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Evaluating Acetaldehyde Synthesis from L-[¹⁴C(U)] Threonine by Streptococcus thermophilus and Lactobacillus bulgaricus¹

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

ABSTRACT

To evaluate the synthesis of acetaldehyde from threonine during growth of yogurt cultures, Streptococcus thermophilus MS1 and Lactobacillus bulgaricus MR1 were grown in defined medium in which 10% of the total threonine was composed of L-[carbon-14(U)] threonine. Acetaldehyde production was monitored by formation of 2,4-dinitrophenylhydrazone followed by separation and analysis using high performance liquid chromatography. After growth for 8 h at 42°C, approximately 2.0% of the total acetaldehyde (780.4 nmol) produced was from L-[carbon-14]threonine. Threonine aldolase activity was determined in cell-free extracts from S. thermophilus and L. bulgaricus grown in Elliker broth. Increasing incubation temperature from 30 to 42°C decreased threonine aldolase activity in cells of the streptococcus harvested after 8 h of incubation. Effect of incubation temperature was more dramatic in cells harvested after 18 h where the activity of cells grown at 42°C was 89% lower than that of cells grown at 30°C. Cell extracts from S. thermophilus MS1 possessed higher threonine aldolase activity than did those from L. bulgaricus MR1. Increased assay temperature from 30 to 42°C increased threonine aldolase activity in S. thermophilus MS1.

¹ Florida Agricultural Experiment Station Journal Series Number 6578.

² Dairyland Food Laboratories, Inc., Waukesha, WI.

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