Making malt whisky is a six-part process – malting, milling, mashing, fermentation, distillation, and maturation. Each part, as well as each of the natural ingredients – barley, water and yeast – can have a profound effect and influence on the final product. This creates the many variants and the wide range of aromas, flavours, and styles of malt whisky.
This process is used to make single malt whiskies, blended whiskies, blended malts, and Irish pot still whiskeys. What happens in each step of the process?
Step 1 – Malting
Barley, and others grains, harvested from the field is packed with starch. Malting turns this starch into sugar, which is needed to produce alcohol. This action is natural and takes a week – even modern technology cannot speed it up. To turn starch into sugar the barley is tricked into thinking it needs to become a plant.
To begin, barley is soaked in cold water for 24 hours. This swells the grains, raising the moisture content from around 5-6% to 55-60%. Traditionally the barley was then spread across a long concrete or stone malting floor to a depth of six inches. It must be turned regularly to distribute heat in the early days and prevent roots and shoots knitting together in the latter days.
After six days the process of sugar conversion is complete. The grain must be dried to prevent it growing into a barley plant and using the sugar itself. Traditionally this takes place in a kiln with the malted barley resting on a mesh floor over a fire. Until modern alternatives were introduced this was fuelled by peat, which burns quickly with plenty of heat and smoke.
Modern commercial malting is completed in a large drum and allows for much greater quantities to be malted. The soaked barley is loaded inside, and the drum slowly turns to prevent the heat and knitting issues. The action creates a sand dune effect with grains tipping from the top back to the bottom. After six days the barley is then blasted with hot air to dry it in a similar way to the kiln.
Step 2 – Milling
Once dried, the barley is milled. Prior to this, it is passed through several filters to remove any stones, debris, or foreign objects. Most single malt distilleries have a two-roller milling system – the first rollers crack the husks, while the second set are closer together to crush the grain. The result is called grist. The ratio of husk, middle and flour (normally 20%, 70% and 10% respectively) must be perfect – too much flour and everything will glue together when water is added, too much husk and the water will drain straight through.
Step 3 – Mashing
The malted barley is packed with soluble sugar. The grist is loaded into a large vessel called a mash tun and warm water added. The mixture is stirred using large interior paddles or rods and the sugars and enzymes in the barley pass into solution.
This is drained through a meshed floor of the mash tun and collected before water is added at a higher temperature. This extracts further sugars and enzymes. This occurs three times at most distilleries with average water temperatures being 65°C, 75°C, and 85°C. The resulting sugary liquid is called wort, which goes to the next stage of the process. The residue is known as draff. This is collected, dried, and turned into animal feed.
Step 4 – Fermentation
The wort is cooled and passed to a huge tank called a washback. These can be metal, usually stainless steel, or wood, usually pine or oak. Yeast is added and feeds on the sugar – this produces alcohol and carbon dioxide. Liquid yeast is commonly used, which speeds up the initial time for this process to begin.
Fermentation is the first part of the process that can have an influential effect on the final spirit. The yeast’s job is done after 48 hours with all sugar turned to alcohol. Many distilleries take the liquid at this point, which is now called wash, to distil. However, many more leave the wash for longer – this creates extra depth, complexity and fruity notes as the yeast cells die and fall to the bottom of the washback. The wash is 7-8% ABV and reminiscent of a Weissbier.
Step 5 – Distillation
For single malt the wash is traditionally distilled twice. This is true in Scotland and Japan. In Ireland it is commonly distilled three times. Each time a spirit is distilled it becomes more pure and higher in alcohol strength. Single malt is made in batches and through copper pot stills. These have a bowl-shaped pot at the base, a neck and a lyne arm coming off the top of the neck – this is connected to a condenser. Copper is great at purifying alcohol vapours.
The stills are another key point where a distiller can control and influence the flavour and character of their spirit. The shape and size of stills are a major factor in this – simply put, tall stills make light and delicate spirit while shorter stills produce heavy and oily spirit. Across the industry there are stills of all shapes and sizes, which provide huge diversity.
First distillation takes place in the wash still – normally the larger of the two still types. The wash is pumped into the pot and heated. Alcohols begin to evaporate at a lower temperature than water (around 65-70°C) and vapours rise, travelling up the neck and down the lyne arm. Once they hit the condenser, which is a series of copper tubes surrounded by cold running water, the vapours return to liquid form. This is now called low wines and has an alcoholic strength of approximately 25% ABV.
Some distilleries operate with a worm tub condenser system. This traditional apparatus has largely been replaced with modern tube condensers. A worm tub is a large tank on the exterior of the distillery into which the lyne arms of the stills are connected. This is filled with cold water and the lyne arms coil sharply down through the water. This resembles a worm, hence the name. Again, once the alcohol vapours hit the cold copper in the tank they condense back to a liquid.
Second distillation takes place in the spirit still. The low wines are heated in the pot and different alcohols rise at different stages. Foreshots come first – these are very high ABV, pungent and full of undesirable compounds. Next come the hearts or ‘middle cut’. They come off the still as a transparent liquid at approximately 65% ABV. Finally come the feints – these are weak, cloudy and contain undesirable compounds.
Step 6 – Maturation
The hearts, or middle cut, from the second distillation are the only part that will eventually become whisky. In Scotland, Ireland and Japan the new make spirit must legally mature in oak casks for a minimum of three years. The most common types of wood used for casks are from America or Europe – American oak (Quercus Alba) and European oak (Quercus Robur).
During maturation the spirit breathes in and out of the wood, extracting natural compounds as it does. Toasting or charring the inside of the barrel prior to filling invigorates these compounds and caramelises the natural sugars in the wood. It also cracks the oak to give a larger surface area to present to the maturing spirit.
The cask is said to give up to 75-80% of the flavour to a whisky, plus its colouration. Over time the level of liquid within the barrel will drop – wood is porous and as whisky breathes in and out of the wood, some will naturally evaporate. This is called the angel’s share. In Scotland and Ireland, the average is about 1-5-2% per year. In America it can be 8-10%, while in hot climates such as India and Taiwan it can be as high as 20% per year. The alcohol level will also drop by approximately 0.5% ABV annually.
The location of a warehouse that a cask is maturing in will also have an impact on flavour and character. Factors such as temperature and humidity will be different in different places – by the coast versus inland, in a valley versus on a hill, in a town or city vs on a remote island and so on. As the whisky breathes the natural surroundings will add influence. This is why some people say that coastal whiskies have a distinct salty tang.
