Sweet sorghum juice can be used for syrup, molasses, sugar and ethanol production with average fermentation efficiencies from 85 to 90% (Almodares & Hadi, 2009; Prasad et al., 2007; Wang et al., 2009; Wu et al., 2010b). The sweet sorghum juice is not commonly used for crystallized sugar production because of the presence of significant amounts of inverted sugars (glucose and fructose) that makes difficult crystallization in large-scale processes. However, the sweet sorghum juice, rich in fermentable sugars, has an excellent potential for yeast fermentation (Turhollow et al., 2010; Woods, 2000).

The sweet sorghum juice is obtained through a mechanical operation with a roller mill composed by a set of cylinders, similar to the ones employed by the sugar cane mills. Water is added during the last stage of the crushing process with the aim to augment the solubilization of residual sugars associated to the bagasse. The sweet sorghum juice yields around 50% in relation to the initial weight of the stems (Wu et al., 2010b). However, these authors describe an extraction process by pressing, which results in lower yields compared to roller mills. Furthermore, pressing is a batch process which is difficult to optimize for industrial conditions.

Approximately 90% of fermentable sugars from sorghum stalks can be obtained after conventional roller-mining, yielding an extraction ratio of 0.7 in relation to the initial plant weight (Almodares & Hadi, 2009). Gnansounou et al. (2005) reported extraction ratios ranging from 0.59 to 0.65 for the sweet sorghum cultivars Kelley, Wray, Rio and Tianza. On the other hand, Kundiyana (1996) observed that extraction percentages varied between 47 to 58%, close to values observed by our research group in central Mexico (unpublished data). After extraction, the sweet sorghum juice is fermented, distilled and the ethanol finally dehydrated (Fig. 1). This is the simplest way to produce fuel ethanol because the grain and fiber processes require the hydrolysis of starch and fiber components into fermentable sugars. These steps are considered expensive, take time and expend energy and other additional resources (i. e. enzymes, chemical reagents, etc.) (Fig. 2 and 3). Despite these benefits, some challenges must be solved in order to efficiently convert the sweet sorghum crop into fuel ethanol. The main setbacks are the relatively higher rate of sugar degradation at ambient temperature and the low nitrogen content for yeast growth (Mei et al., 2009; Wu et al., 2010b). Thus, the logistics of just in time harvesting and the storage of the feedstock in facilities that retard decomposition and degradation of fermentable carbohydrates should be considered and stressed. In relation to nitrogen availability, this disadvantage can be overcome with the supplementation of urea, ammonia or yeast extract in order to avoid sluggish fermentation.

Besides sugar and nitrogen content, fermentation performance of sweet sorghum juice can also be affected with processing parameters and bioreactor configuration. Nuanpeng et al. (2011) observed in a repeated-batch study that very high gravity (VHG) fermentation is a good alternative to produce high ethanol concentrations from sweet sorghum juice when an adequate level of yeast cell concentration, nitrogen, and agitation are used. On the other hand, Laopaiboon et al. (2007) reported better results in fed-batch fermentation compared to batch configuration, in terms of ethanol concentration and product yield but not in productivity (measured as grams of ethanol generated/L/hr). These findings indicate the need to optimize parameters as feeding and withdrawn rate in order to optimize yields.

The microorganism used, as indicated in the next sections, is also a factor that is worthwhile exploring. In the case of sweet sorghum juice, fermentation with different yeast strains has been evaluated and productivity varies significantly, but most of the strains showed an efficiency of more than 90% (Wu et al., 2010b). Liu et al. (2008) reported the use of immobilized yeast in a fluidized bed reactor that shortened process time and increased conversion efficiency. These results can be optimized when parameters as temperature, agitation rate, particles stuffing rate and pH are modified. Liu & Shen (2008) found that fermentation with immobilized yeast at 37°C, 200 rpm, 25% particles stuffing rate and pH of 5.0 in shaking flasks and 5 L bioreactor corresponds to the optimal conditions derived from an orthogonal experimental design.