Re: What effect does heat have on Vitamin C

Vitamin C is one of the least stable of all vitamins in solution and is 
oxidized readily in light, air and when heated. It is also water soluble. This 
means that heating in water, (like cooking broccoli in boiling water) causes 
the vitamin to leach out of the food into the water and also to be oxidized, 
first to dehydroascorbic acid and then to diketogulonic acid. This last 
compound has no Vit C activity at all and is irreversible. However, normal 
cooking usually doesn't affect Vit C levels that much; the longer you cook and 
the higher the temperature you use, the more oxidation.

The high heat used in canning would seem to pose a real threat. However, the 
high heat used kills the enzyme ascorbic acid oxidase, found in fruits and 
vegetables,  before much Vit C is oxidized. Since this enzyme catalyzes the 
oxidation process, this is actually serve to protect the vitamin.

On the other end of the spectrum, freezing can also negatively effect Vit C if 
the tissues of the product have been broken and exposed to air.

Hope this helps.

Jill Irvin, RD

sources:

Nutrition in Perspective, Patricia A. Kreutler

Nutrition Concepts and Controversies, Sizer and Whitney