Optimization of β-mannanase production by Bacillus subtilis US191 using economical agricultural substrates

Abstract

The Bacillus subtilis US191 strain producing highly thermostable β-mannanase was previously selected as potential probiotic candidate for application as feed supplement in poultry industry. Initially, the level of extracellular β-mannanase production by this strain was 1.48 U ml−1. To improve this enzyme titer, the present study was undertaken to optimize the fermentation conditions through experimental designs and valorization of agro-industrial byproducts. Using the Plackett–Burman design, in submerged fermentation, a set of 14 culture variables was evaluated in terms of their effects on β-mannanase production. Locust bean gum (LBG), soymeal, temperature, and inoculum size were subsequently optimized by response surface methodology using Box–Behnken design. Under optimized conditions (1 g L−1 LBG, 8 g L−1 soymeal, temperature of 30°C and inoculum size of 1010 CFU ml−1), a 2.59-fold enhancement in β-mannanase titer was achieved. Next, to decrease the enzyme production cost, the effect of partial substitution of LBG (1 g L−1) by agro-industrial byproducts was investigated, and a Taguchi design was applied. This allowed the attaining of a β-mannanase production level of 8.75 U ml−1 in presence of 0.25 g L−1 LBG, 5 g L−1 of coffee residue powder, 5 g L−1 of date seeds powder, and 5 g L−1 of prickly pear seeds powder as mannans sources. Overall, a 5.91-fold improvement in β-mannanase production by B. subtilis US191 was achieved.