A novel neutral thermophilic β-mannanase from Malbranchea cinnamomea for controllable production of partially hydrolyzed konjac powder.

Partially hydrolyzed konjac powder (PHKP) can be used to increase the daily intake of dietary fibers of consumers. To produce PHKP by enzymatic hydrolysis, a novel β-mannanase gene (McMan5B) from Malbranchea cinnamomea was expressed in Pichia pastoris. It showed a low identity of less than 52% with other GH family 5 β-mannanases. Through high cell density fermentation, the highest β-mannanase activity of 42200 U mL-1 was obtained. McMan5B showed the maximal activity at pH 7.5 and 75 °C, respectively. It exhibited excellent pH stability and thermostability. Due to the different residues (Phe214, Pro253, and His328) in catalytic groove and the change of β2-α2 loop, McMan5B showed unique hydrolysis property as compared to other β-mannanases. The enzyme was employed to hydrolyze konjac powder for controllable production of PHKP with a weight-average molecular weight of 22000 Da (average degree of polymerization 136). Furthermore, the influence of PHKP (1.0%-4.0%) on the qualities of steamed bread was evaluated. The steamed bread adding 3.0% PHKP had the maximum specific volume and the minimum hardness, which showed 11.0% increment and 25.4% decrement as compared to the control, respectively. Thus, a suitable β-mannanase for PHKP controllable production and a fiber supplement for steamed bread preparation were provided in this study. KEY POINTS: • A novel β-mannanase gene (McMan5B) was cloned from Malbranchea cinnamomea and expressed in Pichia pastoris at high level. • McMan5B hydrolyzed konjac powder to yield partially hydrolyzed konjac powder (PHKP) instead of manno-oligosaccharides. • PHKP showed more positive effect on the quality of steamed bread than many other dietary fibers including konjac powder.