Starch is a polymer of glucose in which glucose units are linked with each other via a — 1,4 and a-1,6 linkages. Starchy materials are first hydrolyzed to glucose using amylase enzymes (Eq. (5.2.2)).

(CeHwO5)n + nH2O ^ nC6H12O6 (5.2.2)

molecular weight n(162.14) n(18.02) n(180.16)

100 g 11.11 g 111.11 g

Starchy materials are first cooked at temperatures between 100 and 130oC and then hydrolyzed to glucose by using a-amylase and gluco-amylase.

A large amount of ethanol is produced from corn in USA and from sweet potato in China.

Low temperature cooking procedure for ethanol production from sweet potato which was the practice in Japan until 1990’s is described below. Raw sweet potato is first crushed by hammer-mill, cooked at 80-90 oC for 60min, added with a-amylase to liquefy starch and to reduce viscosity, and then cooled to temperature of about 58oC. Liquefied starch is hydrolyzed to glucose in two hours of hydrolysis by gluco-amylase. Glucose concentration of mash is adjusted at about 15%. Fermented beer of about 8 vol% ethanol is obtained after four days of batch-wise fermentation at 30-34oC. When starch value of raw sweet potato is 24.3% (27% glucose equivalent), and fermentation yield is 92%, the amount of raw sweet potato required to produce 1 m3 (kL) of 95% ethanol is 6.03 t-wet.

Fuel ethanol is produced mostly from corn in USA. In a wet mill process corn is immersed in dilute sulfite solution, fractionated into starch, germ, gluten, and fiber. Starch fraction is hydrolyzed to glucose by the amylases after cooking and then fermented by yeast. One of the popular fermentation process is a continuous fermentation process with several fermentation tanks connected in series in which yeast cells are recycled via centrifuge resulting in high fermentation rate. Traditional batch-wise process is also the practice in some of ethanol plants. Final ethanol concentration of fermented beer is 8 to 11 vol% on average. Wet-mill process of corn to ethanol conversion is indicated in Figure 5.2.2. When starch value is 63% (70% glucose equivalent), and fermentation yield 90%, the amount of corn necessary to produce 1 m3 (kL) of 95% ethanol is about 2.4 t-wet.