I am asking because I know Vitamin C
>and other additives evaporate from juice after it has been openned for a
>long time or sits out all day.
The vit C (Ascorbic Acid) doesn't evaporate. It is broken down by light
(notice good OJ containers are cardboard/ dark). It also can be broken down
by the oxygen (oxidized) in the air. Companies are trying to get every bit
of oxygen and free space out of the (OJ) containers so they'd last longer
to ship overseas and increase their sales.

I don't get lactaid, but my guess is that the only calcium form that may
help the oxygenation would be calcium citrate. This is a much more rare
chemical, and harder to make than the ground up shells or bones, or rocks
that most companies use. I don't even see it listed in my chemical
catalogs. (I work in a university lab). Changing consistancy sounds like
some type of filler.