Effect of Storage Conditions on Carbon-Centered Radicals in Soy Protein Products

Abstract

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.

Effect of Moisture, Lipids, and Select Amino Acid Blocking Agents on the Formation and Stability of Metastable Radicals in Powdered Soy Proteins

“…A sample of the control ISP stored under nitrogen behaved similarly to that observed by Boatright and others (2009) (data not shown). EPR peak shapes and g ‐factors of these ISP samples were similar to that previously reported (Boatright and others 2008, 2009).…”

“…When the water activity was increased in these powdered samples, there was a pronounced decrease in the ability of these materials to maintain their metastable radical content (Figure 2), similar to the observations made by Munday and others (1962). Because elevated levels of water attract microwaves used in EPR spectroscopy, a series of ISP samples were spiked with niobate crystals doped with K 3 CrO 8 (Cage and others 1999; Boatright and others 2009) to determine if the decrease in EPR signal in the higher moisture content samples was due to dielectric loss. Figure 3 shows the decrease in EPR signal with the corresponding increase in moisture content of an ISP sample, while the internal K 3 CrO 8 (Cr 5+ ) signal remained relatively unaffected by the moisture content.…”

“…One consideration in this experiment is that the decrease in radicals from elevated moisture content occurred rather rapidly (within days). However, the increase in metastable radicals in ISP typically occurs over weeks (Boatright and others 2009). In order to extract moisture from the powdered soy proteins samples, and lower the water activity below that of the typical “dried” commercial soy product, we mixed samples in a sealed jar with a molecular sieve (13×) capable of binding moisture and stored these samples at 40 °C.…”

“…Boatright and others (2008, 2009) demonstrated that the metastable free radical content of commercial and laboratory isolated soy proteins (ISPs) and powered drink mixes (made from ISP) ranged from 2.96 × 10 14 to 4.10 × 10 15 spins per gram. The level of these radicals in powdered soy protein is about 10 to 100 times greater than other source proteins.…”

Chemically stimulated luminescence from food proteins

“…Examples of metastable carbon radicals in several different food protein sources have been shown using solid phase electron paramagnetic resonance (EPR) spectroscopy (Boatright, Lei, & Jahan, 2009;Boatright et al, 2008;Fan et al, 2016). The primary sites of these initial oxidative reactions are usually either at the alpha carbon of amino acids or on the various amino acid side chains.…”