4th November 2018

Whisky Production: Mashing and Fermentation

Whisky is made by distilling beer. As such, we first need to produce a beer before we can distil it. This article covers the brewing process for whisky.

Having converted the starch in the barley to fermentable sugars by malting, we now want to extract the sugar. The yeast can then be added in order to produce the alcoholic liquid that will later become whisky.

Brewing is separated into two distinct stages:

Mashing: also known as the hot side. This is where the malted barley is mixed with hot water to dissolve the sugars.
Fermentation: also known as the cold side. Yeast is added to the sugary liquid (called wort) to produce alcohol.

Milling: Preparing the malt for brewing

In order to make the sugar accessible for brewing, the malt must be broken up into grist.

The malt first passes through a dresser to remove stones and other larger objects. A magnet also stops any metal objects from reaching the mill and causing damage. There are a variety of mill types in use but by far the most popular use two sets of rollers. The first set cracks open the husk while the second set shreds the grain.

The milled malt can be classified into three categories based on size: husks; grit; flour. The flour allows for the best extraction of sugars but using too much can quickly clog the mash tun. For this reason husk is also added to help with filtration. The mill is calibrated to achieve a certain ratio of grit:husks:flour, usually around 70:20:10.

The two mills most commonly used in the whisky industry are those manufactured by Porteus and Robert Boby. These mills were made to be so efficient that they rarely needed servicing. In fact they were so good that they never had to be replaced, forcing the producers out of business when they couldn’t secure any repeat custom.

Mashing: Extracting the sugars

Once the malt has been milled, it is mixed with hot water to extract the sugars and any remaining starch. This stage is known as mashing.

The milled malt is initially mixed with water at a temperature of around 64°C. During mashing the grist absorbs water and the sugars begin to dissolve. The remaining starch gelatinises in the hot water, making it easier for the enzymes to hydrolyse it further into fermentable sugars. The first water is then drained off and a second water is added at a temperature of around 70°C. The higher temperature of the second water dissolves further sugars, increasing the extract efficiency. This water is again drained off and a third water is added at temperatures between 80°C and 90°C. The third water ensures that as much sugar as possible is extracted from the mash. The sugar content in the third water is so low however, that it is not used in fermentation but instead recycled and used as the first water for the next mash. Depending on the distillery, occasionally a fourth water is also used.

There are three types of vessels in which mashing can take place: mash, lauter and semi-lauter tuns. Traditional mash tuns use large paddles to mix the grain whereas lauter tuns use a series of revolving rakes. Lauter tuns are thereby able to agitate and apply pressure to the mash to a higher degree, increasing extraction efficiency. All other designs between mash and lauter tuns are categorised as semi-lauter tuns. All three have slots in the floor to filter off the sugary liquid, now called wort.

Once mashing is completed, the remaining grain (called spent grains or draff) is removed from the mash tun. It is often sold on to farmers to be used as cattle feed due to its nutritional value. The wort now moves to the fermentation tanks, or washbacks, for the next stage: fermentation.

Fermentation: Converting the sugars into alcohol

Yeast convert sugars to alcohol and carbon dioxide, as well as a number of additional compounds. These include esters, fusels, sulphur and carbonyls, many of which will lend specific aromas and flavours to the finished whisky. As such, the fermentation stage, including which yeast strain is selected, has a large impact on the characteristics and quality of the final product. Distiller’s yeasts are bred to survive the high sugar concentrations of the wort and produce desirable flavour compounds.

The yeast is added to the wort in a large vessel (up to 30,000 litres) known as a washback. These were traditionally made from Oregon pine, although steel is becoming more popular for its ease of cleaning. Many people within the industry swear that the wooden washbacks lend a flavour to the whisky that steel cannot replicate. Distilleries that use steel claim however that tests on both types of washbacks show no significant difference in flavour.

After pitching there is an initial lag phase when the yeast acclimatises to its new environment. During this time the yeast begin absorbing nutrients from the wort and producing the enzymes necessary for growth. The yeast then begin to consume sugar and produce alcohol, growing exponentially. As already mentioned the yeast also produce carbon dioxide. This causes the wort to foam and can lead to the washbacks overflowing. Many washbacks have rotating blades above the wort to cut the foam and prevent this from happening. The process also produces heat, raising the temperature of the wort from about 20°C to roughly 32°C. This heat increase must be kept in check as excessive temperatures can exert stress on the yeast and negatively affect the fermentation.

Once the nutrients and sugars in the wort have been exhausted, the yeast activity begins to decline. The length of the fermentation process varies between distilleries but is commonly between 48 and 100 hours. The alcoholic liquid, now called wash and very similar to an un-hopped beer, is transferred to the stillhouse for the next stage in whisky production: Distillation.

Want to learn more about whisky production?

Take a look at the next step in the production process: Distillation

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What is Whisky Made From?

