Origin of acetaldehyde during milk fermentation using (13)C-labeled precursors

Area of Science:

• Food Microbiology
• Biochemistry
• Analytical Chemistry

Background:

• Acetaldehyde is a key flavor compound in fermented dairy products.
• Understanding its formation pathways is crucial for controlling flavor profiles.
• Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus are essential yogurt starter cultures.

Purpose of the Study:

• To investigate the origins of acetaldehyde produced by Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus during milk fermentation.
• To quantify the contribution of glucose and L-threonine to acetaldehyde formation.
• To utilize a novel static-and-trapped-headspace technique for accurate acetaldehyde analysis.

Main Methods:

• Fermentation of cow's milk using Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus.
• Addition of (13)C-labeled glucose, L-threonine, and pyruvate as precursors.
• Analysis of acetaldehyde using a static-and-trapped-headspace technique coupled with gas chromatography-mass spectrometry (GC-MS).
• Determination of labeled acetaldehyde originating from different precursors.

Main Results:

• Over 90% of acetaldehyde produced by Lactobacillus delbrueckii subsp. bulgaricus originated from glucose.
• Almost 100% of acetaldehyde produced by Streptococcus thermophilus originated from glucose.
• Both bacterial species demonstrated threonine aldolase activity.
• Labeled acetaldehyde was formed from (13)C-labeled L-threonine by both microorganisms during milk fermentation.

Conclusions:

• Glucose is the predominant precursor for acetaldehyde formation by both Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus in fermented milk.
• L-threonine also contributes to acetaldehyde production due to the threonine aldolase activity in these bacteria.
• The findings provide insights into the metabolic pathways influencing flavor development in fermented dairy products.