Starch Hydrolysis

Starch may be regarded as a long-chain polymer of glucose (i. e., many glucose molecular units are bonded in a polymeric chain similar to a condensation polymerization product [16]). As such, macromolecular starches cannot be directly fermented to ethanol via conventional fermentation technology. They must first be broken down into simpler and smaller glucose units through a chemical process called hydrolysis. In the hydrolysis step, starch feedstock is ground and mixed with water to produce a mash typically containing 15 to 20% starch. The mash is then cooked at or above its boiling point and treated subsequently with two enzyme preparations. The first enzyme hydrolyzes starch molecules to short-chain molecules, and the second enzyme hydro­lyzes the short chains to glucose. The first enzyme is amylase. Amylase lib­erates "maltodextrin" by the liquefaction process. Such maltodextrins are not very sweet inasmuch as they contain dextrins (a group of low molecular weight carbohydrates) and oligosaccharides (a saccharide polymer contain­ing a small number of simple sugars, monosaccharides). The dextrins and oligosaccharides are further hydrolyzed by enzymes such as pullulanase and glucoamylase in a process known as saccharification. Complete sacchari­fication converts all the limit dextrans (complex branched polysaccharides of many glucose molecules) to glucose, maltose, and isomaltose. The mash is then cooled to 30°C, and at this point yeast is added for fermentation.