The presence of glucose in pretreatment liquors was relatively low across most test conditions at about 12% or less (Table 5) and correlated well with increasing pretreatment severities. Only
under extreme pretreatment conditions (high temperature and acid levels) did yields approach 25% theoretical. Although relatively high, these values still fall short of fulfilling the summative
mass closure for glucose recoveries following cellulase digestion.As noted in the preceding section, a substantial proportion of this glucose persists as soluble glucan oligosaccharides and degradation compounds (discussed below) in pretreatment liquors. These observations are consistent with reported findings on steam exploded CGT
ploded CGT (Jeoh and Agblevor, 2001).
It is evident that the formation and accumulation of biomass
degradation products in pretreatment liquors occur predominantly
at higher acid and temperature combinations (Table 5). Of the
three representative degradation products (acetic acid, furfural
and HMF), acetic acid resulting from the hydrolysis of hemicellulose
acetyl groups was the most abundant with levels approaching
21 mg/g (or 2.1 g/L based on 10% solid load). For the most part, acetic
acid levels remained fairly constant over the experimental trials
with midpoint treatments (1.0% H2SO4 at 190 C for 12 min) producing
about 19 mg/g. Acetic acid levels were substantially
reduced at low temperature and acid combinations and correlated
well with poor hemicellulose solubilisation. Furfural levels ranged
between undetected (in the absence of an acid catalyst) and 25 mg/
g, whilst HMF levels were relatively low (c.a. 6 7 mg/g). Surface
response curves for product furans (furfural and HMF) in relation
to pretreatment variables revealed that levels continued to
increase with pretreatment severity, which is consistent with correlating
declining C5/C6 sugars yields and the creation of degradation
products (Fig. 4). Under described conditions for optimal
enzymatic glucose recoveries, total accumulated levels of furans
and acetic acid remained relatively low at