Study on isolation and selection of high amylase and protease microbes in traditional Vietnamese soy sauce

Abstract

The aims of this study were to evaluate the microbial diversity in traditional Vietnamese soy sauce based on the number of isolated strains and to select strains with the highest activity using as starter cultures. From 12 soy sauce samples collected in Hanoi, Hung Yen, Ha Nam, and Nghe An, 85 microbial strains (45 bacteria, 30 fungi, 04 actinomycetes, and 6 yeasts) were isolated. While 54.11% of the strains did not exhibit enzyme production by the spot inoculation method, only 8.23% of the strains (07 strains) produced both enzymes at high levels (D-d >15 mm). These 07 strains were identified through morphological analysis and sequencing of the ITS region for fungi and the 16S region for bacteria, and their enzyme activity was further screened using the agar well diffusion method. Aspergillus oryzae BD3-5 and Bacillus subtilis BD3-7 were selected as starter cultures due to their safety and highest amylase and protease activity: 20-26 mm and 16-18 mm. Adding 1% (v/v) BD3-5 to rice fermentation during the koji stage increased the total sugar content 1.8 times; adding 1% (v/v) BD3-7 to soybean liquid fermentation increased the total protein content 3.0 times compared to the control sample.