Date of Award
Doctor of Philosophy
One strain of oleaginous yeasts, Cryptococcus curvatus (ATCC 20509) has been studied to grow on several substrates including biodiesel production byproduct crude glycerol and sweet sorghum juice. After cultivation, yeast cells were heated under microwave radiation to extract lipid and produce biodiesel through in-situ transesterification. Firstly, the yeast growth with crude glycerol was studied. When cultured in a one-stage fed-batch process wherein crude glycerol and nitrogen source were fed intermittently for 12 days, the final biomass density and lipid content were 31.2 g/L and 44.2%, respectively. When cultured in a two-stage fed-batch operation wherein crude glycerol was supplemented at different time points while nitrogen source addition was discontinued at the middle of the experiment, the biomass density was 32.9 g/L and the lipid content was 52% at the end of 12 days. On the second step, an optimization of yeast fermentation with crude glycerol was conducted. Through Box-Behnken design and response surface methodology, the optimal temperature, pH, and glycerol concentration for yeast growth on pretreated crude glycerol was identified as 30.2 deg C, 6.0, and 19.8 g/L, respectively. Adopting these optimal parameters, the biomass density and lipid concentration obtained were 7.11 ± 0.36 g/L and 38.53 ± 1.88%, respectively, which matched well with the model predicted values of 6.98 g/L and 41.31%.The resulting parameters of the response surface method optimization were used in a fed-batch fermentation where crude glycerol was automatically pumped in responding to exhausted oxygen levels in the fermentor. At the end of 12 days, the biomass density and lipid content were 44.53 g/L and 49%,respectively. Compared with our fed-batch experiment which was conducted under un-optimized condition, the yield of biomass and lipid increased 35.26% and 25.29%. When cultured in a fed batch process where sorghum juice syrup was supplemented at different time points for 3 days, the final biomass density was 23.6 g/L with a lipid content of 51%. To extract lipids from cells in an effective and fast fashion, a domestic microwave oven was used with different solvents. With only methanol, a lipid yield of 33.2% of yeast cells was obtained in 4 min. This was comparable with a lipid content of 51% attained through using a traditional solvent extraction approach. In the end, to convert yeast lipids to biodiesel directly without the step of lipid extraction, the in-situ transesterification method used microwave irradiation on the simultaneous extraction and transesterification of wet yeast biomass to biodiesel. Response surface methodology was used to analyze the influence of the process variables (solvent to biomass (v:w) ratio, catalyst concentration, and reaction time) on the fatty acid methyl ester conversion. Based on the experimental results and RSM analysis, the optimal conditions for this process were determined as: methanol to yeast biomass (v:w) ratio of around 50:1, catalyst concentration about 5 wt.%, and reaction time of 2 min. The biodiesel samples were analyzed with GC and the FAME content in biodiesel is about 50%.
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