Many people have often wondered how they milk those little soybeans, haven’t you?
Soy milk is made by soaking, boiling, grinding, and pressing the liquid out of soybeans. The liquid is called “milk” because it is white, but it bears as little relationship to cow’s milk as does milk of magnesia.
Soy milk is a tempting alternative to cow’s milk because it is higher in protein, lower in fat (and calcium), and free of cholesterol and lactose, which millions of lactose-intolerant people are incapable of digesting properly. When fortified with calcium and vitamins, it can be used as infant formula for the estimated 7 percent of babies in the United States who cannot digest cow’s milk.
Nevertheless, soy milk is far from a substitute for natural milk, either in flavor or in many culinary applications.
For one thing, the soybean-crushing process releases an enzyme, lipoxygenase, that catalyzes the oxidation of the beans’ unsaturated fatty acids into unpleasant-tasting compounds. While that doesn’t seem to bother Asian consumers, the enzyme must be deactivated for most Western palates by heating the “milk” to a temperature near its boiling point for 15 to 20 minutes.
Which takes us back to the stove.
Plants contain various sugar-related chemicals called glycosides that serve a wide range of functions. Some of the glycosides in soybeans are called saponins (from the Latin sapo , meaning soap) because they foam up into suds when boiled.
They are the source of the boiling-over problem. But heat destroys the saponins, so a period of gentle heating will slowly eliminate the foaming tendency. That’s why you can get away with simmering soy milk but not with boiling it, unless you simmer it first.