Understanding Whisky Production Unit 2: Raw Materials
Chapter 5: Distiller's Yeast: Strains, Metabolism, and Cellular Biology
The biology of Saccharomyces cerevisiae, comparing liquid cream, pressed, and dried strains, and their nutritional and oxygen requirements.
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If barley is the fuel and copper is the vessel, yeast is the workforce. Saccharomyces cerevisiae is a single-celled fungus with an astonishing capacity for hard labour and zero interest in anything other than multiplying, absorbing simple sugars, and converting them into ethanol, carbon dioxide, and heat.
Distilleries rely on specialised distilling strains bred for high alcohol tolerance and rapid sugar uptake. During its brief, frantic life cycle, a yeast cell divides by budding, leaving microscopic scars on its outer wall as it populates the washback. Along the way, its internal metabolic machinery generates hundreds of secondary flavour compounds that will define the character of the spirit long after the yeast itself has expired.
Significance of the Name Saccharomyces cerevisiae
Yeast is one of the key raw materials used in spirits production throughout the world. It plays a major role in the fermentation process, converting fermentable carbohydrate to alcohol and other important flavour components.
Yeasts are single-celled micro-organisms. They are members of the fungus kingdom which includes moulds, toadstools and mushrooms and can be readily found as wild yeasts in nature. Commercially available cultured yeast is also globally obtainable from numerous suppliers. These micro-organisms are very small and can only be seen using a microscope at a magnification of around 100 x.
Now that we have introduced yeast, let us quickly learn some interesting facts.
Like all living things, the fungi kingdom has a naming convention. This particular genus of Saccharomyces contains many species including S. cerevisiae . S. cerevisiae is used extensively in the fermentation of distilled spirits. It has been instrumental in winemaking , baking , and brewing since ancient times.
The word Saccharomyces derives from two Greek words σάκχαρον (sugar) and μύκης (mushroom). It means sugar fungus. The Latin translation for beer is cerevisiae .
Many different strains of S. cerevisiae are used in the production of spirits from a variety of raw materials. The strains used depend on the characteristics of the fermentation medium and the required quality of the final product.
Key Operating Insight
Welcome back. So, what were your thoughts? In the bread making process, yeast acts upon the carbohydrates in the dough, producing carbon dioxide and alcohol. These are then driven off in the baking process with the escaping carbon dioxide. This causes the bread to rise.
Structure and Appearance of a Yeast Cell
Yeast is a single cell fungus. A typical Saccharomyces yeast cell varies in size between 5-10 μm (micron) in size (1 μm = 10 -4 cm) and is ellipsoid in shape.
Let us look at the yeast cell structure in some detail.
Yeast Cell Structure
- The outer two layers of the yeast cell (the cell wall and inner plasma membrane) provide shape, structure and a barrier respectively between activities on the outside and inside of the yeast cell.
The cell wall is rigid and made of protein and polysaccharides. The plasma membrane allows passage of molecules into and out of the yeast cell during fermentation. It regulates the unfermented materials taken into the yeast cell.
- The cell wall’s thickness varies from ~ 100 to 200 nm (nano metres) depending on the surrounding environment.
- The cell wall thickens when the cell reaches the post-fermentation phase of the growth cycle (more of this in the Fermentation Theory and Technology lesson).
The other components of the cell include the nucleus, vacuole, mitochondria and nutrients that perform the vital functions for yeast growth and fermentation. Under the microscope, the cells’ appearance gives vital information about the yeast.
- Bud scars indicate cell age. Older cells will have produced more buds and hence have more bud scars (see mechanism of yeast growth by budding on the next page).
- The shape, size and number of vacuoles changes with cell age and condition.
Living organisms use energy released during fermentation for their life processes. There are two types of yeast sugar metabolism: aerobic (which needs oxygen) and anaerobic (which does not need oxygen).
Yeast can function in both aerobic and anaerobic conditions. Under aerobic conditions, with little other microbiological activity and in the presence of suitable carbohydrate (feedstock) and essential nutrients, yeast cells will multiply. This is the essential mode of commercial yeast production.
Under anaerobic conditions, with little other microbiological activity and in the presence of suitable carbohydrate (feedstock) and essential nutrients, yeast converts the fermentable carbohydrate to alcohol, carbon dioxide and other low concentration flavour compounds in an exothermic reaction (releases energy in the form of heat). More about this in the Fermentation Theory and Technology lesson.
Mechanism of Yeast Growth by Budding
Yeast cells multiply through the action of budding. Budding yeast cells can be readily found in a healthy growing population of Saccharomyces cells.
Yeast cells normally reproduce asexually. This means that they do not mate with another cell. Instead, they bud new daughter cells from a mother cell.
Figure: Yeast propagation by budding
The above diagram shows how the daughter cell initially starts as a small bud, which increases in size during the cell cycle. It continues to grow until it is almost the same size as the mother cell, at which stage it cleaves itself from the mother cell.
