Emerging science: onion skins a salmon preserver?

Senem Guner led a team of scientists from the university's Institute of Food and Agricultural Sciences (UF/IFAS) focusing on the preservation of salmon.

First, the researchers extracted bioactive antioxidant compounds – anthocyanins and polyphenols – from onion skins by keeping them in hot water for around 80 minutes and then filtering the skin.

The skin and flesh of red and yellow onions were extracted with water mixed with ethanol and acetone at 25°C, 45°C, 65°C and 90°C. Anthocyanin and polyphenol levels and antioxidant capacity were determined after extraction. Then extract was then spread over the salmon.

MAP use​

Next, the scientists used modified atmosphere packaging (MAP) and the extracts for the experiment. This MAP system, commonly used with meat, fish, poultry and dairy, changes the internal atmosphere of packages to improve shelf life.

Minced salmon samples were mixed with red onion skin extract and packaged under MAP at different oxygen concentrations (0%, 5%, 10% and 15% of total package gas composition) with 40% carbon dioxide and balance nitrogen.

By using increasingly warm water, the scientists found that polyphenol recovery and antioxidant capacity rose for each sample as the extraction temperature went up. What’s more, water extraction at 90°C provided higher extraction yields than the ethanol and acetone in water solutions.

Increased shelf life​

The MAP and red onion skin extract treatment of the salmon reduced lipid oxidation rates, leading the team to conclude that the combination could effectively suppress free radicals. Thus, a skin extract and MAP application might increase the shelf life of salmon.

Marty Marshall, University of Florida food science professor emeritus, supervised Guner’s work and commented: “Her work shows that onion skins contain many bioactive compounds that can be used as value-added ingredients in food-processing systems, especially seafood.​

“If we can extend the shelf life for a grade A product even by a few days, this would be in an enormous economic benefit for fresh seafood markets.​

“Senem’s work demonstrates that in conjunction with MAP technology, the shelf life of salmon can be increased by a few days, thereby aiding the seafood industry and extending the quality for consumers,” ​Marshall added.

Waste reduction​

Guner, who is now an assistant professor of food chemistry at Afyon Kocatepe University in Turkey, believes the research can help minimise waste.

“Consumers and food producers use conventional methods such as cooking, blanching, and sterilising to increase shelf lives, but some important compounds, such as antioxidants, proteins, vitamins, and minerals are lost on the way,”​ she said.

Guner added that the use of water is important because it’s easy to obtain, harmless to humans, and does not interact with the polyphenols.

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