Bioethanol from Lignocellulosic Biomass - Biotechnology in China: Biofuels and Bioenergy - page 31

Once incorporated into lignin, the three major
monolignols are referred to as p-hydroxyphenyl
(H), guaiacyl (G) and syringyl (S) phenylpropanoid
units, respectively. The most frequent inter-unit
linkage is the β -O-4 linkage, which is also the one
most easily broken chemically. The other linkages
such as the β -5 linkage are more resistant to
chemical degradation.

Fig. 4 Schematic diagram of lignin biosynthesis (Adapted from [ 13 ] with modifications)

adequately elsewhere. For example, fermentation nutrients are usually needed to
nourish ethanologenic microorganisms, either S. ceresive or Zymomonas mobilis
that can be engineered for ethanol production from lignocellulosic biomass, due to
insufficient nutrition in the feedstock, which raises a concern about the supple-
mentation of nutritional components to satisfy the basic requirements for cell
growth and ethanol fermentation.
Like cellulose, hemicelluloses and lignin, nutritional components in lignocel-
lulosic biomass also vary with species, variety, climate, soil fertility and fertil-
ization practice. For major agricultural residues including corn stover, wheat and
rice straw that are abundantly available in China, the protein content is approxi-
mately 5% [ 6 ], much lower than that in starch-based feedstocks like corn.
Meanwhile, ethanologenic microorganisms cannot break down these proteins into
assimilable amino acids, and thus protease treatment, which has been practiced in
ethanol production from starch-based feedstocks, particularly cassava chips, may
be supplemented to hydrolyze proteins, providing nitrogen sources to support
microbial growth and ethanol fermentation. Otherwise, a supplementary nitrogen
source from ammonia or urea needs to be provided. Corn steep liquor (CSL) is a
cost-effective nutrient for providing not only assimilable nitrogen sources but also
trace minerals as well as vitamins, particular for ethanol production from corn
stover, since CSL is a by-product of the corn wet-milling process and its reliable
supply is guaranteed. As for macronutrients such as phosphor, potassium, calcium
and other minerals, they are normally sufficient due to the high ash content, up to
10% in lignocellulosic biomass [ 6 ].