Understanding Whisky Production · Unit 8: Distillery Byproducts & Environment

Chapter 19: Distillery Co-products: Draff, Pot Ale, Spent Lees, and CO2

Recovering spent grain (draff) as animal feed, evaporating pot ale into syrup, treating spent lees copper, and carbon dioxide capture, wasting little.

One of the strengths of traditional distilling is that very little of the incoming agricultural raw material is genuinely wasted. The non-alcoholic residues of the mashing and distillation cycles represent valuable resources in their own right.

Spent grain from the mash tun (draff) is packed with protein and dietary fibre, making it an ideal supplemental feed for local livestock. Pot ale can be evaporated into a nutrient-dense syrup or dried alongside draff to produce animal feed (Edwards et al. 2022, Resources, Conservation and Recycling, 179, 106114). Meanwhile, the copious carbon dioxide produced during vigorous fermentation can be scrubbed, compressed and captured for use.

Co-products Recovery and Production

Most industrial processes generate one or more product(s) from the input raw materials. These products form the basis of a company’s business model. There are likely to be one or more side streams, or materials, that can be turned into a co-product. Most of these can have some inherent value to the business.

The spirits industry is no exception. Carbon dioxide is released from all types of spirits fermentation. Other materials besides this will also be recovered, either at raw material intake, cereal conversion, distillation or even possibly from exhausted casks.

The Circular Distilling Equation

In an ideal universe, a distillery would operate with the thermodynamic perfection of a perpetual motion machine: every grain of starch, drop of water, and whisper of carbon dioxide accounted for.

In the real world, distilling is an exercise in applied material separation. When cereal grain enters the mash house, only its soluble starches are converted to fermentable sugars. The insoluble husks, proteins, oils, and minerals pass through untouched. During fermentation, yeast consumes those sugars, transforming roughly half of their mass into ethanol and venting the other half as millions of litres of carbon dioxide.

Finally, distillation separates volatile ethanol from watery residues. What remains after ethanol has flown the coop is not waste, but concentrated organic nutrients: spent grain (draff), pot ale, spent lees, fusel oils, and captured gas.

The fermentation chapters gave us the following equation:

One molecule of glucose is converted to two molecules of alcohol and two molecules of carbon dioxide as heat is released. Obviously, we do not directly process glucose. Instead, we use a starch source (or sugar source), which we will convert and ferment as in the above equation. This raw material will contain other components than starch. These components will not be consumed in the production processes, and will be recovered as solids at some point in the process.

A very simple rule of thumb, which will help us understand the recovery of co-products, is the following: For every 100 t of raw material input, we will produce: approximately one third as alcohol approximately one third as carbon dioxide approximately one third as of solids

This rule of thumb works well for cereals-based spirits producers, but it is only a very rough estimate. It gives indicative, not definite, recovery rates. In particular the actual tonnage of cereal solids recovered is highly dependent on the moisture (or solids) content recovered. Alcohol recovery will be the key focus. Therefore, this rule of thumb provides an overview of the scale of recovery of co-products. The actual rates of recovery for each process will vary according to the input raw material mash bill, production process efficiency, and a business choice regarding whether you wish to recover all or some of these potential co-products.

It is important to note that:

• The recovery of co-products generally requires additional equipment to that used in the main production processes. These material streams come in various forms, from wet carbon dioxide gas with various other impurities, to solids, semi-solids and liquids. Each of these streams requires its own specific equipment.
• We need to distinguish between co-products and effluent, depending on where our production activities are located. Distilling operations in certain regions may find it more economical to dispose of side stream materials as effluent. This avoids incurring the capital cost of building recovery plant and processes to convert them into a commercial product.
• If these materials are disposed as effluent, all operations must comply with local waste and environmental legislation.
• To maintain a consistent output of the main product (alcohol), these materials must be removed from the site in a timely manner. Compliance to these processes is very important. If these streams are not fully recovered, it can severely limit our ability to produce the final distilled product.

Distillers who choose to recover these materials as co-products aim to recoup some of the capital investment and processing costs incurred. It is important to ensure that this process does not add additional cost burden to the actual cost of the product. However, regulation and legislation can sometimes make this impossible to achieve. Where it proves impossible to recover and produce these co-products, these streams can only be discharged to the environment in a manner permitted and controlled by the relevant authority. Alternatively, they can be sent to a licensed effluent treatment provider. We will explore the various disposal options later here.

