells was also inhibited after F1 and quercetin treatments. F1 and quercetin induced CYP1A1 and CYP1B1 gene expression, but only F1 induced CYP2S1 gene expression in the treated cells. Both F1 and quercetin inhibited the proliferation of MDA-MB-231 cells in different ways, but F1 is likely a better potential anticancer agent derived from the green approach towards breast cancer treatment.

Unraveling the metabolic shift in anaerobic digestion pathways associated with the alteration of onion skin waste concentration

Onion skin waste (OSW) is common waste in developing countries, which can cause severe environmental pollution when not properly treated. Value-added products can be chemically extracted from OSW; however, that process is not economically feasible. Alternatively, dry anaerobic digestion (DAD) of OSW is a promising approach for both energy recovery and environment protection. The main hurdles during DAD of OSW can be the hydrolysis and acidification. In batch tests, sludge digestate (SD) rich with methanogens was co-digested with different fractions of OSW for enhancing hydrolysis and raising biogas productivity. The cumulative biogas production (CBP) was 36.6 ± 0.3 mL for sole DAD of SD (100% SD) and increased up to 281.9 ± 14.1 mL for (50% SD: 50% OSW) batch. Self-delignification of OSW took place by SD addition, where the lignin removal reached 75.3 ± 10.5% for (85% SD: 15% OSW) batch. Increasing the fraction of OSW (45% SD: 55% OSW) reduced the delignification by a value of 68.8%, where initial lignin concentration was 9.48 ± 1.6% in dry weight. Lignin breaking down resulted a high fraction of phenolic compounds (345.6 ± 58.8 mg gallic acid equivalent/g dry weight) in the fermentation medium, causing CBP drop (219.0 ± 28.5 mL). The presence of elements (K, Ca, Mg, Fe, Zn, Mn, S and P) in OSW improved the enzymatic activity, facilitated phenolic compounds degradation, shifted the metabolism towards acetate fermentation pathway, and raised biogas productivity. Acidogenesis was less affected by phenolic compounds than methanogenesis, causing higher H₂ contents and lower CH₄ contents, at batches with high share of OSW.

Preparation and Performance Characterization of a Composite Film Based on Corn Starch, κ-Carrageenan, and Ethanol Extract of Onion Skin

Using corn starch (CS) and κ-carrageenan(κC) as the raw material and active composite, respectively, films containing different concentrations of ethanol extract of onion skin were prepared. The effects of different concentrations of ethanol extract of onion skin (EEOS) on the physicochemical properties, as well as the antioxidant and antibacterial properties, of CS/κC films were also discussed. The addition of ethanol extract of onion skin inhibited the recrystallization of starch molecules in the composite films. It affected the microstructure of the composite films. The color of the composite films was deepened, the brightness was reduced, and the opacity was increased. Water vapor permeability increased, tensile strength decreased, and elongation at the break increased. The glass-transition temperature decreased. The clearance of DPPH radicals and ABTS cation radicals increased. Moreover, when the concentration of EEOS was 3%, the antioxidant effect of the films on oil was greatly improved and could effectively inhibit Staphylococcus aureus and Escherichia coli. The above results showed that adding ethanol extract of onion skin improved the physicochemical properties and biological activities of the CS/κC composite films, so CS/κC/EEOS composite films can be used as an active packaging material to extend food shelf-life. These results can provide a theoretical basis for the production and application of corn starch/κ-carrageenan/ethanol extract of onion skin composite films.

Fruit and Vegetable Peel-Enriched Functional Foods: Potential Avenues and Health Perspectives

Fresh fruit and vegetables are highly utilized commodities by health-conscious consumers and represent a prominent segment in the functional and nutritional food sector. However, food processing is causing significant loss of nutritional components, and the generation of waste is creating serious economic and environmental problems. Fruit and vegetables encompass husk, peels, pods, pomace, seeds, and stems, which are usually discarded, despite being known to contain potentially beneficial compounds, such as carotenoids, dietary fibers, enzymes, and polyphenols. The emerging interest in the food industry in the nutritional and biofunctional constituents of polyphenols has prompted the utilization of fruit and vegetable waste for developing enriched and functional foods, with applications in the pharmaceutical industry. Moreover, the utilization of waste for developing diverse and crucial bioactive commodities is a fundamental step in sustainable development. Furthermore, it provides evidence regarding the applicability of fruit and vegetable waste in different food formulations especially bakery, jam, and meat based products.

