Boiling cuts broccoli's anti-cancer benefits
Last Updated: Wednesday, May 16, 2007 | 5:12 PM ET
Boiling vegetables such as broccoli not only destroys vitamins but also compounds that may help prevent some cancers and heart disease, British researchers say.
Prof. Paul Thornalley and his colleagues at Warwick Medical School focused on how cooking habits damage the anti-cancer properties of brassica vegetables such as broccoli, cauliflower, Brussels sprouts and green cabbage.
Brassica vegetables are high in substances known as glucosinolates that studies suggest play a role in preventing some cancers and heart disease.
"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," Thornalley said Wednesday, adding that eating raw vegetables is best.
"You need to consider stir-frying, steaming or microwaving them."
When broccoli and cauliflower were boiled for 30 minutes, the vegetables lost about three-quarters of the glucosinolates, the researchers report in the Journal of Food and Chemical Toxicology.
Boiled cabbage and Brussels sprouts lost slightly less of their total glucosinolate content, 65 per cent and 58 per cent respectively.
Better cooking methods
Steaming for up to 20 minutes, microwave cooking for up to three minutes and stir-frying for up to five minutes made no significant difference to loss of glucosinolate, but storage and preparation did matter.
The vegetables lost minor amounts of the compounds over the first three days in the fridge. By day seven, up to 25 per cent was lost.
Storing fresh vegetables at much lower freezing temperatures, such as –85 C, caused up to 33 per cent of glucosinolates to be lost as the vegetables broke up during thawing.
For people who like coleslaw, it may be best to eat right after the cabbage is prepared. Shredded vegetables lost up to 75 per cent of their glucosinolates after six hours, the researchers found.
Dennis Johnston, owner of the FID restaurant in Halifax, said he uses a variety of cooking methods since vitamins and minerals are lost in the boiled water. Johnston said the findings will help him to make healthier meals for his customers.
"The more information we have to know about the nutritive value of the vegetables, the better off we are," Johnston told CBC News. "Our job is to nourish the public."