Identification of a thermophilic protease-producing strain and its application in solid-state fermentation of soybean meal

BACKGROUND Thermophiles can thrive at 50-80 degrees C and produce some enzymes with special promise for biocatalysis. A thermophilic protease-producing strain YYC4 was isolated from Yunyan cigarette and employed in solid-state fermentation (SSF) of unsterilized soybean meal (SBM). RESULTS The isolate was identified as Bacillus licheniformis based on appearance of colonies, microscopic observation and 16S rDNA sequencing. After SSF, soluble and crude protein contents in SBM increased from 49.24 to 185.73 g kg(-1) and from 404.18 to 479.46 g kg(-1), respectively, under the fermentation conditions of 10(7) cfu g(-1) inoculation of strain YYC4, 1:1.8 (g mL(-1)) SBM to distilled water, 1.2 g kg(-1) magnesium sulphate addition, 55 degrees C and 48 h. During fermentation, pH of the medium increased from 6.30 to 9.09 and protease activity especially neutral protease increased significantly from 13.5 to 181.31 U g(-1). Meanwhile, trypsin inhibitor (TI) activity was decreased from 8.19 to 3.19 mg g(-1). The safety of fermented SBM (FSBM) was verified by acute toxicity animal experiment. Analysis of microbial community in FSBM showed that Bacillus licheniformis YYC4 as a dominant strain inhibited most of the other microorganisms pre-existing in the materials during fermentation. CONCLUSION Increments of soluble and crude protein by 277.19% and 18.63% and decrement of harmful TI by 61.05% in SBM were achieved using thermophilic SSF by Bacillus licheniformis YYC4, providing a basis for the application of thermophiles in fermentation industry in an environmentally friendly and energy-saving way. (c) 2021 Society of Chemical Industry