Yeast immobilization using thin shell silk cocoon for continuous ethanol production

Abstract

A renewable energy, ethanol gains more interest because of its benefits such as clean energy and production from biomass fermentation. As a biocatalyst of ethanol fermentation, immobilized yeast offers many advantages including high ethanol productivity and reuse ability of cells. Since, yeast immobilization using method of entrapment within porous matrix always encounters with a mass transfer limitation problem, therefore, in this study, yeast immobilization using the method of adsorption or attachment to the surface of thin shell silk cocoon was proposed. Under batch fermentations in 500 ml Erlenmeyer flask, the experimental studies were carried out using Saccharomyces cerevisiae M30 and molasses as the ethanol producer and carbon source, respectively with the initial sugar concentration of 220- 280 g/l at shaking frequency of 150 rpm and temperature of 33 oC. The ethanol fermentation using thin shell silk cocoon immobilized cell (TSI) culture was found to be more effective than that using thin shell silk cocoon immobilized cell entrapment within alginate (ETSI) and suspension cell (SC) cultures, resulting in higher ethanol production. Moreover, by using TSI culture with the initial sugar concentration of 240 g/l, the maximum ethanol concentration of 98.6 g/l was obtained after 64 hours of the fermentation. From the evaluation in the 5-cycle repeated batch, the TSI culture demonstrated a good potential of reusability than that of the SC culture. The further continuous ethanol fermentation in a 1-litre packed-bed reactor revealed that the maximum ethanol productivity of 19.02 g/l h with ethanol concentration of 52.83 g/l could be obtained with the feed of 220 g/l sugar concentration at 0.36 h-1 dilution rate, while the highest ethanol concentration of 80.72 g/l was obtained at the dilution rate of 0.034 h-1. Overall, the developed TSI was successfully used as the cell carrier for the ethanol fermentations in batch, repeated batch and continuous processes. Its favorable biocompatible and mechanical properties resulted in high ethanol production, high cell immobilized yield, high density of biomass and high stability for long-term use.