So much can affect the flavour of single malt Scotch whisky. You may love the meaty tones from the use of a worm tub, the light spirit from triple distillation, the rich oak character from cask maturation, or the fierce smoky flavour of peated whiskies. Some flavours are easier to identify than others, but every step impacts the final dram. This article explores the complex processes and variety of flavours created during one of whisky's most fundamental stages: fermentation.
From Barley to Malt
Whereas gin producers most often rely on a pure, clean distillate as a blank canvas for botanical flavours, Scotch whisky distilleries focus heavily on the character of the spirit itself. Before distillation even begins, the wash is a complex mixture of water, alcohol, and various flavour compounds known as 'congeners', which have been developed over the course of production. The foundation of this complexity begins with malting.
Nature's Transformation
Fermentation is a natural process whereby microorganisms like yeast and bacteria convert carbohydrates—such as starch and sugar—into alcohol or acids. Widely used for producing alcoholic beverages, fermentation also plays a crucial role in bread making and in foods like kimchi, cultured yoghurt, miso, and sauerkraut. Nearly every civilisation and culture has used fermentation to make food and beverages for millennia.
In whisky making, fermentation is critical as it transforms the wort (the sugar-rich solution created during mashing) into alcohol that can then be concentrated through distillation. Without fermentation, there would be no whisky—just sweet barley water.
The Fermentation Process
After milling and mashing have created a sugar-rich solution from the barley, this 'wort' is pumped into washbacks—large vessels usually made from stainless steel, Oregon pine, or larch—and specially selected strains of yeast are added. During fermentation, the yeast splits the sugar molecules into ethanol (alcohol) and carbon dioxide in an exothermic process that generates considerable heat.
If you've ever opened the door into a washback during fermentation, you can feel the powerful rush of carbon dioxide. The traditional tragedy in beer and whisky making was that workers could be overcome by carbon dioxide fumes and fall in while testing the liquid. Nowadays, health and safety at breweries and distilleries is paramount. Washbacks are commonly closed and much of production is controlled electronically, so thankfully such accidents are no longer seen.
Temperature: A Delicate Balance
Careful temperature control is vital to successful fermentation. High temperatures are needed for mashing, but the wort must be cooled before yeast is added to avoid killing it. Fermentation naturally heats the wort again, creating a delicate challenge: the optimal temperature for converting sugar to alcohol is about 34°C, but at temperatures over 35°C, yeast begins to die off.
The length of fermentation varies between distilleries and can last anywhere from 50 to 120 hours. Longer fermentations are said to produce more complex flavours, particularly through a process known as late lactic fermentation. The choice of yeast also varies, with distiller's yeast producing more alcohol and brewer's yeast developing more aromatic flavours. It's common for Scotch whisky distilleries to use a mix of yeast strains to achieve their desired character.
Wood vs Steel: The Washback Debate
Washbacks were traditionally made of wood, often Oregon pine, but more recently distilleries have started using stainless steel washbacks. These are certainly easier to clean and maintain, but they don't support the microorganisms that can survive in wood pores and arguably contribute subtle flavours to the final product.
This is a debated subject. Many people think wooden washbacks are more about heritage and tradition than flavour, while others insist the microbial ecosystem in wood adds complexity. The truth likely lies somewhere in between—wooden washbacks may provide subtle benefits, but the primary factors affecting fermentation are temperature control, yeast selection, and fermentation duration.
Late Lactic Fermentation
The role of microorganisms in fermentation is complex and fascinating. The distiller's chosen yeast—which is a fungus—is added to initiate fermentation, but bacteria present in malted barley, such as lactic acid bacteria, can make their way into the washback along with wild yeasts that may have survived the high temperatures during mashing.
The longer fermentation is allowed to proceed, the greater the influence of bacterial growth. If there are too many bacteria during the early stages, they may compete with yeast and lower the ethanol yield. However, many distilleries welcome late lactic fermentation because the accumulating lactic acid reacts with ethanol to form chemical compounds called esters, which impart fruity notes. Ester ethyl lactate, for instance, contributes buttery, creamy flavours with hints of fruit and coconut.
Each Distillery's Unique Bacteria
Interestingly, there are many different strains of lactic acid bacteria, and a study by scientists from the International Centre for Brewing and Distilling at Heriot-Watt University in Edinburgh has shown that each distillery has its own unique combination of bacteria.
Some strains of lactic acid bacteria (known as "heterofermentative") grow throughout the process and produce both lactic and acetic acid. Other strains ("homofermentative") only begin to flourish in longer fermentations once the yeast starts to die off, and these produce only lactic acid.
With all this microbial activity, it has been suggested that the presence of different strains of bacteria has subtle effects on the wash and might help explain the complexity and variety of flavours found in whisky. Many commentators view this as yet another romantic notion which, whilst unlikely to be the dominant factor, cannot be entirely dismissed. Whisky does seem to be surrounded by such stories.
The Complexity of Flavour Creation
However, with hundreds of flavour compounds present in whisky and the amazing complexity and individuality of many single malts, it's hard to deny that these flavours are influenced by the intricate processes occurring during fermentation. The interplay between yeast strains, bacterial populations, fermentation length, and temperature all contribute to the character of the wash.
A careful eye is essential. Other bacteria, such as vinegar bacteria, can spoil the contents of the washback, so distillers must pay close attention to cleanliness to control bacterial presence during the early stages of fermentation. It's this balance between encouraging beneficial microbial activity and preventing contamination that makes fermentation both an art and a science.
From the sugar-rich wort to alcoholic wash ready for distillation, fermentation transforms simple ingredients into a complex liquid brimming with flavour precursors. The decisions made during this stage—yeast selection, fermentation length, temperature control—will echo through distillation and maturation, ultimately shaping the whisky in your glass.
Further Reading
- Managing Microbes, Ensuring Quality and Valorising Waste Dr Annie E. Hill
- Difford's Guide to Single Malt Scotch Whisky Difford's Guide