Functional Uses for Large Amounts of Onion Waste from Food Processing

Onions, the second most produced vegetable worldwide (behind tomatoes), are widely used across different food industries, but a large amount of the onion is wasted during processing. The aim of this study is to understand and explore the means by which such wastes may be made useful. The results showed that both the brown skin of the onion as well as the bottom and top part of the vegetable bulb could provide dietary fibers that are rich in minerals like calcium and compounds like phenolics and flavonoids, which possess antioxidant properties. Both the outer and inner scales of the bulb can provide various nutritional compounds, in addition to being good sources of dietary fibers.

Onions are the second most important vegetable to be grown and utilized globally. The worldwide annual onion production is close to 66 million tons. Much of the onions produced are used by the processed food industry. Such commercialized and mechanized chopping and use of onions has led to a large amount of onion waste being generated. In the European Union alone, more than 500,000 tons of onion waste is generated yearly. Most of this consists of the outer papery brown skin, tops and bottoms of the bulbs, outer and inner scales that are white and fleshy, and diseased or damaged samples. These onion wastes are unlike other vegetable wastes. Onions are unsuitable for fodder or organic fertilizer because of their strong odor and pungent chemicals. One possible method to use these nutritionally rich waste parts is to convert them to food ingredients that can be used by humans. This study attempted to explore the possibilities of this happening.

* For this study, 20 kg samples of two varieties of onions were obtained — cvs Recas and Figueres.
* The samples were cut into sections similar to those generated in industrial peeling, such as dry outer brown skin, top and bottom sections, and fleshy scales. The top and bottom sections were actually the top or bottom 5 to 10 mm of the vegetable. The outer scales were the outer two fleshy layers of scales while the inner scales were the rest of the bulb.
* All the sections were individually analyzed chemically and the nutrients in each section were observed and compared statistically.

Key findings
* This study showed that the dry, outer brown skin as well as the top and bottom parts of the onion can provide dietary fibers and can provide flavonoids and phenolics. These are known to possess antioxidant properties.
* The brown skin also has high levels of the compound quercetin aglycone and minerals like calcium. On the other hand, the top and bottom parts are rich in minerals.
* Furthermore, the outer scales of onion are rich in flavonoids, phenolics, and dietary fiber. It was found that these could provide compounds of antioxidant value.
* The inner fleshy parts of the bulb can provide fructans and alkenyl cysteine sulphoxides.

Next steps/Shortcomings
The authors agree that unlike laboratory studies, collecting all the industrial onion wastes for processing might be a difficult prospect. They add that further studies should be conducted to explore the exact activity of the nutritional ingredients in onion wastes on humans. Furthermore, toxic effects, utility, and other pharmacological parameters of these compounds should also be analyzed.

This study shows that the parts of the onion like its dry outer skin, top and bottom sections, outer fleshy layers, etc., which are generated as industrial waste in the processed food industry may have nutritional potential. This study is important because onion waste has always been difficult to utilize because of the chemical composition and pungent aroma. It is seen that apart from dietary fibers, various parts of the onions are also rich in other compounds that possess antioxidant properties and minerals like calcium. Referring to these nutrients that may be processed into nutritional supplements from onion waste, the authors conclude that these “functional foods represent an important, innovative and rapidly growing part of the overall food market.”

For More Information:
Characterization of Industrial Onion Wastes (Allium cepa L.): Dietary Fibre and Bioactive Compounds
Publication Journal: Plant Foods Human Nutrition, February 2011
By Vanesa Benítez; Esperanza Mollá; Campus de la Universidad Autónoma de Madrid, Spain

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