Brassica vegetables such as broccoli, Brussel sprouts, cauliflower and green cabbage are known to contain substances that reduces the risk of cancer. But, researchers have now warned that boiling these veggies makes them to lose its anticancer properties.

Brassica vegetables have a high concentration of substances known as glucosinolates which are metabolized to cancer preventive substances known as isothiocyanates.

However before this research it was not known how the glucosinolates and isothiocyanates were influenced by storage and cooking of Brassica vegetables.

As a part of the study, researchers Prof Paul Thornalley from Warwick Medical School at the University of Warwick and Dr Lijiang Song from the University of Warwick's Department of Chemistry bought Brassica vegetables, (broccoli, Brussel sprouts, cauliflower and green cabbage) from a local store and transported them to the laboratory within 30 minutes of purchasing.

The effect of cooking on the glucosinolate content of vegetables was then studied by investigating the effects of cooking by boiling, steaming, microwave cooking and stir-fry.

Boiling appeared to have a serious impact on the retention of those important glucosinolate within the vegetables. The loss of total glucosinolate content after boiling for 30 minutes was: broccoli 77 percent, Brussel sprouts 58 percent, cauliflower 75 percent and green cabbage 65 percent.

The effects of other cooking methods were investigated: steaming for 0-20 min, microwave cooking for 0-3 min and stir-fry cooking for 0-5 min. All three methods gave no significant loss of total glucosinolate analyte contents over these cooking periods.

Domestic storage of the vegetables at ambient temperature and in a domestic refrigerator showed no significant difference with only minor loss of glucosinolate levels over 7 days.

However the researchers found that storage of fresh vegetables at much lower temperatures such as -85 degrees Celsius (much higher than for storage in a refrigerator at 4-8 degrees Celsius) may cause significant loss of glucosinolates up to 33 percent by fracture of vegetable material during thawing.

The researchers found that preparation of Brassica vegetables had caused only minor reductions in glucosinolate except when they were shredded finely which showed a marked decline of glucosinolate levels with a loss of up to 75 percent over 6 hours after shredding.

The boffins' conclusion - cut out boiling if you want the maximum benefit from these veggies.

"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," Professor Thornalley said.