Corn is received at the plant and separated from the chaff, and the kernels are milled to a coarse flour. Particle sizing is a compromise between grinding fine enough to provide increased surface area (to make starch granules available for swelling and hydrolysis), yet large enough to allow separation of residual solids from the liquid. The corn meal is mixed with water, and the resulting mash is adjusted to pH 6 and then mixed with alpha-amylase. The mash is heated above 110°C in a jet cooker using direct steam. A jet cooker is in essence a pipe with a narrowing and a steam inlet directly upstream. The narrowing is carefully engineered to provide maximum mixing of the starch slurry with steam, and also to cause shearing, which aids in thinning the starch. Upon exiting the jet cooker, the corn slurry enters a holding column where the mixture is kept at 110°C for 15 minutes. From the holding column, the slurry enters a flash tank at atmospheric pressure and 80-90°C. Additional alpha-amylase is added and the mash is lique­fied for approximately 30 minutes. The jet cooking and liquefaction steps break apart the starch granules and reduce the size of the polymers. The shorter mole­cules, termed dextrins, contain approximately five to ten glucose molecules [21]. Subsequently, the liquefied mash is cooled to 32°C and the pH is lowered to 4.5-5.0 using phosphoric acid and recycled backset from the bottom of the ethanol distillation column.

Fermentation

Dry yeast is hydrated or conditioned and then added to the mash along with glucoamylase, to initiate simultaneous saccharification and fermentation (SSF). Glucoamylase cleaves the dextrins at a-1,4-glucosidic linkages, releasing glucose and maltose for yeast fermentation. The SSF process reduces the extent of micro­bial contamination because glucose is consumed by yeast as it is formed. The SSF process also reduces osmotic stress, because the yeast cells are exposed to a relatively lower sugar concentration. The dry-grind ethanol fermentation process lasts for 48 to 72 h and yields approximately 2.7 gallons of ethanol per bushel of corn.