A. Jennie Chen, an intern at Tufts’ Frances Stern Nutrition Center, answers: “Antioxidants are sensitive to heat, and freezing seems to preserve antioxidant activity. However, different antioxidants are heat sensitive at different temperatures. For example, in tomatoes, 30 minutes of heating at 190 degrees Fahrenheit causes a loss of 29% of vitamin C, but total antioxidant activity increases by 62%. The decline in antioxidant activity for vitamin C is offset by an increase in antioxidant activity of phytochemicals such as lycopene.
“Actually, antioxidant loss varies much more by cooking and preparation method. Boiling generally decreases antioxidant levels of vegetables as the antioxidants leach into the water from the plant tissue during cooking. Stick with steaming or microwaving to retain more nutrients.”