Soybean this. Soy protein that. It is now part of yuppie conversation. Soybean, also known as sou, soi, soja and soya, is not just some food ingredient or magical nutritional compound that has recently fallen from the tree. As a matter of fact, it is an annual leguminous plant native to eastern Asia where it has been a component of the human diet for over 5000 years. However, it wasn't until the 1800's that it was introduced to the U.S. During the 1930's, the U.S. Department of Agriculture along with state agricultural experiment stations developed improved varieties of soybean through hybridization and selection. These new, improved varieties had higher yield and oil content as well as growth habits more suitable to modern farming methods than the Asian varieties.
Since then, U.S. farmers' commitment to the soybean has grown tremendously. Domestic farming acreage has increased from 10.2 million acres in 1944 to 71.6 million in 1998, which is very close in acreage to corn (73.8 million acres in 1998). This dramatic increase can only be directly linked to the successful commercialization of numerous products derived from this legume. For today, each component is commercially isolated and processed for a variety of uses ranging from industrial grade adhesives and printing inks to food grade meat extenders, beverage mixes and high value nutritional supplements.
To make matters more interesting for the bean, medical researchers have been studying its natural constituents (soybean oil, protein, lecithin, isoflavones, tocopherols and phytosterols) extensively for health and nutritional properties-and getting results. One recent example is the approval of the soy protein health claim for cholesterol reduction by the U.S. Food and Drug Administration. This research, complemented by the ever growing media attention, is being fueled by the general population's increased desire to take control of their own health. However, what is not fully understood by all, including the nutraceutical and food industries, is that the soy products discussed in the media are very complex materials that have been derived through the use of advanced chemical and mechanical processing technologies. The result is a wide spectrum of materials that vary in functionality, composition, taste and potential healthful properties. Any of this can make a consumer product a success or failure.
Getting Down To Bare 'Beans'
To get to the heart of the facts, all soy products begin with the same processing steps, which includes the bean being cleaned, conditioned, cracked, dehulled and rolled into flakes. These flakes are then subjected to a solvent (hexane, a petroleum ether) bath to extract the crude oil. The solvent containing crude oil is then removed and the flakes are dried, creating the "meal." This defatted material is the basis for the three major soy protein product categories: flours, concentrates and isolates. The crude oil, which was removed, is further processed, creating products such as retail/cooking oil, lecithin, tocopherols and phytosterols. Isoflavones, on the other hand, are a co-product of the protein purification process.
Soybean Oil. The crude soybean oil removed via solvent extraction is refined further to produce retail oil. Soybean oil is a good source of essential fatty acids and tocopherols. Essential fatty acids can range between 55-65% of total fatty acids. Linoleic acid typically comprises approximately 50% of total fatty acids while linolenic is approximately 7% of total fatty acids. Soybean oil on the market today has commercial uses as a cooking and salad oil and serves as oilstock for conversion to semi-solid fat for frying, margarine and baking applications. Its conversion to semi-solid fat is necessary to achieve greater stability and functionality for heat seasoning applications such as frying and baking.
Soybean oil possesses a natural profile of unsaturated fatty acids, lecithin and plant sterols, all of which are desirable for lowering blood cholesterol levels. The cholesterol-lowering properties of soybean oil has been well-documented in animal and human studies. However, because of its relatively high content of polyunsaturated fatty acids, the health benefits of soybean oil as well as other highly polyunsaturated vegetable oils are being challenged by the more monounsaturated vegetable oils such as olive oil due to their greater stability. In addition, the recent concern about the greater susceptibility of low density lipoprotein (LDL) particles to in-vitro oxidation in humans consuming high polyunsaturated vegetable oils such as soybean oil has led to increased consumption of the more monounsaturated vegetable oils1,2.
Lecithin. Lecithin is obtained during the processing of crude soybean oil. Two commercial forms of lecithin are most widely available: lecithin and deoiled lecithin. The primary components of lecithin are phosphatidylcholine followed by phosphatidylethanolamine and phosphatidlyinositol. Lecithin serves to impart a variety of functional properties in numerous finished products as well as to provide a source for dietary choline and essential fatty acids.
Soybean lecithin, containing predominantly unsaturated fatty acids compared to egg lecithin, has been shown to reduce blood cholesterol levels in both animal and human studies although these studies generally attribute soy lecithin's cholesterol-lowering properties to its linoleate content3-7. More recent studies in animals8 and humans9 suggest that these cholesterol-lowering properties are not completely explained by its fatty acid composition. In addition, most of these studies indicate that the reduction in blood cholesterol levels is confined to decreases in low density lipoprotein cholesterol (LDL-C) with increases or no effects on high density lipoprotein cholesterol (HDL-C)3,8-9. This is an important finding since LDL-C levels are positively correlated with coronary heart disease (CHD), whereas HDL-C is negatively associated with CHD.
Tocopherols and phytosterols. Tocopherols (vitamin E) can vary to a significant extent in finished retail soybean oils depending on the degree of deodorization. On average, commercial retail brands of soybean oil contain anywhere from 5-20% of the Daily Value for vitamin E per serving. Tocopherols removed during oil deodorization can be recovered as natural-source vitamin E that can be used in a variety of applications ranging from food antioxidants to food ingredients to vitamin supplements. Vitamin E serves primarily as an antioxidant in lipophilic regions in living systems. Higher levels of dietary vitamin E have been associated with decreased risk for diseases linked with oxidant stress.
