Kruisbloemige groenten zoals broccoli en koken?*
Uit veel onderzoeken is al vastgesteld dat kruisbloemige groenten de kans op kanker sterk doen verminderen. Dit komt vooral door in deze groenten aanwezige bioactieve stoffen, de glucosinolaten. Uit deze studie blijkt dat het verkeerd bereiden van deze groenten de belangrijke stoffen grotendeels kunnen verdwijnen. Het gewoon koken in water doet in 30 minuten bij broccoli 77%, bij spruitjes 58%, bij bloemkool 75% en bij groene kolen 65% van deze glucosinolaten verdwijnen. Tot 20 minuten stomen of tot 5 minuten roerbakken doet nauwelijks glucosinolaten verdwijnen. Een paar minuten in de magnetron zou ook weinig effect hebben. Een week opslaan van de groenten bij kamertemperatuur of in de koelkast doet ook nauwelijks de hoeveelheid gezonde stoffen verminderen. Het advies van onderzoeksleider Prof. Thornalley is dan ook deze groenten niet koken doch stomen of roerbakken.
Boiling
Broccoli Ruins Its Anti-Cancer Properties
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 or steaming them."
(Juni 2007) (Opm.
Er
zijn wel meer dan 120 natuurlijke glucosinolaten bekend, een belangrijke is sulforafaan.
Bij het snijden (en het eten) van kruisbloemige groenten worden na een paar
dagen eerst verschillende
glucosinolaten vermeerderd en kunnen ook omgezet worden in
isothiocyanaten.)