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| Author(s): |
Thamiris Guerra Giacon
Total Authors: 1
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| Document type: | Master's Dissertation |
| Press: | São Paulo. |
| Institution: | Universidade de São Paulo (USP). Escola Politécnica (EP/BC) |
| Defense date: | 2021-05-05 |
| Examining board members: |
Thiago Olitta Basso;
Felipe Senne de Oliveira Lino;
Sarita Cândida Rabelo
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| Advisor: | Thiago Olitta Basso |
| Abstract | |
Second generation ethanol (2G) is produced taking advantage of agro-industrial, forestry waste and even municipal waste to obtain lignocellulosic biomass, which is previously treated to release fermentable sugars to obtain ethanol. However, the fermentation of these lignocellulosic hydrolysates still faces many scientific and technological challenges. Pretreatment processes generate a variety of compounds that act as inhibitors of the metabolism of microorganisms that produce ethanol, and thus reduce the efficiency of fermentation. In addition to inhibitors, another problem faced both in the production of first generation (1G) and 2G ethanol is the presence of contaminating microorganisms, especially lactic acid bacteria. In this context, the present study aimed to evaluate the impact of inhibitors present in lignocellulosic hydrolysates from pre-treatment of sugarcane on the physiology of lactic acid bacteria (LAB) contaminating alcoholic fermentation, in the absence and presence of yeasts. Initially, the growth of homo- and heterofermentative LAB was evaluated in the presence of different sugars relevant to the biofuel industry.In these experiments, it was observed that the highest growth rates were obtained in a mixture of glucose and fructose, and only heterofermentative LAB grew in the presence of xylose. In general, the phenolic compounds and organic acids showed negative effects on the two metabolic types of LAB, decreasing the maximum specific growth rate (µmax) compared to the control, without the inhibitors. However, in the presence of furan derivatives (furfural and HMF) a growth stimulus was observed in heterofermentative LAB, with an increase in µmax, from 0.296h-1 of the control to 0.309h-1 in the presence of HMF and 0.414h-1 in the presence of furfural. On the other hand, in homofermentative LAB an inhibitory effect was observed, with a consequent reduction in µmax from 0.358h-1 of the control to 0.297h-1 in the presence of HMF and 0.207h-1 in the presence of furfural. It is likely that both furfural and HMF act on heterofermentative LABs as external electron acceptors, reoxidating enzymatic cofactors and consequently releasing them for biosynthesis reactions, resulting in an increase in µmax. In the experiment in static flasks, in addition to the increase in µmax, the decrease in lactate production and the shift towards acetate production were evidenced, which in view of the inhibitors became a more energetically favorable route for heterofermentative LAB, with a conversion factor to acetate (Yac/s) varying from the control of 0.032g/g to 0.101g/g and 0.068g/g with HMF and fufural respectively. In the presence of furan inhibitors, the co-culitve between LAB and an industrial yeast Saccharomyces cerevisiae, showed that homofermentative LAB had a greater deleterious effect on yeast than heterofermentative LAB, since the yeast cell viability was 50.3% in the first case and 71.1% in the second, apparently due to the higher concentration of lactic acid in the presence of homofermentative. Assessing the performance of these contaminating bacteria against the inhibitory compounds and sugars generated in the pre-treatment is extremely important, as these bacteria are already resistant to the process conditions such as temperature, pH and high ethanol concentration. If, in addition to being resistant to the process conditions, they are also resistant to the inhibitors generated in the pretreatment and consumers of pentose, additional care for the control of these microorganisms should be taken in the production processes of 2G ethanol. (AU) | |
| FAPESP's process: | 19/13826-0 - Physiology of lactic acid bacteria contaminating alcoholic fermentation in the presence of lignocellulosic inhibitors |
| Grantee: | Thamiris Guerra Giacon |
| Support Opportunities: | Scholarships in Brazil - Master |