Re: Does orange juice lose it's vitamin c by evaporation if not covered?

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)