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 was first used in China around 200 B.C. Although the discovery of the process for making tofu is lost to the ages, Chinese legend has it that the first batch of tofu was created by accident. A Chinese cook added nigari to flavor a batch of pure¨¦d, cooked soybeans; the nigari produced the curd that we know today as tofu.

        Today, tofu is a dietary staple throughout Asia. This delicate food is made fresh daily in thousands of tofu shops and sold on the street.

        In recipes, tofu acts like a sponge and has the miraculous ability to soak up any flavor that is added to it. Crumble it into a pot of spicy chili sauce and it tastes like chili. Blend it with cocoa and sweetener and it becomes a double for chocolate cream pie filling. Cubes of firm tofu can be added to any casserole or soup.

Types of Tofu

        Three main types of tofu are available in American grocery stores.

        Firm tofu is dense and solid and holds up well in stir fry dishes, soups, or on the grill... anywhere that you want the tofu to maintain its shape. Firm tofu also is higher in protein, fat and calcium than other forms of tofu.

        Soft tofu is a good choice for recipes that call for blended tofu, or in Oriental soups.

        Silken tofu is made by a slightly different process that results in a creamy, custard-like product. Silken tofu works well in pureed or blended dishes. In Japan, silken tofu is enjoyed "as is," with a touch of soy sauce and topped with chopped scallions.

Tofu Nutrition Facts

        Tofu is rich in high-quality protein. It is also a good source of B-vitamins and iron. When the curdling agent used to make tofu is calcium salt, the tofu is an excellent source of calcium. While 50 percent of the calories in tofu come from fat, a 4-ounce serving of tofu contains just 6 grams of fat. It is low in saturated fat and contains no cholesterol. Generally, the softer the tofu, the lower the fat content. Tofu is also very low in sodium, making it a perfect food for people on sodium-restricted diets.

Buying & Storing Tofu

        Tofu most commonly is sold in water-filled tubs, vacuum packs, or in aseptic brick packages. Tofu is usually found in the produce section of the grocery store, although some stores sell tofu in the dairy or deli sections. Tofu is sometimes sold in bulk in food cooperatives or Asian markets. Unless it is aseptically packaged, tofu should be kept cold. As with any perishable food, check the expiration date on the package.

        Once the tofu package is open, leftover tofu should be rinsed and covered with fresh water for storage. Change the water daily to keep it fresh, and use the tofu within a week.

        Tofu can be frozen up to 5 months. Defrosted tofu has a pleasant caramel color and a chewy, spongy texture that soaks up marinade sauces and is great for the grill.

Tips For Using Tofu

        Tofu is for everyone. The soft consistency of tofu and its mild taste make it a perfect food for anyone. It is a good source of protein for elderly people who prefer dishes that are easy to chew and digest. Soft tofu that has been pureed with fruits or vegetables is a good first protein food for infants. Toddlers can enjoy chunks of cooked tofu for snacks or meals.

        Try some of these ideas for introducing tofu to your family.

        Add chunks of firm tofu to soups and stews.
        Mix crumbled tofu into a meatloaf for a pleasant light dish.
        Mash tofu with cottage cheese and seasoning to make a sandwich spread.
        Create your own tofu burgers with mashed tofu, bread crumbs, chopped onion and your favorite seasonings.
        Marinate tofu in barbecue sauce, char it on the grill and serve on crusty Italian bread.
        Add a package of taco seasoning to pan-fried, crumbled tofu, or a mixture of tofu and ground beef to tofu tacos.
        Blend dried onion soup mix into soft or silken tofu for a cholesterol-free onion dip.
        Stir silken tofu into sour cream for a reduced-fat baked potato topper.
        Blend tofu with melted chocolate chips and a little sweetener to make a chocolate cream pie.
        Replace all or part of the cream in creamed soups with silken tofu.
        Make missing egg salad with tofu chunks, diced celery, mayonnaise and a dab of prepared mustard.
        Substitute pureed silken tofu for part of the mayonnaise, sour cream, cream cheese or ricotta cheese in a recipe. Use it in dips and creamy salad dressings.

