Does boiling cauliflower destroy its antioxidants and other nutrients? To a degree, yes. If you look at the ORAC value of fresh cauliflower you will see it's 870, which is 18% higher. That may not seem like a big difference but consider that 100 grams (the amount used in the test) of boiled is much more dense than the raw fresh vegetable, since boiling eliminates much of the water content. Therefore the boiled version is measuring a higher amount of the vegetable.
USDA Database for the Oxygen Radical Absorbance Capacity (ORAC) of Selected Foods, Release 2 - Prepared by Nutrient Data Laboratory, Beltsville Human Nutrition Research Center (BHNRC), Agricultural Research Service (ARS), U.S. Department of Agriculture (USDA) - May 2010