1.3.2 Bioethanol production from sweet sorghum

Sweet sorghum is a crop for producing energy which not only produce food, but also energy, feed and fiber (Almodares & Hadi, 2009). The chief sugars present in sorghum are monosaccharides: glucose and fructose, and disaccharides: sucrose. Fermentable

carbohydrates in sweet sorghum stalks comprise approximately 80% soluble sugars and 20% starch. To optimize production of ethanol from sweet sorghum grain requires both liquefying and saccharifying enzymes (Rooney and Waniska, 2000). Therefore, it seems that using carbohydrates in the stalk (sucrose and invert sugar) is suitable for ethanol production for biofuel production because these carbohydrates are easily converted to ethanol (Fig 2). Although, ethanol can be produced from sweet sorghum grain (Fig. 2) but it needs more process for converting it’s starch to glucose that later will be converted to ethanol (Jacques et al., 1999). In addition, the produced baggase after juice extraction can be used for ethanol production (Jacques et al., 1999) or animal feed. However, presently it is not economically feasible to produce ethanol from sweet sorghum baggase (Drapcho et al., 2008).

Biomass

Fig. 2. Proposed layout for ethanol production and by-product from sweet sorghum (Almodares & Hadi, 2009).

3.2.2 The important of ethanol in biofuel

One method to reduce air pollution is to oxygenated fuel for vehicles. MTBE (Methyl tert — butyl ether) is a member of a group of chemicals commonly known as fuel oxygenates (Fischer et al., 2005). It is a fuel additive to raise the octane number. But it is very soluble in water and it is a possible human carcinogenic (Belpoggi et al., 1995). Thereby, it should be substituted for other oxygenated substances to increase the octane number of the fuel. Presently, ethanol as an oxygenated biomass fuel is considered as a predominant alternative to MTBE for its biodegradable, low toxicity, persistence and regenerative characteristic (Cassada et al., 2000). In most countries, gasoline supply is an ethanol blend, and the importance of ethanol use is expected to increase as more health issues are related to air quality. Ethanol may be produced from many high energy crops such as sweet sorghum, corn, wheat, barely, sugar cane, sugar beet, cassava, sweet potato and etc (Drapcho et al.,

2008) . Like most biofuel crops, sweet sorghum has the potential to reduce carbon emissions. Therefore, it seems that sweet sorghum is the most suitable plant for biofuel production than other crops under hot and dry climatic conditions. In addition, possible use of bagasse as a by-product of sweet sorghum include: burning to provide heat energy, paper or fiber board manufacturing, silage for animal feed or fiber for ethanol production. However, since
sweet sorghum is at a relatively early stage of its development, continued research was needed to obtain better genetic material and match local agro-economic conditions. The challenge is to harvest the crop, separate it into juice and fiber, and utilize each constituent for year-round production of ethanol.

Sweet sorghum juice is assumed to be converted to ethanol at 85% theoretical, or 54.4 liter ethanol per 100 kg fresh stalk yield. Potential ethanol yield from the fiber is more difficult to predict (Rains et al., 1993). The emerging enzymatic hydrolysis technology has not been proven on a commercial scale (Taherzadeh and Karimi, 2008). One ton of corn grain produces 387 L of 182 proof alcohol while the same amount of sorghum grain produces 372 L (Smith and Frederiksen, 2000). Sorghum is used extensively for alcohol production (Gnansounou et al., 2005), where it is significantly lower in price than corn or wheat (Smith and Frederiksen, 2000). The commercial technology required to ferment sweet sorghum biomass into alcohol has been reported in china (Gnansounou et al., 2005). One ton of sweet sorghum stalks has the potential to yield 74 L of 200- proof alcohol (Smith and Frederiksen, 2000). Therefore, it seems that because ethanol can be produced from both stalk and grain of sweet sorghum (Fig. 2), so it is the most suitable crop for ethanol production using for biofuel comparing to other crops such as corn or sugarcane.