Research Accomplishment Reports 2007

Hexanal Synthesis in Isolated Soy Proteins

W.L. Boatwright
Department of Animal and Food Sciences

 

Project Description

Based on our preliminary data, this project was designed to investigate the enzymatic synthesis of hexanal in isolated soy proteins (ISP). After extensive efforts to isolate such an enzyme(s) and repeatedly losing the hexanal-producing activity during the process, we discovered two non-enzymatic pathways to account for our initial observations.

The first mechanism involves the transition metals found in ISP. We found that free sulfite and manganese, at levels found in ISP (17-33 ppm and 17 ppm, respectively), react with oxygen to produce sufficient free-radicals to degrade methionine to methanethiol and other compounds. Sulfite free-radicals produce sulfate free-radicals which have a one-electron reducing potential similar to hydroxyl radicals and are capable of degrading methionine and unsaturated fatty acids. While the degradation of methionine can be easily monitored in ISP with the addition of sulfite, elevated hexanal levels resulting from added sulfite cannot be detected by headspace-gc/ms techniques due to the formation of hexanal-sulfite adducts.

Further investigation of transition metal catalyzed reactions found that reducing agents dithiothreitol (DTT) and erythorbate when added to ISP slurries can react with the endogenous iron to catalyze the formation DTT-free radicals and hydroxyl radicals, respectively, and the subsequent formation of hexanal from linoleic acid. Further investigation of the free-radicals in ISP using solid-state electron paramagnetic resonance (EPR) spectroscopy found a symmetrical free-radicals signal typical of carbon-centered radicals (g=2.005) ranging from 2.96e+14 to 6.42e+14 spins per gram of ISP. 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 up to 4.10e+15 spins per gram of drink mix, or up to 6.4-times greater than the highest free-radical content found in commercial ISP. When ISP was hydrated in 2.3 millimolar sodium erythorbate or 8.3 millimolar l-cysteine, frozen and dried, the level of trapped free-radicals increased by about 17- and 19-fold, respectively. The ESR spectrum of defatted soybean flakes contained overlapping signals from the primary free-radical peak (g=2.005) and a sextet pattern typical of manganese-II. The manganese signal was reduced in the laboratory ISP and very weak in the commercial ISP. We are currently investigating the contribution that existing free-radicals in ISP contributed to the production of hexanal when ISP is hydrated.

Impact

This is the first time that the paramagnetic centers in commercial soy protein products have been quantified. Not only can hexanal synthesis be catalyzed by the release of stable free-radicals in soy protein once it is hydrated, but such high levels of free-radicals can potentially cause damage to human tissue. These findings should lead to the development of soy protein products that have improved flavor and are less likely to harm humans when consumed.

Publications

Boatright, W.L. and G. Lu, 2007. Hexanal Synthesis in Isolated Soy Proteins, J. Am. Oil Chem. Soc., 84(3):249-257.

Lei, Q. and W.L. Boatright, 2007. Sulfite Radical Anions in Isolated Soy Proteins, Journal of Food Science, 72(5):C302-307