When it comes to the flavour of single malt scotch whisky, which one reigns supreme - water, malted barley, or yeast? Scotch whisky is bountiful with tales of oak casks and impressively polished copper stills, and while these are hugely influential processes, what about the raw ingredients?

In our previous discussion in this series we tackled malted barely. If you’ve not yet read it you can find it here: How Barley Shapes The Soul Of Scotch. This time however, we are taking the conversation up a notch. If malted barley shapes the soul of scotch, what role does yeast play in creating those distinctive nuances in Scotland’s liquid national treasure?

To find out for yourself, we are going to answer these fundamental questions plus many more. Join us as we journey not just into the fascinating world of yeast and whisky flavour, but also as we take you to the very heart of the scotch whisky industry itself. So pop some bread in the toaster and crack open a fresh jar of Marmite, because we are about to venture into how yeast shapes the flavour of whisky.

What Does Yeast Do In Whisky?
Fermentation is what yeast does. These tiny microorganisms of the fungi family feast on sugars. In the case of single malt whisky these sugars are derived from malted grains of barley. As yeast consumes the sugars it releases many different bi-products. The most well-known ones being carbon dioxide, heat and alcohol. 

The fermentation process, with yeast at its helm, offers the most significant chance to distinguish one whisky from another, a pivotal aspect of whisky production. However, given the vastness of this topic, our exploration will be split into two parts. In this article, we are taking you deep into the world of yeast. All you could possibly want to know about fermentation will be covered next time.

The vastness of the topic is brought about by the superhero nature of yeast cells. In fact, until the 19th Century no-one even knew what they were – animal, plant or mineral. With the in-depth understanding we have today, we now know that alongside the all-important alcohol part, yeast cells hold the keys that unlock most of the doors to whisky flavour. But are all yeast strains equal.

What Yeast Type Is Used In Single Malt Scotch Whisky?
There are a myriad of different strains of yeast that are all around us all of the time. In terms of the scotch whisky regulations, any type of yeast can be used. However, many types of yeast will not ferment maltose, which is one of the sugar types obtained when barley is malted. Yeast strains have therefore been developed specifically for brewing, with others designed for distilling, the most common family for both being Saccharomyces Cerevisiae.

While brewer’s yeasts were once commonplace in the whisky industry, more specialised distiller’s yeasts have been selectively cultivated. One of the most popular of these special distiller’s yeasts is simply called M and has been produced by Kerry Ingredients since the mid-1960’s. These distillers yeasts have been no. 1 in the Scottish spirit charts for a long time now, but why is this? What’s the big deal?

Why Do So Many Distillers Use Distiller’s Yeast?
On the surface this may seem like a somewhat daft question, however, it’s really quite an interesting topic. Especially when we begin to unravel what attributes a scotch whisky distiller looks for in a strain of yeast. Without getting too bogged down in the technicalities, it’s time to focus as we take a look.

Firstly the yeast must be able to ferment all of the different sugars from the barley, which includes sucrose, glucose, fructose, and our good friend maltose. It’s a simple matter of keeping the accounting department happy. The more sugar that can be converted to alcohol, the greater the yield per tonne of barley. In an ideal world all of the sugar will be transformed into alcohol.

Secondly, the yeast needs to be a hardy little blighter. Not all strains of yeast can survive if there is too high a concentration of sugar. With all of these sugars floating about, the yeast cells must stay alive and multiply. As the sugar levels decrease and alcohol increases, the yeast must also be able to handle this too. Typically the wash (fermented barley juice) will go up to 8–9% abv for making malt whisky. It may not sound much to you and I, but for some yeasts it could be worse than listening to Justin Bieber. (Sorry Justin)

If you recall from earlier, one of the bi-products of fermentation is heat. Like all of us, when these critters get to work they also get hot. Unlike us though, they cannot go Taps-Aff and crack open a cold beer. To yeast that would be like drinking… well let’s not go there! As most distilleries cannot control the temperature during fermentation, the yeast cells are left to race up the temperatures, in a rather uncontrolled fashion.

