Fermentation in Scotch Whisky
So much can effect 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 tones from the cask or the fierce smokey flavour of peated whiskies. Some flavours are easier to identify than others, but every step impacts the final dram, and this article explores the complex processes and variety of flavours that may be created during long fermentations.
Our Exploring Whisky blog series delves into the fascinating subjects around the production and history of Scotch whisky. We have already touched upon the impact of peat, the origins of distillation and the first steps on the road to making whisky. Now we arrive at fermentation, a natural occurring process which nearly every civilisation and culture has used to make food and beverages for millennia.
Nature’s Way
Fermentation is a natural process whereby microorganisms like yeast and bacteria convert carbs — such as starch and sugar — into alcohol or acids. Widely used for the production of alcoholic beverages, fermentation is also used in bread making and in foods like kimchi, cultured yoghurt, miso or sauerkraut.
In whisky making, fermentation is a critical part of the process as this turns the wort (made in the mashing process) into alcohol that can then be extracted during distillation;
After milling and mashing has created a sugar-rich solution from the barley, this sugar-rich ‘wort’ is pumped into washbacks (large vessels usually made from steel, pine or larch) and special strains of yeast are added. During fermentation, the yeast splits the sugar molecules into ethanol (alcohol) and carbon dioxide in a process which produces heat.
If you’ve ever open the door into a washback during the fermentation process, you can feel the powerful slap of carbon dioxide. The traditional tragedy whilst making beer and whisky, is that brewers could be overcome by carbon-dioxide fumes and fall in while testing the fluid. Nowadays health and safety at breweries and distilleries is paramount. As washbacks are commonly closed and much of production is controlled electronically, thankfully we don’t see this anymore!
Careful temperature control is a vital part of the fermentation process. High temperatures are needing for mashing, but the wort is cooled to avoid killing the yeast when it is added. Fermentation naturally heats the wort again, but the challenge is that the optimal temperature for converting sugar to alcohol is about 34 degrees C, but at temperatures over 35 degrees C yeast starts to die off.
The length of fermentation varies between distilleries and can last anywhere between 50 to 120 hours, with longer fermentations said to produce more complex flavours (read below for our notes on late lactic fermentation). The choice of yeast also varies, with distillers yeast producing more alcohol and brewers yeast developing more aromatic flavours. It is common for Scotch whisky distilleries to use a mix of yeast strains.
Late Lactic Fermentation
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 which can survive in wood pores and arguably contribute subtle flavours to the final product. This is a debated subject and many people seem to think the wooden washbacks are more about heritage and tradition than flavour.
Nevertheless, the role of microorganisms is a complex and fascinating aspect of fermentation. The distillers choice of yeast, which is a fungus, is added to the wort to initiate fermentation in the washback, but some bacteria present in malted barley, such as lactic acid bacteria, can make their way with the wort into the washback along with any wild yeasts that may have also survived the high temperatures during mashing. The longer the fermentation is allowed to proceed the greater the influence of bacterial growth.
If there are too many bacteria during the early stages of fermentation they may compete with the yeast and lower the ethanol yield, but many distilleries welcome late lactic fermentation as the accumulating lactic acid reacts with ethanol to form chemical compounds called esters, which impart fruity notes. Ester ethyl lactate, for instance, imparts buttery, creamy flavours with hints of fruit and coconut.
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 lactic and acetic acid, whilst other strains (“homofermentative”) only begin to flourish in longer ferments once the yeast starts to die off. These only produce lactic acid.
With all this going on 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 seem to think this is yet another romantic notion which, whilst unlikely, cannot quite be dismissed. Whisky does seem to be surrounded by such stories!
However, with hundreds of flavour compounds and the amazing complexity and individuality of many single malts, it’s hard to deny these flavours are impacted by the complex processes going on during production. A careful eye is key, other bacteria, such as vinegar bacteria, can spoil the contents of the washback and distillers must to pay close attention to cleanliness to control the presence of bacteria during the early stage of fermentation.
Further Reading;
Managing Microbes, Ensuring Quality and Valorising Waste, Dr Annie E. Hill
Diffords Guide to Single Malt Scotch Whisky - Diffords Guide
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