Yeast may be a minor factor in terms of production costs, but its contribution to the production process is far more significant. Converting sugars into alcohol is a vital though practical role, while producing various flavours during fermentation is more creative.And with brewer’s yeast now largely absent from the industry, the focus is on distiller’s yeast.“Distiller’s yeast tends to be grown on molasses, and it takes about three weeks to have the finished product, which includes two weeks to culture the yeast in the lab,”says Grant MacKenzie of Kerry Ingredients and Flavours, a leading yeast supplier which is part of the Kerry Group.Distiller’s yeast is available as creamed yeast (also known as liquid yeast), pressed yeast (also referred to as caked yeast),and dried yeast.“Creamed yeast is around 20 per cent dry matter and behaves like a liquid. At 23 to 24 per cent dry matter you start to get a paste,and at 28 per cent dry substance you have a cake, with pressed yeast having about 28 per cent dry matter. You need relatively more creamed yeast compared to pressed, but it’s only a very small amount,”says John Ross of Wm Grant & Sons.Individual strains of yeast are also available in different formats.“We offer five strains of distiller’s yeast, the classic is M,dating from the mid 1960s. We believe, since then it’s made more Scotch whisky than any other yeast. We also offer MX,which has been available for about 25 years, as well as MS-1, SAForte and M+. We give choice to the distiller and can tailor this choice of yeast(s) to particular fermentation requirements for flavour and yield. MX for example tends to be a bit quicker than M to begin fermentation. M and MX are available in the form of pressed yeast, with all the strains available in the form of creamed yeast,”says Grant MacKenzie.During the past 10 years there’s been a significant move to creamed yeast among distillers, away from pressed yeast.“Of our 13 distilleries 12 use creamed yeast. We went through extensive trials to introduce a single culture of creamed yeast which worked very well, and the chief blender pronounced that the spirit was what we wanted,”says Alan Winchester of Chivas Bros.Delivered to distilleries by refrigerated trucks, creamed yeast is piped directly into stainless steel tanks, avoiding the need for manual handling. Running two temperature-controlled tanks, at around four degrees centigrade, specific amounts of creamed yeast can be piped from one tank to the washbacks. Meanwhile, the other tank can be cleaned and made ready for the next delivery, with samples also reaching the lab.“Our checks on creamed yeast cover viability, which looks at the cell count and how many are actually alive,we’re looking for a viable yeast with more than 99 per cent of cells being alive. The vitality of the yeast is also important, as the beginning of fermentation is a very stressful situation for yeast.Another important factor is the lactic acid bacteria count, which you don’t want to be too high, 100-5,000 lactic acid bacteria cells per ml is a typical range,whereas you’re typically pitching 20-25 million yeast cells per ml,”says John Ross.Various distilleries continue using pressed yeast.“Glenglassaugh had been silent for 22 years when we took over, so we could decide what yeast we wanted, which was pressed yeast, and we had production records so we knew the amount of yeast used, and the pitching temperature,”says Stuart Nickerson of Glenglassaugh.Pressed yeast is produced using a filtration process.“We use a rotary vacuum filter wheel, like a bicycle wheel with spokes, and a cotton cloth placed over the wheel. Yeast is sprayed onto the cloth, and a vacuum applied to the spokes which draws water out of the yeast. This leaves de-watered yeast on the wheel,” says Grant MacKenzie.Ordering yeast may seem entirely pragmatic, but location can play a role in the amounts required.“We use pressed yeast, there’s normally one delivery a week every week to last seven days, though we have eight days supply of yeast just in case. Yeast arrives on the ferry, but being an island that’s not guaranteed. The ferry can be delayed and sometimes it’s been cancelled,”says Laphroaig’s John Campbell.Pressed yeast, in 20kg or 25kg bags,entails some handling, but this also enables a hands-on method of quality control.“When unloading the boys have gloves on but they still pick up on the feel of the yeast. If the bags feel too soft it’s a problem, the yeast should compress a bit then come out again when you hold it, if it remains compressed there could be a problem,”says Ardmore’s Alistair Longwell.Additional checks on the texture, and aroma,provide further information.“I rub yeast between my fingers, and if the yeast has grittiness it is an indication that there are less living cells, the grittiness is a result of the yeast cells budding or reproducing. This reproduction causes the cellular structure of the yeast to change from being malleable and soft to the touch to being stiff and gritty. I’m also looking for a typical yeast smell, and if it’s off you know straightaway. We stick a temperature probe in the yeast to make sure it’s below eight degrees centigrade,having been delivered in a refrigerated truck,”says Springbank’s Stuart Robertson.Once delivered yeast requires specific storage conditions. How the choice of yeast evolves remains to be seen,but there’s certainly plenty of scope.“We’re continually looking at developing new strains of yeast, which takes at least 12-18 months to commercialise. During the past 10 years wehavedeveloped three strains of distillers yeast,and nowoffer five. We have several thousand yeast strains catalogued in our collection,which we review. It’s a complex process to get a new strain of yeast intocommercial production,and its potential has to bemapped out because it requires significant investment in time and resources,”says Grant MacKenzie. “We store the yeast in a big walk-in fridge, fitted with wooden racks, which is kept at about four degrees celsius.“ The best storage method would be to keep each bag separate, so as much air as possible can circulate around each bag to prevent any potential heat buildup,” says John Campbell.Russell Anderson of Highland Park adds,“The temperature is ideally four degrees centigrade in our walk-in yeast fridge, yeast doesn’t like to be frozen so the temperature should be no lower than zero.“Opening and closing the fridge door, and storage capacity also affect the temperature, if the fridge is empty it’s easier to keep it at four degrees centigrade,whereas two tons of yeast stacked in bags will in itself generate heat.” There are various methods of adding yeast to the washback.“By combining yeast with water at the ambient temperature we make a slurry in a stainless steel tank, it’s the same production water as for mashing.“An agitator mixes the yeast and water which we leave for an hour or two, then it’s pumped into the wort line from where it goes to the washbacks,”says Alistair Longwell.But there’s also a more straightforward option.“We don’t slurry the yeast,we crumble it up and scatter it across the washback, and don’t drop it in just one area,” says Stuart Robertson.Another option is dried yeast, used at distilleries such as Old Pulteney and Scapa.“We did trials with dried yeast and have used this at Old Pulteney for the past 18 months.“A remote location is one reason why we decided to do this. We take three or four deliveries a year,and dried yeast comes in boxes containing three bags of seven kilos each, so for storage it’s great, with dried yeast having a shelf life of six months or longer,”says Derek Sinclair of Inver House.Dried yeast initially needs to be rehydrated before being added to the washbacks.“At Scapa we add water to dried yeast in a small stainless steel tank and an agitator helps to mix everything,before being pumped into the worts line.“Dried yeast has a good shelf life and doesn’t need refrigerated storage,”says Alan Winchester.Derek Sinclair adds,“Dried yeast needs to be rehydrated,and instead of adding dried yeast directly into the washback we have an in-line mixer, and as wort go’s through this the yeast is drawn in and rehydrated, which means the yeast is rehydrated when reaching the washback.“Using dried yeast produces a more estery spirit which is what we wanted, with a more fruity character compared to using pressed yeast.” When choosing distiller’s yeast the strain is considered more significant than whether it’s creamed or pressed.Consequently, differences between using pressed or creamed yeast are essentially practical, including storage and manual handling.But the differences between using creamed or pressed yeast compared to dried yeast is the beginning of a more detailed dialogue, which will be covered in a feature on how yeast behaves during fermentation in the next issue.