Production

Yeast of Eden

The influence of yeast on the final flavour of a whisky is hotly debated within the industry. Ian Wisniewski takes a closer look at whisky's most active ingradient.
By Ian Wisniewski
The hardest worker in any distillery has always been yeast, according to traditional staff humour. But then yeast’s job description has always entailed two vital functions performed simultaneously: converting fermentable sugars into alcohol, while also creating a range of aromas and flavours beyond wort’s integral cereal characteristics. Needless to say, the influence of the yeast also depends on other contributory factors, including the sugar concentration in the wort, the pitching temperature and rate of fermentation, not to mention distillation and maturation. As different types of yeast are believed by some distillers to promote varying flavour compounds, deciding which to use can be significant. This means a choice of either distiller’s yeast, with Quest M developed in the 1960s for example, or the more historic option, brewer’s yeast. Being a cultured yeast, often cultivated using sugar such as molasses, principal strains of distiller’s yeast include Quest M, Quest MX and Mauri Pinnacle, with so-called ‘fast acting’ and ‘slower acting’ strains available. These behave as you would expect, with some distillers
combining both to promote more sustained action during fermentation. Whether to use fresh or dried distiller’s yeast is another consideration. Fresh yeast is typically used in a ‘pressed,’ also known as ‘caked,’ format, with the yeast drained of liquid and pressed into a cake. This keeps for up to two weeks when refrigerated, at an optimum temperature of around 2 to 4ºC.Practical considerations, such as heavy snowfall interrupting deliveries of fresh yeast to an isolated location, was one reason for stocking up on dried yeast. This is a convenient safeguard as there’s no need to refrigerate, it keeps for up to a year, and blending with warm water is all the preparation that’s required. Some distillers believe combining distiller’s and brewer’s yeast promotes a fuller fermentation. Others say this combination can actually reduce the yield by 0.5-1%, or even have other undesirable issues. At Aberlour for instance, where brewer’s yeast has been history for around 10 years: “Distiller’s yeasts resulted in much more consistent fermentations than when using brewer’s yeast,” says Aberlour’s Distillery Manager Alan Winchester. In terms of flavour, one school of thought is brewer’s yeast can add a heavier, fattier, mealier quality to new make spirit. Similarly, a more complex ‘recipe’ of yeasts, which may comprise two distiller’s and two brewer’s yeasts, is often believed to create a more complex flavour profile. Another opinion is that brewer’s yeast makes no difference to the flavour. Other factors affecting the use of brewer’s yeast are concerns over consistency of supply (particularly as some breweries increasingly sell yeast to food manufacturers rather than distillers), as well as quality. Being recovered at the end of the fermentation process, brewer’s yeast is termed a secondary strain, with a different genetic profile to distiller’s yeast (which is a primary strain being cultured). Brewer’s yeast also varies according to the brewery and the style of beer involved (typically ale rather than lager). Another practical issue is the level of live yeast cells present. This could be around 95% in a batch of distiller’s yeast, compared to a possible level of 70-90% in brewer’s yeast, which obviously has a cost implication. Consequently, there has been a general reduction in the amount of distilleries using brewer’s yeast. Once the yeast line-up has been established, the next question is, of course, how much to use. Two formulae can be utilised, based on the total litres of wort, or the original weight of malt, which typically works out at around 2% of the weight of the malt. These totals are obviously based on experience, backed up by lab checks, in order to maximise flavour creation and efficiency. As adding too little yeast carries a risk of not converting all the sugar to alcohol, it’s usual to add a little bit extra just to be sure of a full yield. However, adding too much yeast adds extra cost for no additional benefit, and can also intensify the rate of fermentation. The method of adding yeast to the wort may involve reconstituting it with water before pumping it into the washback, on the basis that this initiates the beginning of fermentation more rapidly. A simpler approach is tipping the yeast straight in over the side of the washback, as the wort is being pumped in, which creates enough motion to disperse the yeast. An historic alternative was to combine yeast with a quantity of the first water from mashing, to help get it going.As the fermentation process raises the temperature and yeast expires at around 34ºC, all the fermentable sugars within the wort need to be converted into alcohol by that stage (yeast also expires once all the sugars are fermented). Correspondingly, distillers cool the wort to a specific ‘pitching temperature,’ at which the yeast is added. This ensures that fermentation is completed before the yeast reaches its temperature limit. As an additional precaution, this often means reaching a peak temperature of 30 to 32ºC during fermentation.The rate and length of fermentation are also set, or adjusted by, the pitching temperature, which in turn influences the flavours created by the yeast. A lower pitching temperature, around 16 to 19ºC, means a more leisurely start and rate of fermentation, with a faster start and finish at 20 to 22ºC.A more gradual rate of fermentation is typically believed to yield a finer result, with 40 to 44 hours often quoted as a
minimum. At some distilleries around 48 hours is a guideline for a clean, estery style of new make spirit, though fermentation time can also extend to 100 hours or longer. A short, rapid fermentation can be prone to nutty tones, while too slow a fermentation can introduce grainy or solventy notes.While there are rare instances of washbacks that have been accessorised with cooling jackets (through which cold water is conducted), the fact remains that the pitching temperature is effectively the only form of ‘heat control’. Consequently, this temperature may be lowered by a few degrees in warmer weather.During fermentation yeast converts sugar into alcohol and carbon dioxide by feeding on a series of increasingly complex sugars, essentially breaking the sugar down into other compounds which enable it to grow. First on the menu is glucose, before moving onto maltose, then maltotriose. Depending on the strain of yeast, these sugars may be tackled at different rates, and not always strictly in sequence. Although sugars account for the majority of flavours, yeast works on various other compounds, including amino acids and fatty acids, which also contribute flavours.Another significant factor is the level of bacterial presence (essentially lactic acid) in the washbacks, with wood hosting a higher level than stainless steel. While a certain level can enhance the resulting flavour profile, it’s the case of finding a balance as lactic acid bacteria can challenge the yeast for the fermentable sugars, and as they convert them into lactic acid rather than ethanol, the alcohol yield suffers.While some distilleries keep the same fermentation period, at some a mashing cycle late in the week may undergo a longer fermentation period over the weekend, ready to be distilled when staff return on Monday. As this type of short- and long-hour fermentation can produce differences in new make spirit, distillers may blend batches of new make distilled from each fermentation cycle, to ensure consistency.Once yeast is added to the wort it is technically known as wash, although this term generally applies to the fermented liquid. The action of yeast on sugars tends to be as much credit as yeast gets in terms of flavour. However, some distillers say that as yeast expires at the end of fermentation, the membrane of the cells ruptures and releases additional flavours into the wash.Expired yeast cells, which also contain a certain amount of alcohol, are typically retained within the wash during the first distillation. A consideration when using direct fire wash stills is the risk of
caramelising the yeast, which may carry over into the new make spirit. However, depending on the house style, this could be a deliberate and positive factor. How influential yeast is in the flavour profile of a mature malt varies, though it could be around 5%, says The Macallan’s Master Distiller David Robertson. While these flavours can be apparent in longer-aged malts, they will show far more readily in a younger malt, with a ‘milder’ oak influence, such as a second- or third-fill bourbon barrel. A higher peating level is another key factor in the ‘visibility’ of these flavours, with Laphroaig a prime example. “Flavours in the yeast will be overpowered by the peating,” says Laphroaig’s Distillery Manager Ian Henderson.