Soybean (U.S.) or soya bean (UK) (Glycine max) is a species of legume native to East Asia that may occur in various sizes, and in many hull or seed coat colors. Its high oil content (around 20%) has led to its classification as an oilseed rather than a pulse. It is also characterized by a high content of high-quality protein (40%), which explains its use as a meat substitute. Soybean seeds are not directly consumed by humans, but only as various products obtained by fermentation, soaking, filtration, frying or boiling. Soy products have been traditionally consumed in Asian countries. Their consumption has increased during the recent years in Western countries due to their documented health effects and to those of isoflavonoids in particular.
Three different isoflavonoids have been described in soybean and soy products, two isoflavones, daidzein and genistein, and a methoxyisoflavone, glycitein, found in a typical ratio of 45:45:10 (403). Daidzein and glycitein are concentrated in the hypocotyl portion of the soybean, whereas genistein is more evenly distributed throughout (404). These three isoflavones are present in four different forms: aglcyone, glucosides (respectively called daidzin, genistin and glycitin), malonylglucosides and acetylglucosides. In soybean seed, 68% of the isoflavones are in the form of the highly thermally labile 6’’-O-malonylglucosides and less then 5% are aglycones. The 6’’-O-acetylglucosides are absent in soybean and only found in some soy products and derive from 6’’-O-malonylglucosides (405, 406). Labile isomers of malonyldaidzin, malonylgenistin and malonylglycitin with the malonyl group attached at the 2’’, 3’’ or 4’’ positions have also been described (407). The structures of other derivatives of genistein, daidzein and glycitein have been described, although not fully characterized (408). Low amounts of lignans have also been determined in soy products (most often lower than 0.05 mg/100 g).
The different methods for extraction and analysis of isoflavones were recently reviewed (409, 410). Different hydrolysis procedures, either acid or enzymatic, have been used to estimate isoflavonoids in soy foods as aglycones (410, 411, 412). Basic hydrolysis only cleaves ester bonds, transforming malonyl and acetylglucosides into glucosides (413). Estimates using basic hydrolysis have not been included in Phenol-Explorer.
Total isoflavonoid content in soybean seeds is around 200 mg/100 g DW, but these concentrations vary depending on genetic, environmental and processing factors (405, 414). Content in soy products depends on the process used (415). Boiling does not usually affect the total isoflavonoid content (403), in contrast to frying, roasting or fermentation which may decarboxylate the conjugates and degrade the aglycones (405, 414, 416, 417). The extent of the conversion of malonylglucosides into aglycones in fermented soy foods depends on the inoculum (418). Defatting results in a higher concentration of isoflavones without altering isoflavone conjugation, while extrusion causes a degradation of isoflavones (166). Storage of different soybeans cultivars during three years at room temperature resulted in a 50% reduction of the total isoflavonoid content, while storage at low temperature reduced this loss to a 35% (419).
Polyphenol contents have been determined in both the so-called first and second generation soy products. First generation soy products refers to minimally processed products made from whole soybean. Second generation soy products to products in which extracted soy proteins have been incorporated in small proportions into complex food systems.
Soy flour has a high isoflavonoid content, of 467 mg/100 g. As in the seed, the malonylglucosides are the predominant forms, followed by the glycosides. Other isoflavonoids, such as biochanin A, coumestrol and formonetin, have not been detected in soy flour (420). Different phenolic acids, such as caffeic, p-coumaric, ferulic acid and syringic acid have been detected in soybean flour, with concentrations ranging from 6 to 30 mg/100 g (341). Soybean flour produced from dehulled beans has a higher isoflavonoid content than flour produced from the whole soybean, due to the lower isoflavonoid content in the hulls, as compared to cotyledons and hypocotyls (421).
The traditional Japanese dish edamame consists of green immature soybeans boiled with their green pod in salted water (422). Isoflavonoid contents in immature soybeans are lower than those in mature soybean, with an average of 45 mg/100 g. Again, the malonylglucosides, followed by the glucosides, are the most common forms. 6’’-O-acetylgenistin has been detected in edamame, with a mean concentration of 0.52 mg/100 g frsh weight.
