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 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|
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)|
|van Beek (2002)|
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|