Tofu Characteristics Influenced by Soybean Crop Year and Location

Introduction
        Processors that manufacture tofu from soybean (Glycine max (L.) Merr.) desire both a high quality product and high tofu yield. Firmness and whiteness are two important quality attributes of tofu. However, little is known regarding the effect of environment, genotype, and the interaction of these factors on tofu quality.

        Wang et al. (1983) reported that tofu made from a cultivar with a black hilum had a grey tinge and was less attractive than tofu made from other cultivars. The yields of fresh tofu differed among 10 cultivars due to differences in water content, but not dry product. They stated that "neither the size of the beans nor the amount of water absorbed by the beans was significantly associated with the yield and quality of tofu." They found that tofu produced from different cultivars varied in hardness and that cultivars high in protein content produced tofu high in protein content. Although differences among cultivars affected the texture and yield of tofu, they concluded that these differences were small.

        Lim et al. (1990) reported that increased total solids of soymilk was associated with increased firmness of tofu. Firmness was not associated with protein content of tofu or soybean seed. Fresh tofu yield increased as the total solids content of soymilk increased. Soybean seed size was not associated with the yield of fresh tofu.

        Smith et al. (1960) stated that tofu is typically 6% protein, 3.5% fat, 1.9% carbohydrates, 0.6% ash, and 88% water. They compared five soybean cultivars developed in Japan to 15 U.S. cultivars and one from India. Cultivars were not grown in the same environment and were not a composite of samples grown in different environments. Therefore, cultivar differences were confounded with the influence of the different environments in which the seed was produced. Japanese and U.S. cultivars had similar volume and water content. However, tofu produced from cultivars of Japanese origin were mostly a greyish white color, whereas tofu produced from cultivars of U.S. origin were light yellow. They stated that "the greyish white color is preferred in Japan and appears to be favored because of custom..." They concluded that "because uncomposited samples of soybean were used, observed differences may have been caused by climate and soil at the location of their growth." Tofu produced from two U.S. cultivars was equal in quality to tofu produced from Japanese cultivars.

        Smith et al. (1960) stated that "yield of tofu varies with variety (location), but the general average yield from U.S. soybeans is about the same as from Japanese beans." However, due to the confounding of the influence of cultivar and environment, they were unable to determine whether environment influenced tofu quality and yield.

        Murphy and Resurreccion (1984) reported that environment had a greater influence on glycinin concentration than genetic differences between two cultivars. Soybean protein is comprised primarily of glycinin and beta-conglycinin fractions. Glycinin is commonly considered the major component of the 11S sedimentation fraction and beta-conglycinin is the major component of the 7S fraction. Although genetic differences among cultivars for glycinin content were observed, they concluded that both glycinin and beta-conglycinin content were affected more by environment than genetic differences.

        Tofu manufacturers desire high yield as well as firm, white tofu, but the influence of the growth environment on tofu firmness, color, and yield has not been evaluated. If differences among locations within a year affect tofu quality and yield, processors may decide to test soybean samples produced in different locations before purchasing the seed.

        Plant breeders need to determine whether a relative or absolute standard should be used to evaluate experimental lines for tofu quality and yield. An absolute standard would be based on minimum tofu quality and yield of a genotype, without considering the environment in which the seed was produced. However, if environment affects tofu quality, breeders should compare new experimental genotypes to a standard cultivar grown in the same environment. Under these circumstances, breeders using an absolute standard might discard genotypes suitable for making high quality tofu. Our objective was to determine the influence of locations within a year and differences between years on tofu yield, quality, and color.

Materials and Methods
        Two soybean genotypes, 'Proto' (Orf et al., 1991) and M86-356 were compared. Both are Maturity Group 0, high protein genotypes developed at the University of Minnesota. They were evaluated in a randomized complete block design with three replicates per environment. Environments included: 1993 Northwood, ND; 1993 Hendrum, MN; 1993 Kragnes, MN; 1993 Dwight, ND; 1994 Northwood, ND; 1994 Hendrum, MN; 1994 Casselton, ND; 1994 Great Bend, ND; and 1994 Mantador, ND.

