April 1, 2004
by BY CLYDE E. STAUFFER, PH.D
Soy is about health … and so much more.
In many parts of the world, poorer people do not get an adequate intake of protein. Meat is expensive, and cereal grains (wheat or corn, known internationally by the term maize) have a relatively low content of low-quality protein, being insufficient in lysine. Legumes, comparatively rich in lysine, can partially offset this deficiency, and soy is the leading member of this plant group.
Soy is readily available at a modest price. Extensive efforts are being made by the American Soybean Association to promote the addition of soy to foods in countries around the world. A major part of these international promotion efforts is focused on adding soy flour to bread.
In the United States, however, soy’s main benefits to the baker are the functions it brings at an economical cost. Recently, though, the public’s fascination with high-protein, low-carb diets prompts American food processors to take another look at soy, this time for its protein content.
Protein quality. Addition of soy protein to wheat products enhances the nutritional value of the protein in finished products. This enhancement arises from the complementary amino acid makeup of the two kinds of protein. The protein in soy is deficient in sulfur amino acids (methionine) but relatively rich in lysine. Gluten, on the other hand, is deficient in lysine but rich in cysteine (which the body can convert to methionine). Thus, the combination of the two protein sources creates a better balance of these two essential amino acids, ones that our bodies cannot synthesize and must obtain from our diet.
A 1979 study found the Protein Efficiency Ratio (PER) of gluten to be 0.7 and soy protein 1.3, but PER for an 88:12 blend of wheat and soy flours was 2.0. Since then, nutritionists have turned to a different way of measuring protein nutritional adequacy, the Protein Digestibility Corrected Amino Acid Score (PDCAAS). In this test, casein (the protein in milk) is accorded the top score of 1.0; wheat gluten comes in at 0.25 and soy protein at about 0.90 to 0.95, depending on the form. The protein in the 88:12 blend scores 1.0. By either measure, the improved nutrition achieved by combining the protein sources is apparent.
Other health benefits. Soy confers major health benefits besides its contribution to protein nutrition. After evaluating relevant research, the Food and Drug Administration approved in 1999 a food label health claim for soy protein. Four benefits from the consumption of soy have been well documented:
• It lowers blood cholesterol, thus decreasing chances of coronary heart disease.
• It decreases osteoporosis, resulting in less weakening of bones during aging.
• It alleviates post-menopausal symptoms such as “hot flashes.”
• It cuts the incidence of some kinds of cancer, particularly prostate and colon.
While soy protein is considered to be the main causal agent for these effects, other constituents of soy may also be involved. The most widely publicized is a group of phytoestrogens called isoflavones. Soy phytosterols have been shown to decrease blood cholesterol levels, and soy’s native antioxidants (less well characterized) may play a role in cancer inhibition.
The model health claim suggested by FDA reads: “25 g of soy protein a day, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease. A serving of [name of food] supplies [number of] g of soy protein.”
To qualify for use of this claim, a serving of the food must contain at least 6.25 g of soy protein, and the product must also meet the regulatory requirements for “low saturated fat” and “low cholesterol.”
To support its decision, FDA cited a number of scientific studies. The agency published the proposed rule in the Federal Register of Nov. 10. 1998, Pages 62977-63015, buttressed by 72 scientific references and 18 tables of data. In support of the final rule, published in Federal Register of Oct. 26, 1999, Pages 57700-57733, FDA discussed the many comments it received on the topic and gave 167 additional references to scientific literature. Both publications are available online at www.gpoaccess.gov/fr/index.html . Since then, research into the health benefits of soy has continued and has appeared in many different publications and forums.
Phytochemicals. Isoflavones are present in soy as glucosides, that is, they are linked to a glucose molecule. Bacterial action in the gut or in fermented soy foods such as tofu breaks the glucosidic linkage. The freed isoflavones (see the Figure) have weak estrogenic (hormonal) activity. Defatted soy flour contains 70 mg to 170 mg of isoflavones per 100 g of flour, consisting of about 60% genistein and 40% daidzein proteins. As weak estrogens, they compete with normal mammalian estrogens for binding sites in the body, contributing to the observed health benefits.
Saponins, another phytochemical in soy, are natural emulsifiers consisting of sugar molecules linked to a sterol or triterpene hydrocarbon chain. They are effective in lowering blood cholesterol levels, binding mutagenic compounds in the gut, eliminating digestive toxins and, perhaps, strengthening the immune system. Defatted soy flour contains about 2.2% natural saponins.
