Onion Peels and Agricultural Waste: An Unexpected Source of Natural Antioxidants

Did you know that onion peels, which we usually throw away, actually hold many health benefits? Beneath their dry and tough appearance, onion peels are rich in bioactive compounds (especially polyphenols) that have strong antioxidant effects. Unfortunately, since this part is not edible, onion peels often end up as kitchen waste.

However, more and more scientists and environmental enthusiasts are now recognizing the extraordinary potential of this agricultural and kitchen waste. One of the latest and most interesting approaches is the use of biochar—charcoal produced from burning plant waste such as grapevine prunings (a byproduct of vineyards)—to absorb and collect valuable compounds from onion peels.

Onion Peels: A Forgotten Nutritional Treasure

Onions (Allium cepa L.) have been consumed by humans for thousands of years. Besides their distinctive taste and aroma, onions are known to offer many health benefits, such as cancer prevention, heart health support, and even aiding in weight loss.

Surprisingly, it is the peel that contains the highest concentration of antioxidant compounds such as quercetin, rutin, and various polyphenol glycosides. In fact, research shows that onion peels can contain up to 10 times more polyphenols than the onion flesh itself!

Unfortunately, because they are not directly consumable, onion peels often end up in the trash. This is quite unfortunate, especially given the growing public interest in functional and natural foods.

Biochar: An Eco-Friendly and Multifunctional Charcoal

Biochar is a type of charcoal produced from burning biomass, such as plant branches, under low-oxygen conditions. This charcoal has a porous structure and is rich in organic functional groups, making it a natural adsorbent capable of capturing and storing certain compounds from liquids or the environment.

In this context, biochar made from grapevine waste is used to absorb polyphenols from onion peel extracts. Interestingly, biochar demonstrates a high capacity to "capture" these compounds, particularly quercetin and its derivatives.

Environmentally Friendly and Effective Extraction

To extract polyphenol content from onion peels, three environmentally friendly methods are used: maceration, ultrasound-assisted extraction (UAE), and microwave-assisted extraction (MAE), all using deionized water without harmful chemicals.

The best results are achieved through ultrasound extraction at 90°C with a solid-to-liquid ratio of 1:100. This method efficiently yields a high amount of antioxidant compounds.

Towards a Zero-Waste Future

With this approach, two types of waste that were previously unused—onion peels and grapevine prunings—can be transformed into high-value products. In addition to reducing organic waste and greenhouse gas emissions, this innovation also opens new opportunities for the food, cosmetic, and natural health industries.

Imagine, from kitchen scraps that are usually discarded, we can now obtain highly nutritious compounds that can be further processed into natural antioxidants, dietary supplements, or even active ingredients in skincare products.

Onion peels are not mere waste. Beneath their dry layers lies an incredible natural power for human health. Combined with the use of biochar as a green technology, this serves as proof that nature always provides solutions—if we are willing to seek them wisely and sustainably.

Discover more about SAWA’s innovative biochar production and its impact on sustainable farming.

Explore SAWA Biochar