The Secret Life of Yeast: How a humble fungus shapes your favourite whisky!
When we talk about the great elements that create the character of a single malt whisky, we often praise the quality of the barley, the purity of the water, or the magic of the cask. But there is another, often overlooked, ingredient that plays a fundamental role in shaping the very soul of the new make spirit: yeast.
This single-celled microorganism is the engine of fermentation, the unseen workforce that turns sugary wort into alcohol. But its contribution goes far beyond just creating booze. The specific type and strain of yeast used can introduce a universe of flavour and aroma, defining the spirit long before it ever touches oak. Let's take a closer look at this humble but mighty fungus.
What exactly is yeast?
Yeast is a single-celled microorganism, and there are hundreds, if not thousands, of different types in the world. When we talk of 'yeast' in distilling, we're interested specifically in the ones that help us make alcohol.
Think back to school biology and how living things are classified: Kingdom > Phylum > Class > Order > Family > Genus > Species.
For human beings, our full title is Homo sapiens. We belong to the kingdom Animalia, the genus Homo, and the species sapiens. The same system applies to yeast. A name you have likely come across is Saccharomyces cerevisiae, which belongs to the Fungi kingdom.
A lot of the yeast responsible for making our favourite tipples belong to the genus Saccharomyces (which literally means ‘sugar fungus’). They are named ‘sugar fungus’ for a good reason: they are incredibly efficient at converting carbohydrates, like sugars, into alcohol and carbon dioxide.
Saccharomyces cerevisiae is the most well-known companion to humankind for all sorts of leavening and fermentation, key in making everything from our daily bread to our favourite wines, beers, and spirits for millennia.
It's worth noting, however, that S. cerevisiae can't take all the credit.
Lager: The yeast that makes your favourite lager is typically a variant of the Saccharomyces pastorianus species.
Wine: A large number of wine yeasts are of the species Saccharomyces bayanus.
Rum: Saccharomyces pombe teams up with other species in wild fermentations to make rum.
But for whisky, our old friend S. cerevisiae is the most frequently used.
From sugar to spirit: How yeast creates flavour?
Within the S. cerevisiae species alone, there exists a huge variety of different 'strains' or genetic variants. It's from this variance that we get different new make spirit characteristics. In essence, different yeast strains will produce a different number and proportion of chemical compounds, called metabolites, when they convert sugars into alcohol.
In whisky making, these sugars are present in the wort—the liquid created when malted barley is mixed with hot water during mashing. Two key examples of flavour and aroma compounds produced by yeast during fermentation are higher alcohols and esters.
There are thousands of these metabolites, each with different characteristics. For example:
Isoamyl acetate (an ester) can give off aromas of banana and honey.
2-phenylethanol (a higher alcohol) can produce the scent of roses.
Furthermore, during particularly long fermentations, a large number of yeast cells can die and burst, releasing their contents. This process contributes flavours known collectively as 'autolytic' notes, often described as bready, yeasty, or meaty (think of the 'sur lies' process in winemaking).
Overall, yeast produces a whole cocktail of metabolites. The most volatile of these compounds—the ones that evaporate easily—carry over through the distillation process and into the final new make spirit, contributing hugely to its overall flavour and aroma.
How is distiller's yeast produced?
Yeast can switch its metabolism based on the environment. It can respire aerobically (with oxygen) or anaerobically (without oxygen). Understanding this is key to producing commercial yeast. The focus is on encouraging the yeast to make more healthy cells, which happens in the presence of oxygen. The alcohol production we want in the distillery, however, happens in the absence of oxygen.
Yeast for alcohol production is typically grown aerobically and then used anaerobically.
Both liquid and dried yeast start as tiny cultures, usually no more than a few millilitres. These are progressively grown on substrates like molasses. By carefully controlling oxygen and nutrients, the yeast is forced into an aerobic state where it doesn't produce ethanol. Instead, all its energy goes into making more cells, exponentially increasing the volume. This creates a liquid yeast slurry. For dried yeast, this slurry is then separated, dried, and vacuum-packaged.
References and info:
Brewing yeast and fermentation, Chris Boulton and David Quain.
This blog was kindly written for WWS by: Vicky Muir-Taylor EU Distilling Sales Manager, WHC Lab Brewing & Distilling.
(Introduction and graphics Sean Williams, WWS.)
