Improving the performance of a continuous process for the production of ethanol from starch

In a previous work, a continuous simultaneous saccharification and fermentation process to produce ethanol from cassava starch was studied, using a set of fixed-bed reactors. The biocatalyst consisted of glucoamylase immobilized in silica particles and co-immobilized with S. cerevisiae in pectin gel. Using 3.8 U mL(-1) (reactor) and 0.05 g(wet yeast) mL(-1) (reactor) at start-up, starch hydrolysis was the rate-limiting step. Maximum ethanol productivity was 5.8 g(ethanol) L(-1) h(-1), with 94.0% conversion of total reducing sugars (TRS) and 83.0% of the ethanol theoretical yield. In this work, the molar mass of the substrate and the biocatalyst particle size were reduced in an attempt to improve the bioreactor performance. The diameters of silica and pectin gel particles were reduced from 100 mum and 3-4 mm, respectively, to 60 mum and 1-1.5 mm, and the degree of substrate prehydrolysis by alpha-amylase was increased. The bioreactor performance was assessed for different loads of immobilized glucoamylase (2.1, 2.8, and 3.8 U mL(-1) (reactor)), for the same initial cell concentration (0.05 g(wet yeast.)mL(-1) (reactor)). Feeding with 154.0 g L(-1) of TRS and using 3.8 U mL(-1) (reactor), fermentation became the rate-limiting step. Productivity reached 11.7 g L(-1) h(-1), with 97.0% of TRS conversion and 92.0% of the ethanol theoretical yield. The reactor was operated during 275 h without any indication of destabilization.