"Immediately after pitching, the yeast adjusts to the wort and, during this time, there is a lull or 'lag phase 'before it starts to metabolise and use up the small amount of dissolved oxygen available in the wort,"says Grant MacKenzie of Kerry Ingredients and Flavours, a leading yeast supplier which is part of the Kerry Group.
Even before the end of the lag phase, which typically lasts several hours, the yeast experiences significant growth.
"By the time yeast goes into the exponential phase it may already have doubled in quantity," says Dennis Watson of Chivas Brothers. The first sugar on the menu for the yeast is glucose. This is the simplest and most easily digestible sugar within the wort, comprising one glucose unit.
"Yeast would be feeding on sugars before the lag phase ends, and as soon as yeast is digesting glucose it's producing alcohol.Even before the washback is filled yeast starts to ferment the wort," says Dennis Watson.
Working its way through glucose quite rapidly, yeast moves onto a more complex sugar, maltose, which comprises two glucose units linked together. As maltose is the main sugar in fermentation, yeast spends the longest time digesting maltose compared to other sugars. Yeast then moves onto maltotriose, comprising three glucose units.
A consequence of yeast feeding on sugars is the production of alcohol and carbon dioxide.
"All of this fermentative activity releases large quantities of carbon dioxide. Once the wort is saturated with this gas, it starts to come out of solution and rises rapidly to the surface. If production is too fast, the washbacks can literally 'bubble over' and valuable alcohol is lost. It's important not to promote too fast a fermentation for this reason," says Grant MacKenzie.
Additionally, aromas emerging from the washbacks deliver a clear message about the alcohol being created.
"You get lovely sweet aromas in the tun room of pear drops and apples, coupled with the yeast aromas. Most of the flavour of the new make spirit is produced at the fermentation stage," says Highland Park's Russell Anderson.
In addition to sugars, yeast feeds on other compounds present within the wort, including minerals. While yeast begins to feed on minerals during the lag phase, this mainly occurs in the exponential phase.
"Once the lag phase is over and the active or 'exponential phase' starts, yeast growth is stronger, and alcohol production is quicker and more efficient, if the fermentation medium has a good supply of nitrogen. In malt whisky fermentations, the wort is rich in nitrogen derived from the breakdown of barley proteins; the yeast uses this nitrogen to make the proteins that it needs during growth, particularly for the enzymes which metabolise sugar to release energy. Similarly, minerals from the mashing water and the barley play an important role in the well-being of the yeast by promoting the action of these enzymes," says Grant MacKenzie.
Glenmorangie's Dr Bill Lumsden adds:"Co-factors like zinc, magnesium and calcium are very important for yeast vitality, without them yeast would struggle to metabolise. Apart from sugars, other foods are important for the health of the yeast and others for producing flavours.
"A lot of lovely fruity flavours are produced through the metabolism of nitrogen compounds."
Another consideration is that the make up of minerals in the wort can influence the flavour profile of the wash.
"Our wort is bound to have elevated levels of calcium and magnesium because of our water source, the Tarlogie Springs, which will have some impact.
"It makes logical sense that some of the fruity, estery notes of the new make spirit come from the minerals in Tarlogie Springs water, but it's difficult to prove scientifically," says Dr Bill Lumsden.
By the time the exponential phase is drawing to an end, the yeast has achieved a significant level of growth.
However, as fermentation continues the yeast finds itself in an increasingly adverse environment. This includes a growing concentration of alcohol, and a lack of nutrients. The yeast also has to contend with a rising temperature, whereas yeast is unable to survive above about 34 degrees centigrade. Similarly, the changing ph of the wort (ie. the level of acidity) sees the wort become more acidic, whereas yeast cells are unable to live within an acidic environment.
As yeast cells start to die off the cell walls begin to undergo autolysis (ie. they rupture),and release their contents into the wort. This release includes enzymes, amino acids and fatty acids, which can make a significant contribution to the character of the wash.
Rather than being completed within the washback, the process of autolysis continues within the wash still.
PITCHING TEMPERATURE AND AMOUNT OF YEAST
The rate and length of fermentation are set by the pitching temperature, which in turn influences the flavours created. As the fermentation process sees the temperature rise, and yeast expires at around 34 degrees centigrade, fermentable sugars within the wort need to be converted into alcohol by that stage.
Correspondingly, wort is cooled to a specific 'pitching temperature, at which yeast is added.
This ensures that fermentation is completed before the yeast reaches its temperature limit.
A lower pitching temperature, around 16 to19 degrees centigrade, promotes a more leisurely start and rate of fermentation, with a faster start and finish when pitching at 20 to 22 degrees centigrade. A more gradual rate of fermentation is typically believed to yield a finer result, with 40 to 44 hours often quoted as a minimum. At some distilleries around 48 hours is a guideline for a more fruity style of new make spirit, though fermentation time can also extend to 100 hours or longer.
Meanwhile, a short, rapid fermentation can be prone to nutty tones, while too slow a fermentation can introduce grainy notes.
The amount of yeast added is calculated using one of two formulas, allowing either a certain amount of yeast per tonne of malt, or per litre of wort in the washback.
There's no standard industry figure, and comparing totals at various distilleries shows a broad range.
Adding a little less yeast than usual results in a slightly slower fermentation, while adding slightly more merely increases the rate. However, adding significantly less yeast gives an inefficient fermentation, and a reduced yield of alcohol.
Adding significantly more yeast can also reduce the yield, by intensifying the rate of fermentation, with a practical factor being the waste incurred by using unnecessary amounts of yeast.