74 Introduction to Renewable Biomaterials
cell. Although in most crops sucrose is used only a short-term storage compound and
converted into other forms of storage as more reserves accumulate, in the so-called
sugar crops sucrose is the ultimate form and is accumulated in storage parenchyma
throughout the vegetative season of the plant [17]. This makes sucrose a very accessible
pool of carbohydrates for extraction and conversion. Most crops however store majority
of their easily accessible forms of chemical energy in seeds. When plants mature and are
ready for reproduction, new tissues are synthesised to promote the spread of genetic
material to new territories. The process and methods or reproduction vary between
plant species but ultimately after successful pollination plants produce seeds that
propagate into a new territory. The seeds contain long-term storage compounds: starch
and triglycerides. From the chemical point of view, these are the same substances as the
ones use for the short-term storage; these reserves however are much more abundant.
The biological function of long-term reserves is to fuel the entire germination process
of the plant from the embryo until the development of photosynthetic tissues is
completed. Such process requires a significant amount of chemical energy; therefore
the pool of energy available in seeds is significantly larger than that accumulated in
short-term storage compounds. When plants germinate from their seeds, these storage
compounds are broken down into their respective building blocks by the action of
specific enzymes and are conveyed to the embryo to fuel the germination process [13].
Energy reserves stored in seeds are also accessible for other uses. During thousands of
years of agriculture development, people aimed to maximise the content of these energy
reserves, collect them and use for food and feed before they reach the germination
stage. The pursuit of grain yield has resulted in modern high yielding food varieties
of wheat, corn, soybean, rapeseed and so on where these pools of carbohydrates
and lipids are much larger than they were in the ancestral species. Because of their
original function as energy reserves storage compounds such as starch and triglyc-
erides are relatively easy to break down into their respective building blocks, which c
an be assimilated and converted to other molecules by other organisms like bacteria or
yeast.
Depending on the predominant chemistry present in biomass, feedstocks can be
divided into the following groups:
Sugar feedstocks– This group includes plants that store their reserves in soluble
form as sucrose, such as sugarcane, sweet sorghum and sugar beet. Sucrose is easy to
extract and can be either used directly or hydrolysed to simple sugars with enzymatic
treatment.
Starchy feedstocks– These feedstocks store their reserves as starch granules mainly in
seeds and also fruits or tubers. These include the most important cereal crops such as
corn, wheat, rice and so on as well as tuberous crops: potato or cassava. Starch can be
easily isolated from seeds and tubers and converted into simple sugars with enzymatic
treatment.
Oily feedstocks– These feedstocks store their reserves as triglycerides in oil bodies
situated within the seeds. These include the most important oilseeds such as soybean,
rapeseed, sunflower and oil palm; oils can be extracted from oilseeds with an array of
methods used by edible oil industry and used directly for conversion process into fuels
or chemicals.
Lignocellulose feedstocks– They are composed of coalesced polymers of cellulose,
lignin and hemicelluloses packed together to create a very robust structural biomaterial.
