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A. Zhou, Department of Animal Sciences, University of Kentucky, 412 W.P. Garrigus Building, Lexington, KY 40546-0215 and W. L. BOATRIGHT.

JUSTIFICATION: The flavor of soy protein products is a major impediment to their expanded use in human foods. A major contributor to the flavor of soy protein isolates (SPI) is 2-pentyl pyridine, which has the largest flavor value of any compound reported in SPI.

OBJECTIVE: To use stable-isotope labeled compounds to confirm the precursors for the synthesis of 2-pentyl pyridine in SPI.

METHODS: Both 13C-2,4-decadienal and 15N-ammonia were added individually during SPI processing followed by extraction of lipids and analyses by gas chromatography/mass spectrometry (GC/MS). The effect of adding individual amino acids during SPI processing was also evaluated.

RESULTS: With the individual addition 13C-2,4-decadienal or 15N-ammonia, the corresponding stable-isotopes were incorporated into 2-pentyl pyridine. The addition of either unlabeled 2,4-decadienal alone or with ammonia increased 2-pentyl pyridine levels from an average of 0.181 ppm in the control SPIs to 2.303 ppm and 4.405 ppm, respectively. Ammonia alone increased 2-pentyl pyridine to an average of 0.229 ppm. Adding individual amino acids during SPI processing resulted in no significant increase in 2-pentyl pyridine. Adding arginine, lysine, asparagine or glutamine together with 2,4-decadienal increase the level of 2-pentyl pyridine compared to the addition of 2,4-decadienal alone. No significant increases were observed with aspartic acid, glutamic acid, histidine or glycine.

SIGNIFICANCE: The results provide conclusive evidence that both the 5th carbon of 2,4-decadienal and the nitrogen of ammonia are incorporated into 2-pentyl pyridine during SPI processing. Because lower levels of 2-pentyl pyridine occurring in SPI prepared from soybeans null for lipoxygenase enzymes 1, 2, and 3, and with the addition of ammonia alone, the limiting factor in the synthesis of 2-pentyl pyridine appears to be 2,4-decadienal.