Xylanase contribution to the efficiency of cellulose enzymatic hydrolysis of barley straw.

In this study, different enzyme preparations available from Novozymes were assessed for their efficiency to hydrolyze lignocellulosic materials. The enzyme mixture was evaluated on a pretreated cellulose-rich material, and steam-exploded barley straw pretreated under different temperatures (190, 200, and 210 degrees C, respectively) in order to produce fermentable sugars. Results show that xylanase supplementation improves initial cellulose hydrolysis effectiveness of water-insoluble solid fraction from all steam-exploded barley straw samples, regardless of the xylan content of substrate. The mixture constituted by cellulase: beta-glucosidase: endoxylanase of the new kit for lignocellulose conversion at a ratio 10:1:5% ([v/w], enzyme [E]/substrate [S]) provides the highest increment of cellulose conversion in barley straw pretreated at 210 degrees C, for 10 min.

Effect of binary combinations of selected toxic compounds on growth and fermentation of Kluyveromyces marxianus.

The inhibitory effects of various lignocellulose degradation products on glucose fermentation by the thermotolerant yeast Kluyveromyces marxianus were studied in batch cultures. The toxicity of the aromatic alcohol catechol and two aromatic aldehydes (4-hydroxybenzaldehyde and vanillin) was investigated in binary combinations. The aldehyde furfural that usually is present in relatively high concentration in hydrolyzates from pentose degradation was also tested. Experiments were conducted by combining agents at concentrations that individually caused 25% inhibition of growth. Compared to the relative toxicity of the individual compounds, combinations of furfural with catechol and 4-hydroxybenzaldehyde were additive (50% inhibition of growth). The other binary combinations assayed (catechol with 4-hydroxybenzaldehyde, and vanillin with catechol, furfural, or 4-hydroxybenzaldehyde) showed synergistic effect on toxicity and caused a 60-90% decrease in cell mass production. The presence of aldehydes in the fermentation medium strongly inhibited cell growth and ethanol production. Kluyveromyces marxianus reduces aldehydes to their corresponding alcohols to mitigate the toxicity of these compounds. The total reduction of aldehydes was needed to start ethanol production. Vanillin, in binary combination, was dramatically toxic and was the only compound for which inhibition could not be overcome by yeast strain assimilation, causing a 90% reduction in both cell growth and fermentation.