Optimization of fermentation conditions for producing γ–glutamyl transpeptidase by Bacillus licheniformis and enzymatic synthesis of characteristic volatile sulfur-containing compounds in Toona sinensis

γ-glutamyl transpeptidase (GGT, E.C.2.3.2.2) has great applications in the food industry, but there is no commercially available bacterial GGT on the market. In this paper, response surface methodology was used to optimize the yield of Bacillus licheniformis GGT (BlGGT). The enzyme was extracted and used to catalyze the synthesis of volatile sulfur compounds (VSCs) in Toona sinensis (TS). The results showed that the optimal conditions for fermentation of GGT production were: lactose 8 g/L, tryptone: soybean meal (1:1) 15 g/L, MgSO₄·7 H₂O 0.5 g/L, K₂HPO₄ 1 g/L, inoculum volume of 5 % v/v, initial pH 8.0, loading volume of 50 mL, and fermentation temperature of 37.0 ℃. Under this condition, the enzyme activity of GGT reached 918.38 ± 0.012 U/L, which was 19 times higher than the activity under the initial medium conditions. In addition, when the GGT was added to the precursor extract from TS, it was noteworthy that VSCs in TS were regained, with the total amount reaching 30.38 μg/mL. The optimization of the fermentation medium to increase the yield of BlGGT may also provide valuable data for the large-scale production of GGT. The results of this study provide an effective pathway for the synthesis of unique VSCs in TS.