Beer. At least that’s the short answer. Much like brandy is made by distilling wine, whisky is made by distilling beer, or at least something very close to beer – I wouldn’t recommend drinking a pint of it. The longer answer, which is still fairly short is that whisky is made from cereals (grains such as barley, corn and rye – not Cornflakes). These grains are combined with water to produce a sugary mixture known as wort, which is then fermented to produce an alcoholic liquid (similar to beer) called wash. The wash is distilled to increase the alcohol content, while reducing unwanted and concentrating desired aromas before being aged, or matured, in wooden casks. A single malt whisky is a malt whisky produced by a single distillery, i.e. not a blend of several distilleries’ malts. A single malt however does not have to come from a single barrel (which would be a single cask malt whisky) but is commonly a mix of several casks from the distillery, allowing the manufacturers to achieve a higher level of consistency in their product despite small variations between batches and casks. Not quite. Each whisky style or whisky producing region has their own set of regulations dictating when the distilled spirit is allowed to be labelled as whisky. For example, the Scotch Whisky Regulations 2009 set forth that “Scotch Whisky” means a whisky:

produced entirely in Scotland;
that has been distilled at an alcoholic strength by volume of less than 94.8 per cent so that the distillate has an aroma and taste derived from the raw materials used in its production;
that has been matured entirely in an excise warehouse or a permitted place in Scotland in oak casks of a capacity not exceeding 700 litres for a period of not less than three years;
to which no substance has been added except water and/or plain caramel colouring; and
that is bottled at a minimum alcoholic strength by volume of 40%.

We will cover the regulations of each style/region (Bourbon Whiskey, Rye Whisky, Japanese Whisky) when we look at each of these whiskies in more detail in future articles. Take a look at the first step in the production process: Malting barley

Whisky Production: Distillation

Distillation is the process of separating the components from within a liquid by taking advantage of their different boiling points. The wash that was produced in the previous step – mashing and fermentation – contains roughly 10% alcohol, a bunch of flavour compounds and a whole lot of water. The aim of distillation is to increase the alcohol content while concentrating the desired aromas and filtering out unwanted compounds. Water boils at 100°C, ethanol at 78.37°C. By keeping the area at the top of the still between these two temperatures, the water will condense and fall back into the pot, whereas the alcohol vapour will be allowed to continue around the head and into the lyne arm. This vapour is then collected and condensed back into liquid. Single malt Scotch whisky is generally distilled twice, although there are of course exceptions. Several distilleries distil their whisky three times, which is more typical for Irish whiskey. Springbank famously distil their product 2.5 times! Grain whisky, the basis for blended whisky, is distilled using a continuous distillation process. We will cover blended whisky fully in a future article.

The stills in which single malt Scotch whisky is produced are, by law, made from copper. They consist of a large pot base with a tall, thin neck (known as a “swan neck”), which ends in an angled turn into a pipe called a lyne arm. This pipe is connected to a condenser which will cool the vapours coming off the still back into liquid form. The liquid coming off the stills then flows through a receiving vessel called a spirit safe. This is a locked glass box where the stillman can check the spirit and decide when to start collecting the liquid that will become whisky. The spirit safe is kept locked by the local excise official to prevent alcohol from being siphoned off before it can be measured for tax purposes. The first still, into which the wash is filled, is called the wash still. When heated, the ethanol, as well as various compounds, will begin to vaporise and rise in the still. Much of the vapour will condense on the sides of the still and fall back into the pot. This reflux ensures that only the lighter, desirable compounds reach the top of the still. The copper in the stills also helps to strip away the heavier compounds, such as sulphur, ensuring a lighter spirit. Once the vapour has made it around the head and into the lyne arm, it reaches the condensors. Here cold water flows along the sides of the pipes to cool the vapour back into liquid form. The liquid, now called low wines passes through the spirit safe and is collected in the low wines receiver. The liquid coming off the still first is higher in alcohol (roughly 45% ABV); as the distillation progresses the alcohol content falls. When the alcohol content of the low wines coming off the still reaches around 1% the distillation is deemed complete. The collected low wines will have be approximately 25% ABV. The second distillation takes place in the spirit still. It is similar to the first, however the resulting liquid is separated into three “cuts”: heads, hearts and tails. The heads, also called foreshots, are the first runnings of the spirit still. They contain all of the lighter compounds that vaporise first, including volatile and aromatic compounds such as ethyl acetate. These are not deemed worthy of collection and are rerouted via the spirit safe back to the low wines receiver. They will be distilled again with the next batch. After approximately 10 – 30 minutes, once the alcohol content has fallen to roughly 75% ABV, the stillman will turn a handle in the spirit safe and begin collecting the hearts. The hearts, also called the middle cut, contain all of the desirable flavours and aroma compounds. This liquid will be collected in the spirit receiver and will later, after maturation, become whisky. For now though it is called new make spirit. The choice of when to begin and stop collecting the hearts has a major impact on the level of compounds in the final spirit. Therefore this decision has a significant effect on the final character and is a major contributor to the differences between whiskies from separate distilleries. Another contributing factor to the differences between distilleries is the shape of the still and the angle of the lyne arm. Shape and height both influence the level of copper contact and reflux. Taller stills with upward reaching lyne arms generally result in lighter whiskies. Collection of the new spirit lasts roughly 3 hours, when the alcohol content falls to roughly 60% ABV, before the tails begin to run off the still. The tails, also called the feints, contain all of the heavier compounds, such as fusel oils, that are not desirable in the spirit. The stillman will again turn the handle on the spirit safe and redirect the tails to the low wines receiver. Again these will be distilled in the next run. The collected new make will now be transferred to casks for maturation. Take a look at the next step in the production process: Maturation and Bottling.

