Warwick, United Kingdom
May 15, 2007
Researchers at the
University of Warwick
have found that the standard British cooking habit of boiling
vegetables severely damages the anticancer properties of many
Brassica vegetables such as broccoli, Brussel sprouts,
cauliflower and green cabbage.
Past studies have shown that consumption of Brassica vegetables
decreases the risk of cancer. This is because of 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 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 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 °C (much higher than for
storage in a refrigerator at 4–8 °C) may cause significant loss
of glucosinolates up to 33% 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% over 6 hours after
shredding.
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. You need to
consider stir frying steaming or micro-waving them." |
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