Why Long Fermentation Times are Important for Ester Formation in Malt Whisky

One fact I noticed during my trip to Scotland was that the average weekday fermentation time at scotch whisky distilleries was about 55 hours, but some went as short as 48 hours while others went as long as 75 (weekend fermentations sometimes reaching 120 hours).

It was generally the case that the larger distilleries used shorter fermentation times while the smaller ones had longer fermentation times (Caol Ila used to be an outlier, but appears to have shortened their fermentation times since replacing their old wooden washbacks with stainless steel a few years ago). In addition to Caol Ila, other distilleries such as Ardbeg have also decreased their fermentation times over the last decade, likely in an effort to increase the output of the distillery. But will shorter fermentation times produce the same kind of spirit?

After doing a bit of reading on the subject, I'm willing to say that the answer is probably 'no'. While shorter fermentation times can extract the same amount of alcohol out of a mash as a longer fermentations, there are other processes that need more time.

Lets begin with what happens during the production of malt whisky.

Malted barley is ground in a mill to produce grist, a mixture of flakes, finer flour, and hulls. This is then added into the mash tun, where it is mixed with hot water to extract the simple sugars from the grain. after soaking for some time, the liquid is drained off and progressively hotter water is added each time, usually three or four times total. The water is rather hot, with the first water added at ~65º C, the second at ~75º C, and subsequent waters are between 85-95º C, which extract the last bits of sugar from the grist and are generally recycled to be used for subsequent first and second waters.

Semi-lauder mash tun at Auchentoshan Distillery

The key point in that process is that while temperatures are high, they are not, unlike mashing done at breweries, heated above 100º C. This means that while the microbial cultures living in the malt are significantly thinned by the heat, they are not all killed.

The sugary liquid from the first two waters is cooled to 18ºC and piped over to the washbacks, where cultured yeast is added. As the dissolved oxygen in the wort is quickly consumed, the yeast begin to grow and divide anaerobically, converting the sugars in the liquid into alcohol, carbon dioxide, and other compounds (for more details, see Whisky Science). Because the yeast is pitched at fairly high concentrations, it can out-compete the remaining residual bacteria for the first 30-40 hours of the fermentation. At that point, the yeast begin to run out of steam as they start to choke on their own waste products - alcohol and heat.

Highly active fermentation at Laphroaig Distillery

While the starting temperature of 18-22º C is a bit below the optimal temperature for yeast, it is necessary to start that low because no whisky distillery I have seen has active cooling systems in its washbacks. This means that the liquid will absorb all of the heat produced by the yeast as they multiply and divide, which, over time, ends up being a lot of heat. After 48 hours, the temperature of the wort can rise by 10-20º C. While wine yeasts are sometimes tolerant up to 32º C, the S. cervisiaie strains used by distilleries will begin to suffer above 25º C or so.

Additionally, the end product of fermentation, ethanol, is toxic to the yeast that produce it. Final alcohol concentrations range from 5-8%, which is approaching the upper limit of survivability for S. cervisiaie. While the yeast will attempt to sequester the alcohol by converting it into esters, this is not a long-term strategy.

Both heat and alcohol end up creating the conditions for autolysis. While you may have heard of this process as something that brewers attempt to prevent, it may actually be an important step in developing the flavors of malt whisky (and champagne). As the yeast become stressed, they begin, in essence, to digest themselves. Cells are exquisitely organized to keep different functions in distinct compartments. When those compartments begin to lose coherence, degradative enzymes are loosed upon the rest of the cell, leading to almost complete breakdown. Large polysaccharides, including the major constituents of the yeast's cell wall, are broken down into smaller mono- and oligosaccharides; proteins are broken down into peptides and free amino acids; triglycerides are broken down into free fatty acids and glycerol.

All of those compounds released during yeast autolysis provide fodder for the bacteria that have been lurking in the background during the initial phases of fermentation. A study by van Beek & Priest (2002) found that bacterial communities, primarily lactic acid bacteria, only begin to thrive after 30-40 hours of fermentation and hit their maximum growth after 70 hours.

van Beek (2002)

While the bacteria are important in and of themselves, the intermediate period between 30 and 50 hours is critical because the yeast begin to defend themselves against the growing bacterial communities. Yeast and bacteria have coexisted for billions of years and yeast have developed a number of defensive mechanisms to suppress bacterial competition for resources. One of these defense mechanisms is the synthesis of acids.

van Beek (2002)

As you can see from the table above, acetic acid concentrations rise almost 10X between 40 and 50 hours. From the previous figure you can see that this is where the bacterial community enters an exponential growth phase and starts to present real competition to the yeast. In response, the yeast produce acetic acid to suppress that growth. As noted above, this is also partially a strategy to reduce the concentration of alcohol by converting it into ethyl acetate, though that never rises above mg/L concentrations. The rise in acetic acid has effectively ceased by 65 hours, at which point the yeast have almost all undergone autolysis and the bacteria are dominant.

The major constituent of the bacterial communities during malt whisky fermentation are strains of Lactobacilli. As the name suggests, these bacteria tend to produce lactic acid. This is the end produce of lactic acid fermentation, which breaks down sugars anaerobically. Why is this important to the flavor of whisky? Lactic acid can form esters, primarily ethyl lactate, which has a creamy or buttery flavor. Additionally, Wanikawa et. al. found that lactic acid bacteria hydroxylate unsaturated fatty acids from yeast, which can be esterified into lactones, which have fruity or coconut odors and flavors. Lactic acid bacteria also continue the process of ester synthesis started by the yeast, producing new acetate derivatives of fusel oils. Additionally, the action of lipases continuing to break down the triglycerides from the yeast to produce free fatty acids, which are then available for esterification and the production of fusel oils is continued from the free amino acids released by yeast autolysis via the Ehrlich pathway, which provide the two necessary raw materials for esterification.

To add to the importance of lactic acid bacteria, Simpson et. al. found that there are differences in the strains of bacteria present in the worts of different distilleries in Scotland. These populations are relatively stable, though they do change to some degree depending on time of year and the types of malt being brought into the distillery. Especially in distilleries with longer fermentation times, these bacterial communities may represent one part of their 'terroir'.

Microorganisms growing on the washbacks at Springbank Distillery

So while yeast may get center stage when it comes to the production of whisky, there are other microorganisms that also play important roles in developing the flavors we associate with the spirit but need more time than is usually given to exert their influence. This does not bode well for distilleries that have reduced their fermentation times over the last decade in an effort to increase output. They may find that this spirit is less complex and flavorful than it was before.