Following on from the recipe for steamed greens , Deborah was right to be suspicious about a recipe which suggested boiling them for 35 minutes. The University of Warwick, in the UK, which used to be another source of badly overcooked vegetables, has published a paper explaining why greens should be microwaved, steamed briefly, or stir-fried.
press release 15 May 2007
The 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%, Brussel sprouts 58%, cauliflower 75% and green cabbage 65%. The boiling of Brassica vegetables for 5 minutes leads to losses of 20 - 30% and for 10 minutes losses of 40 - 50%.
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.