Influence of Water Activity on Production of Bacillus thuringiensis by Solid State Fermentation

Bacillus thuringiensis can be produced by a fermentative process using agro-industrial residues as substrates.The culture medium in solid state fermentation (SSF) is composed by moist nutrients.The water is a limiting factor in the SSF, differently from that in submerged fermentation.The solid culture medium must have a minimum moisture content to allow microbial growth but the concept of water availability in the substrate is best explained in terms of water activity (Aw).This study investigated the water activity influence on the solid state fermentation of Bacillus thuringiensis using rice husk, bran and grits as substrates.Three culture media were tested: (i) rice grits; (ii) rice grits and rice husk; (iii) rice bran and rice husk.In order to study hygroscopic characteristics of each medium, isotherms were constructed.For each culture medium, two processes were carried out: one with low Aw and the other with high Aw.The fermentations with B. thuringiensis were made in plastic bags containing the culture medium.The results showed differences in productivity.The best results were obtained for the medium composed by rice grits.The productivity ((spores/g medium)/h) for the process using initial Aw of 0.620 was 3.0  10 4 and for the initial Aw of 0.965 was 1.0  10 6 .