Acetic acid was undetectable by HPLC in water-washed samples that underwent hydrolysis for 48 h (Table 4), except for the
substrate that was produced under the lowest pretreatment sever- ity. This indicates that elephant grass hemicelluloses were almost completely deacetylated above a severity factor of 3.13, which cor- responds to 180 C for 6 min, and this is consistent with the low acetyl group content of these substrates as reported elsewhere (Scholl et al., 2015). Hence, the amount of acetic acid that was detected in all enzymatic hydrolysates derived from unwashed samples corresponds to the amount of this analyte in the fraction of the pretreatment liquid stream (C5 fraction) that was retained in the pretreated substrate after filtration. Not surprisingly, acetic acid was not found in the enzymatic hydrolysates of Avicel and Celuflock because the former is a hemicellulose-free commercial form of microcrystalline cellulose while the latter is originated from a pulping process in which acetyl groups are known to be completely hydrolysed and washed away from the cellulose fibers.