BIOCHEMICAL CONVERSION: BIOFUELS BY INDUSTRIAL FERMENTATION - Biomass as a Sustainable Energy Source for the Future - page 420

hemicellulose. The production of inhibitors is limited, although this method is not as
efficient as acid hydrolysis or acid-catalyzed steam explosion.

13.3.1.8 Biological Pretreatment This method employs microorganisms such as
white-, brown-, and soft-rot fungi to degrade hemicellulose and lignin. Although the
advantages of this method are that it has a low energy requirement and mild operation
conditions, the rate of biological hydrolysis is usually too low for industrial
implementation.

The best pretreatment method depends on the type of lignocellulosic feedstock.
Current industrial activities for the production of ethanol show a preference toward
acid-based pretreatment methods, probably due to the available information for a
broad range of feedstocks. In acid-based methods, lignin is only removed after
hydrolysis of the cellulose, which is a disadvantage for the hydrolysis step: lignin
binds to the hydrolysis enzymes, reducing their activity and hampering enzyme recov-
ery. In summary, the following aspects are important in the choice of pretreatment
methods: efficiency, generation of inhibitors, neutralization requirement, need for
solvent recovery, technical equipment demands (e.g., corrosion, operating tempera-
ture and pressure), use of chemicals and energy, separability of the resulting fractions,
and waste generation.

13.3.2 Inhibitors and Detoxification

Next to the target fermentable sugars, during pretreatment, other products are formed
to a varying extent depending on the process conditions used. Those products can
have an inhibitory effect on the fermentation process, the level of toxicity depending,
among others, on the concentration in the fermentation medium and on the cell
physiological conditions. The main types of inhibitors are (see also Table 13.8):

TABLE 13.8 Main inhibitors produced during pretreatment

Degradation
products Fraction Hydrolysis products Inhibitory effect on fermentation

Cellulose Hexoses (glucose) HMF, levulinic

acid

Affects sugar assimilation, cell
growth, and respiration

Hemicellulose Pentoses

(xylose, arabinose)

Furfural Affects sugar assimilation, cell

growth, and respiration

Hexoses (mannose,
galactose, glucose)

HMF, levulinic
acid

Affects sugar assimilation, cell
growth, and respiration

Acetic acid Diffuses across cell membranes.

Lowers cell pH

Lignin Phenolic compounds

(among others)

Affects the integrity of cell
membranes. Affects cell
growth and sugar assimilation