FERMENTATION

Fermentation is a very interesting transformative process: yeast feasts on sugars and reproduces, in the meantime it triggers production of alcohol and flavour compounds.
In whisky production, fermentation starts when yeast is added to the wort, which is the liquid resulting from the mashing process. Yeast starts to adapt to its new environment, feeding on the oxygen present in the wort, oxygen that fuels initial growth and reproduction.
During fermentation, yeast cells grow and replicate: each yeast cell is elliptical and transparent and produces a small bud that grows in size. When it reaches half the size of its parent cell, the bud is freed and continues growing independently. When the bud has reached its full size becoming a full yeast cell, it begins to reproduce.
The “log” phase of fermentation starts after several hours when oxygen runs out. At this point yeast looks for another source of nourishment to continue growing and reproducing: sugars and other nutrients lodging inside the wort.
Yeast cells, called organelles, metabolise sugars absorbing through the cell membrane and creating energy, but also a product: alcohol.
Alcohol is toxic to yeast and it’s released into the wort through the membrane of the cell.
Fermentation has different steps: first yeast feasts with simplest sugars, glucose and maltose; when these run out it tucks into more complex sugar, maltotriose. Meanwhile, other organelles within the yeast cells metabolise nutrients present in the wort, including protein and fats. This process creates flavour compounds.
In this phase yeast grows and reproduces at its fastest rate, producing the maximum amount of alcohol and flavour compounds.
As fermentation continues, sugars diminish while the alcoholic strength of the liquid increases becoming more acidic, with the temperature also rising.
This stresses the yeast making it unable to cope and this is when the terminal “stationary” phase begins.
At this stage, the fermentation process can be ended and the fermented liquid (called wash) can be drained from the fermentation vessel.
On the contrary, distillers can decide to continue and begin a subsequent malolactic fermentation, when lactic acid bacteria compete with surviving yeast cells for any remaining sugars or nutrients.
Yeast cells rupture releasing their content into the liquid to provide part of the nourishment.
One benefit of the malolactic fermentation is increasing the level of esters, responsible for the fruit notes of whisky.
80% of the character of a new-make whisky is given by the fermentation and the flavour compound created during the process.

The amount of time yeast spends inside the wort defines fermentation time and results.
Short fermentation is around 48 hours and typically produces a wash with more distinct cereal, malty biscuit notes.
Longer fermentation is up to 100 hours or beyond and gives the whisky a wider range of characteristics including various fruit notes.
Temperature is another key factor about fermentation: when yeast is added to the wort, temperature is typically around 18°C and then rises during fermentation. Many yeast expire at 34°C, but at the same time, the rise of temperature intensifies the metabolic rate of yeast. It’s a fine balance between time and temperature, to let yeast complete its tasks before it’s too late.
Some distilleries, for example Kyrö in Finland, Zuidam in the Netherlands and Milk&Honey in Israel, use temperature-controlled fermentation vessels.
Temperature-controlled fermentation vessels have a cooling jacket wrapped around it, which is filled with water to decrease temperature inside the vessels when necessary. Lower temperatures let yeast continue working without stressing and with this technique, fermentation can last up to seven days, including malolactic fermentation.

Vessels are the containers where fermentation takes place: they’re called washbacks in Scotland and fermenters in Kentucky but the main difference is their material.
Vessels can be wooden or stainless steel and both host a bacterial population, which is higher in first type.
The ideal scenario for a good whisky is for yeast to get a good start and bacteria to grow gradually. If the bacteria goes too fast it starts competing with yeast for nutrients and potentially leads to an infection that reduces alcohol production, compromising the quality of the whisky-to-be.
Infections are under control thanks to rigorous cleaning regimes, for example with steam.
Stainless steel is easier to clean compared to wood, which provides natural havens for bacteria.
This said, the aim is to create a clean but not sterile environment for whisky to ferment properly and gain character during the process.