Finally there are other considerations that we will briefly mention. A whisky distiller will want a yeast which doesn’t clump together (called flocculating). It must be able to survive well during transit and storage. And it must add other compounds that ultimately make the whisky taste good. This final point is in fact probably the most important (depending on your priorities of course), which we shall delve into in a moment. But first, how did these specialised yeast strains come about?

How Did Distiller’s Yeasts Come about?
Brewer’s yeasts have been phased out almost entirely amongst scotch whisky distillers. One reason for this is that since breweries began to consolidate in the 1960s, the manufacturing of yeast also started to become more centralised and dominated by a handful of companies. This was also partly driven by the rising popularity of lager (lager needs a different type of yeast). 

Also as whisky production began to increase, efficiency became an increasing priority for Scottish distillers. The decisions around flavour creation were shifted to other parts of the process, such as fermentation time and cask maturation. While brewer’s yeasts were still being used up to a point, it was in combination with distiller’s yeast.

Hence mass-produced distiller’s yeasts became readily available, reliable, efficient and the popular choice. Interestingly the approach to yeast selection by distillers is slightly different in the U.S. and even Japan – keep reading to find out more as we begin to look at yeast’s impact on flavour. Starting with a common question.

Does Yeast Affect The Flavour Of Single Malt Whisky?
Bourbon producers in the U.S. are renowned for using strains of yeast that are unique to their distillery. Kept under lock-and-key, these closely guarded microorganisms are considered central to the distillery’s style. In fact, one such distillery, Four Roses, keeps five different strains of yeast to create different styles of whisky.

In Japan too, yeast is viewed as a major contributor to the creation of certain flavours in the spirit. Not to say that there’s a right and a wrong approach here, it’s merely a case of priorities. With only three ingredients being allowed in single malt scotch whisky, when pushing the quality and character of the spirit to the highest levels, it is also our approach to maximise each and every ingredient.

It’s important to maintain that yeast is inseparable from fermentation. By manipulating fermentation it’s possible to greatly alter the flavour compounds that are created by the yeast, regardless if it’s a brewer’s yeast or distiller’s yeast. Through our own experiments at Crafty Distillery we have come to understand this first-hand. But, by trialling different yeast strains also, we too appreciate the difference that yeast alone can make. Differences in flavour congeners that are not only detectable on the nose, but are also supported by scientific analysis too.

What Whisky Flavours Does Yeast Create?
The obvious one is of course alcohol, and when we talk about alcohol we generally mean ethyl alcohol (ethanol), because this is the largely flavour-limited type that we drink in everything from wine to vodka. But yeast cells produce many different types of alcohols with complicated names. Some of which sound more like products used for degreasing engines. The aromas of these alcohols range from scented roses to pharmacy counters, and fortunately for health reasons many of these will be removed or changed through distillation.

Then there are other compounds such as aldehydes and phenols, which in this case are associated with green apples and spicy aromas respectively. Plus many different types of acids that are important for flavour creation, some of which you may have heard of such as lactic, acetic and butyric acid. While many of these will not become flavour themselves, they play important roles in shaping other flavour producing molecules. But esters on the other hand... 

The Importance Of Esters For Scotch Whisky
Esters are very special because they are responsible for most of scotch whisky’s fruity aromas. A common example being banana which classically comes from an ester called iso-amyl acetate (in case you want to impress your guests at your next soirée). But there are many, many more with equally technical names. To keep things simple we are talking about aromas of mango, apples, pineapple, under-ripe banana and apricot. How about butter, cream, aniseed or nail varnish remover? All of which are quintessentially ester derived.

In basic terms, yeast cells take a relatively simple liquid of sugars, enzymes, proteins and amino acids extracted from barley, and transform it into a complex primordial soup that is the first indication of whisky flavour. The balance of all of these compounds will be determined by the interplay between the length of the fermentation and, specifically for this article, the strain of yeast used. What about our own experience at Crafty Distillery?

