Fermentation

Mashing the malt produces a sugar solution called wort. To convert this sugar into alcohol, a fermentation process is required, which is why yeast must be added. Since yeast cannot tolerate hot temperatures, the wort is cooled down to about 20 °C before the yeast is added in the fermentation vat. The yeast can either be added directly to the wort as it is, or it can be dissolved and prepared in a small additional stirrer to initiate a faster fermentation.

There are different types of yeast used by brewers and distillers, but they all belong to a species called "Saccharomyces cerevisiae". Yeasts are fungi and not bacteria as some may think. Brewer's yeast develops more aromatic flavours, distiller's yeast can produce more alcohol. Most whisky distillers use a mixture of brewer's and distiller's yeast because experience has shown that this gives the best results in several respects. For example, a particular strain of yeast ensures rapid and complete fermentation, produces positive flavours and minimises foaming.

The amount of yeast accounts for 2 - 2.5 per cent in the fermenter, but distiller's yeast can convert a tonne of malt into 4,000 litres of alcohol.

Traditionally, fermenting vats are made of wood. You often see them made of Oregon pine, which grows very tall, has only a few branches and has a very fine wood structure. But you also see fermentation vats made of larch or cypress. They are covered with lids to prevent bacteria from entering and to prevent the mash from overflowing if it rises too much due to fermentation. Rotating blades are placed in the fermenting vats as an additional tool to combat foaming.

Wood is an organic material and an ideal host for bacteria. Therefore, wooden fermenting vats must be cleaned particularly carefully and thoroughly to avoid undesirable side effects. Cleaning is done both by heavy steaming and by using chemical solutions. This is not only necessary for traditional wooden fermenting vats, but also for those made of iron or steel. However, the risk of stubborn bacterial "guests" is particularly high with wooden vats.

So why keep wooden vats at all? Some say that fermentation in wooden fermenting vats delivers flavours due to interactions. But when you consider how often the vats are used, how small the surface area of the wood in contact with the mash is, and finally how short the fermentation time is, you can imagine how infinitesimal this influence may ultimately be.

But there is one argument in favour of wooden fermenting vats that trumps all others, however logical and competent they may be: wooden vats really do look better to our eyes and fit in with our ideas of traditional whisky making. Or do they? In the end, it's a question of feeling!

In the 19th and 20th centuries, some distilleries were looking for alternatives. They wanted to use materials that were easier to clean and had a longer life. Cast iron was the answer at that time and in some distilleries you can still see these heavy fermenting vats.

Stainless steel was the great invention of the early 20th century that soon revolutionised most industries. You will see stainless steel in every distillery you pass through, even if traditional methods are still used. Some use it only as a tank for the raw spirit, others appreciate its considerable advantages and have replaced many other tanks and equipment, such as the fermenting vats with stainless steel vats.

Stainless steel vats are the optimal alternative to wooden fermentation vats when it comes to enabling cleaning solutions and keeping bacteria and fungi out through a closed tank. Modern stainless steel vats use automatic cleaning systems and thus fewer chemical cleaning processes are necessary.

Some modern distilleries use refrigerated fermentation vats that can regulate the temperature of the mash. These are closed systems in which the fermentation process and the activity of the yeast as well as the formation of by-products can be influenced.

What happens inside the fermenting vats when yeast is added to the wort? Let's call it chemistry! It is a very simple reaction that follows, but it produces a fascinating result: C6H12O6 -> 2 x C2H5OH + 2 CO2 + heat.

Putting this formula into words: There are glucose molecules in our wort. The yeast now spits them out and each molecule yields two ethanol molecules (this is the alcohol we need), two carbon dioxide molecules and heat.

You can observe this reaction and the three elements produced with your senses:

You can smell the alcohol, but it is not advisable to stick your nose too deep into the vat because of the second product, the carbon dioxide.

The carbon dioxide rises to the top, which causes the bubbles in the mash and creates the foam, so you can't see the carbon dioxide itself, but the reaction it causes. But it doesn't stay there, it evaporates into the air. Some distilleries that have closed fume cupboards collect these gases.

And then there is the resulting heat that you can feel when you put your hand on the fermenting vat.

But: Besides these three main products, there are many other by-products of fermentation. Distillers know that they can influence the aroma of the whisky by the length or temperature of the fermentation. Especially in the second part of the fermentation, when the activity of the yeast decreases, an aromatic complexity can be achieved.

The duration of fermentation varies from distillery to distillery. After 48 to 96 hours, it is complete and the mash has an alcohol concentration of about 6 to 10 per cent. That is enough for a good beer, but not enough for a whisky. So it's time for the next step:

the distillation