Anyone out there still boiling your broccoli – stop now! Research from the University of Warwick says that this treatment seriously reduces the cancer fighting glucosinolate content- up to 77% if you do it for half an hour. They also suggest that freezing is not the best way to store these vegetables as it also reduces glucosinolate levels.
My reaction to this bit of news was – yes interesting about glucosinolate content after cooking and also interesting that storage in the freezer appears not to be good for glucosinolate content either especially as some vitamins are preserved by freezing, but – half an hour? Who does that these days? The broccoli would be falling apart! A straw poll round the office shows 10 minutes would be more usual. According to the research however, this still inflicts substantial losses of glucosinolates- up to 50%.
What cooking method is best to maximise one’s glucosinolate intake then? Well the researchers say steaming and frying for a few minutes are OK and this in agreement with some previous research (see search below). However a paper by Conway et al.1 suggests brassicas like broccoli and cabbage are actually better eaten raw because myrosinase, the enzyme needed to convert the glucosinolates into cancer fighting isothiocyanates is then more active – heating inactivates it.
Why did people get into the habit of boiling their green vegetables for so long anyway? Apart from perhaps being uninterested in their food or too busy, that is. This paper by Niti Sharma et al.2 is giving the clue – bitter taste. Glucosinolates taste bitter and boiling reduces the bitterness. You can’t win. Plant breeders and food scientists are getting headaches over the problem of reducing bitter taste but keeping the phytochemicals. For a review on consumers and the bitter taste in vegetables see Drewnowski et al.3
If you do still want to boil your greens to death, perhaps you should drink the cooking water too or use it to make something else, then you’ll get the leached glucosinolates back. My grandmother was right to press that ‘cabbage water’ on me. maybe she got her advice from this BMJ paper from 1941.4
1 Disposition of glucosinolates and sulforaphane in humans after ingestion of steamed and fresh broccoli
Conaway, C. C. , Getahun, S. M. , Liebes, L. L. , Pusateri, D. J. , Topham, D. K. W. , Botero-Omary, M. , Chung, F. L. / Nutrition and Cancer, 2000, Vol. 38, No. 2, pp. 168-178, 38 ref.
2Effect of maturity and boiling on glucosinolate content of cruciferous vegetables
Niti Sharma , Swati Bansal / Indian Journal of Nutrition and Dietetics, 2004, Vol. 41, No. 10, pp. 421-425, 17 ref.
3Bitter taste, phytonutrients, and the consumer: a review
Drewnowski, A. , Gomez-Carneros, C. / American Journal of Clinical Nutrition, 2000, Vol. 72, No. 6, pp. 1424-1435, 137 ref.
4 The Preparation and Cooking of Green Vegetables.’ BRITISH MED. J, 1941 pp. 26-7.
Search CAB Direct (subscribers only) Glucosinolates and cooking