Lactose is the sugar found in milk. It is a composite of two simple sugars, glucose and galactose. Cooks seldom encounter it in pure form. Because it’s much less sweet than table sugar, manufacturers use it much as they do the sugar alcohols, more for its physical bulk than for its sweetness.
From the book On Food and Cooking (2nd edition) by Harold McGee. © 2004 Harold McGee. By permission of Scribner, a division of Simon & Schuster, Inc.