Cooking by boiling severely damages the anticancer properties of many Brassica vegetables such as broccoli, Brussel sprouts, cauliflower and green cabbage, say researchers at the University of Warwick.
Research in the past has shown that Brassica vegetables decrease the risk of cancer thanks to their high concentration of substances known as glucosinolates. In the body, these are metabolized to cancer preventative substances known as isothiocyanates.
In the new study, the researchers investigated the effects of boiling, steaming, microwave cooking and stir-frying on the glucosinolate content of vegetables. While they found that steaming, microwave cooking and stir-frying resulted in no significant loss of total glucosinolate analyte, boiling resulted in a loss of up to 75 percent of the total glucosinolate content.
"If you want to get the maximum benefit from your five portions-a-day vegetable consumption, if you are cooking your vegetables boiling is out. You need to consider stir frying steaming or micro-waving them," said researcher Paul Thornalley.
Interestingly, the researchers also found that storage temperatures played an important part in total glucosinolate content. Domestic refrigerator temperatures showed no significant effect with only a minor loss of glucosinolate levels over 7 days, but much lower temperatures (below-freezing) caused up to 33 percent of glucosinolates to be lost during thawing.
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Source: University of Warwick