At the end of fermentation, the beer contains 10% or more ethanol by volume. Separation of ethanol from the whole fermentation mixture begins with distilla­tion on a beer column [22]. Further removal of water is accomplished in subse­quent distillation steps using a rectifier and/or stripper. Conventional distilla — tion/rectification methods yield 95% pure ethanol, at which concentration of ethanol and water form an azeotrope. The remaining 5% water is removed by molecular sieves, which rely on size exclusion to separate the smaller ethanol molecules from water [23]. Finally, anhydrous (100% or 200 proof) ethanol is denatured, typically with 5% gasoline or with higher-chain alcohols formed in the fermentation, to exempt the ethanol from beverage alcohol taxes.

Stillage Processing and Feed Products

The slurry remaining after distillation of ethanol, known as stillage, is concen­trated by centrifugation. The solids cake is referred to as corn distillers grains. Up to one third of the liquid fraction, known as thin stillage, is recycled (backset) into the mash. The remaining liquid is concentrated by evaporation and mixed with the corn distillers grains. The mixture is either sold as wet distillers grains or dried to generate DDGS. The moisture content and correspondingly short shelf — life of wet distillers grains limit use of this feed product to the immediate vicinity of ethanol plants, though the shelf-life can be lengthened by adding organic acids as preservatives.