Differences in Antioxidant Potential of Allium cepa Husk of Red, Yellow, and White Varieties

The effective management of agro-industry organic waste for developing high-commercial-value products is a promising facet of the circular economy. Annually, more than 550,000 tons of waste that is potentially rich in biologically active substances is generated worldwide while processing onions (Allium cepa L.). The antioxidant potential of red, yellow, and white onion husks was studied using FRAP, ORAC, chemiluminescence, and UPLC-ESI-Q-TOF-MS analysis methods. The extraction of phenolic compounds from onion husks was more effective when using an aqueous solution of 70% ethanol as compared with water. Ethanolic extract from red onion husks exhibited the highest TAC_ORAC and TAC_FRAP values, averaging 2017.34 µmol-equiv. Trolox/g raw material and 2050.23 µmol-equiv. DQ/g raw material, respectively, while the white onion exhibited much lower levels of antioxidants. According to the chemiluminescence results, it was determined that the red and yellow onion husks contained antioxidants of three types of power, while white onion husks only contained medium and weak types. The highest content of flavonoids was found in red onion husks, averaging 1915.90 ± 9.92 µg-eq. DQ/g of raw material and 321.42 ± 2.61 µg-eq. DQ/g of raw material for ethanol and water, respectively, while yellow onion husks exhibited 544.06 ± 2.73 µg-eq. DQ/g of raw material and 89.41 ± 2.08 for ethanol and water, respectively. Quercetin and its glycosides were the most representative flavonoids, and a number of substances with different pharmacological and biological properties were also identified.

Value-Added Crackers Enriched with Red Onion Skin Anthocyanins Entrapped in Different Combinations of Wall Materials

The objective of this study was to encapsulate anthocyanins from red onion skins into different biopolymeric matrices as a way to develop powders with multifunctional activities. Two different variants of powders were obtained using a combination of gelation and freeze-drying techniques and characterized by encapsulation efficiency, antioxidant activity, phytochemical content, and color. Stability during storage and the bioavailability of anthocyanins in the in vitro simulated digestion were also examined. Powder V2, with a higher concentration of polysaccharides than V1, allowed a better encapsulation efficiency (90.53 ± 0.29%) and good stability during storage. Both variants had a high content of phytochemicals and antioxidant activity. In vitro investigations proved that an increased polysaccharides concentration offers the best protection for anthocyanins. Thus, a controlled release of the anthocyanins in the intestinal medium was achieved. The powder with the highest encapsulation efficiency was added to crackers, followed by phytochemical characterization to assess its potential added value. The addition of the micro-particles improved the functional characteristics such as antioxidant activity, showing its suitability for the development of bakery products. The attained results may bring implicit benefits to consumers, who can benefit from improved bioactive concentrations in foodstuffs, with significant health benefits.

Valorization of ‘Rossa di Tropea’ Onion Waste through Green Recovery Techniques of Antioxidant Compounds

The aim of this work was to maximize the recovery of the bioactive components from an important solid waste derivate from Tropea onion processing. To achieve this, three different extractive procedures (conventional for maceration, ultrasound, and microwave-assisted) were employed, using only food-grade extraction solvents such as water and ethanol:water (50:50). Solvent, temperature, microwave power, time, and their interaction were studied as the principal factors that might affect the extractability rates. The obtained data suggest that the hydroalcoholic mixture proved to be the best for each of the techniques developed and at each time and temperature considered. In particular, the best results were achieved by conventional extraction for 60 min at 40 °C (total flavonoids content: 25.64 ± 1.40 mg QE g−1 d.w.; total anthocyanins content: 0.78 ± 0.01 mg C-3-GLUC g−1 d.w.). The UHPLC analysis of the optimally obtained extract revealed that the principal phytochemicals recovered were quercetin (5322.61 ± 0.32 mg kg−1) and quercetin 3-4′-diglucoside (1023.80 ± 0.34 mg kg−1) after conventional and ultrasound-assisted extraction, respectively. In this perspective, the implementation of sustainable, food-grade extraction processes to recover value-added substances from solid onion waste could play a crucial role both in reducing the waste load and in formulating natural food additives with functional properties, with a potential direct industrial impact.

