Abstract
TheBacillus subtilisUS191 strain producing highly thermostable beta-mannanase was previously selected as potential probiotic candidate for application as feed supplement in poultry industry. Initially, the level of extracellular beta-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 beta-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 degrees C and inoculum size of 10(10) CFU ml(-1)), a 2.59-fold enhancement in beta-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 beta-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 beta-mannanase production byB. subtilisUS191 was achieved.