Carbon Dioxide

Fermentation produces carbon dioxide alongside alcohol. Recovery depends on the gas available, the equipment and the required purity; not all of the gas generated can necessarily be collected. The Scotch Whisky Association describes projects testing ways to capture and use fermentation carbon dioxide.

The economics of recovery depend on the scale of the plant, the equipment needed and the available use for the gas. Recovery, purification and liquefaction require energy and capital. The main principles involved will include:

• Raw gas purification
• Equipment e.g. pipework
• Primary and secondary compression
• Silica gel towers to remove trace levels of water
• Activated carbon towers to remove any unwanted off-odours
• Refrigeration to produce liquid carbon dioxide
• Refrigerated bulk storage of the liquid product under pressure

This liquefied or solid product is commonly used in the food and beverage industries. It has also been used as a coolant in the nuclear power industry: the UK’s Magnox and advanced gas-cooled reactors are carbon dioxide cooled. The product must therefore meet stringent quality conditions using sophisticated analytical techniques, which incur additional costs to the distiller. It is important to ensure there is no cross contamination of product during loading and emptying of tankers used to transfer the liquefied product to the customer.

As with all products, the sales value of the product is the key to understanding whether it is commercially viable to recover this natural product. Historically, only the larger grain or neutral spirits businesses would provide the scale of opportunity to make using this type of plant a viable proposition. This is because they recover such large tonnages on an annual basis.

At the larger distilleries, gas and water scrubbers are often employed to remove concentrate and recover the entrained alcohol from the fermentation gas stream. Carbon dioxide is a colourless, odourless gas that is heavier than air; at high concentrations it can collect in tanks, sumps and cellars and displace oxygen, and people have died from asphyxiation in such spaces. This poses a significant risk to those involved, or to those entering production activities. It also restricts entry to certain areas of the plant, requiring access only under very strict conditions.

Co-products from Cereals Residues (Whisky) Production

During the production of alcohol from cereals fermentation, the starch in the cereal endosperm is largely converted into fermentable sugar, though a small residue of unfermented dextrins carries over into the co-products. This sugar is fermented by yeast action, to produce alcohol, carbon dioxide, heat, yeast biomass and other flavour compounds.

Before we take a look at the co-products from cereals-based distillation, we should consider the streams within malting processes that generally end up as co-products:

• Cut barley direct from the field is unsuitable for malting immediately. The delivered barley will be dried, cleaned and dressed to remove barley dust, small corns and any other extraneous materials that could adversely affect the malting process.
• Barley dust and screenings will be recovered.
• Stones and other materials will be discarded.
• Malt rootlets will be recovered.

Barley grows large numbers of small roots or rootlets. During the malting process, the majority of these are removed when the malt is moved from the kiln and dressed prior to bulk storage. The accumulation of this material is combined with the barley dust and small dressings. It is usually pelletised and sold within the animal feeds market. This, of course, will only be relevant to those distillers who own and run their maltings. It is irrelevant to those businesses that are supplied malt on a contract basis from external commercial maltings businesses.

Cereals used in spirits production are also generally cleaned to remove dust, stones and other extraneous materials and thus avoids these materials entering the process where they can cause significant damage. The grain dust can also be sold onwards as an animal feed.

The cereals chapter covered that the largest component in the cereal is starch. There are other important components within its structure such as vegetable oils, proteins, fibre and minerals. These all play a minor role in the various reactions taking place during the production processes. However, they remain largely unchanged at the end of the process.

The following diagram represents the composition of the raw materials feed as it enters the process. It also shows the composition of the dried solids recovered at the end of the production processes. This is a very generic model, but it clearly illustrates the effect of starch removal during the production of spirits.

