Ethanol from Starch

Starch is a one of the best and most high yielding feedstock for ethanol production, but yeast S. cereviciae cannot utilize it directly. Hydrolysis is required to produce ethanol from starch by fermentation. Starch was traditionally hydrolyzed by acids, but the specificity of the enzymes, their inherent mild reaction conditions, and the absence of secondary reactions have made the amylases to be the catalysts generally used for this process. There are two steps present in hydrolysis of starch using amylases. First, these starch suspensions should be brought to high temper­atures (90-110 °C) for the breakdown of starch kernels. The product of this first step, called liquefaction, is a starch solution containing dextrines and small amounts of glucose. In second step, the liquefied starch is subject to saccharification at lower temperatures (60-70 °C) through glucoamylase obtained generally from Aspergillus niger or Rhizopus species (Pandey et al. 2000; Shigechi et al. 2004). Apar and O zbek (2004) provide information about the effects of operating conditions on the enzymatic hydrolysis of corn starch using commercial a-amylase. In previous years, the possibility of hydrolyzing starch at low temperatures for achieving energy savings is being investigated (Robertson et al. 2006).

Potential Starchy Substrates for Ethanol Production Corn:

Ethanol is produced almost exclusively from corn in the USA. Corn is milled for extracting starch, which is enzymatically treated for obtaining glucose syrup. Then, this syrup is fermented into ethanol. There are two types of corn milling in the industry: wet and dry. During the wet-milling process, corn grain is separated into its components. Starch is converted into ethanol and the remaining compo­nents are sold as co-products. During dry-milling, grains are not fractionated and all their nutrients enter the process and are concentrated into a distillation co­product utilized for animal feed called Dried Distiller’s Grains with solubles (DDGS) (Gaulati et al. 1996).

Wheat:

Generally in Europe, ethanol is mostly produced from beet molasses; in some countries like France it is also produced from wheat by a process similar to that of corn. Some efforts have been made for optimizing fermentation conditions. For example, Wang et al. (1999) have determined the optimal fermentation tempera­ture and specific gravity of the wheat mash. Soni et al. (2003) have optimized the conditions for starch hydrolysis using a-amylase and glucoamylase obtained by solid-state fermentation of wheat bran.

Cassava:

Cassava is an important alternative source of starch for ethanol production and for production of glucose syrups. Cassava is the tuber that has gained most interest due to its availability in tropical countries being one of the top ten more important tropical crops. Ethanol production from cassava can be accomplished using either the whole cassava tuber or the starch extracted from it. Starch extraction can be carried out through a high-yield large-volume industrialized process as the Alfa Laval extraction method (FAO 2004), or by a traditional process for small — and mid-scale plants. This process can be considered as the equivalent of the wet­milling process for ethanol production from corn. The production of cassava with high starch content (85-90 % dry matter) and less protein and minerals content is relatively simple.

Others:

Besides corn and wheat, ethanol can be produced from rye, barley, triticale (Wang et al. 1997), and sorghum (Prasad et al. 2007). For these cereals, some pretreatments have proved to be useful. Abd-Aziz (2002) suggested the utilization of sago palm for ethanol production in the case of Malaysia. The ethanol pro­duction from bananas and banana wastes using commercial a-amylase and glu — coamylase has been studied by Hammond et al. (1996). In their work, an ethanol yield of 0.5 L EtOH/kg dry matter of ripe bananas was obtained. The processing of starch-containing food wastes by adding malt to the pulverized feedstock has been patented (Chung and Nam 2002). One of the most promising crops for fuel ethanol production is sweet sorghum, which produces grains with high starch content, stalks with high sucrose content and leaves, and bagasse with high lignocellulosic content. In addition, this crop can be cultivated in both temperate and tropical countries requiring only one-third of the water needed for cane cropping and half of the water required by corn. Moreover, it is tolerant to the drought, flooding, and saline alkalinity (Winner Network 2002). Grassi (1999) reports that from some varieties of sweet sorghum, the following productivities can be obtained: 5 ton/Ha grains, 8 ton/Ha sugar, and 17 ton dry matter/Ha lignocellulosics. The estimated price for fuel ethanol production from this feedstock is US$200-300/m3, whereas the corresponding one for sugarcane ethanol is 260, for corn ethanol is 300-420, and for lignocellulosic ethanol is 450.

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