The cell nucleus, which contains the cell’s genetic material, divides as the bud is produced. The new nucleus, some of the cytoplasm (the liquid filling the cell comprising around 80% water), and its constituents, move into the new cell, which separates from the mother cell. A bud scar is left on the mother cell, while the new cell has a smaller birth scar. Count the number of bud scars on a yeast cell to determine its age.
Distilling Yeast Types
Yeast as a raw material can be sourced from commercially available suppliers or can be grown in the production process from a single healthy cell isolated under the microscope. This should take place in a clean environment. Any exposure to potential microbial contamination at this first stage can seriously affect the efficient growth of healthy yeast.
At the end of this large aerobic propagation in the yeast factory, the biomass produced can be harvested as cream yeast, pressed yeast or dried yeast. It is harvested at various solids concentrations through rotary vacuum filtration or other dewatering technologies. All three of these yeast product forms are purchased on an agreed specification between producer and supplier. The specifications will include solids content (dry matter - % w/w), yeast viability and total counts for bacteria and wild yeasts:
- Cream yeast ~ 18 % dry matter (w/w)
- Pressed yeast ~ 25 % dry matter (w/w)
- Dried yeast ~ 95 % dry matter (w/w)
The choice of yeast product form (cream, pressed, dried) depends on:
- The scale of the spirits production process
- The distillery location (supply chain implications)
- Whether there is automated or manual control of the production processes
- Actual on-site storage capability
- The availability of the required strain in each form
Cream yeast
Cream yeast can be delivered in large tankers. It lends itself to large-scale production processes with:
- Suitable on-site cream yeast storage
- Automated delivery
- Rapid stock turn-over
Dried yeast
Dried yeast is best suited to those smaller scale production processes with:
- Limited delivery options
- Manual reactivation into a liquid stream
- Manual addition
- Climatic conditions that strongly affect the storage of compressed or cream yeast
Pressed yeast
Pressed yeast is more suitable for:
- Small to medium production processes
- Sites with a refrigerated storeroom
Some distillers may use cereal based unfermented liquor, while others may use molasses-based liquor. It will depend on the type of raw material used in their spirits production.
Other distillers may employ small-scale yeast propagation processes. These are where wort is used to grow yeast before pitching aliquots (a small volume of the yeast rich liquor) of this wort into subsequent fermentations. A continuous fermentation process can also be used for yeast optimisation.
Historically, recovered brewers’ yeast was used in many areas of the spirits industry to augment the use of commercially produced strains. This type of yeast is still used, but only by a few distillers. The brewing industry’s global contraction and consolidation has very much reduced the availability of recovered yeast for distilled products.
Finally, not all spirit producers use commercially available yeast supplies. Some culture their own yeast from laboratory stock cultures or can adopt natural (spontaneous) fermentation. This is where naturally occurring yeasts are used in the fermentation process. More of this later on in the lesson!
Requirements for Selecting a Yeast Strain
Yeast selection will be determined by:
- Raw material usage
- Type and flavour of spirit
- Local processes used
- Ambient conditions
Global distilling industries use a variety of raw materials in the production of their products. Each type of raw material produces a wort specific for its own use. Globally, spirits industries are governed by legislation and regulation specific to that country or group of countries. This may limit:
- The type of raw material used
- Whether you can add process chemicals or process aids
- The composition of the mash bill
Key Operating Insight
Welcome back. So, what were your thoughts? An example of this would be the Scotch whisky industry. Here, regulations define the type of raw materials and processes to be used. They restrict the use of anything not included within the legislation.
Local climatic conditions can be quite different, depending on the location of each distillery. This particularly applies to ambient temperature and general climatic conditions. For example, the weather conditions throughout the year in a remote whisky distillery in the highlands of Scotland will be very different to the conditions experienced in rum distillery in Australia, the Caribbean or in a US Bourbon distillery. These huge differences in ambient temperatures need to be carefully considered when selecting yeast storage facilities. They can affect shelf life and subsequent yeast performance.
Below is a list of factors that should be considered in the selection of yeast for your industry:
- The desired fermentation rate (e.g. fermentation time)
- The original gravity and maximum alcohol concentration of the wash
- Consistency of performance
- Plant capability, yeast process and storage conditions
- The requirement for any particular flavour enhancing yeast strain
- Reliability and performance throughout the supply chain
- The commercial cost of your yeast
As you may be aware, this is not an exhaustive list. You may wish to add your own thoughts or points relating to any specific requirements for your own facilities.
The key to successful and efficient global fermentation is to select the most appropriate yeast strain for your particular needs. You must also maintain and optimise local process conditions to support the strain of yeast selected. With all of the above in mind, each distiller requires a strongly active yeast that produces a relatively clean, efficient and fast fermentation. The majority of these commercially available strains belong to the S . cerevisiae species.
