4.1.1 Genetic modified sorghum

Nowadays, advances in transformation and genetic modification in plants make the development of special sorghum cultivars one of the best tactics to overcome the various known factors that reduce ethanol yields. Previous research works have concluded that fermentation efficiencies and ethanol yields are influenced by genotype and chemical composition (Wu et al., 2007, 2008; Zhao et al., 2008). These investigations have determined important traits that enhance or reduce yields. Starch, protein and tannins are the principal components related to ethanol production from sorghum grain and these characteristic can be associated to genotype and also, in the case of starch and protein, to environmental factors as sowing season and location (Wu et al., 2008). Starch composition, specifically the amylose:amylopectin ratio, is related to fermentation efficiency. Raw materials with less amylose are more efficiently converted into ethanol (Wu et al., 2006). The improvement is related to digestibility of starch, reported as higher in waxy types (Rooney & Pflugfelder, 1986). Wu et al. (2006) also attributed the increased efficiency to the lower content of amylose-lipid complexes in mashes.

The DSC thermograms of starches from waxy sorghum and waxy maize are essentially the same: both display a single, smooth endothermic peak, with approximately the same onset, peak, and ending temperatures in the range of 60-80°C. However, in normal sorghum a second peak appears around 85 to 105°C corresponding to an amylose-lipid complex that reduces the availability of starch. Waxy starches are thereby easily gelatinized and hydrolyzed, giving high conversion efficiencies (Wu et al., 2007). Thus, the waxy characteristics improved the susceptibility of the endosperm matrix for low-energy gelatinization, enzymatic hydrolysis and total ethanol production (Wu et al., 2010a).

In the case of proteins, Wu et al. (2010a) indicate that high-lysine, high-protein-digestibility (HD) sorghum lines have been developed. These genotypes have several potential advantages for their use as feedstocks in biorefineries. First, the starch granules swells and pastes more easily at lower temperatures; second, the proteins have improved feed value with higher bioavailability even for monogastrics. Interestingly, these high-lysine genotypes can contain 60% more of this essential amino acid compared to regular counterparts and similar content compared to quality protein maize (QPM) genotypes (Wu et al., 2010a). The enhanced protein digestibility of these lines is attributed to an improved kafirin digestibility as a result of the unique, abnormal and highly invaginated protein bodies. Segregated progeny with HD population lack the kafirin protein body matrix that surround starch granules and restrict swelling and pasting.

While modification in starch and protein digestibility affects ethanol production, one of the most important traits in starch conversion is total starch harvested per area. The primary goal of sorghum breeding programs has been and continues to be the development of high — yielding, drought-tolerant and pest-resistant hybrids. This effort will continue and additional gains in yield can be expected which will result in higher ethanol production from each hectare dedicated to sorghum (Rooney et al., 2007).