What Type Of Yeast Do We Use At Crafty Distillery?
As with all our spirits, when Crafty Distillery’s master distiller, Craig Rankin, set out to create our Billy & Co single malt whisky new make spirit (there must be a shorter term for it), he stripped everything back. To maximise each individual variable that could impact flavour and quality, of course yeast came under much scrutiny. In the end after looking at dozens of yeasts, and their profiles, Craig narrowed the trials down to three strains of yeast, one of which was a more standard distiller’s yeasts as a benchmark while the other two were beer or brewer’s yeasts.

Each type of yeast was tested at its extremes to accentuate the differences between them. This meant trialling both short and hot fermentations and also long and cool fermentations.  A very unusual approach was taken to find strains of yeast to test, and that was yeast, that could provide compounds that could mimic classic food combinations. One yeast on the table, from the offset, produced big banana notes, which of course would ultimately sit with grain and mayard caramel notes from the Cask. Those notes are the nuts and bolts of Banoffee pie for instance This approach of finding classic flavour combinations, recognising the baseline points involved was thrown into the mix with great success.

While the distiller’s yeast clearly delivered more alcohol, Billy & Co is an exercise in excellence rather than volume. It’s a push towards crafting a new make spirit of exceptional quality and character in its own right – something we call a ‘spirit first’ approach. The results were analysed by the most sensitive equipment available – the human nose. And not just our own noses, but the hundreds of noses of our Founder’s Club Members whose feedback helped to shape our choices.

In addition, the sensory feedback was substantiated by laboratory analysis conducted by clever people in white coats at University College Dublin. The results from all of this data, both olfactory (the nose) and lab-based, highlighted very clear information about the fermentation process too. But as mentioned, that’s for next time. Now how about the yeast.

No matter the fermentation time, both of the beer yeasts presented much higher concentrations of those fruity esters that we previously discussed. However, the feedback from ourselves at the distillery and from our Founder’s Club Members pointed in one direction. By following a simple process of trialling and evaluation, Craig understood that the path would become apparent. It was just a case of following it to see where it would lead.

The clear winner at the end was massively banana forward, with hints of red apple and tropical fruit . This yeast was in fact Munich beer yeast. Now it just so happens that beer yeasts cost a lot more than distiller’s yeast. This is because brewers can re-use their yeast, whereas distiller’s yeast does not last long enough. The upshot for us is that our Munich yeast costs around thirteen times the price of distiller’s yeast. That’s right 13 x as much.

This may seem crazy because normally for distillers yeast is one of the most inexpensive parts of the process. But for us it’s the most expensive. With smaller distillers having the flexibility to experiment with different yeast strains, brewer’s yeasts are becoming more commonplace in the craft area of the industry. It’s still commonplace though to combine brewer’s yeast and distiller’s yeast together whereas we are 100% specialist beer yeast.

Is It All Worth It?
Why not just use standard distiller’s yeast as it clearly produces incredible results for many, many Scottish distilleries? The Crafty ethos is the spirit of innovation, questioning and pushing the limits of quality at every opportunity. This comes before all else. So for us it was a natural decision to follow this path, not just based on our own preferences but also those of our Founder’s Club Members. Creating flavours for the people, by the people. We’ll worry about the costs later.

On your next visit to Crafty Distillery, if you have not done so already, have a try of our Billy & Co new make spirit. It’s the liquid embodiment of the importance of yeast selection and quite unlike any other new make spirit out there. And when you try it, see how many ester aromas you can pick out.

After reading this, I’m sure you’ll agree that these single cell organisms are truly remarkable. And perhaps the champions of scotch whisky’s flavour. The mysterious dark art that is oak cask maturation adds further complexities for sure (we’ll need some candles, full moon and a black cat to discuss that one). But yeast takes what is provided by the barley and shapes it into something tangible, complex and ultimately tasty. So the next time you pour a dram (or any alcoholic tipple for that matter), say a little cheers to yeast. The gatekeeper to whisky flavour.