Brewer’s yeast used to enjoy a monopoly in malt whisky production, until distiller’s yeast became more established in the early 20th Century, and using a combination of both types of yeast became standard practice.
Being recovered at the end of fermentation, brewer’s yeast is termed a secondary strain, and has a different genetic profile to distiller’s yeast, which is a primary strain (being cultured).
The method of harvesting brewer’s yeast depends on the design of the fermenters.This could mean raking yeast from the top of the fermenter, then pressing the yeast into a cake. Alternatively, fermenters with cone shaped bottoms provided an area in which the yeast could settle, and be drawn off for filtration, then pressed into a cake.
Sourcing brewer’s yeast traditionally meant either dealing direct with large breweries, who employed dedicated sales staff, or more usually, merchants acting as middlemen between brewers and distillers.
“Yeast was originally delivered in barrels by train in liquid form, in warm weather the bung would almost be popping out.Then some breweries moved to centrifugal filtration and making a pressed yeast,”says Derek Sinclair of Inver House.
Dennis Watson of Chivas Brothers adds:“In the 1970s there were still some breweries shipping brewer’s yeast in small pressurised tanks, which could go on the back of a lorry, and once unloaded at the distillery these tanks were linked up to pipe work from where the yeast could be sent to the washbacks.”
More recently brewer’s yeast was delivered in clear plastic sacks, which provided an immediate insight into the condition of the yeast.
“You could reject it just by looking through the sacks.When you opened a sack and put your nose in, you knew it was in good condition if you got a nice yeast smell, and if there were strong vinegar notes you knew it wasn’t,”says The Edrington Group’s Dr Bill Crilly.
Watson adds:“There was a high level of ‘blown bags,’ meaning bags in which the yeast had started to die off, and bacteria had grown which produced a lot of gas and the bags could pop, which meant you’d have a dead yeast slurry all over the storage facility. Bags in this condition would be rejected.”
The viability of brewer’s yeast (the level of live yeast cells present) varied enormously.This could range from 70 to 90 per cent on arrival at the distillery, and within two to three days of being stored the viability could have reached 20 to 30 per cent. This had an immediate consequence.
“If you were concerned about the viability of the yeast you’d have to use more of it,”says Derek Sinclair.
Using larger amounts due to lower viability obviously increased costs.
Another consequence of lower viability could be a reduced yield of alcohol.
Crilly adds:“The yield of alcohol depended on the condition of brewer’s yeast. If it had started to autolyse (the cell walls had started to rupture), the material coming out made an ideal growth medium for any bacteria present, and this would compete with yeast and affect the spirit yield. Sometimes brewer’s yeast could be autolysing on arrival.
“This could reduce the yield by 10 litres per ton, or more, which is a significant financial loss, though you wouldn’t have added the yeast if it looked bad.”
Even when it was standard practice to use brewer’s yeast in malt whisky distilleries, there were periods during the year when it was more difficult, or even impossible to obtain. This is because production cycles in breweries and distilleries weren’t exactly compatible. Breweries generally increased production levels in time for the summer, when beer consumption peaked, whereas distilleries have a silent season during the summer to conduct essential maintenance and repairs.
Similarly, beer production decreased for the winter, reflecting lower consumption levels, whereas distilleries were at full production during this period.
Consequently distilleries that continued producing without the usual level of brewer’s yeast, or without any brewer’s yeast at all, were able to see whether this made any difference to the fermentation process.
“Fermentation times are more consistent without brewer’s yeast,” says Derek Sinclair.
A key question was also whether, and to what extent, omitting brewer’s yeast affected the character of the new make spirit. However, as brewer’s yeast would have been sourced from various breweries, making definitive comparisons between different batches of spirit could in itself be a challenge.
“Because no single brewery was producing enough yeast to service the malt whisky industry, a distillery might have different brewer’s yeast week to week with different activities, and could have a slightly different flavour profile which resulted in inconsistency in performance and the quality of spirit produced.
“Not huge, just a slight variation,”says Dennis Watson.
Another factor in the debate about brewer’s yeast was consistency of supply, which became more of an issue during the 1990s when the brewing industry consolidated, and the number of breweries in Scotland declined. Sourcing yeast from breweries in England was one alternative, but the greater distances involved also raised the question of transportation costs, and refrigeration.
Meanwhile, there was also a change of focus in the brewing industry, from producing ale to lager.
Consequently, the past 10 years has seen a significant decline in the number of distilleries using brewer’s yeast.
“We stopped using brewer’s yeast at our distilleries 12 years ago.We went through a process of trials to see if dropping brewer’s yeast had any effect on the new make spirit, but it didn’t.We phased it out over the course of a year, doing a course of experiments and getting feedback from our sample room, where no changes were found in the new make spirit.
“The fear was that we would loose estery notes, but we didn’t find this at all,”says Crilly.
Sinclair adds:“We stopped using brewer’s yeast eight years ago, with Balblair and Old Pulteney the last sites to stop.We did trials with and without brewer’s yeast that showed it didn’t have any impact on spirit quality.”
Meanwhile, the traditional ‘recipe’ of different yeasts has been maintained at Benromach.
“We originally used a mix of distiller’s yeast in a pressed form, and brewer’s yeast.When the supply of brewer’s yeast dried up, we replaced it with Anchor dried brewing yeast.
“We did trials on the length of fermentation and types of yeast, and came to the conclusion that two types of distiller’s yeast and brewer’s yeast in a longer fermentation gave us a far richer new make spirit, with complexity and that extra esteryness that we require,”says Keith Cruickshank of Benromach.