Effects of steaming on soybean proteins and trypsin inhibitors

Steaming of defatted soybean flakes decreases both protein solubility and trypsin inhibitor (TI) activity as measured by nitrogen solubility index (NSI) and TI assay, respectively. NSI can be Iowered from 80–90, in raw flakes, to 10–20 with sufficient steaming. Concurrently, about 90% of TI activity is destroyed. Differential scanning calorimetric (DSC) analysis of raw soy flour demonstrated transition temperatures (T_max) of 58.2, 72.8 and 96.4°C for TI, 7S and 11S proteins, respectively. Lowering the NSI to 42.4 by steaming caused the TI transition to disappear, even though half of the TI activity remained. DSC analysis of steamed soybean flakes revealed that undenatured 7S and 11S storage proteins remain, even when the NSI is decreased to 10.     

Effect of feeding soy products with varying trypsin inhibitor activities on growth of shrimp

Six isonitrogenous, isocaloric diets containing commercially defatted, toasted and lightly toasted soy flours (SF) (diets 1 and 2) and four soy protein concentrates (SPC) (diets 3–6) as replacements for 40% of animal protein were fed to satiation to juvenile shrimp (Penaeus vannamei) for 10 weeks. The SPCs used in diets 3 and 5 were chemically modified products with reduced trypsin inhibitor (TI) content. The chemical modification of SF in diet 2, which resulted in an SPC for diet 3, and of SPC in diet 4 consisted of heating at 70°C for 1 hr with 50 mM Na₂S₂O₅, followed by dialysis to remove salt residues. To keep all diets isocaloric, cornstarch was added to replace the oli-gosaccharides lost during processing to an SPC. The TI contents, in mg TI/g diet, were 0.77, 6.14, 0.64, 1.40, 0.92 and 1.72 for diets 1–6, respectively. Shrimp fed lightly toasted SF had the highest weight gain, which was significantly higher than shrimp fed SPC diets 4, 5 and 6, but not significantly higher than shrimp fed diets 1 and 3. No significant difference was observed in survival rates. Shrimp fed diet 3 (with lowest TI) had the highest body percentages of crude protein, while toasted soy flour diet 1, also with low TI, had the lowest content of this constituent. In general, a high body protein reflects good health of the animal and excellent utilization of the feed. At the replacement levels of soy evaluated, TI content did not affect overall weight gain. Presented at the 1991 AOCS Meeting, May 12–15, 1991, Chicago, IL.     

Sterol metabolism studies in rats: Effects of taurodeoxycholic acid feeding on sterol metabolism

Sterol metabolism studies using isotopic and chromatographic techniques were carried out in: (a) control rats fed stock chow +0.1% cholesterol (control group), and (b) rats fed stock chow +0.1% cholesterol and supplemented with 0.5% sodium taurodeoxycholate (taurodeoxycholate group). Feeding the bile acid enriched diet led to decreased acidic steroid synthesis, decreased cholesterol turnover, and cholesterol balance compared to nonsupplemented controls. There were no significant differences in fecal neutral sterol output, endogenous neutral sterol output, or cholesterol absorption between bile acid fed animals and controls. Tissue cholesterol levels (liver, plasma, and bile) in the two groups were also similar.     

