1. The Yeast
The result of mashing the malt is a sugar solution called wort. To convert this sugar into alcohol a fermentation progress is necessary and therefore yeast has to be added. As yeast can’t stand hot temperatures the wort is cooled down to about 20°C before it is married with the yeast in washbacks. The yeast can either be put directly into the wort in the condition it arrives in the distillery or it is dissolved and prepared in a small extra mixer to start a faster fermentation.
There are different kinds of yeast that are used by brewers and distillers, but they all belong to a species called "Saccharomyces cerevisiae". Yeasts are fungi, not bacteria as some may think. Brewer’s Yeast develops more aromatic flavours, distillers yeast can outreach more alcohol. Most whisky distillers use a hybrid of brewer’s und distiller’s yeast, because experience and experiments have shown that this will show best results regarding several aspects: This strain of yeast ensures a quick and entire fermentation, creates positive aromas and it minimizes the development of foam.
The amount of the yeast makes up 2 - 2.5 percent of the wash, but distillers yeast can convert one tun of malt in 4.000 litres of alcohol.
2. The traditional wooden washbacks
Traditional washbacks are made of wood. You often see them made of Oregon pine that grows very high, has only few knots and a very close-grained wood structure. But you also see washbacks made of lark or cypress. The washbacks are covered with lids to prevent bacteria from coming in and avoid the overflow of the mash if fermentation brings it up too high. As an additional instrument to combat the upcoming foam rotating blades are common in the washbacks.
Wood is an organic material and an ideal host for bacteria. Therefore wooden washbacks must be cleaned extremely carefully and hearty to prevent undesirable side effects. Cleaning is done by high-steaming them as well as by using chemical solutions. This is not only necessary in traditional wooden washbacks, but of course in iron or steel washbacks, too. But because of the open wood structure the danger of persistent bacterial “guests” is high.
So why keep wooden washbacks at all? Some say fermenting in wooden washbacks delivers aromas because of interactive processes. But if you consider how often washbacks are used, how small the surface of the wood is that is in contact with the wash and at last how short the fermenting time is you can imagine how vanishing small this influence (eventually) might be.
But there’s one argument for wooden washbacks that beats all others, how logical and competent they may be: Wooden washbacks really look better in our eyes and fit into our ideas of traditional whisky-making. Don’t they? Finally it’s a question of emotions.
3. The old cast iron washbacks
In the 19th and 20th century some distilleries looked for alternatives. They wanted to use materials that are easier to clean and effect a longer operational life span. Cast iron was the answer of these centuries and in a few distilleries you still can see some of these heavy washbacks.
4. New stainless steel washbacks
Stainless steel was the big invention of the beginning 20th century that soon revolutionized most industries. It’s stainless steel you will see in each distillery you walk through, even if they still work with traditional methods. Some only use it for their spirit receivers, some value its considerable advantages and have replaced many other tanks and devices such as the washbacks.
Stainless steel washbacks are the optimal alternative to wooden wasbacks regarding cleaning solutions possibility and to keep bacteria and fungi out of it by having a closed tun. Modern stainless steel washbacks use automatic cleaning systems and less chemical treatment is required than it is for cleaning wooden washbacks.
5. Cooled washbacks
Some modern distilleries use cooled washbacks that can regulate the temperature of the mash. They are closed systems in which fermentation process and activity of the yeast can be influenced as well as developing byproducts.
6. The fermentation process
What happens inside the washbacks when yeast is added to the wort? Let’s call it chemistry! It’s a very easy reaction that follows, but it affects a fascinating result: C6H12O6 -> 2 x C2H5OH + 2 CO2 + heat
To put this formula in words: In our wash there are glucose molecules. The yeast now spits them and each molecule gives two ethanol molecules (which is the alcohol we need), two carbon dioxide molecules and heat.
One can watch this reaction and the three resulting elements by using one’s senses:
The alcohol can be smelled, but it’s not advisable to put the nose too deep into the washbacks because of the second product, the carbon dioxide.
Carbon dioxide rises up, what causes the bubbles in the mash and produces the foam so the carbon dioxide itself cannot be seen, but the reaction it causes. But it doesn’t stay there, it evaporates into the air. Some distilleries that have closed washbacks collect these gases.
And then there is the resulting heat that can be felt by putting a hand on the washback.
But: Besides of these three primarily products there are many other byproducts of fermentation. Distillers know that they can influence the aroma of the whisky by the length or the temperature of the fermentation. Especially in a second part of fermentation, when the activity of the yeast calms down, aromatic complexity can be reached.
Primary fermentation will come to an end when there is no more sugar for the yeast to transform and the alcoholic concentration of the wash rises to high. At this stadium, when the alcohol hinders the yeast from being active, several bacteria will start to work, especially lactic acid bacteria. They were “inhabitants” of the malt before it was mashed, so different from the yeast they don’t have to be added separately. The chemical reactions these bacteria push will end up with new compounds. These are acids, aldehydes, esters and long-chain alcohols. They make up a small amount, but they have influence the aroma and the body of the wash. As most of them are not lost during the following distillations it would be a big mistake to pay no attention to them.