The antioxidant-rich extracts were found to inhibit the growth of a range of bacteria, including Listeria, according to findings published in the International Journal of Food Science and Technology.
“The antioxidant and antimicrobial properties of the flavonoids of the raw onion make it a good candidate for use in food preservation,” said lead author Jonathan Santas from the University of Barcelona.
“The onion can be effective for delaying lipid oxidation in emulsions of oil and water; a model system of foods like margarines and mayonnaises, and it also inhibits the growth of microorganisms that alter foods,” added Santas.
If further studies support the apparent activity, and the extracts can be produced in industrial quantities, it could see the onion compounds join the ever-growing list of natural alternatives to more established synthetic preservatives such as like butylhydroxyanisole (BHA) and butylhydroxytoluene (BHT).
According to a 2003 report by Frost and Sullivan, the synthetic antioxidant market is in decline, while natural antioxidants, such as herb extracts, tocopherols (vitamin E) and ascorbates (vitamin C) are growing, pushed by consumer desire acceptance and easier market access.
The Barcelona-based researchers prepared various extracts of three varieties of Spanish onions. The activity of the flavonol-rich extracts was then investigated against a range of microbes, including gram positive bacteria like Bacillus cereus, Staphylococcus aureus, Microcroccus luteus and Listeria monocytogenes, and gram negative bacteria Escherichia coli and Pseudomonas aeruginosa.
Results showed that the extracts from the yellow onion varieties had the highest flavonoid content and antioxidant capacity.
Up against the bacteria, the ethyl acetate subfraction of the extracts showed significant antimicrobial inhibition, particularly against the gram positive bacteria, said the researchers. Of the particularly compounds analysed, the greatest activity was for quercetin and kaempferol.
“These results suggest that the use of onions as a spice, can be useful not only to increase the organoleptic quality of food, but also to enhance the stability and preservation of food systems,” wrote the researchers. “Among the varieties tested, the highest expectations can be focused on Grano de Oro with the highest flavonoid content,” they concluded.
Source: International Journal of Food Science and Technology
Volume 45, Issue 2, Pages 403-409
“Antimicrobial and antioxidant activity of crude onion (Allium cepa, L.) extracts”
Authors: J. Santas, M.P. Almajano, R. Carbo