flavor. I don't understand it 100%, but Kedem clearly does this in the grape beverage product ("light" grape juice list water and gj concentrate in the ingredients) I asked one of the Herzogs who works @ the Marlboro plant and he said they add for flavor (the concentrate added is from California). He also told me that there is no water added to regular grape juice as did R' Nochum Rabinowitz.
True. So I guess adding water to the "lite" grape juice is risking your
hafafen much more then when adding water to the regular grape juice.
As a side note, I once was sitting at the
Shabbos seuda with my brother in law and came across a grape juice (or maybe it was a wine) that on one side of the bottle it said
mevushal and on the other side it said
non-mevushal. It took us a while, but my brother-in-law came up with the answer. Supposedly, there is a degree which when boiled to that point, according to some
poskim the wine is considered
mevushal, whereas, according to others it is not. Apparently, this bottle was boiled to that degree and no more. I later confirmed this speculation from another source.