        Each plot consisted of two 6.4-m long rows with 0.76 m between them and two adjacent plots. The center 4.6 m of each plot was harvested with a self-propelled combine. Tofu characteristics, protein, and oil content were evaluated on two replicates in the 1993 environments and three replicates in the 1994 environments.

        Soybean (139 g) were soaked in tap water at room temperatures (20-22ˇăC) for 8 hr. The soaked bean was ground with 625-ml water for 4 min at high speed in a blender (Model 908-2, Hamilton Beach Co., Washington, NC). After grinding, the slurry was filtered through a cheese cloth and squeezed by hand to obtain soymilk. The residue was mixed with water to produce an 8:1 water-to-bean ratio. The slurry was filtered again to recover soymilk solids. After adding 0.2 g antifoaming agent (Kaoh Co., Wakayama, Japan), the soymilk was heated to boiling and maintained for 5 min while stirring. When cooked milk was cooled to 85ˇăC, modified nigari (a mixture of CaSO4?2H2O and natural nigari, Taiwan Salt Workers Co., Tainan, Taiwan) suspended in 15-ml water at 3% of raw bean weight or glucono-delta-lactone (Fujisawa Pharmaceutical Co., Osaka, Japan) at 0.5% of soymilk weight was added. The mixture was stirred at 150 rpm by a Caframo stirrer (Model R2R1, Wiarton, Ontario, Canada) for 30 sec. After standing for 10 min, the bean curd was cut into pieces with a scoop and transferred to a wooden mold (12.5 x 12.5 x 5.5 cm) for pressing. The bean curd was pressed sequentially at 8.7 g/cm2 for 10 min, 14.6 g/cm2 for 10 min and 20.4 g/cm2 for 15 min. After pressing, tofu samples were weighed and placed in cool water before performing textural and color analyses.

        Tofu samples were freeze-dried. The crude protein, moisture, and crude oil contents of soybean and freeze-dried tofu were determined by near-infrared reflectance (NIR) (Infra-Alyzer 400, Technicon Instruments Corp., Elmhurt, IL). The NIR readings were calibrated with standard curves determined by AOAC Methods (1990).

        The solid content of soymilk was determined as a degree of Brix using a refractometer (Auto Abbe, Model 10500, Buffalo, NY) at room temperature. Tofu yield is the fresh weight of tofu per weight of dry soybean seed. The color of fresh tofu was measured on a Hunter colorimeter (Model XL-23, Gardner Lab Inc., Bethesda, MD). The instrument was standardized with a standard white tile (L=91.94, a= -1.03, b=1.14).

        Soybean were examined for storage protein mass distribution (7S and 11S) by sodium dodecyl polyacrylamide gel electrophoresis (SDS-PAGE) using a gradient gel of 8-16%, based on the procedure of Laemmli (1970). Samples were extracted on a magnetic stirrer with 0.05 M sodium phosphate buffer (1.2 g: 14 ml) at pH 7.5 for 90 min. The slurry was centrifuged at 1,960 x g for 10 min to remove residues. The protein concentration of supernatant was determined by the Biuret method and adjusted to 2 mg/ml with distilled water. An equal volume of SDS-sample buffer containing 10% of 2-mercaptoethanol was added to the protein solution. After boiling for 2 min, 40 microliters of the cooled solution containing 40 micrograms protein was loaded into the gel. Electrophoresis was performed in a BioRad Protean II chamber at 100 volts for 8 hr. The gel was stained with Coomassie Brilliant Blue R-250. The 7S and 11S proteins were quantified according to Wang and Chang (1995).

        The firmness of tofu samples was measured using an Instron Universal Testing Machine (Model 1011, Instron Co., Canton, MA) equipped with a 5-kg load cell. A 5-cm diameter cylindrical plunger was used to compress the interior sample of tofu cake (1.5-cm height with 5-cm diameter). Four tofu cakes from each batch were compressed twice to 25% of original height at a crosshead speed of 20 mm/min. The recording chart speed was 20 mm/min. Firmness was measured from the highest point of the first compression cycle of the Texture Profile Analysis curves (Bourne, 1978).