Abstract: In order to improve the activity of isomannanase, Bacillus subtilis K-6, a strain capable of secreting isomannanase, was subjected to mutation breeding through UV induction. A positive mutant strain was obtained and fermentation conditions for the production of isomannanase by it were optimized by one-factor-at-a-time and orthogonal array design methods. The isomannanase activity of the original strain was 206.0 U/mL, while that of the high-yield mutant strain K-6-9 was 349.3 U/mL, showing a 69.6% increase compared with Bacillus subtilis K-6. The isomannanase activity of the fifth generation of K-6-9 varied from 343.0～350.3 U/mL, indicating good genetic stability. The optimal fermentation conditions for isomannanase production by K-6-9 were found as 72 h of fermentation time, 3% of inoculum size, initial pH 7.5, and 25 g/250 mL of medium filling percentage. The optimal fermentation medium was composed of 40% wheat bran, 8% yeast wall and material-to-liquid ratio 1:1.2.The maximum activity of K-6-9 strain reached up to 601.6 U/mL under the optimal fermentation conditions.

Key words: isomannanase, UV mutation, solid-state fermentation