Evaluating the in vitro bioaccessibility of isoflavones (aglycones, non-conjugated glucosides, and conjugated glucosides) from different soy products under simulated gastrointestinal digestion

Epidemiological studies suggest that soy consumption may be associated with prevention of certain chronic diseases such as cancer, cardiovascular disease, osteoporosis and postmenopausal symptoms. It has been reported that the health benefits of soy are largely attributed to isoflavones known as phytoestrogens, and soy isoflavones have gained considerable attention over the past two decades. However, these physiological effects by soy isoflavones are quite variable in different studies. The food matrices, processing procedures, and their isoflavone profiles are believed to be important factors for this inconsistency. In the present study, to examine the influence of these factors on isoflavone bioavailability, in vitro bioaccessibility of 6 isoflavones (daidzin, genistin, malonyl-daidzin, malonyl-genistin, daidzein, and genistein) was estimated from 7 different soy products: dried soy (Glycine max) powder, boiled soybean, soybean ethanol extract, tofu, soy milk, isolated soy protein, and cheonggukjang (a fermented soybean paste). To simulate in vitro digestion, digestive steps are carried out sequentially; that phase was separated into five parts (mouth, stomach, duodenum, jejunum, and ileum). Each soy product showed different isoflavone profile while each isoflavone form exhibited diverse release and stability level according to the soy products. Addition of glucose and malonyl group to isoflavone elevated release in digestion. Malonylglucoside conjugates showed the highest release level, but in a stomach step, it was sharply decreased. The aglycon forms were little released in digestion. The stability of isoflavone in soy milk or extract was relatively low, which suggests food matrix could be an important factor for isoflavone stability during the digestion process. These results demonstrate that the release and stability of soy isoflavones in digestive tract could be strongly influenced by food matrices and isoflavone forms.

Keywords: soybean, isoflavone, bioaccessibility, simulated gastrointestinal digestion