The bulk of data available about the processing of phytoestrogens again reflects the primary interest in the isoflavones subclass of phytoestrogen.
Before 1990 the composition of isoflavones in soy foods was thought to be largely determined by whether the food had been fermented. Fermented foods (e.g., miso and tempeh) contain the unconjugated isoflavones agycones, while non-fermented food (e.g., tofu, soy flower, and soy milk) contain the conjugated glucoside. Subsequent experiments have demonstrated that fermentation of soy decreased the isoflavone content of the food product, but increased the urinary isoflavonoid recovery, suggesting that fermentation increases availability of isoflavones in soy (Slavin et al., 1998).
The principal chemical forms of isoflavones in soybean are their 6"-O-malonyl-p-glucoside (6OMalGlc) conjugates. Hot alcohol extraction of ground soybean yields deesterified 6OMalGlc conjugates. Although room temperature extraction slows the conversion, extraction at 4°C for 2 to 4 hours led to the highest yield of 6OMalGlc conjugates and the lowest proportion of p-glucoside conjugates. Analysis of soy food products by reversed-phase high performance liquid chromatography (HPLC)-mass spectrometry reveals that defatted soy flour that had not been heat treated consisted mostly of 6OMalGlc conjugates; in contrast, toasted soy flour contained large amounts of 6"-O-acetyl-p-glucoside conjugates, formed by heat-induced decarboxylation of the malonate group to acetate. Soymilk and tofu consist almost entirely of p-glucoside conjugates; low-fat versions of these products were markedly depleted in isoflavones. Alcohol-washed soy-protein concentrate contains few isoflavones. Isolated soy protein and textured vegetable protein consisted of a mixture of all three types of isoflavone conjugates. Baking or frying of textured vegetable protein at 190°C and baking of soy flour in cookies does not appear to alter total isoflavone content, but produces a steady increase in p-glucoside conjugates at the expense of 6OMalGlc conjugates (Coward et al., 1998).
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