Whisky Magazine Issue 90
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Ian Wisniewski asks what role does yeast play during fermentation, and how influential can this be in determining the flavour profile of mature malt whisky?
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 (c...