Q. Lei

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.

Metastable Radicals and Intrinsic Chemiluminescence from Soy Proteins

UNLABELLED Chemiluminescence from various powdered food proteins were examined without the addition of any external source of free radicals or luminescent agents. In the solid-state, soy and whey proteins produced more intrinsic chemiluminescence than casein, sodium caseinate, or egg albumin. However, when these same food proteins were hydrated, intrinsic chemiluminescence from soy proteins was about 4- to 8-times greater than other source proteins. Quenching the alkyl-radicals in the powdered soy proteins with hydrogen sulfide reduced the typical electron paramagnetic resonance spectra from soy proteins below detectable levels, and reduced the chemiluminescence from the hydrated soy proteins by about 65%. Antioxidants also reduced chemiluminescence in hydrated soy proteins by about 50% to 92%, with ellagic acid being the most effective. The reduction in chemiluminescence from both quenching radicals in the solid state, and by the addition of antioxidants to aqueous mixtures, indicate that the chemiluminescence produced when soy proteins are hydrated is a free radical catalyzed event. Based on the production of chemiluminescence, the radicals from soy protein were largely released within 30 min of hydration at 23 °C. Elevating the hydration temperatures increased chemiluminescence by as much as 280% at 70 °C, and decreased the half-life of the light-emitting reaction by about 9-fold. PRACTICAL APPLICATIONS Levels of metastable radicals in powdered soy proteins typically range from to 10 to 100 times greater than free radicals from other food protein sources. This research focuses on the types of reactions these radicals catalyze when soy proteins are hydrated, and the radicals suddenly become reactive. The findings suggest that a portion of the energy released from metastable radicals when powdered soy proteins are hydrated is involved in the generation of chemically-induced light.

Thermally Stimulated Luminescence in Powdered Soy Proteins

Heating powder isolated soy proteins (ISPs) in a N₂ environment produced thermally stimulated luminescence (TSL), in 2 major temperature regions, 50 to 250°C (region R1) and 250 to 350°C (region R2). In soy protein 7S fraction, strong TSL was detected in both regions with glow peak maximum (T(m)) at 150 ± 15°C and at 300 ± 10°C. Two additional satellite or shoulder peaks were detected from the ISP and 7S protein fraction within region R1 at T(m) = 90°C and T(m) = 210°C. The soy protein 11S fraction produced a broad, poorly defined TSL peak in the low-temperature region. Electron paramagnetic resonance spectroscopy data from the control ISP sample, deuterium sulfide-treated ISP, ISP stored in either N₂ or O₂, and defatted soy flour, indicated that the trapped radicals present in ISP is associated with the production of the primary TSL peak at 150 ± 15°C. Activation energies required to release the trapped charges (for luminescence to occur) are approximately 0.70, 0.78, 1.50, and 1.8 eV for TSL at Tm = 100, 150, 200, and 300°C, respectively. The reaction mechanism that leads to the release of the trapped charges for TSL to occur followed a mixed order kinetic, between 1.5 and 1.8. The frequency factor varied between 10⁷/s and 10¹⁷/s.