April 2015 Issue

Tofu's Many Faces
By Virginia Messina, MPH, RD
Today's Dietitian
Vol. 17 No. 4 P. 22

This versatile, ancient food comes in a variety of tastes and textures and is used in recipes ranging from smoothies to vegetable stir-fries.

Tofu has been appearing on menus throughout Asia for centuries. Although the first written record of tofu dates back to around 950 AD, some historians believe it was in existence at least 1,000 years earlier. One theory is that it was first produced when some long-forgotten cook seasoned warmed soymilk with sea salt, causing it to curdle and accidently producing the soybean curds we now call tofu.

Tofu made its way to Japan by 1100 AD, probably introduced by Buddhist monks whose diets were vegetarian. In fact, early tofu shops may have been located in temples and monasteries. Travelers eventually brought tofu to all parts of Asia. Today, it's perhaps the most important soyfood consumed in Hong Kong, Korea, and Singapore. In Japan, where adults eat one to two servings of soyfoods per day, tofu accounts for about one-half of all soy consumed.1

Tofu shops were established in large North American cities with Asian populations in the 19th century. By the mid-1900s, Seventh-day Adventist-run food companies also were producing tofu. But it wasn't until the 1970s that tofu began to appear on menus outside of these communities, largely in response to an interest in natural foods and meatless diets.

Today, tofu is found in grocery stores in nearly every corner of America. It's also the basis for a new crop of commercial meat and cheese analogs. Tofu hot dogs, cream cheese, sour cream, and mayonnaise represent a whole new generation of this ancient food.

Tofu Types
The procedure for making all types of tofu follows the same general principles. First, a coagulant is added to soymilk—the liquid expressed from soaked soybeans. The coagulated milk can be consumed as is for the Japanese product called silken tofu. Or the soymilk can be allowed to form curds, which are then drained and pressed into blocks.

Block tofu usually is sold as extrafirm, medium-firm, or firm depending on its water content and density. Some companies also produce super- or ultrafirm tofu.
While recipes generally specify the type of tofu to use, there are no standard definitions to describe these different block tofu categories. One company's firm tofu may be the same as another's extrafirm variety. So while choosing the exact tofu to fit a recipe isn't a science, it's helpful to know that firmer types of tofu hold up well to sautéing and grilling while less firm types can work well for blending in soups, smoothies, and desserts. Sometimes it takes a little trial and error to find the right block tofu for the job.

Block tofu usually is water-packed in sealed containers. It should be kept refrigerated, and once opened any unused tofu should be stored in fresh water.
Japanese silken tofu is a delicate product in which the soymilk is coagulated but doesn't form into curds and isn't pressed. The result is a consistency similar to poached egg whites, which is a good choice for sauces, puddings, or smoothies. Silken tofu sometimes is used as an egg substitute in baking. Both soft and firm silken tofu are available. Some brands of silken tofu are sold in shelf stable aseptic packs. These should be refrigerated once the packs are opened.

Moreover, there are a number of specialty tofu products on the market. Black tofu is made from black soybeans. It's usually fairly firm and looks and tastes similar to regular block tofu except there are black specks throughout the flesh. It may be somewhat higher in antioxidants than the more traditional tofu made from yellow soybeans.

Sprouted tofu is made from sprouted soybeans. It's usually firm and can be used interchangeably with regular block tofu.

Fermented or pickled tofu is dried tofu that's pickled with salt, rice wine, or vinegar. Sometimes it's flavored with chili peppers. Fermented tofu most often is found in Asian food stores where it's sold immersed in brine in jars. Its strong flavor makes it most suitable for use as a condiment.

Tofu Nutrition
The nutrient content of tofu varies widely depending on how it's made. Generally, the firmer the tofu, the higher it is in calories, protein, and fat. Protein content ranges from about 4 g in 3 oz of soft silken tofu to 10 g in 3 oz of extra firm tofu.
Calcium content depends on the type of coagulant used to make the tofu. Traditionally, a naturally derived salt of magnesium chloride called nigari has been used to make tofu. Today, tofu more often is made using calcium sulfate as the coagulant, sometimes in addition to nigari, which can produce a calcium-rich tofu. Studies show that calcium absorption from tofu is comparable to that from cow's milk.2

Tofu also provides about 1 mg of well-absorbed iron per serving3 and is a source of alpha-linolenic acid, the essential omega-3 fatty acid. Some brands of tofu are fortified with vitamin B12 and vitamin D and sometimes with the omega-3 fats DHA and EPA from a vegetarian source. (Table 1 shows the calorie, protein, fat, calcium, and iron contents of several tofu varieties.)