When considering the image above, we must consider these key points:

• The lower pie chart shows the composition of the input cereal raw material. With around 61% w/w, starch is the main constituent. Fibre, oil and protein make up the balance.
• These charts will not accurately represent 100% of cereal mash bills. The feed composition will depend on the mash bill recipe, variations between harvest year and the types of cereals used.
• During the spirit production process, the distiller uses almost all of the starch component, concentrating the other components (protein, fibre and oil) and adding substantial quantities of yeast and yeast cell debris.
• The cereal co-products will be enriched in the other components, but deficient in the highly digestible starch and sugar.
• Both sets of data in the pie charts are expressed on a 100% dry weight basis.
• The chemical and physical characteristics of these cereal residues suggest that they are best used as animal feeds or fertilisers. They are now more commonly used as the carbon source in the production of biomass or biogas. This is then used as fuel in the production of bioenergy, as either thermal or electrical power. Disposing them as waste is very expensive and is usually avoided.
• These are a rich source of nitrogen (protein), energy (vegetable oil), amino acids and minerals such as phosphorus and potassium; pot ale and spent wash carry soluble phosphate and potassium (Edwards et al. 2022).
• They can be compounded (mixed) with other feed materials to increase their overall financial and nutritional value.
• Cereal solids, semi-solids and liquors are removed from the production process at various stages.
• Copper concentrations can increase due to pick up in the copper distillation equipment. This can be a cause for concern, particularly if it is fed to sheep: some breeds are relatively susceptible to copper poisoning, and NADIS lists dark grains from copper stills among the feeds to avoid for them. Malt dark grains are no longer made in Scotland: the last remaining malt dark grains plant closed in early 2018 (Scottish Government study).
• Water is added at various stages in the production processes, but primarily at the cereal conversion stage. It will then be removed during these treatment processes to produce dried or wet products for further processing.
• Regardless of the treatment option used, there will always be some final liquid effluent, which will be discharged under permit conditions.

Tracking Distillery Residues

In batch pot still malt distilling, residues exit the system at two distinct points:

• From the Mash Tun: Insoluble grain husks and solids are discharged as wet draff (about 23% dry matter in a Scottish Government estimate).
• From the Wash Still: After boiling off volatile alcohol, the de-alcoholised residue is discharged as pot ale (containing dead yeast cells, unfermented dextrins, and dissolved proteins).

In continuous grain distilleries, all insoluble cereal solids and spent yeast exit the base of the stripping column together as whole stillage (spent wash), which is subsequently separated using decanter centrifuges into wet cake and thin stillage.

These distillery side streams will have a very high BOD.

BOD stands for Biochemical Oxygen Demand. This is a measure of the oxygen consuming capacity of the organic matter contained in these side streams. During decomposition, these materials will have an oxygen requirement that will deplete the oxygen supply in any waterway. It would result in the death of fish and any other aquatic life. The BOD is normally measured based on the weight of oxygen consumed per unit of any effluent.

Cereal Residues

As already seen, these side streams of cereal residues contain proteins, vegetable oils and fibre.

These residues can have high BOD and COD. Their composition varies by distillery and process, as shown in a study of pot ale and spent wash from 22 Scotch whisky distilleries. They are also very acidic in nature. They contain trace metals from the original cereal, as well as those picked up by erosion/corrosion of the process plant. The removal of these streams from any distillery will likely follow one of the following routes:

• Discharged to the sea or estuarial waters if there is sufficient tidal dispersal. This is likely to be under permit conditions.
• Discharged to the public, domestic effluent treatment plant if capacity is available. Again, this will be under permit conditions.
• Discharged to a dedicated effluent treatment plant for treatment prior to disposal, again under permit conditions.
• Used as feed for a bio-energy plant.
• Discharged over-land, again under permit conditions.
• Used as feedstock for an animal feeds production plant.

Cereals residues have various names depending on where the spirits are produced.

• In North America: thick and thin wet stillage evaporator syrup wet distillers grains with solubles distillers dried grains with solubles distillers solubles and many more…
• In Great Britain and Ireland: draff pot ale spent lees spent wash centrifuge cake syrup dried grains various other names depending on whether they are sold as wet or dry animal feeds

Cereals residues may have alternative names in other parts of the distilling world, but they all refer to the cereal solids and yeast debris left after the starch has been consumed.

Cereal residues can also be used as fertilisers. Pot ale and spent wash, for example, are a good source of soluble phosphate and contain potassium, which researchers see as a route to fertiliser products (Edwards et al. 2022).

The distiller, having removed the readily available starch from the cereals during processing, will leave a residue that is best suited to digestion by ruminants (cattle and sheep), who consume and process high fibre materials most effectively.

