In a recent study conducted in Spain, researchers examined and compared the antioxidant and antibacterial properties of three types of onions available in Spain. The study showed that onions are a rich source of flavonoids and have good antioxidant activity. This antioxidant activity was highest in the Grano de Oro variety; and subsequently, the Fuentes de Erbo and the Calcot de Valls types of onions. “In the present study, onion extracts inhibited the growth of some important bacteria commonly associated with food deterioration.”

Since ancient times, onions have been used as herbal medicine. Many of the studies in the past have shown that eating onions reduces the risk of heart disorders and cancer. Some of the studies have shown that onions have “antiallergic” properties, and they also help to reduce inflammation. In this study, researchers have evaluated the amount of flavonoids and phenols in different varieties of onions. They also examined the antioxidant activity of onion extracts. In addition, they extracted quercetin and kaempferol from the onions, and examined their antibacterial properties, especially against food-spoiling bacteria. This information was thought to help in the discovery of a new group of natural food preservatives.

* Three varieties of Spanish onions, Fuentes de Erbo, Grano de Oro and Calcot de Valls, were purchased from the local market. An extract was prepared from them by treating a paste made from the onions with various chemicals, such as ethyl acetate and methanol.
* Total flavonoid and phenol content was measured by using the Folin-Ciocalteu method. Subsequently, trolox equivalent Antioxidant Capacity was also measured.
* Quercetin and kaempferol were extracted from the onions and were separately added to cultures of Bacillus cereus, Staphylococcus aureus, Microcroccus luteus and other bacteria; and inhibition of growth of these bacteria was noted. All the bacteria used in the study are known to spoil food.

* The extracts of the Grano de Oro variety showed the highest phenol and flavonoid content (44 and 34 units, respectively), followed by Fuentes de Erbo variety (12 and 8 units, respectively), while Calcot de Valls contained the least phenols and flavonoids (1.3 and 0.4 units, respectively).
* The antioxidant capacity was in accordance with the phenol and flavonoid content.
* Quercetin had an inhibitory effect on all the strains of bacteria that were studied. Kaempferol was effective only against S. aureus and M. luteus bacterias.

Next steps/shortcomings
The authors of this study agree that extracts from onions inhibit the growth of some bacteria, which are commonly associated with food deterioration. They also suggest that “odourless lyophilized onions can be a good source of non-volatile antimicrobial compounds and in consequence may be useful to improve food preservation without undesirable changes of organoleptic properties.”

This study highlights the rich antioxidant capacity of onions. Being a good source of flavonoids, onions can reduce the risk of coronary heart disease, which is one of the major contributors of death in the United States. The study has also confirmed the presence of antibacterial agents in onions. These natural compounds can be used in the food industry as an alternative to synthetic chemical preservatives. Many of the earlier studies attributed the beneficial effects of onions to sulfur-containing chemicals, such as thiosulfinates. As these compounds are very volatile and degrade fast, many people questioned the biological utility of onions. This study has shown that onions also contain flavonoids, which are stable compounds and possess rich antioxidant capacity.

For More Information:
Antimicrobial and Antioxidant Activity of Crude Onion Extracts
Publication Journal: International Journal of Food Science and Technology, 2010
By Jonathan Santas; María Pilar Almajano; University of Barcelona, Spain, and The Technical University of Catalonia, Barcelona, Spain

*FYI Living Lab Reports Are Summaries of the Original Research.