Significance of soya trypsin inhibitors in nutrition

Although recent evidence clearly indicates that trypsin inhibitors (TI) and low protein digestibility are the major factors responsible for the pancreatic hypertrophic and growth inhibitory effects of raw soybeans, there was uncertainty regarding the biological threshold level of TI at which these biological effects occur. To obtain such data, dehulled defatted flakes (10% dietary protein) containing graded levels of TI were fed to weanling rats for 4 weeks in two feeding trials. Normal pancreas weights were obtained in rats fed samples in which only 54 to 68% of the original TI of raw soya flour was inactivated. In partially toasted flakes with a nitrogen digestibility value of 77%, the average tolerance level of dietary TI activity that did not cause pancreatic hypertrophy was calculated to be 385 mg TI/100 g diet. TI tolerance level at maximum nitrogen digestibility of 85%, which did not significantly lower weight gain and reduce protein efficiencey ratios, was 260 mg TI/100 g diet. Continuous ingestion of high levels of TI (459 mg TI/100 g diet) in a 20% protein diet for 215 days did not inhibit growth nor cause pancreatic hypertrophy when compared to rats fed toasted soya flour diets. Pancreatic hypertrophy that occurs in rats fed raw soya diets containing up to about 1300 mg TI/100 g diet for 35 days was reversed by switching the rats to control diets or to 30% toasted flour. In long-term feeding studies, no pancreatic hypertrophy occurred in rats fed commercial edible-grade soya flour, concentrate, or isolate from time of weaning to adulthood (ca. 300 to 330 days). TI content of the diets ranged from 178 to 310 mg/100 g diet. Microscopic examination of the pancreas revealed no abnormalities. Gross appearances of heart, kidney, spleen and liver were normal. In long-term feeding, vitamin B-12 supplements were needed to provide optimum growth and to maintain body weight. Results of numerous chemical analyses, relatively short-term human tests and long-term animal feeding studies indicate that with proper control of manufacturing processes, soya protein products can be produced that, in mixed diets, have protein nutritional value approaching that of animal protein.     

Effects of consumption of processed soy proteins on minerals and digestion in man

Most of our knowledge of digestion and utilization of minerals and other components of foods is derived from model experiments with animals. Results indicate that digestion and utilization depend not only on type of foodstuff, but also on, e.g., animal species, nutritional status and overall diet composition. Despite these limitations, such model experiments help us in delineating problem areas and in finding solutions. To verify the conclusions drawn, tests in human beings, preferably under normal living conditions, are needed. Results of animal experiments with soy protein materials indicate that no major effects from the consumption of heat-treated soy proteins are to be expected. As systematic human experiments are scarce, we carried out a large scale experiment to assess acceptation and the possible physiological effects of substituting soy proteins for conventional proteins in the usual diet. We compared two diets in a 4+4 week crossover design with 92 healthy volunteers. One diet contained a wide variety of soy protein foods, mainly made with concentrates (test diet), about 25% of the protein intake being from soy. The other diet (control) contained products made from conventional protein sources. Although most of the 100 parameters investigated showed no difference, statistically significant reactions to soy were observed in mineral metabolism and digestion. All these changes are well within normal physiological ranges, do not indicate unfavorable trends, and can be explained by adaptation to shifts in dietary composition due to substitution of soy protein materials for conventional protein sources. In a separate experiment with 12 subjects, the effect of processing soy proteins on flatulence was investigated. It was found that refining can remove up to two-thirds of the oligosaccharides stachyose and raffinose, with a parallel decrease in volume of intestinal gas production. We conclude that these results confirm the prevailing view that soy protein materials are acceptable for our daily food.     

Functional properties of soy proteins

Soy protein ingredients must possess appropriate functional properties for food applications and consumer acceptability. these are the intrinsic physicochemical characteristics which affect the behavior of protein in food systems during processing, manufacturing, storage and preparation, e.g., sorption, solubility, gelation, surfactancy, ligand-binding, and film formation. These properties reflect the composition and conformation of the proteins, their interactions with other food components, and they are affected by processing treatments and the environment. Because functional properties are influenced by the composition, structure and conformation of ingredient proteins, systematic elucidation of the physical properties of component protein is expedient for understanding the mechanism of particular functional traints. The composition and properties of the major components of soy proteins are summarized, and the functional properties of soy proteins of importance in current applications (e.g., hydration, gelation, emulsifying, foaming and flavorbinding characteristics) are briefly reviewed.     