Along with tocopherols, soybean oil deodorization removes phytosterols, which are plant versions of cholesterol. This structural similarity led to the study of phytosterols on cholesterol metabolism in humans. Research has shown that phytosterols are absorbed significantly less in the gut compared to cholesterol. In part, this phenomenon of phytosterols may contribute to interference of cholesterol absorption/reabsorption. At levels of consumption greater than that present in the refined oil, phytosterols demonstrate the ability to lower serum cholesterol levels in certain individuals. An example of this is beta-sitosterol, an isolated soybean phytosterol that is subsequently esterified to fatty acids and incorporated into margarines, which has been recently shown to have significant LDL-C cholesterol-lowering properties without accompanying significant changes in HDL-C in humans10,11.
Soy Protein. The simplest and least expensive form of soy protein is soy flour. It is produced by screening and grinding the defatted flakes (meal). The flour composition is approximately 50% protein, 10-15% insoluble carbohydrates (fiber) and ~15% soluble carbohydrates (oligosaccharides). It carries a "beany" flavor due to the high concentration of oligosaccharides (sucrose, stachyose, raffinose, others). Soy flours have many commercial uses in products such as baked goods, snack foods and pet foods. The health benefits of the soy flours are the same as the soy protein products; however, the commercial desirability for them in consumer food products is diminished due to the flatulence associated with the digestion of the soluble carbohydrates (a minor problem that should not be ignored).
The most expensive form of soy protein is the isolate, which was first developed in the 1940's. These isolated soy proteins are manufactured from defatted soy flakes by separation of the soy protein from both the soluble and insoluble carbohydrate of the soybean. Isolated soy proteins are probably the most versatile of the soy proteins and thus find use in a broad range of food products. These high protein (90%+), spray-dried products are typically light in color, bland in flavor and possess functional properties.
Isolated soy protein functionality is determined, in large part, on the specific processing parameters for the given product. Heat, homogenization and pH are three factors that greatly influence the functional characteristics of the finished proteins. Gelation, emulsification and viscosity are important functional characteristics for isolated soy proteins. Viscosity is important in a wide range of beverage applications. Enzyme-modification is used to produce very low viscosity isolated soy protein for production of beverages and infant formula. Viscosity and gelation properties are critical in the manufacture of soy yogurt. In cream soups and high fat sauces, emulsification and viscosity are important to the stability and texture of the finished products. Processed meat applications require isolated soy proteins with good emulsification and gelation properties. In all, understanding the specific desired characteristics of the finished food product that is being developed is essential when selecting the appropriate isolated soy protein. Improper selection of the isolated soy protein often ends in product failure.
In the 1960's, soy protein concentrates were developed to overcome the problems of flours at a lower cost than isolates. This soy protein product is approximately 70% protein and 20% insoluble dietary fiber. Soy protein concentrates are traditionally manufactured using aqueous-alcohol to remove the soluble sugars from the defatted soy flakes (soy flour). This process results in a protein with low solubility and a product that can absorb water but lacks the ability to gel or emulsify fat. Traditional alcohol-washed concentrates are used for protein fortification of foods as well as in the manufacture of textured soy protein concentrates. Functional soy protein concentrates bind water, emulsify fat and form a gel upon heating. Functional soy protein concentrates can be produced from alcohol-washed concentrate using heat and homogenization followed by spray-drying or produced using a water-wash process at an acid pH to remove the soluble sugars followed by neutralization, thermal processing, homogenization and spray-drying. Functional soy protein concentrates are widely used in the meat industry to bind water and emulsify fat. These proteins are also effective in stabilizing high fat soups and sauces.
As for health benefits, there is a large body of evidence that indicates that replacing animal protein with soy protein lowers LDL-C in animals12-15 and humans (see review articles 16-18) without affecting HDL-C levels. One review paper suggests that up to 60% of the cholesterol-lowering properties of soy protein is largely attributed to the isoflavone content16. However, in a recent Letter to the Editors by Sirtori et al.19, it was argued that the upregulation of LDL receptors, a potential mechanism of action for soy protein could not be explained by isoflavones and also that the major Italian studies demonstrating the significant cholesterol-lowering properties of soy protein in Type II hypercholesterolemic patients was with soy protein preparations essentially devoid of isoflavones. Thus the mechanism of action for the cholesterol-lowering properties of soy protein has not yet been established with any certainty. The cholesterol-lowering properties of soy protein concentrates have not been evaluated in humans but have been demonstrated to reduce LDL-C in animals13,14.
Isoflavones. The mighty soybean also yields important phytochemical compounds known as isoflavones. Isoflavones are present in varying degrees in each class of soy proteins but to a lesser extent in proteins prepared using alcohol washing (concentrates). Subsequently, the alcohol wash can be used as a source for obtaining an isoflavone-enriched soy extract. Isoflavones have received significant attention due to their actions as phytoestrogens.
