06 July 2012
Cell mates: The quest for whisky enlightenment continues
By Gary Atkinson
If there were an awards ceremony for mankind’s favourite microorganisms, it’s likely yeast would take home the top prize. Not only does it make leavened bread, wine and beer possible, it’s also a key part in making whisky.
This single-celled helper is the star of the show during the fermentation stage of whisky production – when it takes a dip in the sugary soup “wort”, which is made from mashing hot water and malted barley together.
The scientific process
Fermentation involves the yeast cells consuming the sugars – mostly maltose, but also glucose and maltotriose – from the malted grain, producing ethanol (alcohol) and carbon dioxide as the major by products.
Specifically for whisky making, this is usually done in the absence of oxygen – so that the yeast concentrates on using the sugars to create alcohol rather than for reproducing. This is because without oxygen, the cells can’t create any more lipids, which are necessary for yeast cells to multiply. However, the supplied yeast comes pre-loaded with lipids, and will reproduce three or four times, but after that, the cells will concentrate on converting the sugars into alcohol, instead of new cells.
As well as creating alcohol, the fermentation process also creates flavours, for example through the production of esters, which give the spirit a floral and fruity taste. David Quain, professor of brewing at the International Centre for Brewing and Distilling at Heriot-Watt University and co-author of the book Brewing Yeast & Fermentation, said: “The yeast wants to grow more yeast cells and the distiller wants to convert sugary liquid into alcohol and flavour. Their ambitions are very different, but the distiller has the ability to control the environment for his own ends.”
Origins of the yeast
The species of yeast that is used for producing whisky, as well as beer making and baking, is Saccharomyces cerevisiae (S. cerevisiae). Only a few strains are used in whisky making with the M Type the most popular. Typically, yeast manufacturers will grow their yeast on molasses and other nutrients, from stocks kept in deep freeze.
“They use liquid nitrogen following the same principles as cryogenics to keep the separate yeast strains preserved before they need to be brought back to life,” David explained.
However, it wasn’t always just these two yeast strains that made their way into the whisky-making process. Distilleries used to draw on the yeast from nearby brewers.
It’s not all about the yeast, you know!
So, if the vast majority of yeast used in whisky making comes from just two main strains, surely the pot ale produced by all distilleries will taste the same, right? Wrong. Bacteria from the malted barley and present on surfaces has a beneficial impact on taste, contributing fruity and “estery” aromas. The main type is lactic acid bacteria, which, unlike many types of bacteria, doesn’t die in relatively high concentrations of alcohol.
It’s a delicate balancing act, however – too much bacteria will reduce the alcohol yield. The type of washback also has a role to play in the contribution of this type of bacteria. A wooden washback will host more colonies, whereas stainless steel ones won’t. “If you change your washback from wood to steel, you’ll notice a change in flavour,” David explained. “You’ll lose many of those fruity notes from the spirit. If that’s part of your flavour profile, the change can have a knock-on effect in the mature whisky.”
The difference between beer and whisky fermentation
Although the process of making beer and making pot ale for distillation is largely the same, there are some key differences.
Firstly, brewers introduce a bit of oxygen into the fermentation process to grow the yeast a little. That’s because they’ll reuse the same yeast, as many as 15 times in different fermentations. However, a single batch of yeast is used only once in whisky production. The yeast is brought over in the pot ale to distillation, increasing the alcohol strength of the liquid (to 8-9 per cent). That’s because the distillation releases the alcohol trapped in the yeast cells. The yeast doesn’t make it into the final spirit though; it’s left behind with the other waste products during the distillation process.
Another difference is the yeast strains the brewers use. David pointed out: “Yeast for the beer industry is one of the main flavour differentiators, which isn’t the case with the whisky industry, with distilleries adding more individual flavour characteristics further down the process. That’s probably one of the reasons there are only a few strains of yeast for distillation, compared with hundreds for brewing.”
Also, another crucial difference is that the liquid isn’t boiled in whisky fermentation, in contrast to the practice in beer making. “Many enzymes that would have been killed through boiling, live on to break down more starch, create more sugar, which in turn means more alcohol is produced, as well as other flavours from the surviving bacteria.”