Chemical inactivation of soybean trypsin inhibitors

Sodium metabisulfite and glutaraldehyde were used alone and in combination to inactive Kunitz trypsin inhibitor (TI) in model systems and TI in lyophilized alkaline soy meal extract. Reaction of glutaraldehyde (0.1 to 3.0%, based on total volume of the reaction mixture) with Kunitz TI (3 mg/ml buffer) at 25 C resulted in 60–75% reduction in activity. Treatment of soy meal extract containing 0.08 mg TI/mg at a concentration of 10 mg sample/ml buffer under similar reaction conditions reduced TI activity only 40%, however. A reaction temperature of 75 C had little additional effect on TI inactivation. On the other hand, sodium metabisulfite (0.05 to 1.0 mM) inactivated >96% Kunitz TI within 1 hr at 75 C and 75–94% of the TI in soy meal extract. The combination of 0.6 mM metabisulfite and 0.5% glutaraldehyde at 75 C inactivated >99% Kunitz TI, and combination of 0.6 mM metabisulfite and up to 3% glutaraldehyde inactivated 88% of TI in soy meal extract vs 91% inactivation with metabisulfite alone. Thus, metabisulfite plus glutaraldehyde inactivated Kunitz TI better than either one alone, while bisulfite alone best inactivated TI in soy meal extract. When the reactions were performed at neutral pH, protein solubility of 80% or better was retained. Presented in part at the 77th annual meeting of the American Oil Chemists’ Society, Honolulu, May 1986.     

Nutritional aspects in textured soy proteins

Textured soy protein products have found extensive use in pet foods and many human foods. They extend and complement meats in formulated foods. Numerous feeding tests with people and experimental animals show that biological values are high and protein efficiency ratios compare favorably with pure meat products. By careful processing of the textured soy products, high quality, safe, low cost protein food can be prepared. Textured soy proteins are an excellent example of man’s creative ability to engineer a supplementary food that is nutritious, palatable, and economical.     

Use of soy proteins in baked foods

Technology for the utilization of soy products in bakery foods is well established and reasonably simple. We can expect the functional properties and flavor of soy products to be continually improved through major research efforts in the soy industry. Large scale protein fortification programs will be forthcoming as the world population continues to grow, and economics dictate more and more efficient sources of nutrients. Before this will be realized, however, careful evaluations of the nutritional requirements and the technical, economic and political situations in a country will have to be made and the constraints removed before widespread use of fortified bakery foods in the commercial sector can be realized. In developed as well as developing countries, the near term constraints for usage of soy proteins in bakery products are represented by food regulations or laws which must be changed before the full nutritional and functional assets of soy proteins can be realized to the benefit of the baking industry.     

Use of soy proteins in Swedish foods

A positive action program has been instituted in Sweden. A new food law recognizing soy protein as a food ingredient and not an additive was obtained, a discriminatory tax was reduced by 75%, and the image of soy proteins was changed in newspapers and journals. Many soy-containing foods are now on the market as a result of these changes.     

Soybeans and soy products in the feeding of children

Soya products are used in infant formulas, hypoallergenic foods and vegetable mixtures mainly because of their good protein quality. They have good potential in feeding children from birth to adolescence and seem to satisfy the needs for total nitrogen and essential amino acids when ingested in adequate amounts. Other factors, however, must be considered for their use. The availability of calcium and phosphorous from soy formulas may be inadequate to support for any length of time the rapid bone mineralization of growing premature infants and should thus be fed to such infants only for a few weeks as therapeutic agents rather than as routine feeding formulas. Allergy to soya exists and may be serious, especially among infants with severe allergy to cow’s milk. Industrial processing may affect the quality of soya protein and it can produce allergenic substances in some protein foods. Thus, it may be necessary to evaluate the overall nutritional quality of new soya products before recommending their use. The effect of methionine supplementation on soya protein quality is not clear but it may be advisable in infant formulas. Soybean protein has great advantages when it is part of a food or a food system, particularly those based on cereal grains. Finally, the prevention by soya formulas of diarrhea induced by phototherapy is interesting, but its overall metabolic implications need further study.     

