Repeated batch conversion of raw starch to ethanol using amylase immobilized on a reversible soluble-autoprecipitating carrier and flocculating yeast cells.

Abstract

A raw-starch-digesting amylase, Dabiase K-27, was immobilized covalently on an enteric coating polymer (hydroxypropyl methylcellulose acetate succinate: AS) as a carrier which is autoprecipitating in an insoluble state below pH 4 as well as reversibly soluble-insoluble depending on pH. Dabiase immobilized on AS (D-AS) showed a sharp response of solubility to slight changes of pH without decrease in enzymatic activity. Moreover, D-AS in an insoluble state had good properties of sedimentation and a large portion of D-AS spontaneously precipitated after 10 min at pH 4. D-AS was used successively for repeated ethanol production from raw starch, in which D-AS and flocculating yeast cells were separated simultaneously from a product solution by sedimentation in a reactor with a conical bottom. In the five batches of 10% raw starch, the total amount of ethanol produced from 150 g of raw starch was 61 g, a value of which corresponds to the average ethanol productivity of 0.85 g/l/hr. The repeated ethanol production by a combination of D-AS and flocculating yeast cells is a promising procedure for effectively using the enzyme and recovering the product solution economically in a heterogeneous culture system containing a solid substrate.