Efficient saccharification of lignocellulosic biomass requires concerted development of a pretreatment method, an enzyme cocktail and an enzymatic process, all of which are adapted to the feedstock. Recent years have shown great progress in most aspects of the overall process. In particular, increased insights into the contributions of a wide variety of cellulolytic and hemicellulolytic enzymes have improved the enzymatic processing step and brought down costs. Here, we review major pretreatment technologies and different enzyme process setups and present an in-depth discussion of the various enzyme types that are currently in use. We pay ample attention to the role of the recently discovered lytic polysaccharide monooxygenases (LPMOs), which have led to renewed interest in the role of redox enzyme systems in lignocellulose processing. Better understanding of the interplay between the various enzyme types, as they may occur in a commercial enzyme cocktail, is likely key to further process improvements.
Background: A possible future shortage of feed protein will force mankind to explore alternative protein sources that can replace conventional soymeal or fishmeal. Several large industrial organic side-streams could potentially be upgraded to feed protein using a fermentation process to generate single cell protein. Yeast is the most widely accepted microorganism for production of single cell protein, because of its superior nutritional quality and acceptability among consumers. Here, we have assessed the growth of four different yeasts, Cyberlindnera jadinii, Wickerhamomyces anomalus, Blastobotrys adeninivorans and Thermosacc ® Dry (Saccharomyces cerevisiae), on media composed of enzymatically saccharified sulfite-pulped spruce wood and hydrolysates of by-products from chicken, and we have characterized the resulting yeast biomass. Results: Generally, the yeast grew very well on the spruce-and chicken-based medium, with typical yields amounting to 0.4-0.5 g of cell dry weight and 0.2-0.3 g of protein per g of sugar. B. adeninivorans stood out as the most versatile yeast in terms of nutrient consumption and in this case yields were as high as 0.9 g cells and 0.5 g protein per g of sugar. The next best performing yeast in terms of yield was W. anomalus with up to 0.6 g cells and 0.3 g protein per g sugar. Comparative compositional analyses of the yeasts revealed favorable amino acid profiles that were similar to the profiles of soymeal, and even more so, fish meal, especially for essential amino acids. Conclusions: The efficient conversion of industrial biomass streams to yeast biomass demonstrated in this study opens new avenues towards better valorization of these streams and development of sustainable feed ingredients. Furthermore, we conclude that production of W. anomalus or B. adeninivorans on this promising renewable medium may be potentially more efficient than production of the well-known feed ingredient C. jadinii. Further research should focus on medium optimization, development of semi-continuous and continues fermentation protocols and exploration of downstream processing methods that are beneficial for the nutritional values of the yeast for animal feed.
The conversion of nonedible biomass to protein for use in feed is an attractive strategy toward improved sustainability in aquaculture. We have studied the possibility to produce protein-rich yeast Candida utilis on a medium consisting of enzymatically hydrolyzed sulphite-pulped spruce wood, mainly providing glucose, and enzymatically hydrolyzed brown seaweed, supplemented with ammonium sulfate. The results show that this blend constitutes a complete fermentation medium that enables good growth rates and cell yields. Results from a salmon feeding trial showed that the yeast can replace parts of a traditional fishmeal diet without harmful effects, although the apparent protein digestibility coefficient for the yeast was suboptimal. While further optimization of both the fermentation process and downstream processing is needed, the present proof-of-concept study shows a path to the production of microbial protein based on a simple, local and sustainable fermentation medium.
The recent discovery that impregnation with a carbocation scavenger may improve the enzymatic saccharification of steam-exploded softwood has brought a softwood-based biorefinery closer to reality. However, the nature of the impregnation effect remains unresolved, and its impact on process efficiency and product quality in high-dry matter reactions remains underexplored. Here, we show that 2-naphthol impregnation enables the complete saccharification of spruce cellulose by lytic polysaccharide monooxygenase (LPMO)-containing Cellic CTec2, but not by an LPMO-poor cellulase cocktail (Celluclast), in 10% dry matter reactions with an industrially feasible enzyme dose and reaction time. Importantly, we show that this remarkably high saccharification yield correlates with increased LPMO activity, which is due to the impact of 2-naphthol on the ability of lignin to drive the LPMO reaction. These findings show that impregnation improves saccharification not only by reducing cellulase adsorption and inactivation but also by boosting oxidative cellulose depolymerization by LPMOs. Pyrolysis of the lignin-rich saccharification residues revealed that 2-naphthol impregnation had little effect on lignin-derived components in the resulting bio-oil, which, due to the efficient saccharification, showed reduced levels of carbohydrate-derived components that reduce oil storage stability. These results bring closer the prospect of a spruce-based biorefinery that combines biochemical and thermochemical conversion routes.
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