Effect of green extraction techniques on phenolic recovery and bioactivities of onion peel extracts
This study demonstrates that ultrasound-assisted extraction (UAE) is particularly effective for recovering high phenolic content and antioxidant activity from yellow onion peels, while the optimal green extraction method for antimicrobial activity depends on both the onion variety (red or yellow) and the target bacterial type.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Every year, the world grows nearly 48 million tonnes of onions, a staple crop that feeds billions. Yet, for every bulb that ends up in a kitchen or a market, a significant amount of waste is generated. In the European Union alone, roughly 450,000 tonnes of onion skins are discarded annually. These outer layers, usually thrown away as trash, are actually packed with powerful natural chemicals known as phenolic compounds. These substances are famous for their ability to fight off the damage caused by oxidation in the body and to stop harmful bacteria from growing. While the edible part of the onion is nutritious, the skin contains even higher concentrations of these protective compounds, particularly a type of flavonoid called quercetin. The challenge for scientists has been finding the best way to pull these valuable chemicals out of the tough, dry skin without using harsh chemicals or wasting energy, turning what was once waste into a useful resource for food and medicine.
Researchers in Croatia set out to solve this puzzle by comparing two modern, eco-friendly methods for extracting these compounds from onion peels. They focused on two distinct varieties of onions: the common yellow onion and a local red variety. The team tested two specific techniques: one that uses high-frequency sound waves to shake the compounds loose from the plant material, and another that uses microwave energy to heat the mixture rapidly. They wanted to see which method worked better for each type of onion and whether the choice of technique changed the final chemical makeup of the extract. By analyzing the results, they could determine not just how much of the good stuff was recovered, but also how effective the resulting liquid was at neutralizing harmful free radicals and killing common foodborne bacteria.
The study began by collecting fresh peels from yellow and red onions grown on a local farm. After drying and grinding the skins into a fine powder, the researchers treated them with a mixture of water and ethanol, a safe alcohol commonly used in food processing. They split the samples in half, subjecting one batch to the sound-wave treatment and the other to the microwave treatment. Once the extraction was complete, they dried the liquids and began a detailed chemical analysis. They measured the total amount of phenolic compounds in each sample and identified the specific types of molecules present, such as quercetin, rutin, and various acids. They also put the extracts through a series of tests to see how well they could act as antioxidants, which are substances that protect cells from damage, and how well they could stop the growth of bacteria like Listeria, Salmonella, and E. coli.
The results revealed that the best method depended entirely on the type of onion being processed. For the yellow onions, the sound-wave technique proved superior. It extracted the highest amount of total phenolic compounds, reaching nearly 370 milligrams per gram of dried extract, and produced the strongest antioxidant activity. In contrast, the microwave method was less effective for yellow onions, yielding about half as many compounds. However, the story was different for the red onions. For this variety, both methods performed similarly, with the microwave technique actually extracting slightly more total phenolics than the sound waves. The researchers found that quercetin was the most abundant compound in all the samples, confirming its status as the primary active ingredient in onion skins. Yet, the specific mix of other compounds, such as protocatechuic acid and epicatechin, varied significantly based on both the onion variety and the extraction method used.
When the team tested the extracts against bacteria, the differences in performance became even more distinct. The extracts were generally more effective at stopping Gram-positive bacteria, which lack a tough outer membrane, than Gram-negative bacteria, which have an extra protective layer that is harder to penetrate. The most sensitive target was Listeria monocytogenes, a dangerous foodborne pathogen. The extracts were able to stop its growth at very low concentrations, as low as 0.31 milligrams per milliliter. Interestingly, the microwave-extracted yellow onion peel was particularly potent against this specific bacterium, while the sound-wave extracted red onion peel showed the best results against other types of bacteria. The study also showed that the antimicrobial power was not just about the total amount of phenolics present, but rather the specific combination of compounds. For instance, the presence of certain flavonoids like kaempferol and rutin seemed to correlate strongly with the ability to fight Listeria, suggesting that the quality of the chemical mix matters more than just the quantity.
Ultimately, this research demonstrates that there is no single "best" way to extract value from onion waste. The effectiveness of the process is a partnership between the plant's genetic makeup and the technology used to harvest its chemicals. For yellow onions, the sound-wave method unlocks the most antioxidant potential, making it the ideal choice for creating natural preservatives or health supplements. For red onions, the microwave method offers a viable alternative that is particularly good at targeting specific bacteria. These findings suggest that by carefully matching the extraction technique to the specific crop, industries can turn a massive amount of agricultural waste into a sustainable source of natural ingredients. This approach not only reduces environmental impact but also provides a new, eco-friendly way to enhance food safety and develop natural health products.
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