Q: What's the nutritional content of tofu? How long can it be kept in the refrigerator and how can I tell if it's gone bad?
A: Tofu is made from soybeans that have been soaked, crushed, cooked and filtered. Calcium sulfate is then added, causing the soy pulp to form a gel. The water is removed as the soy is pressed into cakes.
Unlike other legumes, the soybean is relatively high in fat content. About half the calories in tofu come from fat. One 4-ounce piece of tofu supplies 20 percent of the day's protein requirement, 12 percent of the U.S. Recommended Daily Allowance for iron, 14 percent for phosphorous and 15 percent for calcium.
Because of it's relatively high content of poly-unsaturated fat (PUFAs), tofu will quickly turn rancid when exposed to the air -- the warmer the air, the more rapid the change. As such, most tofu is kept refrigerated under 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. 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 every day, fresh tofu can last up to two weeks in the refrigerator.
Q: Do you have information on the safety of non-stick cooking sprays, such as PAM? What is the propellant used and how do the sprays work? I stir-fry vegetables for my family using these non-stick cooking sprays and soy sauce. Is this practice as healthy as I think it is?
A: Cooking sprays contain vegetable oil, such as corn or soy, plus lecithin, an ingredient from soybeans traditionally used to keep oil and water in solution together. Most of the non-stick sprays use compressed hydrocarbon gas, such as propane or iso-butane as the propellant. The main safety concern with these products is that the gases are highly flammable. Because of this danger, only use these sprays on cold surfaces, away from all flames.
Because the sprays use a minimum amount of oil, little fat is contributed to the meal.
Because the ingredients have to be thoroughly mixed to work right, shake well before using.
With soy sauce, its sodium content is the main caution. One tablespoon of regular soy sauce contains over 3,000 milligrams of sodium -- over six times the recommended daily allowance. Even the new low-sodium soy sauces are relatively high in this mineral, 1 tablespoon containing over 1,000 milligrams of sodium.
Ed Blonz is a nutrition scientist based in Berkeley, Calif.