Whisky Production: Maturation and Bottling

After distillation the spirit is not yet whisky. It must first be matured in oak casks for at least 3 years before bottling. Whisky was originally consumed straight after distillation, without the mandatory wood maturation. Nobody knows for sure how this practice of oak maturation began. It has been suggested that because whisky was traditionally a seasonal product, it would be stored in containers for consumption throughout the year. What these early whisky drinkers will have noticed is that after some time in contact with wood the spirit became much mellower. This is attributable to the reaction of the spirit with the wood and is one of the major contributing factors to today’s mandatory maturation period. Compounds within the wood, such as lactone, eugenol, and vanillin (responsible for the typical vanilla flavours), are pulled into the maturing spirit, contributing a variety of flavours and aromas. Additionally, the sharp aromas within the new make spirit are lost over time, resulting in a smoother whisky. It is not only flavour that the wood contributes. The new make spirit, as it comes off the stills, is almost completely clear in colour. Whisky’s typical, golden colour is a direct result of the tannins from the wooden casks. What the cask was previously filled with can also have a large impact on the final product. Due to laws governing its production, new barrels must be used for aging bourbon whiskey. This leaves an abundance of barrels that can’t be reused for bourbon maturation. Since there is no such law demanding Scotch whisky be matured in new barrels, many of the bourbon casks end up in Scotland. These casks then lend flavours of vanilla and caramel to the maturing Scotch whisky, typical characteristics of bourbon whiskey. Ex-sherry butts are also a popular choice, although the decline in the sherry industry over the past years makes them rarer and therefore more expensive. Sherry casks generally lend sweet, fruity flavours to the whisky. Temperature changes in the warehouse cause the pores in the wood to expand and contract, allowing the spirit to flow in and out of the staves. This leads to the extraction of flavour compounds as mentioned above. Additionally, many of the casks are charred on the inside, providing a layer of carbon that acts as a filter, cleansing the spirit of unwanted compounds. As oak is porous, the whisky is also able to breath. Over time, some of the alcohol will evaporate from the cask, known as the angels’ share, leading to a change in flavours as the concentrations of alcohol, water and other compounds changes. Once the spirit has spent at least three years maturing, it is legally allowed to be called whisky and can be bottled. The flavour development doesn’t stop after only 3 years however, and the majority of whiskies will be matured for much longer, with periods of 10 or 20 years not being uncommon. As mentioned at the beginning of this series about whisky production (“What is Whisky Made From?”), a single malt does not have to come from a single barrel. The bottled whisky is commonly a mix of several casks from the distillery, allowing the manufacturers to achieve a higher level of consistency in their product despite small variations between batches and casks. It is the role of the blender to design these recipes and manage the inventories in such a way to ensure brand consistency in the future. Of course, distilleries also release single cask expressions, which as the name suggests do come from a single cask and therefore do not require blending. Prior to filtration it is also common that the whisky is reduced in strength. Depending on how long the whisky has been maturing, the strength could be anywhere between 40% and 70%, as a result the whisky is mixed with water to bring it down to a reasonable drinking strength, a minimum of 40% but commonly also 43% or 46%. Of course, it is also possible to purchase cask strength whiskies, which are not diluted at all. Additionally, it is common for the producer to add caramel colouring prior to filtration in order to ensure that the colour is consistent over multiple batches. After filtration the whisky is stored in a whisky vat ready for bottling. Due to the difference in size between individual distilleries, there is no one size fits all approach when it comes to bottling. In the industry everything is observable from hand-filling of bottles, through semi-automatic bottling lines where the bottles are hand fed onto the line at one end and retrieved at the other, all the way to fully-automatic systems where even this step is processed by machines. It is important to note that whisky maturation is dependent on the wooden casks. Once the whisky has been bottled, maturation no longer occurs and the whisky will not improve in flavour over time (in fact, once opened oxidation can quickly degrade the quality of the whisky). That is to say that a 20 year old single malt kept on a shelf for 10 years (unopened or otherwise) is not a 30 year old whisky but a 20 year old whisky that has been kept on a shelf for 10 years.