Sorghum grain is a naked caryopsis composed of three major anatomical parts: pericarp, germ, and endosperm. The pericarp is composed of epicarp, mesorcarp and endocarp (cross and tube cells). Among cereals, sorghum is the only one that can contain significant amounts of starch granules in the mesocarp cells. The starch-devoid germ is rich in fat, soluble sugars and proteins (albumins and globulins) whereas the endosperm is divided into the single layered aleurone and the starchy endosperm cells positioned in the corneous and floury or chalky regions of the endosperm. The endosperm constitutes the largest fraction of the kernel and where almost all the starch is contained. Similar to maize, sorghum contains 60 to 70% of starch. The endosperm texture and hardness are highly related to the performance of the grain during several stages of ethanol production. In general terms, composition of sorghum is similar to maize with a few small but significant differences mainly in protein and fat concentrations. Sorghum for instance, has an average 1% less fat and 1.5 to 2.0% more crude protein compared to maize. Both sorghum and maize have more than 50% of this protein as prolamins named kafirins and zeins, respectively. In sorghum, approximately half of the prolamin fraction is bound. In contrast, approximately 70% of the maize prolamins are free or alcohol-soluble. There are some sorghum varieties that contain significant amounts of condensed tannins in the testa. These sorghums are classed as type III and have a lower nutritional value compared to other sorghums and maize. This is due to the presence of tannins that bind proteins and inactivate enzymes. As a result, high tannin sorghums may have reduced ethanol yields (Serna-Saldivar, 2010).

One of the most noteworthy differences between sorghum and maize is its starch granule — protein matrix interaction that negatively affects the susceptibility of both proteins and starch to enzyme hydrolyses. These structural differences affect protein digestibility and the speed of dextrins and glucose production during liquefaction and saccharification and thereafter the efficiency of yeast fermentation. Kafirins, despite the high sequence homology with zeins, tend to be less digestible especially after wet-cooking indicating the change in conformational structure attributed to formation of disulphide bonds. This is due to its high hydrophobicity which also makes possible the formation of additional protein aggregates that enhance the formation of more covalent bonds compared to zeins (Wong et al., 2009). Prolamins in the kernel are concentrated in protein bodies arranged among starch granules. The protein body composition in maize and sorghum is also similar, with alpha kafirin in the inner core surrounded by beta and gamma kafirins. The difference with maize is that during wet thermal processes the external part of protein body seems to form a net that makes difficult to access the alpha portion that is in turn more digestible than the beta and gamma counterparts. This phenomenon affects starch digestibility because in sorghum is 15 to 25% less digestible compared to maize. Taylor & Belton (2002) indicate that in sorghum, a complex rather than a simple obstruction mechanism between kafirins and starch is more likely to occur. This is the main reason why sorghum has lower susceptibility to hydrolysis and fermentation and yields less fuel ethanol compared to maize. Besides the starch-protein relationship, some other factors such as mash viscosity, amount of phenolic compounds, ratio of amylose to amylopectin and formation of amylose-lipid complex in the mash, limit the rate of enzymatic hydrolysis and fermentation efficiency during bioethanol production. For instance, starch in amylose-lipid complex cannot be converted into fermentable sugars, reducing conversion rate and final ethanol yield (Wang et al., 2008).