DEAR DR. BLONZ: We have been shifting away from meat and making more vegetarian dishes lately. We're considering eating tofu, and I have a few questions. How much fat and other nutrients are in tofu, and how long should soybean tofu be expected to last? What is going on when it starts to change color, and is it no longer safe to eat at that point? Does tofu interfere with minerals in other foods? -- S.T., Sun City, Arizona
DEAR S.T.: Unlike other legumes, the soybean is relatively high in fat content; about half the calories in tofu come from fat.
There are a lot of steps to get from the soybean to the tofu. After soybeans have been soaked, crushed, cooked and filtered, calcium sulfate is often added to the soy pulp, which causes it to form a gel. Excess water is then removed as the soy gets pressed into cakes.
There are minor differences among different tofu makers, and there will also be variations between degrees of tofu firmness. But as a general benchmark, just under half the calories in tofu come from fat. A half-cup serving of firm tofu made with calcium sulfate (nigari) supplies 88 calories, 10 grams of protein, 2 grams of carbohydrate and 5 grams of fat (43 percent polyunsaturated). Other nutrients include 2 milligrams of iron (11 percent of the Daily Value), and 253 milligrams of calcium (25 percent of the Daily Value for adults). The calcium comes primarily from the calcium sulfate used to form the tofu into cakes.
Because of its relatively high content of polyunsaturated fats (PUFAs), tofu is susceptible to oxidation when exposed to the air for an extended period of time; the warmer the air, the greater the risk. As such, most tofu is kept refrigerated in water, or is vacuum-packed. When you purchase tofu, make sure it's stored in a similar manner. Tofu should be discarded if the storage water becomes cloudy, if a slippery film develops on the surface, or if you notice any unusual smells or tastes. Also, the tofu should be discarded if it takes on a pinkish tinge -- usually the effect of exposure to air. If the water is changed daily, fresh tofu can last up to two weeks in the refrigerator.
Regarding minerals, as long as it is consumed in moderation, tofu and soy milk are versatile, nutritious foods. Soy does contain phytic acid (phytate), as do other legumes and grains. This substance has anti-nutritional properties, in that it can bind with certain nutrients like iron and zinc and inhibit their absorption. This is a direct, physical effect that takes place in the digestive system; the ability to bind is limited by the milligrams of phytate present.
Tofu and soy milk contain phytates, and regular cooking doesn't affect the phytic acid that much. One way to blunt the phytate effect is via fermentation. This can occur in grains, as a yeast changes flour into bread. With soybeans, phytate gets broken down when the beans are fermented into products such as natto or tempeh.
Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to email@example.com. Due to the volume of mail, personal replies cannot be provided.