Formation and Reactivity of Carbon-Centered Radicals in Isolated Soy Proteins

Soybeans are the second largest food crop in the U.S.A. with about 87 million metric tons produced in 2006. Solid-state electron paramagnetic resonance (EPR) spectroscopy of commercial samples of isolated soy proteins (ISP) revealed a symmetrical free-radicals signal typical of carbon-centered radicals (g=2.005) ranging from 2.96 x 1014 to 6.42 X 1014 spins per gram. The level of free-radicals in ISP was 14-times greater than similar radicals in sodium caseinate, 29-times greater than egg albumin and about 100-times greater levels than casein. Nine soy protein powdered drink mixes contained similar types of free-radicals, but at levels up to 4.10 x 1015 spins per gram of drink mix, or up to 6.4-times greater than the highest free-radical content found in commerciallSP. Laboratory-prepared ISP samples contained trapped radicals similar to the levels in commercial ISP samples. Preliminary stability studies of the ISP stored in the dark at 23°C in sealed containers found that the level of free-radicals increase by as much as 35-fold during the first 9 weeks of storage. The high levels of free-radicals in the commercial soy protein products examined can contribute to numerous degradation reactions once the protein is hydrated, and consuming such high levels of free-radicals may have harmful consequences in humans. This is the first time the levels of free-radicals in commerciallSP, and food products high in ISP, has been reported. It is also the first time the large increase in free-radicals during ISP storage (which appears to account for the majority of free-radicals) has been reported. This research will investigate the mechanism that free-radicals in ISP are formed and stabilized within the "dry" protein. The type and rates of reactions that are catalyzed by the release of free-radicals from ISP once the protein is hydrated will also be determined. Because the reactions catalyzed by freeradicals can potentially have harmful effects to humans as well as contribute to the degradation of food components during processing and storage, minimizing the occurrence of these radicals in ISP will improve the safety and nutritional quality of ISP. A basic understanding of free-radicals chemistry in ISP is necessary to develop commercially practical solutions to minimizing the occurrence of the observed free-radicals.