The way you prepare and cook vegetables can influence the amount of antioxidants available to you at mealtime. The healthiest methods overall appear to be microwaving and “griddling” or cooking veggies on a flat metal surface without any oil.
Spanish researchers looked into how different cooking methods affect antioxidant content by analyzing the nutrient value of 20 different vegetables after boiling, pressure-cooking, baking, microwaving, griddling and frying.
Results of the study published in the Journal of Food Science showed that the greatest antioxidant losses occurred in cauliflower after boiling and microwaving, in peas after boiling and in zucchini after boiling or frying. Antioxidant levels remained high in green beans, beets and garlic after most cooking methods and they actually increased in celery, carrots and green beans after cooking, no matter what method was used (although no increase was seen in green beans after boiling).
Artichokes were the only vegetable that maintained antioxidant content no matter what cooking method was used. Given all methods tested, the greatest antioxidant losses stemmed from boiling and pressure-cooking.