Last year, a group of researchers from the University of Warwick studied vegetable cooking techniques and the loss of nutrients. Their work was published in the Food and Chemical Toxicology journal. In the study, scientists cooked broccoli, cauliflower, Brussels sprout, and green cabbage by boiling, microwaving, stir-frying, and steaming. After cooking, they tested the loss of their cancer-protective compounds called glucosinates. Check out more on Health Food
The techniques that did NOT cause any significant nutrient loss were steaming (up to 20 min.), microwaving (up to 3 min.), and stir-frying (up to 5 min.). But, when the vegetables were boiled for 30 minutes, the level of glucosinates dropped significantly. Broccoli lost 77%, cauliflower lost 75%, and cabbage lost 65%. However, the researchers tested the cooking water, and about 90% of the glucosinates were found in there.
I think 30 minutes is an excessive amount of time to cook some vegetables, but this study goes to show that steaming, microwaving, and stir-frying are the best cooking methods for preserving nutritional content. Steaming is really great because your vegetables keep that great crispiness instead of getting soggy. Check out more on Health Food