|MadSci Network: Biochemistry|
The vitamin C content of orange juice is less in juice that has been exposed to the air, but not because it has evaporated. Vitamin C is one of the most unstable of the vitamins, and it is readily oxidized when exposed to the air. This oxidation converts vitamin C (also called ascorbic acid) into a form that the body cannot use. If orange juice is stored, covered and in the refrigerator, there will not be much loss of vitamin C. However, if it is stored at room temperature or uncovered, more of the vitamin will be oxidized and the juice will, in a couple of days, contain very little vitamin C. Reference: The Concise Encyclopedia of Foods and Nutrition (Emsinger et al., 1995, CRC Press)
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