From both culinary and health perspectives, individual soyfoods have varying attributes, making them easy to incorporate into diets of people with different tastes and nutritional needs.
Soyfoods are often classified as either traditional Asian foods, like tofu and soymilk which have been consumed for centuries by some Asian populations, or as modern soyfoods, like plant-based meat alternatives made from soy protein. Among traditional Asian products, soyfoods are further divided into 2 groups: fermented and unfermented. However, these types of classifications can’t fully describe differences among these foods because even different forms and brands of a single type of soyfood can vary.
As with most plant foods, nutrient composition depends on growing conditions, location and variety of the parent soybean.1 For example, the protein content of soybeans ranges from 28.7-44.5% of calories.2 Generally, soybeans are around 38% protein making them higher in protein than other legumes.3 Many traditional soy products such as tofu, soymilk, and tempeh are good sources of protein. Modern soyfoods, including textured vegetable protein (TVP) and foods made from soy protein isolate (SPI) and soy protein concentrate (SPC), often provide even more of this macronutrient. SPI and SPC are about 90% and 65% protein, respectively.4
These concentrated sources of soy protein can be especially helpful for people who want to maintain a high protein intake while eating a diet comprised of more plant foods. Modern soy products can make it easier to achieve intake of 25g/d of soy protein, which has been shown to reduce LDL-cholesterol levels5,6 and possibly to benefit patients with chronic renal disease.7
Whereas most beans are nearly fat-free, soybeans derive almost 50% of their calories from fat.3 The fat – soybean oil — is predominantly polyunsaturated (~62%) and includes both the essential omega-6 and omega-3 fatty acids.8 Soybean oil was awarded a health claim by the U.S. Food and Drug Administration in 2017 that states “supportive but not conclusive scientific evidence suggests that eating about 1 ½ tablespoons (20.5 grams) daily of soybean oil, which contains unsaturated fat, may reduce the risk of coronary heart disease.”9
Differences in processing affect the isoflavone composition of soyfoods. In traditional Asian unfermented soyfoods, each gram of protein is associated with about 3-4mg isoflavones primarily in the glycoside form.10 In contrast, in fermented soyfoods the isoflavones occur in the aglycone form to varying degrees.11-15 The health implications of the different forms of isoflavones continue to be investigated.16
Not only can processing affect the form of isoflavones, but it can also reduce isoflavone content. The isoflavone content of SPI and SPC is typically ≤25% of the isoflavone content of traditional Asian soyfoods expressed on a mg/g protein basis.17 To this point, the Impossible burger provides 19g protein per patty, most of which comes from SPC, and contains only about 2mg isoflavones,18 whereas a cup of soymilk made from whole soybeans provides about 25mg isoflavones.10
Overview of Common Soyfoods
Traditional Non-fermented Soyfoods
Because these foods are minimally processed, their nutrient and phytochemical content is preserved. Whole soybeans are an excellent source of protein, fiber and iron, are predominantly polyunsaturated fat, have isoflavones and are a source of choline, a nutrient that can be low in plant-based diets such as flexitarian, vegetarian, and vegan.19,20
Mature soybeans can be purchased in dried form or as a canned product. Most often though, whole soybeans are consumed as edamame (green soybeans), which are soybeans harvested at about 80% maturity, accounting for their higher moisture and sugar content.21 They are usually preferred to whole, mature soybeans for their milder, sweeter flavor and lower content of oligosaccharides.22
Oligosaccharides can cause intestinal discomfort and flatulence in some individuals but may also have health benefits. These poorly digested sugars are prebiotics that stimulate the growth of beneficial bacteria in the colon.23,24
Both edamame and mature soybeans can be roasted to create soynuts, a snack item that undergoes minimal processing and therefore, like all whole soyfoods, retains all of the nutrients found in whole soybeans. Soynuts may be made with added oils and flavorings.
