I'm going to suggest that milk powder is the best way to add MSNF, short of reverse osmosis. The trick is to find a good brand like Now, which 100% skim milk solids, and which is spray dried at low temperatures. If your goal is to concentrate milk solids while exposing them to the minimum amount of heat, you're not going to beat an industrial spray drier with your stove.   I've found that with the package sealed in an additional ziploc bag in the freezer, this kind of dry milk lasts months witho