Page 99 - Biofuels Refining and Performance

82 Chapter Three

Substrate properties, cellulase activity, and hydrolysis conditions (e.g.,
temperature and pH) are the factors that affect the enzymatic hydroly-
sis of cellulose. To improve the yield and rate of enzymatic hydrolysis,
there has been some research focused on optimizing the hydrolysis
process and enhancing cellulase activity. Substrate concentration is one
of the main factors that affect the yield and initial rate of enzymatic
hydrolysis of cellulose. At low substrate levels, an increase of substrate
concentration normally results in an increase of the yield and reaction
rate of the hydrolysis. However, high substrate concentration can cause
substrate inhibition, which substantially lowers the rate of hydrolysis,
and the extent of substrate inhibition depends on the ratio of total sub-
strate to total enzyme [12].
Increasing the dosage of cellulases in the process to a certain extent
can enhance the yield and rate of hydrolysis, but would significantly
increase the cost of the process. Cellulase loading of 10 FPU/g (filter
paper units per gram) of cellulose is often used in laboratory studies
because it provides a hydrolysis profile with high levels of glucose yield
in a reasonable time (48–72 h) at a reasonable enzyme cost. Cellulase
enzyme loadings in hydrolysis vary from 5 to 33 FPU/g substrate, depend-
ing on the type and concentration of substrates. -glucosidase acts as a
limiting agent in enzymatic hydrolysis of cellulose. Adding supplemental
-glucosidase can enhance the saccharification yield [28, 29].
Enzymatic hydrolysis of cellulose consists of three steps [12]: (1) adsorp-
tion of cellulase enzymes onto the surface of cellulose, (2) biodegrada-
tion of cellulose to simple sugars, and (c) desorption of cellulase.
Cellulase activity decreases during hydrolysis. Irreversible adsorption
of cellulase on cellulose is partially responsible for this deactivation.
Addition of surfactants during hydrolysis is capable of modifying the cel-
lulose surface property and minimizing the irreversible binding of cel-
lulase on cellulose. Tween-20 and Tween-80 are the most efficient
nonionic surfactants in this regard. Addition of Tween-20 as an additive
in simultaneous saccharification and fermentation (SSF) at 2.5 g/L has
several positive effects in the process. It increases the ethanol yield,
increases the enzyme activity in the liquid fraction at the end of the
process, reduces the amount of enzyme loading, and reduces the required
time to attain maximum ethanol concentration [30].

3.7 Basic Concepts of Fermentation

The general reaction for ethanol production during fermentation is
Microorganisms
Sugar(s) ⎯⎯⎯⎯⎯→ Ethanol By-products

In this reaction, the microorganisms work as a catalyst.