Effect of low severity dilute-acid pretreatment of barley straw and decreased enzyme loading hydrolysis on the production of fermentable substrates and the release of inhibitory compounds

The objective of this work was to investigate the feasibility of combining low severity dilute-acid pretreatment of barley straw and decreased enzyme loading hydrolysis for the high production of fermentable substrates and the low release of inhibitory compounds. For most of the pretreatments at 160 and 180 °C, the sugar production with 15 FPU (filter paper unit)/g straw was equally high compared to higher enzyme loadings. For the pretreatments at 170 °C an enzyme loading higher than 15 FPU/g straw was necessary to achieve a carbohydrate conversion of 50% or higher. The effect of acid loading on sugar production was discernible only in the experiments with 15 FPU/g straw or higher. The concentration of 5-hydroxymethylfurfural (HMF), levulinic acid and formic acid was kept below 0.7, 0.6 and 0.8 g L−1, respectively, with all experiments. The release of acetic acid and furfural reached toxic levels with experiments at 170 °C and experiments at 180 °C, respectively. Decreasing the enzyme loading did not have a major effect on the release of HMF, furfural and formic acid but resulted in decreased release of acetic acid and levulinic acid at 170 and 180 °C.