Thesis (M.Eng.) - Asian Institute of Technology, 1994

A thesis manuscript submitted in partial fulfillment of the requirements for the degree of Master of Engineering.

Growth kinetics of Saccharomyces cerevisiae (bakers' yeast) in glucose syrup and sugarcane molasses was studied under two different cultivation procedures namely batch and fedbatch cultivation. Possible use of on-line ethanol sensor in automating the feeding of substrate in the fed-batch culture was examined. The optimum temperature and pH require for the growth of S. cerevisiae were found to be 30°C and 5.5 respectively, irrespective of the substrate used. Productivity and overall cell yield of S. cerevisiae were better in glucose syrup than in sugarcane molasses in the batch cultivation. This shows that the inhibitors present in molasses can inhibit the cell growth. The productivity and overall cell yield of glucose syrup were 0.31 g/1-h and 0.23 g cellslg sugar, respectively and of molasses were 0.22 gll-h and 0.18 g cellslg sugar, respectively. In fed-batch culture, the cultivation time could be reduced to obtain high cell concentration with low ethanol production. The productivity and the overall cell yield obtained in glucose syrup cultivation were higher than those in molasses cultivation in both specific growth rate levels without controlling the reducing sugar concentration in fed-batch cultivation. At high level of specific growth rate, glucose syrup gave a productivity of 2.5 gll-h with a cell yield of 0.46 g cellslg sugar whilst molasses yields 1.80 gll-h and 0.4 g cellslg sugar respectively. At low level of specific growth rate, the productivity of 2.52 gll-h and overall cell yield of 0.5 gig sugar were achieved in glucose syrup cultivation whereas molasses gave a productivity of 1.86 g/1-h and cell yield of 0.41 gig sugar. By controlling the reducing sugar concentration at its optimum level, higher productivity and overall cell yields could be achieved in both cultures. A productivity of 3.12 g/1-h and overall cell yield of 0.52 gig sugar were obtained with glucose syrup, and 2.33 gll-h and 0.45 g cellslg sugar were obtained with molasses. On-line ethanol sensor can be successfully used in fed-batch cultivation based on Teflon tubing method to control ethanol concentration constant by manual and automatic operation, however, higher productivity and overall cell yield is achieved in automatic operation than manual operation.