Optimization of solid-state enzymatic hydrolysis of chestnut using mixtures of alpha-amylase and glucoamylase.

Solid-state hydrolysis of starch present in chestnut was assayed in a single step with a mixture of a thermostable alpha-amylase and glucoamylase at three temperatures: 17 and 30 degrees C, for simultaneous hydrolysis and ethanol fermentation, and 70 degrees C, the optimal temperature for these enzymes. Total hydrolysis was only reached at the highest temperature, leading to a more concentrated hydrolysate than in submerged hydrolysis. Mass transfer limitations and starch retrogradation appear as the main causes for the incomplete hydrolysis of chestnut starch in solid-state operation at 17 and 30 degrees C. Even accepting that this limitation causes a 15% reduction of the yield of the hydrolysis with respect to the submerged process or the solid process at high temperature, solid-state hydrolysis at low temperatures seems to be adequate for simultaneous solid-state hydrolysis and fermentation processes.