The idea behind culturing pasteurized milk is that you are replacing enzymes and nutrients that were killed during the pasteurization process, making the milk more nutritious and digestible. I would recommend doing this only with organic grass-fed unhomogenized full-fat milk if you can't get raw. Next best would be organic homogonized milk, and so on down the line. Otherwise, kefir can be made with liquids other than milk. I'm not sure if the benefits of culturing plain old low fat grocery milk would outweigh the risks, what with the pus, chlorine, Antibiotics , hormones, and rancid oxidized cholesterol they add to the stuff.