Abstract
Simultaneous saccharification and fermentation (SSF) is one process option for production of ethanol from lignocellulose. The principal benefits of performing the enzymatic hydrolysis together with the fermentation, instead of in a separate step after the hydrolysis, are the reduced end-product inhibition of the enzymatic hydrolysis, and the reduced investment costs. The principal drawbacks, on the other hand, are the need to find favorable conditions (e.g. temperature and pH) for both the enzymatic hydrolysis and the fermentation and the difficulty to recycle the fermenting organism and the enzymes. To satisfy the first requirement, the temperature is normally kept below 37°C, whereas the difficulty to recycle the yeast makes it beneficial to operate with a low yeast concentration and at a high solid loading. In this review, we make a brief overview of recent experimental work and development of SSF using lignocellulosic feedstocks. Significant progress has been made with respect to increasing the substrate loading, decreasing the yeast concentration and co-fermentation of both hexoses and pentoses during SSF. Presently, an SSF process for e.g. wheat straw hydrolyzate can be expected to give final ethanol concentrations close to 40 g L-1 with a yield based on total hexoses and pentoses higher than 70%.
Highlights
IntroductionBioethanol produced by fermentation of lignocellulosic biomass (second generation bioethanol), from agricultural by-products, forest residues or energy crops, shows many potential advantages in comparison to sugar or starch-derived bioethanol (first generation bioethanol), from both energetic and environmental points of view
Bioethanol produced by fermentation of lignocellulosic biomass, from agricultural by-products, forest residues or energy crops, shows many potential advantages in comparison to sugar or starch-derived bioethanol, from both energetic and environmental points of view
One significant environmental factor is that the reduction in greenhouse gas emission will be larger with lignocellulosic ethanol than for starch-derived ethanol, due to the lower overall oil input required in the process [1]
Summary
Bioethanol produced by fermentation of lignocellulosic biomass (second generation bioethanol), from agricultural by-products, forest residues or energy crops, shows many potential advantages in comparison to sugar or starch-derived bioethanol (first generation bioethanol), from both energetic and environmental points of view. Several compounds present in pretreatment hydrolyzates, which inhibit enzymatic hydrolysis are converted by the fermenting organisms This is a probable explanation behind the higher reported ethanol yields in SSF compared to SHF [51,52]. Xylose-fermenting yeasts, such as Pichia stipitis and Candida shehatae [81,82,83], could potentially be advantageous to use in SSF of materials with high xylan contents Their tolerance to inhibitory compounds in undetoxified lignocellulose hydrolyzates is rather low [84,85], and in addition, a very low and well-controlled supply of oxygen is required for efficient xylose fermentation [86,87,88].
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