In a previous study, feeding an isolated soy protein (ISP)-based diet to rats was found to reduce colon cancer risk as assessed by a reduced number of colonic precancerous lesions. However, this same ISP, after storage at room temperature for >2 years, increased the number of precancerous lesions (Gallaher et al., 1996). We hypothesize that this increase was due to the development of Maillard reaction products in the ISP during storage. Thus, the objective of this study was monitor development of the Maillard reaction during storage of ISP and delactosed whey protein concentrate. Proteins were stored at different water activities (0.22, 0.33, 0.55) and temperatures (22, 30, 45 °C) with and without glucose (5% w/w) and increases in browning (A = 420 nm) and fluorescence (λex 365 nm/λem 475 nm) determined. In the absence of glucose, only soy protein underwent browning; otherwise the rate of browning and fluorescence increased with increasing temperature and water activity. To investigate why ISP underwent browning in the absence of glucose while whey protein concentrate did not, the reaction of genistein was investigated. Genistein is an isoflavone with putative chemoprotective properties found in ISP but not in whey. Genistein (2 mM) was incubated alone or with lysine (2 mM) in buffer. The absorbance (A = 420 nm) of the reaction mixtures and genistein concentration was measured over time. It was found that genistein underwent reaction both alone and in the presence of lysine. The rate of browning was found to parallel the rate of genistein loss, suggesting that genistein plays a role as a reactant in nonenzymatic browning reactions. This suggests that long-term storage of ISP will lead to the loss of genistein and potentially result in the development of carcinogens.
- Isolated soy protein
- Maillard reaction