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.