Steaming and stir-frying win as healthiest ways of cooking cruciferous veggies.

University of Warwick - Researchers have found that the standard British cooking habit of boiling vegetables severely damages the anti-cancer properties of many Brassica vegetables such as broccoli, Brussel sprouts, cauliflower and green cabbage. Fine shredding of the cruciferous vegetables also seriously decreased the anti-cancer phytonutrient content.

Past studies have shown that consumption of Brassica vegetables decreases the risk of cancer. This anti-cancer effect is attributed to the high concentration in Brassicas 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.

The researchers, Prof Paul Thornalley from Warwick Medical School 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%, Brussel sprouts 58%, cauliflower 75% and green cabbage 65%.

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 content over these cooking periods. Prior studies of microwaving vegetables have shown serious loss of vitamins from microwaving, but those studies did not analyze other phytonutrients like glucosinolates.

Professor Thornalley said: “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.”

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.

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% over 6 hours after shredding. So for maximum health benefit and anti-cancer effect, steam or stir-fry your cruciferous veggies, and resist shredding them.