Study of improving nattokinase activity through solid-state fermentation of cold-pressing walnut dregs by compound strains

To improve nattokinase ( NK) activity of fermentation products by Bacillus subtils natto and reduce the ammonia smell, solid-state fermentation of walnut dregs by the mixed strains of Bacillus subtils natto and Torlaspora delbrueckii ( Bacillus subtils natto and Lactobacillus plantarum) was investigated. Results showed that the optimum technological parameters of fermentation by the mixed strains of Bacillus subtils natto and Torlaspora delbrueckii were as follows: inoculation amount of 3%, proportion of inoculums of 2∶ 1, fermentation temperature of 34 ℃, fermentation time of 60 h.In this fermentation conditions, the NK activity was 2040.82 U/g, increasing by 70.42%.The content of volatile base nitrogen was 64.18 mg/100 g, decreasing by84.37%.In addition, the optimum technological parameters of fermentation by the mixed strains of Bacillus subtils natto and Lactobacillus plantarum were as follows: inoculation amount of 9%, proportion of inoculums of 1.5∶ 1, fermentation temperature of 42 ℃, fermentation time of 60 h.In these fermentation conditions, the NK activity was 1966.02 U/g, increasing by 64.18%.The content of volatile base nitrogen was 62.68 mg/100 g, decreasing by 84.74%.The mixed strains fermentation could decrease the ammonia smells significantly, improve the flavor of traditional fermentation products.