Stillage is the brownish liquid left behind after alcohol is distilled from a fermented grain mixture. Whether the goal is fuel ethanol, whiskey, or vodka, the process is the same: grains are fermented, yeast converts their sugars into alcohol, and that alcohol is boiled off in a distillation column. Everything that doesn’t evaporate, including water, leftover grain solids, yeast, proteins, fats, and minerals, flows out the bottom as stillage.
Far from being waste, stillage is one of the most economically important byproducts in the ethanol and spirits industries. Most of it gets processed into animal feed, and a growing share is used to generate biogas energy.
How Stillage Is Created
Alcohol production follows four basic stages: preparing the grain, fermenting it, distilling it, and packaging the final product. During fermentation, yeast (typically a species called Saccharomyces cerevisiae) eats the carbohydrates in the grain mash and produces ethanol. By the end of fermentation, the mixture contains roughly 8 to 10% alcohol.
That mixture then enters a distillation column, which heats it until the alcohol vaporizes and rises to the top for collection. The liquid remaining at the bottom, stripped of its alcohol, is stillage. It retains nearly everything else from the original grain: fiber, protein, fat, minerals, and a large amount of water.
Types of Stillage
The liquid that flows directly out of the distillation column is called whole stillage. It contains both dissolved nutrients and suspended grain particles, with an average moisture content around 87%. From there, it gets separated into distinct products.
Whole stillage is sent through a centrifuge, which spins it into two fractions. The liquid portion is thin stillage, an even waterier stream at about 92% moisture. The solid portion is wet distillers grains (WDG), a heavy, paste-like material packed with fiber and protein.
Thin stillage then goes to an evaporator, where much of the water is boiled off to create a thick, concentrated liquid called syrup, or condensed distillers solubles (CDS). Syrup has a moisture content of roughly 62% and is far more stable than the thinner forms. While whole stillage and thin stillage both spoil rapidly due to extremely high water activity (0.99), syrup’s lower water activity (0.92) resists mold growth and separation during storage.
From Stillage to DDGS
The most commercially valuable product made from stillage is DDGS: Dried Distillers Grains with Solubles. To make it, the syrup (CDS) is blended back into the wet distillers grains, and the combined mixture is drum-dried at high temperatures. The result is a dry, shelf-stable feed ingredient that can be bagged and shipped anywhere.
About 15% or more of the thin stillage never makes it to the evaporator. Instead, it’s recycled as “backset,” meaning it gets added directly into the next batch of grain during the initial cooking and liquefaction step. This recycles water and nutrients, reducing the amount of fresh water the plant needs.
Storage and Spoilage
Wet stillage products spoil fast. Wet distillers grains last only 3 to 4 days in summer temperatures and 5 to 10 days in winter, which limits their use to farms located close to the ethanol plant. Two main strategies extend that window.
Chemical preservatives based on propionic acid are widely used in the U.S. ethanol industry. Depending on the application rate, these can push shelf life out to 14 to 28 days. The other common approach is ensiling: bagging the wet grains with forage material and sealing them to cut off oxygen, similar to how farmers preserve silage from corn or grass crops.
Stillage as Livestock Feed
Stillage-derived feeds are a staple in U.S. livestock nutrition. DDGS delivers a concentrated package of protein, fat, fiber, and phosphorus at a price that often undercuts traditional feed ingredients like soybean meal. Typical inclusion rates in animal diets (on a dry matter basis) are about 20% for beef and dairy cattle, 10% for swine, and 5% for poultry. The lower limits for pigs and chickens reflect their sensitivity to the high fiber and sulfur content in distillers grains.
Beef feedlots near ethanol plants often skip the drying step entirely, buying wet distillers grains directly to avoid the energy cost of drying. The trade-off is that narrow spoilage window, which makes logistics critical.
Stillage in Spirits Production
The spirits industry produces its own version of stillage under different names. In Scotch malt whisky distilleries, the liquid leftover after the first distillation is called pot ale, while the residue from the second distillation is called spent lees. For every liter of alcohol a malt whisky distillery produces, it generates approximately 8 liters of pot ale and 10 liters of spent lees.
Pot ale contains yeast residues, barley fragments, soluble proteins, carbohydrates, and variable levels of copper picked up from the copper pot stills. Its nutrient profile is comparable to thin stillage from corn ethanol plants, with similar concentrations of phosphorus and potassium. Processing methods overlap too: pot ale can be evaporated and combined with spent grain (called draff in Scotland) to produce a feed product much like DDGS.
Energy Recovery From Stillage
Stillage’s high organic content makes it a strong candidate for biogas production through anaerobic digestion, a process where microorganisms break down organic matter in the absence of oxygen and release methane. Research at Iowa State University demonstrated that processing 60,000 gallons per day of thin stillage through a high-temperature anaerobic digester could yield an estimated 130,000 cubic feet of methane per day. That methane can fuel boilers at the ethanol plant itself, offsetting natural gas purchases and reducing the facility’s carbon footprint.
Environmental Concerns
Untreated stillage is a potent pollutant. Its extremely high concentration of organic compounds means that if it enters waterways, microorganisms consuming those organics would strip dissolved oxygen from the water, suffocating fish and other aquatic life. This is why ethanol plants and distilleries cannot simply discharge stillage into rivers or municipal sewers. Converting it into feed, energy, or other value-added products solves both an environmental problem and an economic one, turning a disposal liability into a revenue stream.