BIOMASS COMPOSITION, PROPERTIES, AND CHARACTERIZATION - Biomass as a Sustainable Energy Source for the Future - page 49

difference, starch is not a structure-building compound, but serves as an easily acces-
sible storage sugar for certain plant species (e.g., corn). The bonds in starch are
weaker than the cellulose intra- and intermolecular bonds, which makes starch much
better digestible, e.g., by human enzymes. Two main molecules can be distinguished:
the linear and helical amylase and the branched amylopectin. Starch generally con-
sists of 10 90 wt% amylopectin (Solomons and Fryhle,

2004). Starch is usually stored in the form of tiny granules and is found in tubers (like
maize, potatoes, wheat, and rice), roots, seeds (cereals), and fruits. Starch can be
decomposed into sugar molecules using hydrolysis (discussed in Chapter 13). Con-
version of these sugars into fuel-grade ethanol by fermentation using yeast already
has been practiced on an industrial scale for some time; this is another example of a
first-generation biofuel: a bioresource suitable for food or animal feed is converted
into a fuel.
Some biomasses, in particular sugarcane and sugar beet, contain substantial
amounts of free sugar (saccharose), which can be directly used for consumption or
converted into first-generation bioethanol using fermentation. Nowadays, also other
sugars

- 20 wt% amylase and 80 -

— derived from hydrolysis of the polymeric carbohydrates — can be fermented

to, e.g., ethanol. This topic is addressed in Chapter 13.

2.3.4 Other Organic Extractives

Apart from the aforementioned organic components, biomass contains a rich variety
of minor organic molecules having very specific functions in the species life. These

comprise alkaloids, antioxidants, aromatic amines, chlorophyll, hormones, vitamins,
natural dyes, and terpenes, to name a few. It is beyond the scope of this topic to go
into details concerning these organics. Kamm et al. (2006) in their topic concerning
biorefineries present a comprehensive overview of these compounds and their
possible uses.

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2.4 INORGANIC COMPOUNDS

The inorganic constituents of biomass are usually present in minor amounts and com-
prise elements that are essential for plant growth. Elements that are necessary for
plants to complete their life cycle are called essential plant nutrients.
Table 2.6 gives a brief overview of such critical nutrient functions, which are
required in varying amounts in plant tissue. Macronutrients (nitrogen, phosphorus,
potassium, calcium, magnesium, and sulfur) are nutrients required in the largest amount
in plants. Micronutrients (iron, copper, manganese, zinc, boron, molybdenum, and chlo-
rine) are required in relatively small amounts. Additional mineral nutrient elements,
which are beneficial to plants but not necessarily essential, include sodium, cobalt, vana-
dium, nickel, selenium, aluminum, and silicon. The nutrient elements differ with respect
to the form in which they are absorbed by the plant, their functions in the plant, their
mobility throughout the plant, and the plant ' s deficiency or toxicity symptoms charac-

teristic for the nutrient.