Isoflavones are very weak estrogens resulting in either anti-estrogenic or estrogenic properties depending on the estrogen status of the individual. Activity can vary from 1/1000 to 1/100,000 the activity of estrogen. Isoflavones exist in soy foods with sugar (glucoside) or without a sugar (aglycone) molecule attached. Isoflavones in the aglycone form are absorbed in the human intestinal tract. The sugar fraction is subsequently cleaved by intestinal enzymes to release the aglycone for subsequent absorption. The two major isoflavones in soy foods are the agyclones daidzein and genistein and their respective glucosides, genistin and daidzin. In general, soy foods contain more genistein than daidzein although this may vary depending upon the source of soy food. On a dry weight basis, the amount of isoflavones in soy food can vary from 2-4 mg/gr protein so an average serving of a soy product should provide approximately 30-40 mg/serving.
The biological effects of isoflavones have been excellently reviewed recently by Setchell20,21. These reviews describe biological effects of soy isoflavones in cultured cells, animal and human studies, which include LDL cholesterol-lowering, reduced incidence of osteoporosis, breast and prostate cancer and decreased platelet aggregation. Less well-established are the reported beneficial effects of soy isoflavones on post-menopausal symptoms such as night sweats and hot flashes.
The soybean has been part of man's life for thousands of years. It has been manipulated genetically for decades to provide us with a higher yielding, more easily farmed variety. Recently, it has both contributed to and complicated the lives of many professionals, especially in the agriculture, nutraceutical, and functional foods industries. Expect the soybean, and all its offerings, to get more complicated before the roar dies down.
References:
1Reaven, PD, Witzum JL. "Oxidized low density lipoproteins in atherogenesis:role of dietary modification," Annu Rev Nutr. 1996;16:51-71.
2Parthasarathy S, Khoo JC, Miller E, Barnett J, Witzum JL, Steinberg D. "Low density lipoprotein rich in oleic acid is protected against oxidative modification:implications for dietary prevention of atherosclerosis," Proc Natl Acad Sci U.S.A. 1990;87:3894-3898.
3Childs MT, Bowlin JA, Oglivie JT, Hazzard WR, Albers JJ. "The contrasting effects of dietary soya lecithin product and corn oil on lipoprotein lipids in normolipidemic and familial hypercholesterolemic subjects," Atherosclerosis 1981;38:217-222.
4Greten H, Raetzer H, Stiehl A, Schettler G. "The effect of polyunsaturated phosphatidylcholine on plasma lipids and fecal sterol excretion," Atherosclerosis 1980;36:81-88.
5Kesaniemi YA, Grundy SM. "Effect of dietary polyenolphosphatidylcholine on metabolism of cholesterol and triglycerides in hypertriglyceridemic patients," Am J Clin Nutr 1986;43:98-107.
6O'Brien BC, Andrews VG. "Influence of egg and soybean phospholipids and triacylglycerols on serum lipoproteins," Lipids 1993;28:7-12.
7Prack M, Sanborn T, Waugh D, Simkin H, Bennett Clark S, Small DM. "Effects of polyunsaturated lecithin on plasma and lipoprotein cholesterol and fatty acids in normal men," In: Perkins EG, Visek WJ, editors. Dietary Fats and Health, Champaign IL, American Oil Chemist's Society 1983;689-697.
8Wilson TM, Meservey CM, Nicolosi RJ. "Soy lecithin reduces plasma lipoprotein cholesterol and early atherogenesis in hypercholesterolemic monkeys and hamsters: beyond linoleate," Atherosclerosis 1998;140:147-153.
9Wojcickl J, Pawlik A, Samochawiee L, Katdenska M, Mysliwiec Z. "Clinical evaluation of lecithin as a lipid -lowering agent: Short Communication," Phytother Res. 1995;9:597-599.
10Westrate JA, Meijer GW. "Plant sterol-enriched margarines and reduction of plasma total and LDL-cholesterol concentrations in normocholesterolemic and mildly hypercholesterolaemic subjects," Eur J Clin Nutr 1998;52:334-343.
11Hallikainen MA, Uusitupa MIJ. "Effect of 2 low-fat stanol ester-containing margarines on serum cholesterol concentrations as part of a low-fat diet in hypercholesterolemic subjects," Am J Clin Nutr 1999;69:403-410.
12Terpstra AHM, Holmes JC, Nicolosi RJ. "The hypocholesterolemic effect of dietary soy protein vs casein in hamsters fed cholesterol-free or cholesterol-enriched semipurified diets," J Nutr 1991;121:994-997.
13Terpstra AHM, Laitinen L, Stucchi AF, Nicolosi RJ. "The effect of semipurified diets containing two levels (20% and 40%) of either casien or soybean protein isolate and concentrate on plasma lipids in hamsters," Nutr Res 1994;14/6:885-895.
14Nicolosi RJ, Wilson TA. "The anti-atherogenic effect of dietary soybean concentrate in hamsters," Nutr Res 1997;17(9):1457-1467.
15Wilson TA, Nicolosi RJ. "Addition of guar gum and soy protein increases the efficacy of the American Heart