Traditional and Emerging Feedstocks for Food and Industrial Bioproduct Manufacturing - Food and Industrial Bioproducts and Bioprocessing - page 24

Calendula oficinalis seed oil properties are similar to that of tung oil with high viscosity
and refractive index and an excellent air-drying tendency, which make these oils excellent
feedstock for binders in paints and coatings (Muuse et al ., 1992 ). Although these crops have
been around for a long time further research is needed to exploit their full potential for
bioproduct development.

1.4 LIGNOCELLULOSIC BIOMASS

The plant cell walls of lignocellulosic biomass are a complex mixture of polysaccharides,
pectin and lignin. Lignocellulosic materials are mainly composed of cellulose (insoluble
fibers of

-1,4-glucan), hemicellulose (non-cellulosic polysaccharides, including xylans,
mannans, and glucans), and lignin (a mixture of complex polyphenolic compounds). In
some cases where fermentation is used to convert biomass into bioproducts, lignocellulosic
biomass components, specifically cellulose and hemicellulose, need to be converted to
fermentable sugars that can be readily fermented to commodity or specialty chemicals by
suitable microorganisms. However, chemical bonding between lignin and cellulose and
hemicellulose makes depolymerization of polysaccharides into their respective monomers
very difficult. To mitigate this problem, lignin content, composition, hydrophobicity and
cross-linking may be modified by selective plant breeding and transgenic techniques.
Reducing the lignin content of the plant through biotechnology could be a viable option as
well. However, considering that lignin plays an important role in plant structure, reduction
of lignin in only specific tissues or cell types rather than in the whole plant could be a better
approach (Shadle et al ., 2007). For example, retaining lignin in xylem cell walls but lowering
lignin content in storage parenchyma cell walls may improve digestibility without
compromising xylem function (Byrt et al ., 2011 ; Simmons et al ., 2010 ).
There are two major sources of lignocellulosic biomass: agricultural crops and forest
resources. Plants with C3 photosynthesis generate high yields of biomass, but take longer to
grow and have a higher content of lignin than C4 plants. On the other hand, C4 plants have
high light, water and nitrogen use efficiency compared with C3 species. C3 plants, such as
poplar ( Populus ), eucalyptus, loblolly pine ( Pinus taeda ), willow ( Salix ) and silver maple
( Acer saccharum ), are some of the forest resources that are getting attention as feedstock for
bioproduct manufacturing. Poplar contains 40% cellulose, 14% hemicellulose and 20%
lignin and can be grown as a short-rotation woody crop because it grows relatively rapidly
at high density. Poplar can produce 9-16 dry tonnes per hectare (4-7 dry tonnes per acre)
biomass annually over a 6-10-year rotation (Alig et al ., 2000 ). Willow yields 10-18 tonnes/
ha/year of biomass depending on the growth conditions and has higher cellulose content,
about 60%, than poplar. But hemicellulose and lignin content of poplar and willow are
similar (Labrecque and Teodorescu, 2005). Eucalyptus, native to Australia but also grown
throughout the world including California and Florida, has been evaluated as a lignocellulosic
feedstock and shows great potential (Rockwood et al ., 2008). It is important that residues
generated during forest product harvesting and processing, as well as biomass that could
become available through initiatives to reduce fire hazards and improve forest health, are
exploited to their full capacity to meet the lignocellulosic feedstock needs of a bio-based
industry (Wright et al ., 2005 ).
C4 plants are ideal as feedstock for bioproduct manufacturing. The grass (Poaceae)
species mostly use the C4 photosynthetic pathway. A multi-institutional research project
funded by the US Department of Energy assessed thirty-four herbaceous species on a wide

β