1.1 Juice from sweet sorghum

The mature stems of sweet sorghum contain about 73% moisture and the solids are divided in structural and non-structural carbohydrates. Approximately 13% are non-structural carbohydrates composed of sucrose, glucose and fructose, in variable amounts according to cultivar, harvesting season, maturity stage, among other agronomic factors (Mamma et al., 1996; Phowchinda et al., 1997). Anglani (1998) suggests a classification of sweet sorghums based on proportion of soluble sugars in the juice. The first group with a high content of sucrose (sugary type) and the second with more monosaccharides (glucose and fructose) compared to other soluble carbohydrates (syrup type). Smith et al. (1987) in their evaluation of six sweet sorghum varieties throughout four years in nine different locations did not find significant differences in sugar content or composition. The typical composition indicates that around 70% was sucrose and the rest glucose and fructose in equal parts. In stem dry basis, Woods (2000) reported fermentable sugars content between 41 to 44% in Keller and Wray varieties with 80 and 63% represented by sucrose and the rest by glucose and fructose. A fiber variety analyzed by the same author (H173) reached only 20% fermentable sugars based on the dry stem weight; sucrose, glucose and fructose were found in equivalent

amounts (around 7% for each sugar). Compared to sugar cane, the main difference is that the sucrose content in cane is significantly higher compared to glucose and fructose (90, 4 and 6%respectively) and the total content sugar is 49% of the dry stem weight. In general terms, composition of simple sugars in sweet sorghum juice is 53-85, 9-33 and 6-21% for sucrose, glucose and fructose, respectively (Gnansounou et al., 2005; Mamma et al., 1996; Phowchinda et al., 1997; Prasad et al., 2007).

Beyond the proportion of soluble sugars in sweet sorghum plants, the yield of total sugars per harvested area is a better guide in the analysis for fuel production. Woods (2000) reported for sweet sorghum cultivars (Keller, Wray and H173) an average of 7, 10 and 4 ton of fermentable sugars/ha respectively, significantly lower compared to the 17 ton/ha for sugarcane indicated by the same author. The varieties studied by Davila-Gomez et al. (2011) yielded an average of 1.85 to 2.03 ton of sugar/ha, whereas Smith et al. (1987) in a extensive study performed in several locations of continental United States and Hawaii, obtained from 4.5 to 10.6 ton/ha. In other varieties evaluated in China, the best yields reached 18 ton/ha (Zhang et al., 2010).

Sugars in sweet sorghum are very sensitive to microbial contamination especially after crushing stalks for juice production. In data reported by Davila-Gomez et al. (2011), the percentage of sugars, as °Brix before fermentation, was lower (11 to 24% lower) than the obtained immediately after harvest in summer time, when temperatures easily reached 32°C in Northeast Mexico. The microbial contamination was the most obvious explanation of this phenomenon. Besides, the sucrose proportion in the fermented juices was lower in relation to glucose and fructose (0 to 10% of total). This can be related to invertase activity of contaminating wild yeasts that hydrolyzed sucrose into glucose and fructose. These monomers are quickly metabolized by means of facilitated diffusion into the yeast cell. Wu et al. (2010b), working with cultivars with 16 to 18% of fermentable sugars, found that as much as 20% of substrate can be lost in 3 days at 25°C. This loss corresponds to approximately 700 L ethanol/ha when a yield of 50 ton of sorghum stems/ha is considered. Daeschel et al. (1981) reported that juices can be preserved during 14 days at 4°C without detectable changes or deterioration (sour odor and foaming). These authors also reported that the dominant spoilage microorganisms were Leuconostoc mesenteroides and Lactobacillus plantarum at 25 and 32°C, respectively and recommended to process the juice within five hours after extraction.