PRODUCTION OF ETHANOL FROM STARCH BY CO-IMMOBILIZED ZYMOMONAS MOBILIS-GLUCOAMYLASE IN A FLUIDIZED-BED REACTOR

The production of ethanol from starch was studied in a fluidized-bed r eactor (FBR) using co-immobilized Zymomonas mobilis and glucoamylase. The FBR was a glass column of 2.54 cm in diameter and 120 cm in length . The Z. mobilis and glucoamylase were co-immobilized within small uni form beads (1.2-2.5 mm diameter) of kappa-carrageenan. The substrate f or ethanol production was a soluble starch. Light steep water was used as the complex nutrient source. The experiments were performed at 35 degrees C and pH range of 4.0-5.5. The substrate concentrations ranged from 40 to 185 g/L, and the feed rates from 10 to 37 mL/min. Under re laxed sterility conditions, the FBR was successfully operated for a pe riod of 22 d, during which no contamination or structural failure of t he biocatalyst beads was observed. Volumetric productivity as high as 38 g ethanol/(Lh), which was 74% of the maximum expected value, was ob tained. Typical ethanol volumetric productivity was in the range of 15 -20 g/(Lh). The average yield was 0.49 g ethanol/g substrate consumed, which was 90% of the theoretical yield. Very low levels of glucose we re observed in the reactor, indicating that starch hydrolysis was the rate-limiting step.