The 'Gazette'
After the malt is brewed, the resulting wort is a sweet mixture of water and malt. Chilled to about 20-25°C, it is then sent to the fermentation tanks where the alcoholic fermentation takes place.
As the name implies, alcoholic fermentation is the production of alcohol (ethanol) from sugars initially present in the wort (such as glucose, fructose, maltose and maltotriose). This process results in a wash with an ethanol content between 8 and 10%. The conversion of sugars into ethanol, brought to light by Pasteur through his work on wine and beer, is achieved thanks to the effect of the yeasts and their enzymes.
In simple terms, a yeast is a micro-organism, a kind of unicellular fungus that reproduces by budding. Without going into detail about the complexity of the biochemical reactions involved, the alcoholic fermentation process can be broken down into several steps:
1: First of all, glycolysis, that enables the transformation of sugars into pyruvate, through a chain of reactions driven by the action of several enzymes. This leads to the production of glycerol as well as the release of energy in the form of two adenosine triphosphate (ATP) molecules. The temperature of the medium then increases.
2: The transformation of pyruvate into acetaldehyde with release of carbon dioxide. This is made possible by an enzyme called pyruvate decarboxylase.
3: The reduction of acetaldehyde into ethanol by another enzyme, alcohol dehydrogenase. This last step allows the regeneration of nicotinamide adenine dinucleotide, a molecule that will then be used to reset the glycolysis step.
Yeast’s growth curve during alcoholic fermentation
Once the yeasts have been added to the wort, the fermentation process can last between 48 hours and 4-5 days depending on the distillery’s capacity. It can be divided into 3 main phases:
– The lag phase: During this phase, the yeast adapts to its new environment, and activates its metabolism (it synthesises enzymes). This refers to a period of intense aerobic activity, with the synthesis of sterols and unsaturated fatty acids that ends with the first cell division.
– The growth phase: it covers the cell division phase, where the yeasts’ number increases exponentially. It is associated with the production of many aromatic products.
– The stationary phase: When the environment becomes poor in nutrients, the yeasts’ growth enters a stationary phase, and then decreases. This gives the possibility for other micro-organisms to develop, such as lactic acid bacteria, during a second fermentation* phase that may last 48h.
Alcoholic fermentation is the main microbial reaction in whisky production. Whether its main role is to convert the sugars in the malt into ethanol, it also produces aromatic compounds, whose nature and concentration will influence the final product’s organoleptic profile. The choice of the yeast strain is crucial for the successful completion of the fermentation step. Its selection is based on its ethanol yield, its tolerance to stressful conditions, and its production of aromatic compounds.
Watson, D. C. (1981). The development of specialised yeast strains for use in Scotch malt whisky fermentations. In Advances in Biotechnology (pp. 57-62). Pergamon
Bamforth, C. W., & Ward, R. E. (Eds.). (2014). The Oxford handbook of food fermentations. Oxford University Press, USA
Wanikawa, A. (2020). Flavors in Malt Whisky: A Review. Journal of the American Society of Brewing Chemists, 1-19
Wilson, N. (2014). Contamination: Bacteria and wild yeasts in a whisky fermentation. In Whisky (pp. 147-154). Academic Press
Fermentation | May 2025 | Dr. Magali Picard