Cereal Residue Recovery

The two primary uses for these wet and dry cereal residues will be either as an animal feed, or as a carbon source during bioenergy production. The scale of a distillery matters a great deal here:

• Actual solids recovery at any given distillery is driven by the actual mash bill composition, the efficiency of the production processes, and the type of solids recovery processes employed.
• The actual tonnage of recovered solids will be dependent on the moisture (solids) content of the cereals residues
• A small distillery produces far less dry solids each week than a large one, and the solids output scales roughly with alcohol output.
• At small scale it is hard to justify building an animal feed plant, as this type of operation requires high levels of investment in buildings and plant. These dewatering processes also require significant energy input.

Many small distilleries still traditionally supply wet draff or equivalent to the local farming community as a ruminant feed, a practice well-established years ago. The pot ale or stillage from the first batch still process can be spread on land where the environmental regulator allows it, or as in the case of some distillers evaporated to produce a syrup of higher solids which can again be sold as animal feed. In Scotland, where the small malt distilleries are spread across the North of Scotland in particular, many form joint ventures with other distilleries or animal feed plants for centralised treatment of these cereal residues. The Combination of Rothes Distillers, for example, is a joint venture serving several Rothes distilleries: it turns pot ale into pot ale syrup and its partner plant burns pressed, dried draff with wood chip to generate power.

The larger grain and neutral spirit distillers have the economic capacity to install animal feed plants as part of their ongoing distillery processes. The challenge in all of these operations is to find economic and efficient ways of removing water from these cereal residues. Techniques employed will include:

• Use of wedge wire screens as a filter
• Filter presses
• Screw presses
• Centrifuges
• Various evaporation operations
• Various drying operations

Removing water from these cereal residues can prove a difficult and inefficient process. Research and investment in innovative processes often features highly on distillers’ agendas, in order to ensure they remain financially viable and competitive in the animal feed market.

New approaches for handling cereal residues are continually being developed. We have already noted that they are rich in carbon, which makes them suitable as the carbon source for bioenergy production. Low solids liquor streams are processed through anaerobic digestion plants to produce methane gas for conventional boiler applications, or as a co-fuel in large biomass boilers. The solid cereal residues will be further dewatered. They will then be used as fuel along with the methane, or as a single fuel source in a variety of thermal applications. These plants produce significant thermal and electrical output, which can be used as “green” energy within the production processes.

By now, you should be beginning to understand the issues associated with this type of operation. There are many forms of dewatering and drying plants; further information on these plants is beyond the scope of this guide, but is readily available on the internet should you wish to learn more. They are complicated systems that require extensive capital outlay. They also incur ongoing revenue costs for process chemicals, energy, maintenance and labour.

If the plant is not designed to the appropriate scale and capacity, and if the correct method of treatment has not been selected, this could negatively affect primary distilling operation outputs. This situation should be avoided by carefully assessing all the available options prior to installation. Regardless of which route the distiller chooses to take, these types of operations are always governed by regulations that prohibit unauthorised discharges of material to the environment.

Fusel Oil from Whisky Distillation

Fusel oil is a mixture of higher alcohols, mainly iso-amyl alcohol, produced as congeners during the fermentation of cereals-based wort. Iso-amyl alcohol is a colourless liquid with a disagreeable odour. It is only slightly soluble in water and is miscible with alcohol.

We are going to focus mainly on the large continuous distillation operations employed in grain whisky and neutral spirits production. The batch pot still distillation processes in the whisky industry do not yield this fusel oil as a co-product and in fact it has positive attributes in relation to spirit character and quality. This is due to insufficient rectification taking place to separate the amyl alcohol from the main alcohol stream with the higher alcohols staying in the spirit cut or returning with the feints.

During the grain type whisky fermentation, the main products are alcohol and carbon dioxide, along with other higher alcohols, esters, phenols, aldehydes and organic acids. A fraction of these will be recovered in the final product spirit, contributing towards some of the character to this type of whisky. Unlike in the pot still practice just described, most of the iso-amyl alcohol is kept out of the final product here. The continuous distillation process (with its great rectification power) separates it from the other higher alcohols in the rectifier or concentrator column. A side stream will be drawn from the rectifier column where the fusel oil peaks. It will then be further distilled in a separate fusel oil column or water washed and then decanted to remove excess alcohol and water.

If required, you can refer back to the chapters on continuous and whisky distillation to refresh on how this recovery works.

