Scientists at the University of Illinois say certain agricultural practices can increase cancer-preventive chemicals found in broccoli and tomatoes.

According to United Press International, university professor Elizabeth Jeffrey said in the study she and the co-researchers “enriched pre-harvest broccoli with different bioactive components” assessing levels of cancer-fighting enzymes in rats that ate powders made from the produce.

Jeffrey told United Press International the highest levels of detoxifying enzymes were found in rats that ate selenium-treated broccoli.

The amount of one of the cancer-fighting compounds in broccoli was six times higher in selenium-enriched broccoli than in standard broccoli powder, she said.

Now researchers are trying to determine whether selenium compounds are directly responsible for the increase in bioactivity or if selenium acts indirectly by directing new synthesis of the broccoli bioactives called glucosinolates, according to United Press International.

The study that included researchers Ann Liu and Sonja Volker appears in the Journal of Agricultural and Food Chemistry.