Chemical modification of isoflavones in soyfoods during cooking and processing

principal chemical forms of isoflavones in soybean are their 6 "-O-malonyl- beta-glucoside (60MalGlc) conjugates. Experiments were carried out to deter mine the best conditions for extraction of isoflavones from soyfoods and th e effects of commercial processing procedures and of cooking on isoflavone concentrations and composition. Hot alcohol extraction of ground soybeans d eesterified 60MalGlc conjugates. Although room temperature extraction slowe d the conversion, extraction at 4 degrees C for 2-4 h led to the highest yi eld of 60MalGlc conjugates and the lowest proportion of beta-glucoside conj ugates. Analysis of soyfood products by reversed-phase HPLC-mass spectromet ry revealed that defatted soy flour that had not been heat treated consiste d mostly of 60MalGlc conjugates; in contrast, toasted soy flour contained l arge amounts of 6 "-O-acetyl-beta-glucoside conjugates, formed by heat-indu ced decarboxylation of the malonate group to acetate. Soymilk and tofu cons isted almost entirely of beta-glucoside conjugates; low-fat Versions of the se products were markedly depleted in isoflavones. Alcohol-washed soy-prote in concentrates contained few isoflavones. Isolated soy protein and texture d vegetable protein consisted of a mixture of all 3 types of isoflavone con jugates. Baking or frying of textured vegetable protein at 190 degrees C an d baking of soy flour in cookies did not alter total isoflavone content, bu t there was a steady increase in beta-glucoside conjugates at the expense o f 60MalGlc conjugates. The chemical form of isoflavones in foods should be taken into consideration when evaluating their availability for absorption from the diet.