I got some juice from the press a couple days ago and I got a mix of unpasteurized and UV pasteurized so I could experiment with some different clarifiers on the pasteurized stuff. Again, I could not tell the difference in taste, but one thing that I didnt notice before was that the UV pasteurized juice was much warmer than the unpasteurized. The press operator told me that the UV increases the temp by about 25F. So yeah, thats a good bit of energy to heat the juice up this much with a light source.
The reason for the UV pasteurization is to kill e-coli. However this is only a problem with fresh juice - the alcohol will kill any e-coli during the fermentation. Im fairly sure that the UV also knocks out most, if not all of the wild yeast as well, because if I get an extra gallon or two of unpasteurized juice, it will start fermenting in the fridge if I dont drink it in about a week. When I used to get UV pasteurized juice, it never fermented in the fridge, even after several weeks.