What Einstein Kept Under His Hat: Secrets of Science in the Kitchen (16 page)

What is it about soy milk that makes it boil over? One of my recipes requires cooking the soy milk. I found I was able to accomplish this without having it boil over as long as I only simmered it. What is the chemistry behind this?

....

I
have often wondered how they milk those little soybeans, haven’t you?

Sorry. 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 your 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.

There seem to be a dozen forms of tofu in my supermarket. I know that tofu is made from soybeans, but how do they make so many kinds?

....

T
hey begin by curdling soy milk, and then they manipulate the curds in different ways.

A variety of acids, enzymes, bacteria, and salts are able to coagulate the proteins in soy milk, that is, to unwind their long, twisted molecules, allowing them to rebind (
cross-link
) to one another like rungs in ladders, forming a tightly tangled, solid network that separates out from the liquid. As in coagulated real milk, the proteins form curds, from which the tofu is made.

Real milk is usually curdled by rennet, a stomach-lining membrane in the fourth stomach of an unweaned calf, and containing the protein-digesting enzyme rennin, or chymosin. (Makes you wonder how
that
little technique was discovered, eh? But see below.) The rest of the animal presumably ends up as “milk-fed veal.” The curds are then fermented by molds or bacteria, aged and ripened into cheese.

(If only apocryphally, the story of rennet’s discovery goes back to biblical times, when wine, milk, and other essential liquid foods were carried in containers made from the cleaned stomachs of calves or sheep. Perhaps a less than meticulously cleaned calf’s stomach was used to carry milk on a trek across the desert, whereupon some residual rennin coagulated it into curds. Then, with the help of ambient bacteria they fermented, and
voilà!
—cheese.)

Soy milk, on the other hand, is generally curdled by acids or salts rather than by rennin. The Japanese have traditionally used
nigari
, the bitter, salty non-sodium-chloride residue (mostly magnesium chloride) left over from the evaporation of seawater in making sea salt. Today, calcium sulfate is most often used. The curds are then pressed into blocks called tofu.

Purists insist that tofu isn’t “bean curd,” as it is often called in English. The curd itself is called
oburo
in Japanese; it isn’t tofu until it has been pressed so that the whey—the liquid that was left behind when the curdling took place—is squeezed out.

Various degrees of pressure and duration of pressing make tofu cakes of various consistencies from soft to firm and extra-firm. Little cubes of soft tofu can be tossed in salads, whereas the firmer, stronger forms will withstand deep-frying. Tofu is the modeling clay of cooking; it can be mashed, blended, formed, or cut into shapes and sizes to conform to almost any kind of dish, from salad dressings and sauces to sautés, stir-fries, and deep-fries. And it has the almost magical quality of absorbing the flavors of whatever it is cooked with.

Unlike cheese, which has already been attacked by voracious bacteria and/or molds and ripened often beyond olfactory appeal, tofu is a delicate, perishable product. It is sold vacuum-packed, or soaking in plastic containers of water, or in aseptic packages, or in bulk.

“Silken tofu,” a smooth, custardy tofu, is manufactured by a method more like making yogurt than making cheese. Instead of coagulating the soy milk and draining off the whey, makers of silken tofu add a chemical called glucono-delta-lactone (GDL), obtained by the action of an enzyme (
glucose oxidase
) on glucose. The mixture is then packaged in its retail containers and heated mildly at 175 to 195°F (79 to 91°C) for about an hour. During that time the GDL spontaneously turns (
hydrolyzes
) into gluconic acid, which thickens the proteins into a homogeneous gel, with no separation of whey. It can be eaten with a spoon, like yogurt or custard.

All tofu should be refrigerated and used within a week. If it is kept in water, the water should be changed daily. It can be frozen, but the texture upon thawing will be chewier because freezing tightens up the proteins and squeezes out more whey. When it thaws, the tofu has a spongelike texture that enhances its ability to soak up flavorful liquids if cooked with other foods.

I love the flavor of the miso soup I’ve had in Japanese restaurants, but I don’t know what miso is. Can I buy some and experiment with it at home?

....

A
bsolutely. Miso, known as “fermented soybean paste” in English, is one of the most versatile products you can find in Japanese and Korean markets. It’s sold either by itself, to be used in home cooking, or already incorporated into soups, salad dressings, and sauces.

Like soy milk and tofu, miso begins with soaked, steamed, and ground or chopped soybeans. A certain mold, known as
Aspergillus oryzae
in Latin,
koji
in Japanese, and “a certain mold” in English, is added to the heavily salted soybeans, either by themselves or mixed with rice, barley, or chickpeas. (The Japanese use the same mold to make sake.) Traditionally, the fermentation is allowed to proceed for two or three years, until the desired intensity of flavor and color is achieved. Today, the process may be hurried by heating and other accelerating techniques. The role of the salt is to prevent the mixture from spoilage by less friendly microorganisms while the
Aspergillus
, which doesn’t mind salt, works away at it.

There are dozens of kinds of miso, ranging in flavor from salty to sweet to salty-and-sweet, in color from ivory to coffee to dark brown, and in texture from smooth and creamy to lumpy. American chefs are having a field day experimenting with them, and you can, too. Try s
hiro
, a light miso, or
aka
, a darker, heartier version.

Once you discover miso, it won’t be a stranger on the shelf. Because of its intense flavor it needs to be balanced with other ingredients. Add it to a vinaigrette and serve with asparagus, artichokes, or a tossed garden salad. Or make miso soup by stirring a spoonful or two of the paste into vegetable broth and adding
udon
noodles.