Phytosterols have a structure similar to cholesterol but are not metabolized by humans. They are effective either in their native form or in the hydrogenated state (stanols). When added to the diet, they interfere with re-absorption and metabolism of cholesterol, thus lowering blood serum cholesterol levels. Recently, FDA authorized a health claim recognizing this fact (see the Federal Register of Sept. 8, 2000, Pages 54686-54739).
Despite much effort, researchers have not been able to clearly differentiate between the action of isoflavones and saponins and the effects of soy protein itself. Thus, the health claim approved by FDA is defined only in terms of consumption of soy protein. Some studies indicate that health benefits are more pronounced from ingestion of defatted soy flour as compared with consuming isolated isoflavones, saponins and soy protein: A synergistic effect seems to take place.
Many styles. Soy is available to bakers in several different forms: enzyme-active soy flour, defatted soy flour, soy grits, soy protein concentrate and isolated soy protein.
Enzyme-active soy flour is used to bleach crumb and strengthen dough gluten, at a maximum permitted level of 0.5%, flour weight basis (f.b.). Soy protein concentrate and isolated soy protein generally do not contribute additional baking functionality consonant with their increased cost. Defatted soy flour is the product most commonly used in baked foods. When the lipids are removed from soybeans, the meal that remains contains little fat (less than 1%) but is high in protein (greater than 50%). This meal is lightly heated and ground to a mesh size similar to that of wheat flour to produce soy flour.
The meal may be coarsely ground (mesh size USS Sieve 20 to 60 is common) to make soy grits. Lightly heated grits are available, but for addition to breads, toasted grits are more often used. They impart a slight nutty flavor and texture, particularly in “seven grain” variety breads. Soy grits absorb very little water, a consideration when used at a high level.
Water calculation. Defatted soy flour absorbs more than its weight of water. In a typical bread dough application, soy flour at 3% (f.b.) requires an additional 5% (f.b.) of water to keep dough consistency and mixing properties the same as for the base formula product. Thus, in a 100-cwt flour mix, the baker gets an additional 8 lb of dough, yielding seven extra 1-lb loaves of bread. The current price of soy flour is about 15c per lb, so the added cost per dough is 45c. The selling price of the bread varies, but at any reasonable level the income from seven loaves is significantly more than the cost of the soy flour. In overseas markets, where soy flour’s price may be 50% higher than the US price because of transportation costs and bread prices are typically lower than here, the economic advantage persists although the margin is not as great.
Soy flour not only absorbs water, it retains it throughout the bake. Trials with hearth breads report bake loss for soycontaining loaves at 16% vs. 18.5% for regular products, a decrease of 2.5 percentage points. This translates into somewhat higher loaf moisture, which retards staling in these breads. Pan breads generally show a small differential (0 to 1 percentage points), but moisture retention can be a positive factor in products such as Danish and other sweet goods. Adding 3% soy flour increases the elasticity of dough for layered dough products; they are easier to handle with less tearing during the roll-in process.
Media attention. The main push for including soy in bakery products comes from media publicity surrounding the health claim and promoting greater consumer interest in healthier foods. The baker can take various approaches to meeting this demand.
• The addition of soy flour at a reasonable level (3% to 5%, f.b.) enables use of the word “soy” in the ingredients statement, without requiring any significant adjustments in processing.
• By using soy grits (10% to 15%, f.b.), probably in a variety type of bread, the baker would be justified in emphasizing “soy” on the package’s front panel.
• Made with soy grits and a lowabsorption version of isolated soy protein, a bread formula can be developed that delivers 6.25 g of soy protein per 50-g serving, resulting in 12.5% soy protein in the bread.
Soy flour is not recommended for the latter two approaches. For example, the addition of 12% (f.b.) soy flour would require 16% to 18% (f.b.) additional water. This amount of “weight” weakens the gluten, and loaf volume decreases markedly.
Soy grits, because they absorb little water, can be added to a bread formula without encountering the weakening effect of excessive water requirements. They act like any of the many other seeds and specialty flours often used in variety breads. Vital wheat gluten is often needed to carry the extra weight contributed by soy grits.
Making bread that contains 12.5% soy protein, yet with acceptable volume and eating properties, is a challenge. Isolated soy protein, which contains more than 90% protein on a dry basis, is the most applicable, with toasted soy grits added for flavor and texture. Vital wheat gluten and dough strengthening emulsifiers will be needed for volume and textural reasons. Several researchers, both academic and private, have succeeded in making such a formula. Whether one or more of them will be a commercial success, with repeat sales to consumers, remains to be seen.