Strategies to optimize the production of second generation bioethanol by non-conventional yeasts evaluated through mathematical modeling

Author(s)

Luiz Eduardo Biazi

• Document type: Doctoral Thesis
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Faculdade de Engenharia Química
• Defense date: 2020-02-14

Examining board members

Aline Carvalho da Costa; Rubens Maciel Filho; Anderson de Souza Sant'Ana; Cíntia Regina Sargo; Betânia Hoss Lunelli

• Advisor: Jaciane Lutz Ienczak; Aline Carvalho da Costa

Abstract

Sugarcane bagasse is a byproduct obtained from ethanol production process (1G) and it is a promising lignocellulosic substrate for second-generation bioethanol (2G). As hemicelluloses represent about 30 % of bagasse dry weight, the amount of sugars generated after hydrolysis of this material represents a substantial portion for the second-generation bioethanol production. The hydrolysis of hemicelluloses results in a mixture of xylose, glucose and other monosaccharides. Thus, the transformation of pentoses in bioethanol becomes one of the most important challenges to be solved on scientific and technologic scope considering the production of bioethanol from lignocellulosic biomasses. In this context, yeasts that are naturally capable of fermenting pentoses are desired, like Scheffersomyces stipitis and Spathaspora passalidarum. Considering the fermentation process, temperature is an operational variable that plays a key role on the reaction, with influence on ethanol productivity, substrate consumption, cell growth and even on microorganism viability. Estimation of temperature-dependent parameters is one of the key strategies for kinetic comprehension and process improvement. In this context, an unstructured mathematic model for second-generation ethanol production was developed to describe xylose and glucose consumption, cell growth and ethanol production for both yeasts in function of temperature, when batch fermentations were performed under high cell density strategy. For the yeast S. passalidarum, the proposed model satisfactorily described the fermentation system investigated in the range between 26 and 32 ºC, with a correlation coefficient higher than 0.95 for the model validation. Through model simulation, it was possible to conclude that the highest ethanol productivities can be achieved at temperatures from 30 to 32 ºC. In addition, different strategies were evaluated to increase the fermentative performance of the non-conventional yeasts used in this study, with the aim to confirm their potential to be used in industrial-scale processes. After being submitted to five sequential fed-batch fermentations with cell recycle at 30 ºC using a synthetic medium, the yeast S. passalidarum had significant improvements on its fermentation parameters, reaching an ethanol yield of 91 % at the last fermentation cycle, with a productivity of 1.79 g/L.h, besides increases on specific rates of sugar consumption and ethanol production. Through the application of a mathematical model describing the fed-batch fermentations performed, it was possible to comprehend the effect of the cell recycles on the evolution of the kinetic parameters. Finally, a strategy of adaptation was proposed through successive fermentations with increasing concentrations of a hemicellulosic hydrolysate for the yeast S. stipitis, with the aim to increase the tolerance of the microorganism to the inhibitory compounds present on the hydrolysates. Mathematical models were applied to elucidate the influence of the employed strategy on kinetic parameters, concluding that the greater impact was on parameters related to inhibition by acetic acid. By comparing the adapted and the non-adapted strains, the adaptation strategy was validated, considering the better performance of the adapted strain in increasing concentrations of acetic acid. (AU)

• FAPESP's process: 16/14567-0 - Mathematical modeling of second generation bioethanol production with cell recycle by using the microorganisms Scheffersomyces stipitis and Spathaspora passalidarum
• Grantee: Luiz Eduardo Biazi
• Support Opportunities: Scholarships in Brazil - Doctorate (Direct)