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Background: Robust yeasts with high inhibitor, temperature, and osmotic tolerance remain a crucial requirement for the sustainable production of lignocellulosic bioethanol. These stress factors are known to severely hinder culture growth and fermentation performance. Results: Grape marc was selected as an extreme environment to search for innately robust yeasts because of its limited nutrients, exposure to solar radiation, temperature fluctuations, weak acid and ethanol content. Forty newly isolated Saccharomyces cerevisiae strains gave high ethanol yields at 40°C when inoculated in minimal media at high sugar concentrations of up to 200 g/l glucose. In addition, the isolates displayed distinct inhibitor tolerance in defined broth supplemented with increasing levels of single inhibitors or with a cocktail containing several inhibitory compounds. Both the fermentation ability and inhibitor resistance of these strains were greater than those of established industrial and commercial S. cerevisiae yeasts used as control strains in this study. Liquor from steam-pretreated sugarcane bagasse was used as a key selective condition during the isolation of robust yeasts for industrial ethanol production, thus simulating the industrial environment. The isolate Fm17 produced the highest ethanol concentration (43.4 g/l) from the hydrolysate, despite relatively high concentrations of weak acids, furans, and phenolics. This strain also exhibited a significantly greater conversion rate of inhibitory furaldehydes compared with the reference strain S. cerevisiae 27P. To our knowledge, this is the first report describing a strain of S. cerevisiae able to produce an ethanol yield equal to 89% of theoretical maximum yield in the presence of high concentrations of inhibitors from sugarcane bagasse. Conclusions: This study showed that yeasts with high tolerance to multiple stress factors can be obtained from unconventional ecological niches. Grape marc appeared to be an unexplored and promising substrate for the isolation of S. cerevisiae strains showing enhanced inhibitor, temperature, and osmotic tolerance compared with established industrial strains. This integrated approach of selecting multiple resistant yeasts from a single source demonstrates the potential of obtaining yeasts that are able to withstand a number of fermentation-related stresses. The yeast strains isolated and selected in this study represent strong candidates for bioethanol production from lignocellulosic hydrolysates.

BackgroundExpanding biofuel markets are challenged by the need to meet future biofuel demands and mitigate greenhouse gas emissions, while using domestically available feedstock sustainably. In the context of the sugar industry, exploiting under-utilized cane leaf matter (CLM) in addition to surplus sugarcane bagasse as supplementary feedstock for second-generation ethanol production has the potential to improve bioenergy yields per unit land. In this study, the ethanol yields and processing bottlenecks of ammonia fibre expansion (AFEX™) and steam explosion (StEx) as adopted technologies for pretreating sugarcane bagasse and CLM were experimentally measured and compared for the first time.ResultsEthanol yields between 249 and 256 kg Mg−1 raw dry biomass (RDM) were obtained with AFEX™-pretreated sugarcane bagasse and CLM after high solids loading enzymatic hydrolysis and fermentation. In contrast, StEx-pretreated sugarcane bagasse and CLM resulted in substantially lower ethanol yields that ranged between 162 and 203 kg Mg−1 RDM. The ethanol yields from StEx-treated sugarcane residues were limited by the aggregated effect of sugar degradation during pretreatment, enzyme inhibition during enzymatic hydrolysis and microbial inhibition of S. cerevisiae 424A (LNH-ST) during fermentation. However, relatively high enzyme dosages (> 20 mg g−1 glucan) were required irrespective of pretreatment method to reach 75% carbohydrate conversion, even when optimal combinations of Cellic® CTec3, Cellic® HTec3 and Pectinex Ultra-SP were used. Ethanol yields per hectare sugarcane cultivation area were estimated at 4496 and 3416 L ha−1 for biorefineries using AFEX™- or StEx-treated sugarcane residues, respectively.ConclusionsAFEX™ proved to be a more effective pretreatment method for sugarcane residues relative to StEx due to the higher fermentable sugar recovery and enzymatic hydrolysate fermentability after high solids loading enzymatic hydrolysis and fermentation by S. cerevisiae 424A (LNH-ST). The identification of auxiliary enzyme activities, adequate process integration and the use of robust xylose-fermenting ethanologens were identified as opportunities to further improve ethanol yields from AFEX™- and StEx-treated sugarcane residues.Electronic supplementary materialThe online version of this article (10.1186/s13068-018-1130-z) contains supplementary material, which is available to authorized users.

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Biorefineries are regarded as a key enabler to facilitate the transition toward a sustainable and low carbon bioeconomy. In this study, alternative lignocellulose biorefineries annexed to a typical sugar mill were investigated to co‐produce ethanol (EtOH), lactic acid (LA), and/or electricity, utilizing bagasse and a component of harvesting residues (brown leaves) as feedstock. Studied scenarios included EtOH as the sole product from glucose and xylose (Scenario 1), LA as the sole product from these two sugars (Scenario 2), EtOH from glucose and LA from xylose (Scenario 3), and EtOH from xylose and LA from glucose (Scenario 4), all of which were associated with some level of export electricity production. Aspen Plus® simulations of the scenarios were developed considering all supplementary units, such as evaporation, water treatment, boiler, and steam/power generation in addition to the process units required for the main product streams. Economic evaluation, energy assessment, and environmental life cycle assessment (LCA) were carried out on the developed simulations, in a multi‐criteria analysis of the desirability of each scenario. To service the combined energy demands of a sugar mill and annexed biorefinery, a 35 to 40% bypass of lignocellulose directly to the boiler section was required to achieve integrated scenarios that were energy self‐sufficient, i.e., not dependent on external (fossil) energy sources. Scenario 2 was economically most attractive with the highest internal rate of return (IRR) of 31.1%, whereas Scenario 1 had the lowest IRR of 10.0%. Scenarios 2 and 4 were economically the most robust, with the least sensitivity to variations in the key economic drivers, i.e., EtOH, LA and enzymes. The LCA suggested that LA producing scenarios introduced environmental burdens that were marginally higher than Scenario 1, due to higher consumption of processing chemicals. Overall, Scenario 4 was found to be the most desirable biorefinery scenario, by balancing financial and environmental considerations. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd

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