Biomass Feedstock Storage for Quantity and Quality Preservation - Engineering and Science of Biomass Feedstock Production and Provision - page 178

7.5.1.3 Particle Size

Shredding biomass to smaller particle sizes creates more surface area to be exposed
to enzymatic activity. Particle size reduction also mechanically breaks down the
lignin coat surrounding microfibrils, thus exposing some of the cellulose to cellu-
lase enzymes. Particle size can be reduced either by shredding to a small particle
size during harvesting or by milling (wet or dry) or crushing (dry). Hammer mills
are used for dry crushing. However, smaller particles are more susceptible to ero-
sion by wind and rain. Therefore, from a bale or outdoor storage perspective, larger
particle sizes may be desirable.

7.5.2 Pretreatment

In this section, the possible pretreatment options to reduce the biomass recalcitrance
are presented.

7.5.2.1 Acid Hydrolysis

Acids used as additives include hydrochloric and sulfuric acids. These acids break
down lignin into acid soluble lignin (ASL), but are costly to remove. It is necessary
to rinse acids form biomass to increase the pH back to levels at which sugar-
fermenting yeasts can ferment C6 sugars to alcohol. Raising the pH after acid treat-
ment could result in dry matter losses through leaching of water-soluble sugars
during the temperature increase process. Lignin and lignin residues can affect fur-
ther cellulose hydrolysis by cellulase enzymes. Gregg et al. [ 38 ] showed that cel-
lulases are absorbed on lignin as well as the lignaceous residues from hydrolyzed
biomass. Cellulases then become unavailable for further hydrolysis reaction, and
therefore an increase in lignin lowers cellulose hydrolysis rate (Fig. 7.7 ).

7.5.2.2 Use of Enzymes

Enzymes including laccase, lignin peroxidases, and manganese peroxidases can
degrade lignin. The latter two enzymes are produced by the groups of the white-rot
fungus Phanerochaete chrysosporium . Hatakka [ 39 ] tested several fungi groups
according to the lignin enzymes they produce and found that the lignin-manganese
peroxidase fungi was the most efficient at breaking down lignin.

7.5.2.3 Ultrasound

Khanal et al. [ 40 ] reported that corn mash was sonicated (treated with contact ultra-
sounds) at a peak to peak amplitude of 180-299 μm. With ultrasonic treatment for
40 s at a power output of 475 ± 15 W, cells were almost completely disintegrated and