Factors affecting bacterial community dynamics and volatile metabolite profiles of Thai traditional salt fermented fish

Abstract

Bacterial diversity of the Thai traditional salt fermented fish with roasted rice bran or Pla-ra, in Thai, was investigated using classical and molecular approaches. Pla-ra fermentation could be classified into two types, i.e., solid-state fermentation (SSF) and submerged fermentation (SMF). Bacterial population ranged from 102-106 and 106-109 CFU/g in SSF and SMF, respectively. The rRNA detection revealed that Halanaerobium spp. and Lentibacillus spp. were the main genera present in all types and most stages of fermentation. Tetragenococcus halophillus were dominant during final stage of fermentation in the samples in which sea salt was used as one of the ingredients while Bacillus spp. were found in those that rock salt was used. In contrast, cultural plating demonstrated that Bacillus spp. were the dominant genera. B. amyloliquefaciens were the main species found in all types of Pla-ra whereas B. pumilus, B. autrophaeus, B.subtilis and B. velezensis were specifically associated with the samples in which rock salt was used. The main volatile metabolites in all Pla-ra samples were butanoic acid and its derivatives. Dimethyl disulfide was observed during earlier stage of fermentation under high salt condition with a long fermentation period. Key factors affected bacterial profiles and volatile compounds of salt fermented fish are type of salt, addition of roasted rice bran, and fermenting conditions. Importance Protein hydrolysates with high salt fermentation from soy, fish as sauces and pastes are important food condiments commonly found in Asian food cultures. In Thailand, an indigenous semi-paste product derived from salted fish fermentation also called Pla-ra is well recognized and extensively in demands. In-depth information on Pla-ra fermentation ecosystems, in which roasted rice bran and different types of salt are incorporated, are still limited. In this study, we found that Halanaerobium spp. was the key autochthonous microbe initiating Pla-ra fermentation. Addition of roasted rice brand and rock salt were associated with the prevalence of Bacillus spp. while sea salt was associated with the presence of Tetragenococcus halophillus, The risk of pathogenic Staphylococcus spp. and Clostridium spp. needed to be also concerned. Geographical origin authentication of Pla-ra products could be discriminated based on their distinctive volatile profiles. This research provides novel insights for quality and safety control fermentation together with conservation of its authenticity.