Red sour soup (RSS) is a traditional Chinese fermented food that is prone to microbial spoilage, leading to pellicle formation and off-flavor development. This study compared spoiled and normal RSS to analyze quality differences, identify the potential spoilage microorganism, and explore the underlying spoilage mechanism. The spoiled samples exhibited lower acidity (5.622 g/kg) and higher concentrations of histamine (61.730 mg/kg) and tyramine (44.344 mg/kg), with propanoic acid, butanoic acid, 3-methyl-, and butanoic acid, 2-methyl- as characteristic off-flavor compounds. The spoilage yeast Pichia manshurica was isolated from spoiled samples. Reinoculation experiments confirmed that P. manshurica induced pellicle formation, reducing total acid from 15.734 g/kg to 3.037 g/kg and decreasing lactic acid by over 15-fold. Short-chain fatty acids represented by butanoic acid, 2-methyl-, butanoic acid, 3-methyl-, and propanoic acid, 2-methyl- were the primary off-flavor compounds metabolized by P. manshurica , bringing sour, putrid, sweaty, and foot-like odors to RSS. KEGG enrichment analysis revealed that these off-flavor compounds were closely associated with pyruvate metabolism, glycolysis/gluconeogenesis, nitrogen metabolism, and lipoic acid metabolism. These findings reveal spoilage characteristics and the primary microbial culprit in pellicle-spoiled RSS, providing targets and a theoretical basis for precise monitoring and control.
Tang et al. (Mon,) studied this question.