Scientists in Britain are warning that many cooks are in danger of drastically reducing the disease-busting compounds in vegetables such as broccoli by boiling them in water.
The team are now calling on cooks to abandon boiling vegetables in favour of steaming, microwaving or stir-frying in an effort to improve the nation's health.
The scientists from Warwick University say that boiling severely damages the anticancer properties of many Brassica vegetables such as broccoli, Brussel sprouts, cauliflower and green cabbage.
Broccoli boiled in water for just ten minutes reduces its disease-busting compounds by 40 per cent or more.
In a study supported by the Food Standards Agency, the impact of different ways of cooking brassica vegetables was measured, as research has shown that the consumption of these greens can reduce the risk of cancer, heart disease and stroke.
This is because of their high concentration of substances known as glucosinolates, which are broken down by the body into cancer-preventing compounds called isothiocyanates and it was unclear before how these substances were affected by cooking and storing processes.
The researchers, led by Professor Paul Thornalley from Warwick Medical School, bought vegetables from a local shop and took them to the lab for cooking within 30 minutes of purchase; they experimented by steaming, microwaving, stir-frying and boiling the vegetables for different amounts of time.
The team found that in all the vegetables, boiling significantly reduced the glucosinolate content but with broccoli, boiling for just five minutes cut glucosinolate by 15 per cent, while ten minutes of cooking reduced content by about 40 per cent.
Professor Thornalley says cooking broccoli for up to 30 minutes, which was common in the past, cut the glucosinolate content by 77 per cent.
Boiling brussels sprouts for 30 minutes reduced the compound levels by 58 per cent, in cauliflower it dropped 75 per cent and in green cabbage it was cut by 65 per cent.
The researchers however found that storing vegetables at home at room temperature or in the fridge for seven days had only a minor effect on glucosinolate levels while freezing and thawing resulted in up to a third of the compounds being lost.
How the vegetables were prepared for cooking apparently also led to only a minor reduction in glucosinolates.
Professor Thornalley says people should aim to steam or microwave their vegetables, but not microwave with too much water as this has the same effect as boiling, with the healthy compounds leaching out.
Other experts point out that boiling vegetables was also known to decrease their content of other nutrients, such as vitamin C and folic acid and also recommend steaming and microwaving as the best method for retaining vital nutrients.