Researchers at the University of Warwick have found that boiling vegetables severely damages the anticancer properties of many Brassica vegetables such as broccoli, Brussel sprouts, cauliflower and green cabbage.
Specifically, such vegetables are high in glucosinolates which are metabolized to cancer preventive substances known as isothiocyanate.
More of these glucosinolates are lost the longer you boil. It's been found that you lose approximately 20-30% of it if you boil for 5 minutes, 40-50% if you boil for 10 minutes, and at 30 minutes, you lose roughly 65-75% of the glucosinates, depending on the specific type of vegetable.
Fortunately, other methods like stir frying, steaming and microwaving only led to minimal loses of the glucosinolates.
Do not however, that the research only tested these methods for a certain amount of time. It's probably safe to say that you can still reap most of the benefits if you stick to these times:
Microwave – 3 minutes or less
Stir Fry – 5 minutes or less
Steam – 20 minutes or less
Also, if they have been chopped up or shredded, it's best to consume them as soon as possible as they found that after 6 hours, the vegetables would lose up to 60% of the good stuff.
You need not worry too much about normal storage in the fridge though; After 7 days in the fridge, the drop in glucosinolates was found to be minimal.
So don't steam, don't overcook and eat them fresh.
Hmm…but why didn't test them on a grill. Hopefully it's fine, If the stir-fry numbers are anything to go by XD