Effects of temperature on microbial succession and metabolite change during saeu-jeot fermentation

To investigate the effects of temperature on saeu-jeot (shrimp) fermentation, four sets of saeu-jeot samples with approximately 25% (w/v) NaCl were fermented at 10 °C, 15 °C, 20 °C, and 25 °C. The pH values of the 10 °C and 15 °C samples were relatively constant during the entire fermentation period, whereas those of the 20 °C and 25 °C samples gradually decreased after 25 days of fermentation. Quantitative PCR showed that the maximum bacterial abundance was greater in higher temperature samples, and the bacterial abundance in the 10 °C samples steadily decreased during the entire fermentation period. Community analysis using pyrosequencing revealed that the initially dominant Proteobacteria including Pseudoalteromonas, Photobacterium, Vibrio, Aliivibrio, and Enterovibrio were replaced rapidly with Firmicutes such as Psychrobacter, Staphylococcus, Salimicrobium, Alkalibacillus, and Halanaerobium as the fermentation progressed. However, Vibrio, Photobacterium, Aliivibrio, and Enterovibrio, which may include potentially pathogenic strains, remained even after 215 days in the 10 °C samples. Metabolite analysis using (1)H NMR showed that amino acid profiles and initial quick increases of glucose and glycerol were similar and independent of bacterial growth in all temperature samples. After 25 days of fermentation, the levels of glucose, glycerol, and trimethylamine N-oxide decreased with the growth of Halanaerobium and the increase of acetate, butyrate, and methylamines in the 20 °C and 25 °C samples although the amino acid concentrations steadily increased until approximately 105 days of fermentation. Statistical triplot analysis showed that the bacterial successions occurred similarly regardless of the fermentation temperature, and Halanaerobium was likely responsible for the production of acetate, butyrate, and methylamines. This study suggests that around 15 °C might be the optimum temperature for the production of safe and tasty saeu-jeot.