The marketing of isolated soy proteins

The soy protein industry is an established industry today. Current isolated soy protein technology and know-how are practical and being used today. Isolated soy proteins are comparable in protein quality to traditional proteins. Cost effectiveness of isolated soy proteins will offer an economic advantage to processor and consumer. These products are available and strategically located to support commercial utilization anywhere in the world.     

Studies of long term administration of aflatoxin to rats as a natural food contaminant

The effect of feeding aflatoxin, as a natural food contaminant, to rats over long periods of time was studied using multigeneration and longevity tests. The test animals in the multigeneration study consisted of three groups of rats fed diets containing 0, 1 and 10 ppb of aflatoxin (predominantly B₁) continued over four generations, with animals of the first and fourth generation fed the diets for 104 weeks. These diets were in proper nutritional balance and included 35% ground roasted peanut products; the ration with 0 ppb aflatoxin excluded the peanuts usually discarded; the one with 1 ppb had the roasted discards returned, while the ration with 10 ppb included the discards in amount 10 times that which had been initially removed. Another longevity study was also performed in which rats were fed diets containing aflatoxin at a level of 80 ppb. In this case, the test peanuts, also fed as a simulated peanut butter at 35% concentration, consisted entirely of roasted peanut discards. Control diets provided no peanut components. Animals fed the low levels of aflatoxin grew as well and actually had a higher percentage survival at 104 weeks than did the animals on the control, aflatoxinfree diets. Organ weights, liver total lipid and cholesterol levels were comparable in all groups. Pathological abnormalities, e.g., hemorrhagic and opaque spots and mottling in some of the livers, were attributed to the aging process since the abnormalities appeared in the control as well as the experimental groups. In the animals fed the aflatoxin at 80 ppb, which has been reported by several investigators to produce well-defined hepatomas in rats, there was liver involvement and some biochemical changes occurred that were not noted in the controls. However, no hepatomas were observed in these animals even after 21 months on this diet. The liver lesions, indicative of a toxic effect, have not been associated with the development of hepatomas. It is possible that some components of the diet used in these experiments may have protected the animal against hepatoma formation. Our studies indicate that there may be a tolerance for aflatoxin as judged by results in one species of rats when whole ground roasted peanuts provide the natural contaminant. Presented in part at the AOCS Meeting, New York, October 1968.     

Functionality of vegetable proteins other than soy

In addition to the soybean, many other sources of vegetable protein have potential to provide a broad spectrum of functional properties. Among these sources are cottonseed, peanut, sunflower, and rapeseed. As with soy, the functional characteristics vary with the type of product, e.g., flour, concentrate, or isolate. In this discussion, functionality is defined as the set of properties that contributes to the desired color, flavor, texture, or nutritive value of a product. Utilization of these alternate sources of vegetable proteins will depend upon availability, economics of the product in any given country, and on the uniqueness and desirability of the functional properties of the product.     

Intrinsic sulfite content of isolated soy proteins

Commercial isolated soy proteins (ISP) contain 22 and 31 ppm sulfite as measured by the optimized Monier-Williams method (Sulfites in Foods, Official Methods of Analysis, 16th edn., AOAC, Washington, DC, 1995, Official Method 990.28). A method was developed to cryogenically trap and quantify the sulfur dioxide produced by this method using GC-MS. The same commercial ISP samples were found to contain 17 and 26 ppm sulfite, respectively, with GC-MS. ISP prepared in the laboratory contained 33 ppm sulfur dioxide, and defatted soybean flakes contained only a trace. Adding dithiothreitol after beginning the boiling step of the Monier-Williams assay had no significant effect on the sulfite content of a commercial ISP, whereas adding dithiothreitol prior to bringing the sample to a boil reduced the sulfite content from 17 to about 1 ppm.