Abstract
BackgroundPresence of inhibitory chemicals in lignocellulose hydrolysates is a major hurdle for production of second-generation bioethanol. Especially cheaper pre-treatment methods that ensure an economical viable production process generate high levels of these inhibitory chemicals. The effect of several of these inhibitors has been extensively studied with non-xylose-fermenting laboratory strains, in synthetic media, and usually as single inhibitors, or with inhibitor concentrations much higher than those found in lignocellulose hydrolysates. However, the relevance of individual inhibitors in inhibitor-rich lignocellulose hydrolysates has remained unclear.ResultsThe relative importance for inhibition of ethanol fermentation by two industrial second-generation yeast strains in five lignocellulose hydrolysates, from bagasse, corn cobs and spruce, has now been investigated by spiking higher concentrations of each compound in a concentration range relevant for industrial hydrolysates. The strongest inhibition was observed with industrially relevant concentrations of furfural causing partial inhibition of both D-glucose and D-xylose consumption. Addition of 3 or 6 g/L furfural strongly reduced the ethanol titer obtained with strain MD4 in all hydrolysates evaluated, in a range of 34 to 51% and of 77 to 86%, respectively. This was followed by 5-hydroxymethylfurfural, acetic acid and formic acid, for which in general, industrially relevant concentrations caused partial inhibition of D-xylose fermentation. On the other hand, spiking with levulinic acid, 4-hydroxybenzaldehyde, 4-hydroxybenzoic acid or vanillin caused little inhibition compared to unspiked hydrolysate. The further evolved MD4 strain generally showed superior performance compared to the previously developed strain GSE16-T18.ConclusionThe results highlight the importance of individual inhibitor evaluation in a medium containing a genuine mix of inhibitors as well as the ethanol that is produced by the fermentation. They also highlight the potential of increasing yeast inhibitor tolerance for improving industrial process economics.
Highlights
Presence of inhibitory chemicals in lignocellulose hydrolysates is a major hurdle for production of second-generation bioethanol
To identify the major fermentation inhibitors for 2G bioethanol production present in five lignocellulose hydrolysates from different biomass origin, the fermentation performance of industrial yeast strains GSE16T18 and MD4 was assessed in the hydrolysates spiked with different concentrations of the inhibitors (Table 2)
The relevance of different inhibitors in five lignocellulose hydrolysates was determined by spiking with industrially relevant concentrations
Summary
Presence of inhibitory chemicals in lignocellulose hydrolysates is a major hurdle for production of second-generation bioethanol. Cheaper pre-treatment methods that ensure an economical viable pro‐ duction process generate high levels of these inhibitory chemicals. Methods have been developed for detoxification of lignocellulose hydrolysates and mild pre-treatment technologies have been devised that generate only low levels of inhibitors [12, 15, 17], these technologies all add significant additional cost to the industrial process, making the achievement of economic viability even more challenging. The harsh but cheap pre-treatment technologies generate much higher levels of toxic chemicals than the more expensive, mild technologies [15, 18, 19], which underscores the importance of yeast inhibitor tolerance for reaching economic viability in fermentation processes with lignocellulosic biomass [20]
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