Valorization of Onion Waste by Obtaining Extracts Rich in Phenolic Compounds and Feasibility of Its Therapeutic Use on Colon Cancer

In this study, the total phenolic content, the antioxidant and antiproliferative activities of onion waste extracts were characterized. Some phenolic compounds present in the extracts were also identified and quantified by HPLC-DAD. Additionally, an in-silico analysis was performed to identify the phenolic compounds with the highest intestinal absorption and Caco-2 permeability. The onion extract possessed a high amount of phenolic compounds (177 ± 9 mg/g extract) and had an effective antioxidant capacity measured by ABTS, FRAP and DPPH assays. Regarding the antiproliferative activity, the onion extracts produced cell cycle arrest in the S phase with p53 activation, intrinsic apoptosis (mitochondrial membrane potential modification) and caspase 3 activation. Likewise, onion waste increased intracellular ROS with possible NF-kB activation causing a proteasome down regulation. In addition, the extracts protected the intestine against oxidative stress induced by H₂O₂. According to the in-silico analysis, these results could be related to the higher Caco-2 permeability to protocatechuic acid. Therefore, this study provides new insights regarding the potential use of these types of extract as functional ingredients with antioxidant and antiproliferative properties and as medicinal agents in diseases related to oxidative stress, such as cancer. In addition, its valorization would contribute to the circular economy.

What Worth the Garlic Peel

Plants have two types of reproduction: sexual, resulting in embryo production, and asexual, resulting in vegetative bodies commonly derived from stems and roots (e.g., bulb, tuber). Dead organs enclosing embryos (DOEEs, such as seed coat and pericarp) are emerging as central components of the dispersal unit acting to nurture the embryo and ensure its survival in the habitat. Here we wanted to investigate the properties of dead organs enclosing plant asexual reproductive bodies, focusing on the garlic (Allium sativum) bulb. We investigated the biochemical and biological properties of the outer peel enclosing the bulb and the inner peel enclosing the clove using various methodologies, including bioassays, proteomics, and metabolomics. The garlic peels differentially affected germination and post-germination growth, with the outer peel demonstrating a strong negative effect on seed germination of Sinapis alba and on post-germination growth of Brassica juncea. Proteome analysis showed that dead garlic peels possess 67 proteins, including chitinases and proteases, which retained their enzymatic activity. Among primary metabolites identified in garlic peels, the outer peel accumulated multiple sugars, including rhamnose, mannitol, sorbitol, and trehalose, as well as the modified amino acid 5-hydroxylysine, known as a major component of collagen, at a higher level compared to the clove and the inner peel. Growth of Escherichia coli and Staphylococcus aureus was promoted by garlic peel extracts but inhibited by clove extract. All extracts strongly inhibited spore germination of Fusarium oxysporum f.sp. melonis. Thus, the garlic peels not only provide physical protection to vegetative offspring but also appear to function as a refined arsenal of proteins and metabolites for enhancing growth and development, combating potential pathogens, and conferring tolerance to abiotic stresses.

Biotechnological Applications and Health-Promoting Properties of Flavonols: An Updated View

Flavonols are a subclass of natural flavonoids characterized by a remarkable number of biotechnological applications and health-promoting properties. They attract researchers’ attention due to many epidemiological studies supporting their usage. They are phytochemicals commonly present in our diet, being ubiquitous in the plant kingdom and, in particular, relatively very abundant in fruits and vegetables. All these aspects make flavonols candidates of choice for the valorization of products, based on the presence of a remarkable number of different chemical structures, each one characterized by specific chemical features capable of influencing biological targets inside the living organisms in very different manners. In this review, we analyzed the biochemical and physiological characteristics of flavonols focalizing our attention on the most promising compounds to shed some light on their increasing utilization in biotechnological applications in processing industries, as well as their suitable employment to improve the overall wellness of the humankind.

