Effect of pretreatment severity on the conversion of barley straw to fermentable substrates and the release of inhibitory compounds.

The production of fermentable substrates from barley straw under various process conditions was studied. Pretreatment included chemical pretreatment with dilute-acid followed by enzymatic hydrolysis; the pretreatment conditions were expressed in a combined severity factor, CS, which ranged in the present study from -1.6 to 1.1. Considering the production of fermentable sugars and the release of inhibitory compounds, the optimal pretreatment conditions were 170°C, 0% sulfuric acid and 60 min, corresponding to CS -0.4. Under these conditions, 21.4 g glucose/L, 8.5 g xylose/L, and 0.5 g arabinose/L were produced, while 0.1g HMF/L, 0.4 g furfural/L, 0.0 g levulinic acid/L, 0.0 g formic acid/L, and 2.1g acetic acid/L were released. The ratio of Σ sugars/Σ inhibitors proved to be a good tool for evaluating the suitability of a hydrolysate for fermentation purposes.