Alcoholic fermentation with high sugar and cell concentration at moderate temperatures using flocculant yeasts
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pISSN: 0256-1115 eISSN: 1975-7220
INVITED REVIEW PAPER
INVITED REVIEW PAPER
Alcoholic fermentation with high sugar and cell concentration at moderate temperatures using flocculant yeasts Ana Carolina Tolentino Brandão, Miriam Maria de Resende†, and Eloízio Júlio Ribeiro Federal University of Uberlândia, Faculty of Chemical Engineering, Av. João Naves de Ávila, 2121, Bloco 1K, Campus Santa Mônica, Uberlândia - MG, 38.408-144, Brazil (Received 21 February 2020 • Revised 20 May 2020 • Accepted 27 May 2020) AbstractThis paper studied bioethanol production at very high gravity (VHG) conditions using flocculent Saccharomyces cerevisiae, evaluating the response yield, ethanol concentration, productivity, and residual sugar through a central composite design (CCD). This CCD was evaluated at 12 and 24 h fermentation times. In the CCD evaluated for 12 h of fermentation, the best condition for alcoholic fermentation was 27 oC, 260 g/L substrate concentration and a 30% v/v cell concentration; a maximum overall desirability of 0.937 was achieved. For CCD at 24 h of fermentation, the best condition was 27 oC, 300 g/L substrate concentration, and a 26% v/v cell concentration. The desirability achieved was 0.811. These conditions allowed us to verify, experimentally, that the CCD models described the fermentation behavior well. VHG alcoholic fermentation in fed-batch with the reuse of cells without chemical treatment was performed using the optimum conditions obtained from the desirability function (27 oC, 300 g/L, 26% v/v). This resulted in favorable alcohol content 132.90 g/L in comparison to the conventional fermentation process. Keywords: Batch Fermentation, Ethanol, VHG Fermentation, Flocculating Yeast
For VHG fermentation, the temperature effect on substrate consumption and ethanol production rates should be investigated quantitatively to determine the most appropriate operating conditions, as this process imposes stressful conditions on yeast cells. When using high concentrations of sugar in the process, it is recommended to lower the temperature, minimizing cell death and, consequently, avoiding decreases in cell viability due to the increased production of bioethanol, and to maintain high levels of ethanol production when the temperature is rising in the industrial reactor [5,6]. In the production of ethanol via fermentation, shortly after the end of fermentation, it is fundamental to separate the microorganisms (yeast) from the wine. The most conventional way to promote this separation is to use centrifuges, since natural sedimentation becomes unviable due to time constraints and cell deposition is 6,000 times faster by centrifugation than natural sedimentation [7,8]. Alternatively, to decrease the production costs of sugarcane plants installed in the country, researchers were able to successfully select S. cerevisiae yeast strains with flocculant capacity. The advantage of these yeasts over traditional yeasts is that they do not require centrifugation after fermentation. The elimination of this centrifugation step
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