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Barley straw (BS) is considered a good candidate for fuel-ethanol production due to its abundance and high carbohydrate content. Economic viability of the process requires ethanol titer in fermenta...

A co-fermentation process involving Saccharomyces cerevisiae and Fusarium oxysporum was studied, using hydrothermally pretreated wheat straw as substrate. In the first step of the study, we examined liquefaction of the material in a free-fall reactor. Both the enzyme loading and the dry matter content affected severely the liquefaction efficiency. In the second step (simultaneous saccharification and fermentation (SSF) experiments), we found that the enzymatic system of F. oxysporum contributed significantly to substrate hydrolysis, while its metabolic system played a secondary role in fermentation. SSF in the presence of F. oxysporum cells and enzymes gave 62 g L −1 ethanol. In the third step of the study, a semi-consolidated bioprocess was designed in which F. oxysporum culture (submerged or solid-state) was added at the SSF stage along with S. cerevisiae . The addition of solid F. oxysporum culture increased ethanol production by 19%, leading to a final ethanol concentration of 58 g L −1 . The present study proposes a semi-consolidated process combining two microorganisms for the fermentation at high solids concentration of a liquefied material using an in house free fall mixing reactor. The semi-consolidated process proposed not only increased the ethanol yields significantly, but could also lead to lower overall cost of the process by incorporating in-situ enzyme production.

Cotton stalks (CS) are considered a good candidate for fuel-ethanol production due to its abundance and high carbohydrate content, but the direct conversion without pretreatment always results in extremely low yields due to the recalcitrant nature of lignocelluloses. The present study was undertaken to investigate the effect of various chemical and physicochemical pretreatment methods, i.e., alkali, microwave-assisted acid, organosolv, hydrothermal treatment, and sequentially organosolv and hydrothermal pretreatment, on chemical composition of CS and subsequent ethanol production applying pre-hydrolysis and simultaneous saccharification and fermentation (PSSF) at high solid loading. The best results in terms of ethanol production were achieved by the sequential combination of organosolv and hydrothermal pretreatment (32.3 g/L, using 15% w/v substrate concentration and 6 h pre-hydrolysis) with an improvement of 32% to 50% in ethanol production compared to the other pretreatments. Extending pre-hydrolysis time to 14 h and increasing substrate concentration to 20% w/v, ethanol production reached 47.0 g/L (corresponding to an ethanol yield of 52%) after 30 h of fermentation.

BACKGROUND: During the pretreatment of lignocellulosic materials, molecules such as carboxylic acids, furan derivatives and phenolic compounds, which inhibit the growth and ethanol fermentation by bacteria, fungi and yeasts, are produced. The present work determines the tolerance levels of the C5, C6 fermenting fungus Fusarium oxysporum, towards individual model inhibitory compounds on aerobic growth, on lignocellulolytic activities and on fermentative performance. RESULTS During the growth stage, maximum biomass production was more affected than the specific growth rate by the presence of inhibitors. The presence of high concentrations of inhibitors resulted, in most cases, in prolongation of the lag phase. The fermentative performance of F. oxysporum was significantly inhibited by carboxylic acids, while the lignocellulolytic activities were affected to a lesser extent. CONCLUSION: The toxicity tolerance of fF. oxysporum was high enough for aerobic growth in the presence of significant concentrations of inhibitors, which in most cases were higher than those generated from various treatments of lignocellulosic materials, while its fermentative performance was relatively more affected by the presence of inhibitors. The decrease of ethanol production in the presence of weak organic acids could be the main obstacle to the application of F. oxysporum in large-scale bioconversion processes of hydrolysates containing inhibitors. Copyright © 2010 Society of Chemical Industry