Soymilk is the liquid expelled from soaked soybeans. Many versions have a protein content comparable to that of cow’s milk and, if fortified, have a comparable calcium content as well. Calcium bioavailability from fortified soymilk was shown in healthy young women to be comparable to calcium bioavailability from cow’s milk. 25 Fortified soymilk and soy yogurt are the only plant milk and yogurt included in the milk group of the current Dietary Guidelines for Americans. In addition to calcium, fortified soymilk typically includes vitamin D in the form of ergocalciferol. Although the okara (soybean pulp) is lost during the manufacturing of soymilk, it still contains about 1.5g of fiber per cup whereas cow’s milk provides none.26
Tofu is made by adding a coagulant to soymilk and pressing the resulting curds into a solid block. It’s a good source of protein and can contribute calcium when a calcium-containing coagulant (calcium sulfate) is used. The use of other coagulants like nigari (magnesium chloride) produces tofu that is lower in calcium. Some manufacturers use more than 1 type of coagulant. As is the case for calcium-fortified soymilk, in a study involving premenopausal women, calcium bioavailability from tofu was shown to be similar to the bioavailability of calcium from cow’s milk.27
The degree to which tofu is pressed also affects its nutrient content. Firm tofu is often higher in protein and calcium than soft tofu. Silken tofu is a more custard-like product in which the curds are not pressed, and it tends to be lower in nutrients than its pressed counterpart. Because of the various methods and ingredients used in tofu production, nutrient content varies greatly and it’s best to depend on package labels for nutrient information rather than food databases.
One entry in the USDA Food Central database shows a calcium content of 683mg for 100g of raw firm tofu, but package labels suggest that calcium content is more likely to range between 75-200mg/100g. Tofu is low in total carbohydrate and, therefore, can be included in low-carbohydrate diets.
Traditional Fermented Soyfoods
Consumption of fermented soyfoods varies among Asian countries although globally, most soy is consumed in unfermented form. The 3 most common fermented soyfoods — tempeh, miso, and natto — are all made using the whole soybean. Some research indicates fermentation can increase antioxidant content28,29 and may in general enhance nutrient bioavailability although results are somewhat mixed on this point.30 Typically, these foods are not a source of vitamin B12 as is sometimes promoted.
Miso is produced by fermentation of soybeans and rice or barley with koji mold (Aspergillus oryzae) and added salt. It is widely used as a condiment in Japan where countless variations of this product are available. Miso may have a number of health benefits including that it may improve immune function, inhibit oxidation, and function as a probiotic.31 However, miso is often high in sodium.32
Natto is a traditional Japanese food made from whole soybeans that have been fermented with Bacillus subtilis var. natto. With a distinctive odor and viscous texture, natto has not gained popularity outside of traditional Japanese cooking. In Japan, it is often served with rice as a breakfast food. There has been interest in natto because Bacillus natto produces nattokinase, an enzyme which exhibits a fibrinolytic activity33 and some data suggest it may contribute to the reduction of CVD mortality.34 Natto is also among one of the few plant sources of vitamin K2, which may promote bone health.35
Although tofu is widely consumed in Indonesia, tempeh is considered the country’s staple source of protein.36 Tempeh is produced through a fermentation process that binds soybeans into a cake form. Fermentation is achieved by aging the tempeh with a starter, which is a Rhizopus fungus. In Indonesia, tempeh making is often a home-based art where the soybeans are wrapped in banana leaves and left to ferment. Several studies suggest tempeh has a role in combating anemia.36
Soy flour is made from ground roasted soybeans and is available as either a full-fat or defatted product. It is a good source of protein and has isoflavones. Soy flour is often added to baked goods for its beneficial effects on texture due to its content of both protein and lecithin. It is also sometimes used as an egg replacer in vegan baked products. Soy flour is typically 50% protein on a caloric basis.4
Textured Vegetable Protein (TVP)
TVP is a dried granular product made from defatted soy flour. It is sometimes flavored to taste like beef or chicken and must be rehydrated before using in recipes. Because TVP is an excellent source of protein (11g/half-cup prepared) and has a long shelf life, it is a common ingredient in ready-to-eat meals for camping, survival kits, and the military. It is an especially economical protein source that is valuable for preparing low-cost meals.
Plant-Based Meat Alternatives
An extensive variety of plant-based meat alternatives made from SPI and SPC are available. The composition of these products varies depending upon the main ingredients, source of fat, and micronutrients that are added. They are almost always an excellent source of protein, with some providing as much as 15g/serving. Some are also fortified with micronutrients, such as vitamin B12 and zinc. In Canada, all plant-based meat alternatives are fortified with vitamin B12. Different types of fat are used in their production, and sodium amounts may vary. A recent analysis concluded that despite the degree to which they are processed, soy-based burgers compare well with meat-based burgers with respect to overall nutritional and health attributes.26
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