Biofuels from Lignocellulosic Biomass - Sustainable Biotechnology: Sources of Renewable Energy - page 38

Fig. 5 Effects of pretreatment on different components in biomass and actions of non-complexed
cellulases on celluloses [39, 42]

Factors impacting the activity of cellulases include enzyme source (e.g. organ-
isms and producing conditions), concentration, and combinations. The normal
enzyme dose for cellulose hydrolysis study is 10-60 FPU per gram of dry cellulose
or glucan; glucanases to

-
glucosidase for each FPU of glucanase used [29]. Most commercial glucanases are
produced by Trichoderma reseii and the

β -glucosidase ratio is approximately 1.75-2.0 IU of β

β -glucosidase is typically from Aspergillus

niger [43].
Under research conditions, the reported digestibility or the conversion yield of
cellulose from pretreated lignocellulose can be high (Table 4). However, actual glu-
cose yield may vary greatly depending on the type of biomass, method/condition
of pretreatment, cellulases (composition, source, and dose), solid to liquid ratio
of the hydrolysis mixture, and other unspecified factors. The cellulose digestibil-
ity of corn stover and corn fiber can reach >90% following dilute acid or liquid
hot water pretreatment [44], while the digestibility of rice hulls after similar pre-
treatment was about 50% [45]. Similar low digestibility results were obtained on
dilute acid pretreated sorghum stubble in our lab (unpublished data). The vari-
able digestibility of different biomass sources following dilute acid pretreatment
may be an indication that this particular pretreatment is not universally effec-
tive. Currently, all the reported results for AFEX [44] and alkaline peroxide [44,
46] treated biomass sources showed consistently high cellulose recovery, and high
digestibility, even at lower enzyme concentrations and shorter incubation time (48 h
vs normal 96 h) [47] .
Digestibility, or glucose yield, is high when cellulose load is low (1-3% cellu-
lose load) in the hydrolysis system. Glucose yield from pretreated biomass typically
increases as enzyme load increases [47, 48, 49], while digestibility decreases as the
cellulose load increases [48, 50]. We are unaware of any reports of >20% cellu-
lose load with high digestibility. Starch-based ethanol production involves starch
loadings of 20-25% or higher, that results in finished beers with ethanol concen-
tration around 10-12% (w/v). Most lignocellulosic ethanol fermentation studies
have used hydrolysates with 3-10% cellulose load, which resulted in a finished
mash with ∼ 3-5% (V/V) ethanol. Additional research is required to improve the