Fermentation is the process that creates alcohol.but it is not just important for the creation of alcohol, but also vital flavour development that is refined and enhanced by later processes. To be more specific, when discussing fermentation in a brewing/distilling context we are discussing fermentation as the anaerobic energy metabolism of yeast, producing ethyl alcohol. Let's break down this definition to better understand it. Metabolism is the name given to the chemical reactions that happen within a cell to convert energy in a food source into useable energy for the cells purposes, these chemical reactions give a waste product. Anaerobic means that this happens without the presence of oxygen; the metabolism of yeast in the presence of oxygen results in the waste product being water (H2O) and carbon dioxide(CO2), not so useful to distillers. At the most simple level, alcoholic fermentation uses the formula glucose → ethanol + carbon dioxide (C6H12O6 →2C2H5OH + 2CO2 ). But, it is important to know there are other outputs from the process. Firstly, heat is generated, this is important to remember because if the temperature of the ferment gets over 30°C the yeast becomes less efficient and at 55°C it will cook and kill the yeast. Other kinds of alcohol, namely methanol (CH₃OH) and fusel alcohols are also produced. These other alcohols can be removed in the cuts made during distillation, but I will discus this further when I write about the distillation process. Fatty acids and sulphur compounds are also produced. The fatty acids react with alcohol to create esters, which are vital and desirable flavour compounds, usually giving spirits fruity aromas. Sulphur will also be discussed further in distillation.
We now know too much about the process of fermentation. Let's learn about yeast. Yeasts are single cell organisms that are part of the fungi family; yes these vital creatures that create so much pleasure, needed for bread, whisky, and wine, are related to mushrooms and truffles. Remember to thank your local fungi today for all the good work they do! Yeasts are found everywhere: for example water, soil, leaves, buildings, and on animal skin. You probably have a few species of yeast on and in you right now! Some people even have a condition called auto-brewery syndrome where yeast in the gut converts carbohydrates ingested into alcohol. Yeast was one of the earliest organisms to be domesticated, with Saccharomyces Cerevisiae being used to produce bread, wine and beer for thousands of years. In fact, yeast is so important that Saccharomyces Cerevisiae was the first Eukaryote (organism with a defined cell nucleus) to have its genome completely sequenced, in 1996. Sequencing the genome has given us a strong understanding of the form and function of these miracle creatures. The sequencing was a collaborative effort between 94 laboratories in 19 different countries! A rare level of international collaboration. Yeast reproduces asexually through budding, where the cell nucleus divides, and then the new nucleus and some of the cytoplasm forms a growth on the cell wall then separates to become the new yeast cell.
Within a distillery, fermentation happens in washbacks. These are very large vessels, usually around 12x20 feet with a capacity of 30,000L. Washbacks are typically made of stainless steel, pine, or larch. Stainless steel is the current material of choice. This is for a number of reasons. Firstly, it is easy to control the temperature of a stainless steel washback because of the thermal conductivity of the material. Secondly, Stainless steel is easy to make sterile because it has a smooth surface. Finally, stainless steel is relatively inexpensive to purchase and maintain. The Advantage of using wood, however, is that it is porous and so can hold colonies of bacteria and yeasts that can impart unique flavours into the wash (they are still cleaned vigorously don’t worry!), they are more visually attractive, and are seen as traditional. Most washbacks are covered in order to avoid contamination. Some also include a switcher which is a fan installed to break up the foaming if the fermentation threatens to overspill. Other methods of stopping overspill are also employed, in emergencies a silicone based emulsion is used, it also used to be common practice to use some soap powder to tame particularly aggressive ferments, although if this is still done it is not widely discussed. I even know of one distillery that uses butter as its antifoaming agent. Modern washbacks are now fitted with carbon dioxide extractors to keep the workplace safer, however a few of the largest distilleries have exchanged these for machines that capture the carbon dioxide and then sell it to the soft drinks industry.
The wort is cooled to around 20°C from the mash tun and then pumped into the washback and yeast is added to the wort. The technical term for this is an inoculum. More commonly this is called pitching the yeast. The wort must receive the inoculum as soon as possible so that it isn’t infected with bacteria, which can outcompete the yeast if allowed to propagate at the earliest stages. If the yeast is unsuccessful in dominating the wort, this can result in a stuck fermentation where not all the sugar has been converted to alcohol. There are three kinds of yeast a distillery may use: Liquid cream yeast, pressed semi dry, or dried. Liquid cream yeast is the freshest form of yeast so gives the highest yields, but must be used within a week of delivery. Dried yeast gives the lowest yield but can be stored for a couple of years, so is usually kept on site for use in emergencies. Pressed semi dried yeast is the middle ground between these two forms, but must still be stored in an industrial refrigerator in order not to kill the yeast, and must be used within the month. and so is commonly used by distilleries with less reliable supply chains, or who are too far away from a yeast manufacturer. Certain distilleries are able to grow their own yeasts, but this requires specialist equipment and dedicated space so is not viable in all distilleries.
All distilleries used to use a yeast strain called DCL M-Strain, and while it is still used there are some distilleries that are diverting to even more efficient yeasts. Yeast is considered efficient based on its reliability, yield of ethanol, and speed of propagation and fermentation. Occasionally you will see distilleries experimenting with other strains of yeast, but this is rare and usually found only within the craft space. Examples of distilleries that have used other yeasts are: Glenmorangie with their Allta release, where they isolated and propagated a wild yeast found within the distillery. Glen Elgin who for a special release in 2017 used Schizosaccharomyces Pombe, which is more commonly used in rum production. An example of a craft distillery experimenting with yeast is the Holyrood Distillery in Edinburgh who have released a number of new makes successfully utilising different strains of yeast.
The fermentation goes through four stages. The first stage is known as the lag stage and is where the yeast acclimatises and adjusts to the new environment and starts to grow, it is called the lag phase because it appears nothing is happening. It usually takes less than 12 hours. The second stage is called the budding phase, or log phase. This is where the yeast reproduces rapidly, in this phase there is still oxygen present in the mash but this is used up quickly and the layer of carbon dioxide that sits on top of the ferment stops more oxygen getting in. Once the oxygen is gone the fermentation goes into the third phase, this is when ethanol is created, the fermentation also gets more rapid and aggressive, this can be when there is a risk of bubbling over. Phase four occurs when there is no more sugar left. In this phase the yeast dies and if the wash is left after this point this is when wild bacteria and yeast will impact the final flavour.
Fermentation is typically complete within 24-48 hours, but many distilleries ferment for longer than this for the sake of flavour development. A short fermentation is anything less than 60 hours, and gives malty and cereal notes, An example of this is Glenkinchie. A mid length fermentation (60-75 hours) more floral notes, as seen in Deanston’s spirit and long fermentations (75 hours+) starts giving fruity notes that become more tropical as fermentation gets longer, as shown with Glenallachie’s whiskey. This is because of bacterial infection and the propagation of wild yeasts that happens over time as the wash is left exposed.
The resulting liquid from fermentation is called wash and is usually at an alcoholic strength of between 6% and 8% ABV and is now ready to be distilled.
