Research Accomplishment Reports 2007

Characterization of Enzyme(s) Associated with Sulfur Assimilation Type Reactions in Soy Protein Products

W.L. Boatwright
Department of Animal and Food Sciences

 

Project Description

Soybeans are the second largest food crop in the U.S.A. with about 87 million metric tons produced in 2006. Historically, the consumption of purified soy proteins in the United States has been low. In 1971, less than 1 percent of the U.S. soybean crop was used as a protein source for human foods and in 2000, this value was about 1.5 percent. This is largely due to the undesirable flavor and odor associated with soy products.

This research has expanded our knowledge of the post-harvest reactions responsiible for the major lipid and amino acid degradation products responsible for the undesirable odor of soy protein products. Our research has revealed the presence of a non-enzymatic reaction that produces hexanal from linoleic acid in purified soy protein that is accelerated in the presence of reducing agents. Further analysis revealed that lipoxygenase (LOX) activity was not detected from these ISP, indicating LOX is not involved in the observed hexanal increase. Levels of the induced headspace hexanal over ISP aqueous slurries were proportional to the amount of DTT added in the range of 0 to 65 mM.

Subsequent systematic investigations with model systems revealed that iron was required for the reducing agent-induced hexanal formation from linoleic acid. Erythorbate, another reducing agent, can also induce hexanal formation in both ISP and model systems. As a comparison, the lipoxygenase activity and hexanal synthesis in defatted soy flour were examined. The corresponding results showed that defatted soy flour maintained high lipoxygenase activities and that hexanal synthesis was significantly inhibited by high concentration DTT (above 130 mM).

Data from the current investigation demonstrate the existence of lipoxygenase independent hexanal formation induced by reducing agents in ISP and the potential requirement of iron as a catalyst. The one-electron oxidation of methionine and the subseqent production of methanethiol (a major sulfur containing odorant in soy proteins) is catalyzed in the presence of oxygen. The carbon and sulfur of methanethiol originates from the methyl-carbon and sulfur of methionine. Similar aqueous mixtures of sulfite, manganese and oxygen also produce sufficient levels of free radicals to degrade fluorescein. The degradation of methionine by free radicals generated in the sulfite, manganese and oxygen reaction mixture is inhibited by the free radical spin trapping agent 5,5-dimethyl-1-pyrroline N-oxide. Processing ISP with either L cystine or potassium iodate reduces the free sulfite content of ISP and reduces the headspace methanethiol from aqueous ISP slurries to non-detectable levels. ISP processed without additives contained sufficient levels of free-radicals to generate methanethiol from the oxidation of added methionine. There were no detectable levels of methanethiol produced when methionine was added to ISP processed with iodate.

Impact

This research project was designed to address a basic understanding of the reaction mechanisms responsible for the undesirable flavor compounds associated with soybean protein products. As such, there is limited development of new practices or new techniques directly associated with this project. We have discovered two (2) previously unknown reaction mechanisms in purified soy proteins responsible for the production of undesirable lipid oxidation and methionine oxidation products. The reactions that oxidized unsaturated fatty acids and produces hexanal was found to be catalzed by vitamin C (ascorbic acid), or erythorbate.. This finding was shared with the two largest soybean processors in the United States. And in at least one case, the processor was able to modify a product to improve its stability and flavor. There have also been some novel processing techniques developed to produce soy proteins with improved flavor as an indirect result of this project. Some of these have shown limited success on an industrial scale.

Publications

Boatright, W.L. and Lu, G,. 2007. Hexanal Synthesis in Isolated Soy Proteins, Journal of the American Oil Chemists' Society, 84(3):249-257.

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