Cristina Garcia-Viguera and colleagues measured the levels of three key nutrients in samples of fresh broccoli, then cooked it using four different methods (boiling, high pressure boiling, steaming and microwaving):

• Steaming proved to be the least damaging method, hardly reducing the broccoli’s nutritional content
• Boiling reduced levels of flavonoids by two thirds
• Microwaving reduced levels of flavonoids 97%, of sinapic acid derivatives by 74%, and of caffeoyl-quinic acid derivatives by 87%

Cristina believes that the very high temperatures generated inside the food’s cells by the microwaves are the cause.

Ed.- The experiment’s protocol has been questioned. The researchers steamed the broccoli for three and a half minutes but microwaved it for five minutes. Critics ask whether the study measured the effects of overcooking rather than of microwaving.