Nonfat dry milk is whole milk minus its fat and water. About 35 percent of the dried powder is protein (mainly casein), and about 50 percent is carbohydrate in the form of lactose, a milk sugar. Although sucrose (the common table sugar derived from sugar beets or cane sugar) dissolves rapidly, lactose does not. Unless you give your reconstituted dry milk a rest in the refrigerator, it is apt to taste grainy. The instant dry-milk powders eliminate most, but not all, of the solubility problem.
** Asian Cooking