Dóra Dienes

Sweet Sorghum as Feedstock for Ethanol Production: Enzymatic Hydrolysis of Steam-Pretreated Bagasse

Sweet sorghum is an attractive feedstock for ethanol production. The juice extracted from the fresh stem is composed of sucrose, glucose, and fructose and can therefore be readily fermented to alcohol. The solid fraction left behind, the so-called bagasse, is a lignocellulosic residue which can also be processed to ethanol. The objective of our work was to test sweet sorghum, the whole crop, as a potential raw material of ethanol production, i.e., both the extracted sugar juice and the residual bagasse were tested. The juice was investigated at different harvesting dates for sugar content. Fermentability of juices extracted from the stem with and without leaves was compared. Sweet sorghum bagasse was steam-pretreated using various pretreatment conditions (temperatures and residence times). Efficiency of pretreatments was characterized by the degree of cellulose hydrolysis of the whole pretreated slurry and the separated fiber fraction. Two settings of the studied conditions (190 °C, 10 min and 200 °C, 5 min) were found to be efficient to reach conversion of 85–90%.

Characterisation of Specific Activities and Hydrolytic Properties of Cell-Wall-Degrading Enzymes Produced by Trichoderma reesei Rut C30 on Different Carbon Sources

Conversion of lignocellulosic substrates is limited by several factors, in terms of both the enzymes and the substrates. Better understanding of the hydrolysis mechanisms and the factors determining their performance is crucial for commercial lignocelluloses-based processes. Enzymes produced on various carbon sources (Solka Floc 200, lactose and steam-pre-treated corn stover) by Trichoderma reesei Rut C30 were characterised by their enzyme profile and hydrolytic performance. The results showed that there was a clear correlation between the secreted amount of xylanase and mannanase enzymes and that their production was induced by the presence of xylan in the carbon source. Co-secretion of α-arabinosidase and α-galactosidase was also observed. Secretion of β-glucosidase was found to be clearly dependent on the composition of the carbon source, and in the case of lactose, 2-fold higher specific activity was observed compared to Solka Floc and steam-pre-treated corn stover. Hydrolysis experiments showed a clear connection between glucan and xylan conversion and highlighted the importance of β-glucosidase and xylanase activities. When hydrolysis was performed using additional purified β-glucosidase and xylanase, the addition of β-glucosidase was found to significantly improve both the xylan and glucan conversion.

Cellulase production using different streams of wheat grain- and wheat straw-based ethanol processes

Pretreatment is a necessary step in the biomass-to-ethanol conversion process. The side stream of the pretreatment step is the liquid fraction, also referred to as the hydrolyzate, which arises after the separation of the pretreated solid and is composed of valuable carbohydrates along with compounds that are potentially toxic to microbes (mainly furfural, acetic acid, and formic acid). The aim of our study was to utilize the liquid fraction from steam-exploded wheat straw as a carbon source for cellulase production by Trichodermareesei RUT C30. Results showed that without detoxification, the fungus failed to utilize any dilution of the hydrolyzate; however, after a two-step detoxification process, it was able to grow on a fourfold dilution of the treated liquid fraction. Supplementation of the fourfold-diluted, treated liquid fraction with washed pretreated wheat straw or ground wheat grain led to enhanced cellulase (filter paper) activity. Produced enzymes were tested in hydrolysis of washed pretreated wheat straw. Supplementation with ground wheat grain provided a more efficient enzyme mixture for the hydrolysis by means of the near-doubled β-glucosidase activity obtained.