Tofu, also known as soybean curd, is a soft, cheese-like food made by curdling fresh hot soymilk with a coagulant. Traditionally, the curdling agent used to make tofu is nigari, a compound found in natural ocean water, or calcium sulfate, a naturally occurring mineral. Curds also can be produced by acidic foods like lemon juice or vinegar. The curds then are generally pressed into a solid block. Tofu is an excellent source of cholesterol-free protein, B-vitamins, and iron, is low in sodium, and is full of phytoestrogens, which may protect against some cancers and heart disease, bring relief from some of the uncomfortable symptoms of menopause, and protect against osteoporosis. Depending on the coagulant, tofu may be high in calcium and magnesium.
Tofu comes in several consistencies: soft, regular, firm, extra-firm. It is often sold in a small cardboard box found in the refrigerator section of the grocery store or in the Asian ingredients aisle. In all of my grocery stores, we have sections specifically for refrigerated soy products (such as tofurkey and "chik n nuggets"), and that's where tofu is located. The most common package sizes are 10.5oz, 12oz, and 14oz. If purchased already cold (the only way I've bought it), refrigerate until ready to use; otherwise, store in a cool, dark place. If you use only part of a package of tofu, store the rest in the refrigerator, covered with water, for 1 week.
Tofu can be used in smoothies, to add protein and to thicken them. The best tofu to use for smoothies is soft silken tofu, which is slightly more creamy than soft tofu.