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The next time you’re shopping for burger toppings or salad ingredients, opt for red onions over white. Both types of veggies contain potent antioxidant properties, but a new study suggests that the red variety may be especially powerful at fighting cancer.

When researchers exposed human cancer cells to extracts from five different varieties of onions, they found that the red-onion extract killed three to four times the number of cells compared to extracts of lighter-colored onions, says co-author Suresh Neethirajan, PhD, associate professor of bioengineering. His team’s findings are published in Food Research International.

The study looked specifically at onions grown in Ontario, which have been shown to have higher concentrations of the flavanoid compound quercetin than did other varieties around the world. The authors can’t say for sure that their findings would apply to onions grown elsewhere, but they say it’s likely.

To test different Ontario-grown varieties head-to-head, the researchers used a newly developed technique to extract quercetin and other compounds from five onion species. Then they placed those extracts in direct contact with human colorectal cancer cells. All five species were “excellent at killing cancer cells,” says Neethirajan, but the red variety, Ruby Ring, tested highest for total phenolic content.

They also hope that the extraction technique tested in their study—which uses heated water in a pressurized container—will allow the quercetin in onions to one day be added to variety of fortified foods and even medicines. Unlike other extraction methods, this one does not use toxic solvents or chemicals.

For now, the best way to get those valuable nutrients is to eat onions themselves; for the biggest nutritional punch, choose red and eat them raw, since cooking destroys some of their antioxidant properties.

The findings are important not just to onion consumers, says Neethirajan, but also to onion growers, as well. “There are many types of onion varieties, and farmers need to know which is the best to invest in a crop,” he says. Planting more red varieties could give these growers an edge, not just in the food industry but perhaps in the functional food industry, as well.