During fermentation yeast metabolises (digests) sugars in the wort (the sugary liquid produced by mashing). A small amount of these sugars are broken down by the yeast into acids, a vital process as yeast requires acids in order to grow and reproduce. These are termed either organic acids, as they’re derived from malted barley, or fatty acids, which is a more specific reference to fats in the malted barley from which the acids originate.
Yeast produces different types of acids, beginning with short chain fatty acids, so called because each molecule typically comprises two carbon units linked together. Yeast subsequently produces medium chain fatty acids (around 6 to 10 linked carbon units) and long chain fatty acids (around 12 to 16 units). However, it’s short chain acids that are produced in the greatest quantity.
Meanwhile, most of the sugars are broken down by the yeast into different types of alcohol.
The alcohol produced in the greatest quantity is ethanol, which is a short chain alcohol as each molecule comprises two carbon units linked together.
The consequence of yeast producing alcohol and acid is that molecules of each are present within yeast cells, where complex interactions between these molecules sees them integrating and being transformed into esters.
Ethanol and short chain acids are the first to interact and create a group of short chain fatty acid esters, with these esters typically comprising around four units each. Short chain esters, which contribute banana notes for example, account for the greatest number of esters produced during fermentation. This is because the alcohol created in the greatest quantity is ethanol, and the acids created in the greatest quantity are short chain acids.
Ethanol also reacts with medium and long chain acids (which are present in much smaller quantities) creating medium and long chain esters. Medium chain esters (typically comprising several units linked together) contribute richer fruityness, such as apples and pears. Long chain esters (around 10 to 16 linked units) contribute a different range including beeswax, soap, cheese and other pungent notes.
During fermentation most esters are created by interaction that occurs within yeast cells. However, some esters are also produced in the wort (ie. the liquid, and not within yeast cells). This happens because yeast cells also emit ethanol into the wort, and the wort contains acids. Consequently, ethanol and acid molecules interact within the wort, creating additional short chain esters.
“Fermentation is the source of over 90 per cent of ester production, with a smaller, complementary level of ester formation also occurring during distillation and aging in the cask,” says Douglas Murray, process technology manager, Diageo.
Esters are created during distillation because the wash (ie. fermented liquid) contains alcohol and acids, which continue interacting during the first and second distillation.
“Distillation creates a spread of short, medium and long chain esters, but at a slightly faster rate than during fermentation as the heat of distillation speeds up the rate of reactions between the alcohol and organic acids. These reactions occur throughout the distillation process, but the highest level of reactions occurs just as the vapours leave the still and begin condensing,” says Dennis Watson, director of technical and scientific affairs, Chivas Brothers.
Jane Millar, technical support team leader, William Grant & Sons, adds, “The greatest number of new esters formed during distillation are short chain esters. This stems from the fact that the main type of alcohol in the still is ethanol. Meanwhile, the esters which have been formed during fermentation are pretty stable and are not modified by distillation.”
As the new make spirit contains acids and alcohol, interaction between the two creates additional esters while the spirit is maturing in oak casks. Moreover, casks also provide additional acids.
As oak casks are porous, this allows water and alcohol, together with some esters, to evaporate from the cask during aging.
“Short chain esters are very volatile and so can readily evaporate from the cask, together with the alcohol, and be lost. However, as short chain esters are present in such large levels this is not a significant loss. Medium and long chain esters have a heavier molecular weight than short chain esters, and so are far less prone to loss through evaporation,” says Jane Millar.
“The level of long chain fatty acid esters can remain constant during maturation, though you won’t necessarily detect them in the mature product, it all depends on the level. Moreover, at a lower level long chain fatty acid esters can add complexity, as they can interact with, and enhance, other flavour compounds, while not showing through in their own right. Some long chain fatty acid esters also make a significant contribution to mouth feel, adding to the sense of body and elegant texture of a malt,” says Stuart Harvey, master blender, Inver House Distillers.
The role of esters is frequently discussed, but with malt whisky comprising numerous other influences where do esters fit into the big picture?
“Esters are a significantly important group. They don’t dominate numerically, but because the flavour threshold for esters is usually quite low you don’t need a large amount for them to show through, and they can be an intense contributor in terms of flavour,’ says Douglas Murray.