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

Acetaldehyde formation by Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus during fermentation of cow's milk was investigated using (13)C-labeled glucose, L-threonine, and pyruvate with a recent static-and-trapped-headspace technique that does not require derivatization of acetaldehyde prior to gas chromatography-mass spectrometry. Over 90% and almost 100% of acetaldehyde originated from glucose during fermentation by L. delbrueckii subsp. bulgaricus and S. thermophilus, respectively, taking into account both singly and doubly labeled acetaldehyde. As both microorganisms showed threonine aldolase activity and formed labeled acetaldehyde from (13)C-labeled threonine during the fermentation of milk, this amino acid should also contribute to the acetaldehyde produced.