Milk’s blank appearance belies its tremendous complexity and vitality. It’s alive in the sense that, fresh from the udder, it contains living white blood cells, some mammary-gland cells, and various bacteria; and it teems with active enzymes, some floating free, some embedded in the membranes of the fat globules. Pasteurization greatly reduces this vitality; in fact residual enzyme activity is taken as a sign that the heat treatment was insufficient. Pasteurized milk contains very few living cells or active enzyme molecules, so it is more predictably free of bacteria that could cause food poisoning, and more stable; it develops off-flavors more slowly than raw milk. But the dynamism of raw milk is prized in traditional cheese making, where it contributes to the ripening process and deepens flavor.