Use of Onion Waste as Fuel for the Generation of Bioelectricity

The enormous environmental problems that arise from organic waste have increased due to the significant population increase worldwide. Microbial fuel cells provide a novel solution for the use of waste as fuel for electricity generation. In this investigation, onion waste was used, and managed to generate maximum peaks of 4.459 ± 0.0608 mA and 0.991 ± 0.02 V of current and voltage, respectively. The conductivity values increased rapidly to 179,987 ± 2859 mS/cm, while the optimal pH in which the most significant current was generated was 6968 ± 0.286, and the ° Brix values decreased rapidly due to the degradation of organic matter. The microbial fuel cells showed a low internal resistance (154,389 ± 5228 Ω), with a power density of 595.69 ± 15.05 mW/cm² at a current density of 6.02 A/cm²; these values are higher than those reported by other authors in the literature. The diffractogram spectra of the onion debris from FTIR show a decrease in the most intense peaks, compared to the initial ones with the final ones. It was possible to identify the species Pseudomona eruginosa, Acinetobacter bereziniae, Stenotrophomonas maltophilia, and Yarrowia lipolytica adhered to the anode electrode at the end of the monitoring using the molecular technique.

Onion (Allium cepa L.) peels: A review on bioactive compounds and biomedical activities

Huge quantities of byproducts/wastes generated in onion processing are usually discarded, but they are excellent sources of bioactive compounds and phytochemicals. However, with growing interest in the sustainable use of resources and the circular economy to reduce adverse impacts on the environment, food processing wastes such as onion peel/skin can be extracted and employed as inputs in developing or reformulating nutrient supplements, and pharmacological drugs. This review highlights major bioactive components, especially total phenolics, total flavonoid, quercetin and its derivatives present in onion peel/skin and their therapeutic applications as cardioprotective, neuroprotective, antiobesity, antidiabetic, anticancer and antimicrobial agents. The present review emphasized that onion peel is one of the important agricultural by-products which is rich in bioactive compounds and can be utilized as health promoting ingredient especially in pharmacological and biomedical fields. Thus, with increasing burden of life style disorders/non-communicable diseases, finding suitable natural alternative for their treatment is one major concern of the researchers and onion peel and its extract can be exploited as a prime ingredient.

Effective and Selective Extraction of Quercetin from Onion (Allium cepa L.) Skin Waste Using Water Dilutions of Acid-Based Deep Eutectic Solvents

Deep Eutectic Solvents (DESs) are experiencing growing interest as substitutes of polluting organic solvents for their low or absent toxicity and volatility. Moreover, they can be formed with natural bioavailable and biodegradable molecules; they are synthesized in absence of hazardous solvents. DESs are, inter alia, successfully used for the extraction/preconcentration of biofunctional molecules from complex vegetal matrices. Onion skin is a highly abundant waste material which represents a reservoir of molecules endowed with valuable biological properties such as quercetin and its glycosylated forms. An efficient extraction of these molecules from dry onion skin from "Dorata di Parma" cultivar was obtained with water dilution of acid-based DESs. Glycolic acid (with betaine 2/1 molar ratio and L-Proline 3/1 molar ratio as counterparts) and of p-toluensulphonic acid (with benzyltrimethylammonium methanesulfonate 1/1 molar ratio)-based DESs exhibited more than 3-fold higher extraction efficiency than methanol (14.79 µg/mL, 18.56 µg/mL, 14.83 µg/mL vs. 5.84 µg/mL, respectively). The extracted quercetin was also recovered efficaciously (81% of recovery) from the original extraction mixture. The proposed extraction protocol revealed to be green, efficacious and selective for the extraction of quercetin from onion skin and it could be useful for the development of other extraction procedures from other biological matrixes.

