Biodiesel typically consists of a mixture of fatty acid alkyl esters obtained by transesteri­fication of oils or fats, which are normally composed of 90-98% triglycerides, much smaller levels of mono — and diglycerides and free fatty acids, and residual amounts of phospholipids, phosphatides, carotenes, tocopherols, sulphur compounds, and water (Bozbas, 2008).

The biomass left after biodiesel production takes the form of an oilcake containing glycerol as a byproduct of transesterification. This compound may then be refined and sold to the pharmaceutical industry or else used livestock feed (Shirvani, Yan et al., 2011). The oilcake stores 35-73% of its total energy as carbohydrates and proteins (Hu, Sommerfeld et al., 2008), and three distinct options may be considered: (1) an adjacent coal-fired power system that co-fires the left biomass (Xu et al., 2006); (2) a direct combus­tion of the oilcake in an integrated biomass-heating system (Amaro et al., 2011); or (3) a bio­mass combined heat and power unit (Huang and Wang, 2004) for energy cogeneration (and, indirectly, electricity production) (Bozbas, 2008; Mata, Martins et al., 2010; Yang, Guo et al., 2011).

Despite these possibilities, most oilcakes from microalgal origin are fermented into ethanol or methane or else to H2 via anaerobic digestion. Alternatively, they may be incorporated in livestock feed or simply used as organic fertilizer, owing to a particularly high N:P ratio.

In this section, some aspects of algal biology and biochemistry are introduced in view of their relevance to the underlying economics. The composition of algal biomass in terms of polysaccharides, proteins, lipids, pigments, iodine, phenols, and halogenated compounds is expected to critically determine its overall value.

10.3.1 Polysaccharide Material

Algae contain large contents of polysaccharides, notably as contributors to cell-wall structure but also as storage polysaccharides (Holdt and Kraan, 2011). Polysaccharides are polymers of simple sugars (monosaccharides) linked by glycosidic bonds. They entertain nu­merous commercial applications as stabilizers, thickeners, and emulsifiers in food (including beverages) and feed (Tseng, 2001).

Different groups of algae produce specific types of polysaccharides; for example, green algae produce starch for energy storage, which consists of both amylose and amylopectin, in a way similar to higher plants (Williams and Laurens, 2010). Their total concentrations range from 4-76% of dry weight (Holdt and Kraan, 2011).

On the other hand, macroalgae have a low lipid content, and even though their carbohy­drate content is normally high, most of it is accounted for by dietary fibers that are not taken up by the human body but are rather utilized as a bulking agent (Holdt and Kraan, 2011).

The polysaccharides consist mainly of cellulose and hemicelluloses as well as neutral polysaccharides, yet cell-wall and storage polysaccharides are species-specific: green algae may contain sulphated galactans and xylans, whereas brown algae may have alginic acid, fucoidan (sulphated fucose), laminarin (p-1,3 glucan), and sargassan, and red algae may contains agars, carrageenans, xylans, floridean starch (amylopectin-like glucan), and water- soluble sulphated galactan as well as porphyran (Chandini, Ganesan et al., 2008). Cyanobacteria can produce cyanophycin and multi-L-arginyl-poly-L-aspartic acid (Williams and Laurens,

2010) , but the contents of both common and species-specific polysaccharides undergo seasonal variations (Holdt and Kraan, 2011).

Algal polysaccharides can be classified as dietary fibers and hydrocolloids, as is done in the following sections, but they usually possess more than just one type of functional group.

10.3.1.1 Dietary Fibers

These kinds of polysaccharides are very diverse in chemical structure and in composition in the algal biomass. Edible marine macroalgae contain 33-62% total fibers (on a dry-weight basis), quite a bit higher than in higher plants, and such fibers are rich in soluble fractions (Dawczynski, Schubert et al., 2007). Recall that dietary fibers maybe insoluble (e. g., cellulose, mannans, and xylan) or water-soluble (e. g., agars, alginic acid, furonan, laminaran, and porphyran, addressed in further detail in the next subsection).

These algal fibers are commonly extracted by precipitation, as described by Venugopal (2008), and may be used as nutraceuticals for functional food formulation (Holdt and Kraan,

2011) . Examples of polysaccharides bearing antitumor and antiherpetitic bioactivity (among others) are tabulated in Table 10.2.