Effect of storage conditions on carbon-centered radicals in soy protein products

Using electron paramagnetic resonance (EPR) spectroscopy, the levels of carbon-centered radicals in retail samples of isolated soy protein (ISP), soy protein concentrate (SPC), and powdered soy milk were estimated to contain from 6.12 x 10(14) to 1.98 x 10(15) spins/g of soy product. Roasted soy nuts contained about 5.70 x 10(15) spins/g. The peak to peak line width of the carbon-centered radicals from soy nuts was about 10 gauss, whereas ISP samples with a similar peak height had a peak to peak line width of about 8 gauss. Retail snack bars containing ISP, SPC, and/or roasted soy nuts with a total protein content of either 13, 21, or 29% contained 5.32 x 10(14), 6.67 x 10(14), and 5.74 x 10(14) spins/g of snack bar, respectively. Levels of carbon-centered radicals in newly prepared samples of ISP were much lower than levels in the retail soy protein products and levels previously reported for commercial ISP and laboratory ISP samples. The levels of radicals in ISP samples increased over a 12-25 week period of storage in the dark at 22 degrees C and exposed to air from about 8.00 x 10(13) spins/g immediately after preparation to 9.95 x 10(14) spins/g of ISP. Storing the ISP samples under nitrogen at 22 degrees C greatly reduced the increase in radical content, whereas storing the ISP in 99.9% oxygen at 40 degrees C accelerated the formation of stable carbon-centered radicals. ISP samples hydrated at either 22 or 92 degrees C, rapidly frozen, and dried lost about 92% of the trapped radicals. The level of carbon-centered radicals in these same ISP samples immediately began to increase during subsequent storage exposed to the air and gradually returned to similar levels obtained before they were hydrated.