Antioxidant potential and α-glucosidase inhibitory activity of onion (Allium cepa L.) peel and bulb extracts

Allium cepa L. is a commonly consumed vegetable that belongs to the Amaryllidaceae family and contains nutrients and antioxidants in ample amounts. In spite of the valuable food applications of onion bulb, its peel and outer fleshy layers are generally regarded as waste and exploration of their nutritional and therapeutic potential is still in progress with a very slow progression rate. The present study was designed with the purpose of doing a comparative analysis of the antioxidant potential of two parts of Allium cepa, i.g., bulb (edible part) and outer fleshy layers and dry peels (inedible part). Moreover, the inhibitory effect of the onion bulb and peel extracts on rat intestinal α-glucosidase and pancreatic α-amylase of porcine was also evaluated. The antioxidant potential of onion peel and bulb extracts were evaluated using 2,2-diphenyl- 1-picryl hydrazyl (DPPH), ferric-reducing antioxidant power assay (FRAP), 2,2'-azino-bis- 3-ethylbenzothiazoline-6-sulfonic acid (ABTS) radical scavenging assay, H2O2 radical scavenging activity and Fe2+ chelating activity. Total flavonoids and phenolic content of ethanolic extract of onion peel were significantly greater as compared to that of onion bulb. Ethanolic extract of onion peel also presented better antioxidant and free-radical scavenging activity as compared to the ethanolic extract of bulb, while the aqueous extract of bulb presented weakest antioxidative potential. Onion peel extract's α-glucosidase inhibition potential was also correlated with their phenolic and flavonoid contents. The current findings presented onion peel as a possible source of antioxidative agents and phenolic compounds that might be beneficial against development of various common chronic diseases that might have an association with oxidative stress. Besides, outer dry layers and fleshy peels of onion exhibited higher phenolic content and antioxidant activities, compared to the inner bulb. The information obtained by the present study can be useful in promoting the use of vegetable parts other than the edible mesocarp for several future food applications, rather than these being wasted.

Antioxidant Power on Dermal Cells by Textiles Dyed with an Onion (Allium cepa L.) Skin Extract

In this study, the phenol loading and antioxidant activity of wool yarn prepared with the aqueous extract of onion (Allium cepa L.) skin was enhanced by implementing the dyeing process with the use of alum as a mordant. Spectrophotometric and chromatographic methods were applied for the characterization of polyphenolic substances loaded on the wool yarn. The antioxidant/anti-inflammatory properties were evaluated by determining the level of intra- and extra-cellular reactive oxygen species (ROS) production in keratinocytes and dermal fibroblasts pre-treated with lipopolysaccharide put in contact with artificial sweat. An elevated dye uptake on wool was observed for the pre-mordanted sample, as demonstrated by high absorbance values in the UV-Visible spectral range. Chromatographic results showed that protocatechuic acid and its glucoside were the main phenolic acid released in artificial sweat by the wool yarns, while quercetin-4′-glucoside and its aglycone quercetin were more retained. The extract released from the textile immersed in artificial sweat showed a significant reducing effect on the intra-and extracellular ROS levels in the two cell lines considered. Cytofluorimetric analyses demonstrated that the selected mordant was safe at the concentration used in the dyeing procedure. Therefore, alum pre-mordanted textiles dyed with onion-skin extracts may represent an interesting tool against skin diseases.

Onion Peel Extract Inhibits Cancer Cell Growth and Progression through the Roles of L1CAM, NF-κB, and Angiogenesis in HT-29 Colorectal Cancer Cells

Colorectal cancer (CRC) is an aggressive malignancy. Critical mechanisms that support CRC progression include cell migration, invasion, metastasis, and angiogenesis, which is associated with L1 cell adhesion molecule (L1CAM) and nuclear factor-kappa B (NF-κB) signaling pathways. In this study, viability of HT-29 cells and human umbilical vein endothelial cells (HUVECs) was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays, and cell apoptosis was investigated by flow cytometry assays. HT-29 cell migration and invasion were observed by wound healing and Transwell invasion assays, respectively, and tube formation of HUVECs was observed by tubulogenesis assays. L1CAM and NF-κB protein expressions in HT-29 cells treated with onion peel extract were determined by indirect immunofluorescence. Results showed that high dose treatments of onion peel extract inhibited cell viability of both HT-29 cells and HUVECs, induced HT-29 cell apoptosis, and inhibited HT-29 cell migration and invasion. Moreover, onion peel extract decreased total HUVEC tube length and, at a concentration of 10 μg/mL, showed potential to downregulate L1CAM and NF-κB. In conclusion, onion peel extract inhibits HT-29 cell growth, migration, and invasion through suppressing pathways related to angiogenesis downstream of L1CAM-activated NF-κB.