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Journal of Dairy Science Vol. 72 No. 5 1142-1148
© 1989 by American Dairy Science Association ®
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Effect of Threonine and Glycine Concentrations on Threonine Aldolase Activity of Yogurt Microorganisms During Growth in a Modified Milk Prepared by Ultrafiltration1

Romina M. Marranzini, Ronald H. Schmidt, Rachel B. Shireman, Maurice R. Marshall and John A. Cornell2

Food Science and Human Nutrition Department, University of Florida, Gainesville 32611

ABSTRACT

To evaluate the combined effects of threonine and glycine concentrations during growth on threonine aldolase activity (EC 2.1.2.1) of yogurt microorganisms, Streptococcus thermophilus and Lactobacillus bulgaricus, a modified milk growth medium was prepared using UF to deplete the free amino acid level. Threonine and glycine were added according to a 2 x 2 x 2 factorial design at 5 or 195 µg·ml–1 along with a standard amino acid mixture. Acetaldehyde production and threonine aldolase activity were evaluated utilizing headspace gas chromatography. Results showed that threonine and glycine concentrations did not affect growth or titratable acidity. The high concentration of threonine in combination with low glycine in the growth medium resulted in increased acetaldehyde synthesis by both microorganisms. Conversely, high glycine with low threonine decreased acetaldehyde synthesis. High threonine and low glycine increased threonine aldolase activity of cell-free extracts from S. thermophilus and L. bulgaricus, whereas high glycine and low threonine reduced threonine aldolase activity of both microorganisms.

FOOTNOTES

1 Florida Agric. Experiment Station Journal Series Number 9310.

2 Statistics Department, University of Florida, Gainesville.






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Copyright © 1989 by the American Dairy Science Association ®.