Some key points to note include:

• The final fusel oil should contain very little alcohol and must be monitored
• The cleaned up iso-amyl alcohol can next be sent to bulk storage, where it is filled into drums or bulk tankers for delivery to other chemical processing businesses. These businesses will use it as a raw material for the production of other organic compounds.
• The clean iso-amyl alcohol can also be used in the pharmaceutical, photographic, fragrance and flavouring industries.
• Like all organic chemicals, fusel oil has a calorific value that makes it an effective fuel source for oil-fired boiler operations to raise steam. Alternatively, it could be co-burned within other biomass boiler operations.
• The value of fusel oil is driven by its availability across world markets, where very high production levels of grain whisky and neutral spirits lead to oversupply and consequently depressed sales value. When this occurs, it is shrewd to burn it as a fuel, helping to offset the purchase of other expensive energy fuels.
• HMRC charges no duty on fusel oil with an alcohol concentration below 8.7% ABV; above that threshold the whole product is dutiable at the spirits rate. This is a duty rule, not a product standard.

Recovery of this product itself is not too energy intensive, as the energy provided is primarily consumed in the production of the final spirit. The distiller has no choice in the recovery of this co-product, as failure to do so leads to significant final product quality issues. These are caused by the presence of high concentrations of the congener. If not removed, the fusel oil would build up in the rectifying column and disrupt its operation.

If world prices fall due to oversupply, and there exists no mechanism on site to burn the co-product as a fuel, this liquid stream is handled as waste. This brings with it the ensuing costs and subsequent compliance with waste regulations. The following schematic shows the removal of fusel oil from continuous distillation.

The Second Lives of Barrels: Bureaucracy, Oak, and Garden Furniture

If Scotch whisky has a secret ingredient, it is second-hand American oak: the accidental by-product of an unusually specific piece of federal legislation.

In the United States, bourbon makers are bound by a rigid post-Prohibition decree: every batch of bourbon must be stored in charred new oak barrels. No second chances, no “just one more fill.” The law is often said to have been intended to keep coopers in steady work. It also left the American distilling industry with millions of barrels (Kentucky alone held 14.3 million barrels of bourbon ageing at the start of 2024) that had been used exactly once and were looking for something meaningful to do with the rest of their lives.

And Scotland, ever alert to an agrarian windfall, said: “Aye, we’ll take ’em.”

Bourbon barrels, still smelling of Kentucky sunshine, vanilla, and toasted coconut, arrive in Scotland by the thousand. They are dismantled, rebuilt into larger 250-litre hogsheads by Scottish coopers (who have long believed that if something is good, it should be slightly bigger and full of whisky), and filled with new-make spirit.

The first fill extracts bright vanilla, coconut, and golden honey. The second fill yields gentler, subtler maturation as wood and spirit settle into a comfortable marriage. After several cycles, when the wood grows weary, coopers rejuvenate the staves with aggressive carpentry: shaving away tired surface oak, toasting the exposed wood, and re-charring the interior (the STR process).

Eventually, after many fills, the wood surrenders. But in Scotland, oak does not die; it merely enters a dignified, somewhat bewildered retirement. Redundant casks are sawn in half to become flower planters in garden centres, hollowed out to become rustic pub tables, or chipped into shavings to smoke Scottish salmon. It is a remarkably tranquil end for a vessel that spent decades holding flammable liquid in a dark warehouse.

Key Takeaways

Circular co-product recovery transforms brewing and distilling byproducts from potential environmental liabilities into sustainable agricultural feeds, recovered nutrients, and industrial gases.

Sources and review

<ref>{{cite web |title=Distillery Co-products: Draff, Pot Ale, Spent Lees, and CO2 |url=https://uisgebeatha.co.uk/understanding-whisky-production/19-co-products-draff-pot-ale-and-spent-lees/ |website=Uisge Beatha |author=John Fegan |date=24 October 2026 |access-date=[date you read it]}}</ref>

That is the last chapter of the course. Put all 19 together with the whisky production quiz, 12 questions from grain to glass, or try the harder expert quiz on distilling science. For shorter reads on distilleries, glassware and whisky history, browse the Uisge Beatha whisky blog.

About the author

John Fegan · Editor

Hailing from the land of haggis and deep-fried Mars bars, John was always going to find his way to whisky. He has run Uisge Beatha since 2015 and Whiskipedia.com since 2020.