Soybean sprouts are the fresh sprouts of germinated soybeans. The content in isoflavonoids is much smaller than in the mature soybean. None of the isoflavonoids shows a concentration higher than 5 mg/100 g. The proportions between the different isoflavonoid forms are similar to those in the mature soybean. Cooking of soybean sprouts causes a reduction of the total content of isoflavonoids from 60 to 20 mg/100 g (408).
Soy milk is commonly made by soaking whole beans in alkaline water followed by the removal from the extract of the insoluble materials (okara). Soy milk is also commonly prepared from soy protein isolates. Whole bean soy milks have a total isoflavonoid content significantly higher than that of soy milks produced from protein isolates. They also show a higher variability of content between brands. Soy milk has a total isoflavonoid content of 16 mg/100 ml with the aglycones accounting for 5.5 % of the total. Alternative treatments in the preparation of soy milk omit the separation of insoluble okara, leading to the obtention of so-called entire bean soy milks which show an isoflavonoid content close to that of soy milk produced from protein isolates (423).
Two different heating treatments can be applied to soy milk during manufacturing: a traditional one with 20-30 min boiling, and ultra high-temperature (UHT) treatment with heating at up to 150 °C in less that 1 min (424). Both processes modify the proportions of the different forms of isoflavonoids. Glucosides are the most abundant isoflavonoids in heated soy milks, followed by malonylglcucosides. The longer the treatment, the higher is the proportion of glucosides (425). The UHT process also causes a significant increase in the content of acetylglucosides (424, 426) The influence of these two processes on the total isoflavonoid content is not clear as results obtained by different authors have been contradictory (424, 426, 427).
Tofu or soy bean curd is a food of Chinese origin, made by coagulating soy milk, and then pressing the resulting curds into blocks. The size of the curd and the length of pressing time determine the style of tofu made, soft, regular, firm or dry. The denser the texture of tofu, the more isoflavonoids it contains (428).Tofu processing requires longer heat times than soy milk, leading to the formation of higher amounts of acetylglucosides and free aglycones (426). Losses of isoflavone aglycones during tofu processing have been shown to be temperature dependent (429). Calcium sulfate, calcium chloride, magnesium sulfate and magnesium chloride are some of the coagulants used in the indusrial preparation of tofu. The coagulant used influences the total isoflavonoid content in tofu, the highest concentrations being observed with the more efficient coagulants (e.g. calcium sulfate)(427, 430).
Tofu can be eaten raw but it can also be cooked in different ways. Total isoflavonoid content in boiled tofu was not significantly different to that of raw food (136, 426). Some authors did not either find differences in the proportion between the different isoflavone forms, while others found an 8% reduction in malonylglcucosides and a concomitant increase in the other forms (403). Another way of preparing tofu, common in many parts of East and Southeast Asia involves deep frying in vegetable oils. This results in a two to three fold reduction of total isoflavonoid content as compared to raw tofu, and to an increase of acetylglucosides now accounting for 10 to 18% of the total isoflavonoids (3% in raw tofu) (428).
Tofu can be fermented by Actinomycor elegans to obtain the traditional Chinese product sufu (also called fermented soy cheese). Sufu has a much lower content of total isoflavonoids than raw foods, and the glucosides are largely hydrolysed to their aglycones which account for 97% of total isoflavonoids (a maximum of 15% in raw tofu)(418, 428).
Tempe or tempeh is a fermented product made from the whole soybean, which has been dehulled, cracked and cooked in water, after what it is incubated with the Rhizopus mold giving place to a compact, cake-like product. Tempe is, among the fermented soy foods, the one with the highest content of total isoflavonoids (150 mg/100 g) (418, 426). It is also relatively rich in isoflavone aglycones and acetylglucosides. Tempe is normally consumed fried, boiled, steamed or roasted. A 21 and 58% decrease in the total content of respectively daidzein and genistein has been reported for fried tempe when compared to raw tempe (431). Boiling of tempe did not affect the contents of isoflavonoids (426).
