4.1 Conventional dry grind

The five basic steps in the conventional dry-grind ethanol process are milling, liquefaction, saccharification, fermentation and ethanol distillation/ dehydration (Fig. 2). Mashing goes throughout the entire process beginning with mixing the grain meal with water (and possibly backset stillage) to obtain a mash ready for fermentation. Mashing is a wet-cooking process to turn the gelatinized starch into fermentable sugars first with the use of thermostable alpha- amylase and then with amyloglucosidase (Zhao et al., 2008; Solomon et al., 2007; Wu et al., 2007). Starch is the substrate for grain fuel ethanol. Unlike maize, the starch content of sorghum is not the best indicator of ethanol yield obtained by the dry-grind process because this carbohydrate greatly differs in availability or susceptibility to amylases.

The comparatively higher protein content of sorghum compared to maize should be advantageous because the protein is partially degraded into free amino nitrogen compounds during biocatalysis. These compounds are a source of nitrogen for yeast nutrition. However, the relatively lower protein digestibility and nature of the endosperm proteins associated to sorghum counteracts its higher protein concentration. As a result, sorghum mashes almost always contain less free amino nitrogen compared to maize mashes. The use of proteases during or after liquefaction is a good alternative to increase free amino nitrogen in sorghum mashes (Perez-Carrillo & Serna-Saldivar, 2007). Protein digestibility in wet-cooked sorghum is relatively lower compared to other cereals, mainly because of the cross-linking of prolamins. This phenomenon reduces the availability of nitrogenous compound in sorghum mashes needed to support yeast metabolism during fermentation.

Yeast cannot use proteins as source of nitrogen, instead it utilizes amino acids and short peptides (di or tri), indicating the importance of protein fragmentation altogether with starch hydrolysis in mashing. Beyond yeast nutrimental quandary, there are also issues related to starch digestibility that affects the performance of amylolytic enzymes during liquefaction and saccharification. This trend is also related to proteins because of the interaction between protein and starch that in sorghum reduces the susceptibility of this polysaccharide in both native and gelatinized conditions. Sorghum starch has higher gelatinization temperature compared to maize and more prolamin containing bodies within the endosperm, differences that can restrict gelatinization of starch granules (Zhao et al., 2008).

It has been reported that ethanol yields from sorghum decreases as protein content increases; however, at the same protein level, ethanol fermentation efficiency can vary as much as 8%. The difference is higher than typical experimental variations which indicate that additional factors to protein affects starch-conversion rate. In a work reported by Wang et al. (2008), nine sorghum genotypes were selected and used to study the effect of protein availability on efficiency of ethanol fermentation. The results showed a strong positive linear relationship between protein digestibility and fermentation efficiency, indicating the influence, and at the same time, the usefulness of this sorghum grain features as predictor of ethanol yield (Rooney et al., 2007; Wang et al., 2008; Wu et al., 2007; Wu et al., 2010a).

In Fig. 2 a typical process of dry-grind ethanol production is depicted. An average yield of 390 L of ethanol from 1 ton of sorghum can be obtained, but yields as high as 400 L/ ton with fermentation efficiencies of more than 90% has been achieved and reported (Chuck-Hernandez et al., 2009; Perez-Carrillo & Serna-Saldivar, 2007). The Dried Distillers Grains with Solubles (DDGS) obtained in these processes contribute to the economics of biorefineries. The wet distillers grains can be dried to 12% moisture with the aim to produce a shelf-stable byproduct.

Its nutritional composition (39 and 49% of protein and carbohydrates respectively) makes it an excellent option for livestock feed, especially for ruminants.