Optimization of maize starch fermentation by Saccharomyces cerevisiae using pervaporation

Abstract

Due to the depletion of petroleum reserves and environmental concerns, bioethanol has been identified as an alternative fuel to petrol. Bioethanol is a fuel of bio-origin derived from renewable biomass. Starch and sugar containing materials are the primary sources of carbon for bioethanol production. Starch is firstly hydrolysed into simple sugars which are later fermented to bioethanol using Saccharomyces cerevisiae (S. cerevisiae). The fermentation of sugars to bioethanol is however limited by inhibition of S. cerevisiae by the major product of the process, bioethanol. The challenge is thus in keeping the bioethanol concentration at levels which are not harmful to the fermenting organism. Keeping bioethanol concentration low in the broth will provide a suitable environment for yeast to grow and thus increase the overall production. Currently bioethanol producers use high water dilution rates to keep the bioethanol concentrations in the broth low enough so that yeast is not harmed. This excess water has to be removed in the downstream process, which is expensive. The use of excessive amounts of water in the fermentation can be avoided by continual removal of bioethanol from the broth. During this investigation the experimental conditions for the hydrolysis process were determined. A pH of 5.5 was determined as the best pH for Termamyl SC at 95°C with a pH of 5.0 for Spirizyme Fuel at 55°C during the liquefaction and the saccharification step, respectively. During the fermentation process the influence of yeast concentration on bioethanol production was investigated by varying the yeast concentration between 2 g.L-1 and 7 g.L-1. A yeast concentration of 5 g.L-1 produced the highest bioethanol yield of 0.48 g.g-1 after 48 hours of fermentation using S. cerevisiae. Later during the investigation a coupled fermentation/pervaporation system was employed in a batch system for continual removal of bioethanol in the fermentation broth in a process called simultaneous fermentation and separation (SFS). Through the continuous removal of bioethanol from the fermentation broth, the bioethanol concentration in the broth was kept low enough so that it was not harmful to the fermenting organism but the overall fermentation yield was not improved. Pervaporation is a membrane separation process used to separate azeotropic mixtures such as bioethanol and water. It is highly efficient, cost effective and uses less energy than distillation. During the SFS process a bioethanol yield of 0.22 g.g-1 was obtained. The SFS process yield for bioethanol was low compared to 0.45 g.g-1 of the traditional batch fermentation process. The lower overall bioethanol yield obtained in the SFS process could be attributed to only the supernatant being used in the SFS process and not the entire fermentation broth as in the traditional process. The results from this study proved that the SFS process was less efficient compared to the traditional batch fermentation process with respect to the bioethanol yield, but that the fermentation could be carried out without the necessity for additional process water.