The concentration and the composition of soy isoflavones in soy foods and soy-containing foods vary markedly. These differences may result from variability in formulation, from changes induced by the raw material being used and the processing methods and techniques, post processing changes during distribution and storage. These changes may occur as early as during the storage of the raw materials. Long-term storage of 15 soybean cultivars showed minor decrease in the total isoflavones concentration over a period of 1 to 3 y of storage (Lee et al. 2003). However, the concentrations 6–O–malonyldaidzin and 6–O–malonylgenistin decreased dramatically during the same storage period. Similar decrease in the malonyl derivatives was also reported for soybean seeds stored at 80°C and was accompanied by the formation of acetyls and glycosides (Kudou et al. 1991). Hence, the malonyl derivatives appear to be most labile in the soybean raw material. The effects of processing techniques on the distribution of isoflavones were also investigated in the manufacturing of tempeh, soymilk, tofu, and protein isolate (Wang et al. 1998). Wang and Murphy (1996) found that the manufacturing steps causing significant losses of isoflavones were 12% during soaking and heat processing (49%) in tempeh production, 44% during coagulation in tofu processing, and 53% during alkaline extraction in soy protein isolate (SPI) production. Malonyldaidzin and malonylgenistin decreased after soaking and cooking in the production of tempeh, soymilk, and tofu. Concomitantly, acetyldaidzin and acetylgenistin were generated during heat processing. Alkaline extractions in protein isolate processing caused the generation of daidzein and genistein, probably through alkaline hydrolysis. The preparation of SPI resulted in a loss of 20% of the isoflavones found in the soy flour (Wang et al. 1998). Processing significantly affected the retention and distribution of isoflavones in
food. A recent study examined genistein and daidzein stability in aqueous media at conditions simulating commercial sterilization processes (Ungar et al. 2003). The stability of genistein and daidzein was studied at 120°C in alkaline (pH, 9) and neutral environments (pH, 7). A difference in the stability of genistein and daidzein was detected, whereas in alkaline solution, genistein concentration was reduced by 60%, only a minor 15% reduction was observed for daidzein. At neutral pH, daidzein was less stable than genistein and its concentration decreased by 40%, compared with 22% for genistein. High temperature and pressure reduced the total isoflavones content in corn-soy mixtures (Singletary et al. 2000). The study of Mahungu et al. (1999) on extrusion at high temperatures (110°C, 130°C, and 150°C) reported a decrease in the overall isoflavones content of SPI and corn mixtures. In addition, in their study, the loss of the daidzein and its conjugates (44%) was higher than that of genistein (33%). As in the study of Lee et al. (2003), most of isoflavone degradation in the study of Mahungu et al. (1999) was attributed to loss of the malonyl derivatives. The degradation and modification of soy isoflavones are not limited to high water content products such as soymilk, tofu, and tempeh. Coward et al. (1998) studied the chemical modification of isoflavones in soy foods during cooking and processing. Analysis of soy food products revealed that defatted soy flour that had not been heat-treated consisted mostly of malonyl conjugates. In contrast, toasted soy flour contained many 6-O-acetyl-
β-glycoside conjugates formed by heat-induced decarboxylation of the malonate group to acetate. Baking or frying of textured vegetable protein at 190°C and baking of soy flour in cookies did not alter total isoflavone content, but there was a steady increase in β-glycoside conjugates at the expense of malonyl conjugates. The stabilities of the 3 isoflavones at different heating temperatures in their dehydrated form were investigated by Xu et al. (2002). Daidzin, glycitin, and genistin lost 26%, 27%, and 27% of their original concentration, respectively, after 3 min at 185°C. Heating at temperatures above 135°C produced acetylated daidzin and genistin, daidzein, glycitein, and genistein. The rate of formation of acetyl derivatives was higher than the rate of loss of a glycoside group to form daidzein and genistein. They also found that the stability of
daidzein was higher than that of glycitein or genistein. Changes in isoflavones composition and content may also occur during storage of soy products. An interesting report by Hayes et al. (2001) brings evidence for changes in soy isoflavones after ultra high-temperature processing of chocolate beverages. There was a decrease in the malonyl derivatives and a continuous change in the isoflavone profile within the various families. They also pointed out that these changes continued during storage and were affected by storage temperature. Eisen et al. (2003) followed the changes in isoflavones content during storage of soy milk. A decrease in genistin content was observed at elevated temperatures, and also during 6 month storage experiments at ambient conditions. The mechanism by which isoflavones are lost is somewhat obscure. Wang et al. (1990) reported that when standard genistein was mixed with dextrose, fructose, maltose, and sucrose, it formed conjugates with very high ultraviolet (UV) absorption. The amount of these conjugates formed was proportional to the amount of added sugar. Another potential route for isoflavone degradation in composite solutions was reported by Davies et al. (1998), suggesting that isoflavones can react in Maillard-type reactions. These experiments were performed in model systems. Because Maillard reaction products are known to be potential carcinogens, undesirable products may be produced in this reaction.