Health Effects of Soybeans and Tofu
A serving of traditional soyfoods such as tofu or soymilk provides about 25 mg of isoflavones.1 Although these phytochemicals are classified as plant estrogens, isoflavones are different from the female hormone estrogen. Isoflavones are regarded as SERMs (selective estrogen receptor modulators), which are molecules that can function as both estrogen agonists and antagonists.4 For example, in postmenopausal women, isoflavones appear to offer similar benefits as estrogen in reducing hot flashes5 and perhaps also for preventing skin wrinkling.6 But they don't offer the same benefits as estrogen for bone health.7

Isoflavones also may be protective against breast cancer whereas estrogen is thought to promote it. In China and Japan, women with the highest intakes of soyfoods are less likely to develop breast cancer.8 In addition, in women with breast cancer in both the United States and China, those who consume soy regularly are less likely to suffer a recurrence and die from the disease.9

Epidemiologic studies show health benefits from soy consumption in men as well, since Asian men with higher intakes of soyfoods are only one-half as likely to develop prostate cancer as men who seldom eat these foods.10

Soyfoods also appear to offer a comprehensive package of heart-healthy attributes. First, their isoflavones have been found to improve endothelial function in women who had impaired endothelial function.11 In addition, protein from soyfoods has been shown to directly lower LDL cholesterol. A recent meta-analysis of clinical studies found that consuming 24 g of soy protein per day can reduce LDL cholesterol by 4.3%.12 In 1999, the FDA approved a health claim for soyfoods based on the hypocholesterolemic effects of soy protein.13 Products that provide at least 6.25 g of soy protein per serving are eligible to boast the claim. Since then, 10 other countries have approved similar health claims for soy protein. A 3-oz serving of tofu contains between 4 g and 10 g of soy protein.

Finally, replacing animal proteins with tofu and other soyfoods in the diet lowers saturated fat and cholesterol consumption. All of these benefits may add up to provide significant protection from cardiovascular risk factors. In a prospective study of 40,462 Japanese adults, women who consumed soyfoods more than five times per week were about one-half as likely to suffer a heart attack compared with women who consumed soyfoods less than two times per week, and were more than two-thirds less likely to die from heart disease.14

A further example of the benefits of soyfoods for reducing heart disease risk is seen in the Women's Isoflavone Soy Health (WISH) Research Group study. Among the 350 healthy postmenopausal women aged 45 to 92, isoflavone soy protein supplementation inhibited the progression of subclinical atherosclerosis in the younger postmenopausal women (median age 53) who were less than five years postmenopausal and at low risk of developing cardiovascular disease.15 The women consumed 25 g of soy protein and 91 mg of isoflavones per day (the amount of isoflavones in about four servings of tofu).

One concern about isoflavones has focused on their effect on cognitive function. Researchers raised questions in 2000 when the Honolulu-Asia Aging Study (HAAS) found that cognitive impairment was more common in frequent tofu consumers than nonconsumers.16 A study in Indonesia showed similar findings for tofu, but not for tempeh, which also is rich in isoflavones.17 In fact, tempeh was associated with better cognitive function. However, a follow-up to the Indonesian study found both tofu and tempeh were associated with improvements in some aspects of cognition.18

Due to the conflicting findings, and because HAAS wasn't designed to study cognition, better information on this issue comes from more recent clinical trials. In the WISH study, there was no difference in cognitive scores between women consuming isoflavones and those in the control group, except that those taking the isoflavone supplements had greater improvements in visual memory.19 A 2014 meta-analysis of 10 placebo-controlled trials concluded that soy isoflavone supplementation may improve summary cognitive function and visual memory in postmenopausal women.20 The study suggested that postmenopausal women could experience greater cognitive benefits if they begin soy isoflavone supplementation earlier in menopause rather than later.

Moreover, it's important to note that similar to fiber and many other food components, soyfoods can affect dosage requirements for people taking synthetic thyroid medications. Taking these medications between meals rather than with them can be helpful. In people with healthy thyroid function, soyfoods have no effect on the thyroid.21

Preparing Tofu
Another benefit of tofu is that it's versatile in recipes because of two distinctive features. First, it's relatively bland and doesn't compete with other flavors in recipes. It's also porous, which allows it to absorb the flavors around it. It's because of these features that tofu is found in dishes as diverse as spicy Thai curries, vegetable stir-fries, and chocolate cream pie.

Many recipes calling for firm or extrafirm tofu, especially those using a marinade, recommend pressing the tofu first. Pressing reduces the tofu's water content, makes it firmer, and enhances its ability to absorb marinades. Commercial presses are available, but cooks who are new to tofu may not want to invest in a press right away. And even many who have been cooking with tofu for years are satisfied with homemade "presses" using a stack of books or heavy pots.

To press tofu without a commercial press, wrap the tofu in a clean dishtowel and place it on a plate. Cover with another inverted plate (flat side against the tofu) and pile some books or a tea kettle filled with water on top. Allow the tofu to press for 15 to 30 minutes depending on how firm you want it to be.

Freezing tofu is another popular preparation technique. It creates a spongy texture that takes very well to marinades. Simply put the unopened package of tofu into the freezer overnight. Defrost and squeeze the water out of it before using it in a recipe.
Note that silken tofu can't be pressed and always should be used as is. For those who are new to tofu, adding it to stir-fried dishes with Asian flavors such as ginger, sesame oil, and tamari can be a great place to start. But inventive cooks use tofu in every kind of dish imaginable. See the sidebar "15 Ways to Use Tofu" for some creative ideas.

— Virginia Messina, MPH, RD, is a writer and speaker on vegetarian and vegan diets for the public and health professionals. She's coauthor of Vegan for Life, Vegan for Her, and Never Too Late to Go Vegan, as well as a textbook on vegetarian diets for health professionals. She's also a former coauthor of the Academy of Nutrition and Dietetics Position on Vegetarian Diets.

References
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