Any discussion of flavour development entails a key statistic, that up to 70 per cent of a malt's character develops during aging. That's a significant total, but however influential aging is, it won't be able to deliver unless distillation provides a new make spirit with the right quality and character. This in turn depends on the action of the yeast during fermentation, which dramatically transforms the straightforward cereal character of the wort.During fermentation the yeast metabolises sugars and other nutrients within the wort in order to grow and reproduce. As a by-product of this, the yeast emits alcohol and a significant range of flavour compounds, including various fruit notes.
Yeast experiences three consecutive stages during fermentation: the lag phase (when acclimatising to the wort), the log phase (when key growth occurs) and the stationary (ie winding down) phase. Although these terms refer to yeast they also correspond to flavour development, the more active the yeast the greater the production of alcohol and flavour compounds.
In the initial lag phase, typically lasting several hours, yeast cells begin to reproduce by developing a bud. Once the bud reaches half the size of the mother cell it detaches, continues growing to ‘full’ size, then develops a bud of its own. Yeast cells gain the energy to do this partly by feeding on oxygen dissolved within the wort, but essentially by metabolising sugars. Glucose is first on the menu being the simplest sugar (comprising one glucose unit) and so the most easily digestible. Meanwhile, the yeast begins to produce limited levels of alcohol (of which ethanol is one example), and various flavour compounds.
“Yeast cells feed on the sugars and break them down, with a consequence of this being that carbon dioxide and alcohol are produced. Most of this alcohol is emitted by the yeast as ethanol, while some alcohol, including ethanol, is further broken down by the yeast into flavour compounds such as esters. Ethanol (comprising two carbon units) gives rise to the simplest esters (fruity notes), compared to higher alcohols (comprising three or more carbon units) which are longer chain alcohols that in turn provide more complex esters,” says Douglas Murray, Diageo’s process technology manager.
The subsequent log phase sees the yeast metabolise increasingly complex sugars, initially maltose (comprising two glucose units linked together), then maltotriose (three glucose units). The yeast experiences the greatest growth during this phase, with the process of ‘doubling’ seeing the yeast population increasing.
“The first doubling occurs at the end of the lag phase, and the most rapid rate of doubling is during the log phase when the yeast doubles in number. During this time there is also an exponential increase in the production of alcohol and flavour compounds, including esters,” says Dennis Watson, director of technical and scientific affairs, Chivas Brothers.
With the majority of alcohol produced during the log phase, this principally means ethanol, though ethanol has an essentially neutral character and so doesn’t contribute to the flavour profile of the wort.
Other types of alcohol produced include higher alcohols (ie. more complex, longer chain alcohols than ethanol). However, these higher alcohols are only present in tiny quantities, so any impact on the character of the wort is minimal. Moreover, the yeast continues to break down higher alcohols into esters during the latter stages of fermentation.
Esters range from the simpler fruit notes created during the earlier stages of fermentation, through to richer, riper fruit notes, in the latter stages of fermentation.
As fermentation continues the yeast finds itself in an increasingly adverse environment. This includes a dwindling supply of nutrients, a growing concentration of alcohol, and the wash becoming more acidic. Stressed by this scenario, yeast enters the stationary phase when it stops growing and starts to die. Cell walls also begin to autolyse (rupture) releasing their contents into the wort.
Not all the yeast cells autolyse within the washback (ie. vessel in which fermentation occurs), but that hardly matters as the heat created during the subsequent process of distillation ensures that all the remaining cells autolyse. Ruptured cells release a range of enzymes, amino acids and fatty acids, which make a significant contribution to the character of the wash (ie. the fermented wort). Consequently, it’s vital that the ‘slurry’ of yeast cells remain present within the wash during the first distillation.
Beyond the science, a before and after comparison reveals some key changes during fermentation.
“The wash is similar to a wheat beer, and definitely retains a strong cereal note with a lot of residual sweetness though it’s less sticky sweet than the wort.
“Fruity notes also come through in the wash, apples, together with oranges, and hints of pear and peaches,” says Graham Eunson, Glenglassaugh.
And that takes us back to quantifying the role of yeast and the fermentation process.
“There are two major areas of flavour development, fermentation and maturation, and you need the influence of both to get a truly complex whisky,” says Dr Bill Lumsden, head of distilling and whisky creation, Glenmorangie.