Challenges in Producing a Fermentable Sugar Syrup from Second Generation Feedstocks

In recent years a lot of attention has been paid to biofuel production and the transition from first to second generation feedstocks, mainly from the perspective of reducing the use of fossil resources and avoiding competition with the food chain. These lignocellulosic materials can be used for fermentative production of other biochemicals, building blocks or biomaterials, making them attractive for a biobased economy. The many aspects that need to be considered for process design and industrial implementation for second generation sugar syrup, from technical, cost and sustainability perspective, are reviewed in this paper. The major classes of lignocellulosics, being cereal residues, hardwood and softwood, their compositions, and the main pretreatment methods, in view of efficiency in opening up the cell wall matrix and their potential to minimize the production of inhibitors, are compared. Still, due to naturally present potentially inhibitory sugars, as well as the inevitable production of some inhibitors in pretreatment, a detoxification step is likely necessary to prevent carry-over in the final product. In-depth studies have generated a very detailed view on enzymes required to saccharify the pretreated lignocellulosics efficiently, including the description of a new mechanism by the lytic polysaccharide monooxygenase. The enzyme classification, specificity, and their limitations, either substrate, enzyme or process related, are reviewed. Different ways to deal with minor impurities in the sugar syrup, from fermentation operational point of view, are deliberated upon. Finally, economic analysis in combination with life cycle assessment to value sustainability aspects, is discussed and shows the requirement for an integrated multidisciplinary approach to clear the way for production of universally fermentable sugar syrup from lignocellulosics.