Biomass Feedstock Storage for Quantity and Quality Preservation - Engineering and Science of Biomass Feedstock Production and Provision - page 175

7.4.5 Ensiling

Ensiling consists of fermentation that occurs in anaerobic conditions and in the
presence of molds and yeasts [ 20 ]. The by-products of fermentation are organic
acids, and their dissociation results in protons and a lower pH. Low pH levels inhibit
microbial activity and the resulting dry matter loss. The model adapted from Pitt
et al. [ 22 ] includes the factored effect of pH change on the rate of microbial oxida-
tion and can be developed to predict the effect of pH change on anaerobic microbial
activity. Tanjore et al. [ 28 ] compared the impact of drying, freezing, and refrigera-
tion prior to ensiling on the corn stover quality. They concluded that drying and
refrigeration led to irreversible changes in the biomass quality.

7.4.6 Additives

A number of additives have been investigated for the purpose of reducing dry matter
loss. In particular, salts and acids have been applied to biomass and carbon dioxide
is a commonly used gas for inhibiting microbial activity.

7.4.6.1 Salts

Salt is one additive that would reduce the water activity as it increases osmotic
potential and would avoid the high costs associated with drying. If salt is obtained
as an industry by-product, and from a nearby location with manageable transporta-
tion cost, it might be cost-effective to add it to the stored biomass. Water activity
affects microbial respiration and, therefore, dry matter losses by oxidation. Microbial
activity is reduced as water availability reduces.

7.4.6.2 Acids

Acids are typical additives in biomass storage systems. Lower pH inhibits microbial
activity, both in the case of aerobes and anaerobes. In ensiling, acids are self-
generated by the anaerobic fermentation, leading to a negative feedback and the
inhibition of microbial activity in the ensiled biomass. Sulfuric or hydrochloric acid
is typically used, but they are costly to remove [ 16 ]. Acids need to be removed to
raise the pH and create a suitable environment for cellulase-producing fungi for
hydrolysis and yeasts for fermenting the resulting sugar to alcohol. The pH levels
affect the fermentation of glucose into alcohol by the β-glucosidase enzyme as
shown in Table 7.3 . β-glucosidase was used at its optimal level to obtain the shortest
conversion time of cellulose to ethanol, 0.7-0.8 unit/mL. To maintain the optimal
level of β-glucosidase, pH value is critical at the beginning of ethanol production
process (see Table 7.3 ), and an optimal pH was found to be 4.5.