There is a wide variety of fermented bean pastes in Asian countries, such as miso and natto consumed in Japan, or doenjang and cheonggukjang consumed in Korea. All are commercially products. To produce miso, the cooked soybeans are fermented with Aspergillus oryza (sometimes together with other cereals), to give a paste that is consumed in soups. Natto is produced by fermenting cooked soybeans with Bacillus subtillis until they develop a sticky, viscous coating. Doenjang and cheonggukjang are obtained by fermenting dried soybeans with Bacillus subtilis. Fermentation can be quite long, reaching up to 60 days for miso or years for doenjang.
Differences in manufacture process are reflected in their different content and distribution of isoflavonoids. Natto and cheonggukjang contain predominantly isoflavone glucosides, while miso and doenjang, contain a high proportion of isoflavone aglycones (428, 432). Miso, nato and cheonggukjang have a total isoflavonoid content of 60-250 mg/100 g, while the longer fermentation time of doenjang results in a near total loss of isoflavonoids. Also, three orthodihydroxyisoflavone derivatives (7,8,4’-trihydroxyisoflavone, 6,7,4’-trihydroxyisoflavone and 7,3,4’-trihydroxyisoflavone) were described in five-year old aged doenjang (433).
Soy sauce is produced by fermenting soybeans with the mold Aspergillus oryzae and Aspergillus soyae along with roasted grain, water and salt. Soy sauce has a very low isoflavonoid content, about 1 mg/100 mL (426). It is most likely that isoflavonoids are mainly lost during the defatting step in the manufacture process (434).
They are eaten as other cereal flakes (besides being an ingredient for industry). Their isoflavonoid content is about 30 mg/100 g, with genistein being twice more abundant than daidzein (435).
Soybean oil is produced from soybeans by mechanical or solvent extraction. Crude soybean oil is further filtered or refined to produce salad and cooking oils. Analysis of total polyphenols by Folin assay showed a very low polyphenol content, lower than 1 mg/100 g (436, 437).
Soy nuts are produced by frying whole and hydrated soybeans. They are either eaten as snacks or used to produce a paste similar to peanut butter. Their isoflavonoid content is about 200 mg/100 g. The roasting process causes a modification of the proportions between the different forms of isoflavones when compared to the original seed, with a slight decrease in the glucosides and a slight increase in the acetylglucosides and aglycones (403).
Soy meat susbstitutes
A wide variety of soy meat susbstitutes have been developed, using different soy protein concentrates as ingredients, but also other soy foods such as tofu or tempe. Total isoflavonoid content of soy sausages, burgers, baocn bits or meat are usually lower than 50 mg/100 g with glucosides as the main forms. This relatively low isoflavonoid content of soy meat substitutes has been attributed to the alcoholic extraction of the textured soy protein (a procedure commonly used in the food industry to increase the digestibility and palatability of the soy protein), and to a dilution effect resulting from the addition of other ingredients (404, 438).
Cooking of soy meatballs or burgers, in a conventional or microwave oven, reduced the isoflavonoid content by 6-7%; but it did not modify the distribution of the different forms (408). In another study, cooking of soy meat substitutes did not affect total isoflavonoid content when differences in moisture content were taken into account, but it induced a decrease in the proportion of malonylgucosides and an increase in that of glucosides (426).
Soy yogurt and soy pudding
Soymilk yogurt is made by fermenting pasteurized soymilk with Lactobacillus bulgaricus or other bacteria in a way similar to that used to make cow milk yogurt. Soy yogurt has an isoflavonoid content higher than that of soy milk, of about 80 mg/100 g, with glucosides as the main form of isoflavones.
Another dessert developed from soy milk is pudding. The isoflavonoid content of soy pudding is lower than that of soy yougurt, being around 30 mg/100 g, again with glucosides as the main forms.
A wide variety of cheeses have been produced from soy, imitating many cow milk cheeses, such as mozarella, parmesan or cheddar. They were not differentiated in the database. The isoflavonoid content of soy cheese is quite low, below 15 mg/100 g, what has been related to the fact that other non soy ingredients like animal fat or vegetable oil (403). When different varieties of soy cheese and different brands for a same variety of soy cheese were compared, it was found that the variety influenced the isoflavonoid content as much as the brand (418).