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 (Tm) 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 Tm = 90°C and Tm = 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.
Free radicals are capable of catalyzing oxidative degradation of food components, and powdered soy proteins typically contain from 10 to 100 times more metastable radicals than other protein sources. The research described in this paper provides novel information about the nature of these radicals that can be used to develop processes that can minimize the content of free radicals in foods containing soy proteins.