Production of Bio-Ethanol from Soybean Molasse by Saccharomyces cerevisae
Siqueira, Paula Fernandes de
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Abstract: The non-renewable energy resources can not be replaced as fast as they are consumed, and eventually, they will run out. Therefore, alternative energy sources such as ethanol are being considered. Some biological processes have rendered possible routes for producing ethanol in large volumes. The main objective of this work was to define an economically viable bioprocess to produce bio-ethanol from soybean molasses at laboratory, pilot and industrial scale. In order to increase the reducing sugars concentration of the molasses, acid hydrolysis was tested, but it did not improve the ethanol yield of the fermentation. The enzymatic hydrolysis of the molasses and the acid hydrolysis of the vinasse (waste product) in severe conditions provided an increase of 20% and 17% in ethanol yield, respectively. However, until now, these processes are not considered economic. The bacterial strains of Zymomonas mobilis assayed did not show a good yield in ethanol production from soybean molasses, once these bacteria are more selective to the types of metabolizable sugars. The commercial yeasts Levasaf and Fermol presented the best yields among the four assayed strains; however, Levasaf was chosen considering the lower cost. Among the six wild strains assayed, one showed an interesting result, which will be the subject of further studies. In the fermentative process, the medium was maintained at 30°Brix and the previous separation of the molasse’s proteins showed no good efficiency in the conditions assayed. The supplementation of the medium with nutrients and pH adjustment were not necessary. The recycle of biomass was tested and there were expressive contamination problems and losses in cell viability. Therefore, the growth of bacteria needs to be controlled in order to assure a satisfactory ethanol yield. The antibiotic tested in our experiments (Kamoran HJ) and the reduction of the medium pH were not effective in inhibiting bacterial growth. The main contaminants are probably lactic acid bacteria, which are acid tolerant. Consequently, a more effective antibiotic will be necessary in order to control contamination and maintain a good ethanol yield. On the other hand, in pilot scale, the acid treatment of the inoculum provided a better ethanol yield infermentation. The major problems identified after scaling-up the process were foam formation and contamination with bacteria. The first problem was solved by carrying out the process as fed-batch and using antifoam and dispersant agents. The second problem was solved by the previous treatment of soybean molasses (removal of solids by centrifugation or filtration), in order to facilitate the separation of the yeast biomass at the end of the fermentation and the acid treatment. These solid particles can act as "support" for contaminants. The yield in ethanol production was maintained after scale-up. The main by-product generated by the alcoholic fermentation is the waste produced after distillation and ethanol recovery. This residue (called vinasse) has a high concentration of organic compounds, essentially non-reducing sugars, and it is being tested as substrate for production of two other products of high commercial value: xanthan gum and lactic acid.
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