On Food and Cooking

bacteria.) Lactose is a composite of the two
simple sugars glucose and galactose, which
are joined together in the secretory cell of the
mammary gland, and nowhere else in the
animal body. It provides nearly half of the
calories in human milk, and 40% in cow’s
milk, and gives milk its sweet taste.
The uniqueness of lactose has two major
practical consequences. First, we need a
special enzyme to digest lactose; and many
adults lack that enzyme and have to be careful
about what dairy products they consume (p.
14). Second, most microbes take some time to
make their own lactose-digesting enzyme
before they can grow well in milk, but one
group has enzymes at the ready and can get a
head start on all the others. The bacteria
known as Lactobacilli and Lactococci not
only grow on lactose immediately, they also
convert it into lactic acid (“milk acid”). They
thus acidify the milk, and in so doing, make it
less habitable by other microbes, including