It is important to note that yeast used in distillers’ fermentations is generally only ever used once in one fermentation cycle. This is unlike the brewing industry, where yeast is harvested and reused in many fermentation cycles. This places even greater emphasis on the correct selection of yeast strain.
Yeast Storage
Yeast is an expensive raw material. It must be stored under controlled conditions to ensure that it can be delivered into the fermenter in a satisfactory condition. This will ensure efficient fermentation.
Key Operating Insight
Welcome back. So, what are your thoughts? Fresh foods, meats, vegetables and liquids can readily be frozen and stored at around -19 ° C in your domestic freezer. This relates to both liquid and solid products. It renders these foods inactive and inedible. It takes a considerable time to bring them back to ambient temperature for consumption. Your domestic fridge will be set at a temperature of around 3-4 ° C. It holds foods and liquids at this temperature; they can easily be brought back to ambient temperature for use. These goods have differing shelf life, depending on their natural condition (liquid, solid, semi solid etc.), pH and form of packing. Have you ever found mouldy food in your fridge?
You should remember from the previous section on the mechanism of yeast multiplication by budding, that yeast can be recovered commercially in the yeast factory in three different forms. These forms are: dried yeast, pressed yeast and cream yeast. They will be bought on a general specification including total solids, viability (95%), total bacteria and wild yeasts (< 1 x 10 4 per g).
The stability of these forms of yeast is directly proportional to the level of dry matter. The higher the level of dry matter, the longer the shelf life will be (if stored appropriately).
The principle of yeast storage is very similar to that of food storage in your domestic fridge.
Dried yeast is generally supplied in vacuum packs, under an inert atmosphere in sealed cardboard containers. It will usually be stored at ambient temperature, in a cool dry place. At cool, ambient temperature in dry conditions, this form has a shelf life of around 12 months providing the vacuum seal remains in place.
Pressed yeast is normally supplied in 25 kg bags. It must be stored in a temperature controlled cold store at around 3-4 ° C. The bags are normally stored on shelves with good air circulation around the bags. This prevents localised heat build-up. If these conditions are met, shelf life will be around two weeks.
Cream yeast is delivered in either full- or part-filled 25,000 L tankers (with required certificate of tank cleanliness). It is discharged into a previously cleaned, refrigerated bulk storage tank prior to automatic dosing into the fermentation process. These bulk tanks have temperature control which holds the cream yeast at around 3-4 ° C before it is added to the fermenter.
As is the case with most materials of limited shelf life, the following are essential to maintaining good quality yeast:
- Clear stock replenishment timelines
- Good stock rotation, with older stock being processed first
Pure Culture Yeast and Natural Fermentation
A yeast grown without contamination from other micro-organisms, wild yeast or other yeast strains, is known as a pure culture yeast. Naturally occurring, or wild yeast, is a microorganism present in the natural world on raw materials or process plant.
Most, if not all, commercial yeast companies hold their own yeast culture collection. The yeast is safely stored in sterile conditions to ensure it will not be contaminated by any other microbiological activity. These culture collections can also be used in yeast research, to produce hybrids from these yeasts. These hybrids can be used to produce both new and existing distilled spirits.
There is always a desire to create new, faster acting yeast that shows greater tolerance of higher temperature and alcohol concentrations. Other yeasts are developed for their ability to produce specific flavour compounds.
As we learned earlier here, a few cells are isolated under the microscope and grown up in increasing volume. This makes commercial quantities of product, which are then sold to distillers. The yeast supplier may choose to trial specific strains prior to entering a commercial supply agreement. The choice of strain is based on certain factors, these include:
- The reliability of the yeast, particularly its delivery and quality
- The fermentation rate of the particular yeast strain
- The suitability of the yeast in the process plant. Cream yeast requires installation of expensive refrigerated storage
- The ability of the yeast strain to constantly deliver the required flavour in the product
A commercial decision is then made, based on the cost of this expensive raw material, taking cognisance of the factors above.
If you look back to the earliest alcoholic drinks, you will not be surprised to learn that pure culture yeast was not available to our predecessors. Micro-organisms, naturally occurring in the raw materials or from the process equipment, will have fermented these drinks. The resultant spirits will have been of variable quality resulting from a process with inconsistent efficiency.
Nowadays, some industries still use these natural fermentation processes, as they have been found to:
- Be reasonably reliable
- Meet locally available limited processes
- Produce a good flavoured product at a very low cost (as no yeast has to be grown or purchased)
These conditions will sometimes be favourable in heavily flavoured rum production and wine and fruit fermentations. Production facilities located in remote areas of the world (where commercial yeast strains are not available or commercially competitive) will also favour their use.
Key Takeaways
Distilling yeast strains are optimised to ferment high-gravity worts efficiently up to 8% to 10% ABV. Providing adequate initial dissolved oxygen and zinc ensures robust cell membranes and prevents sluggish or stalled fermentations.
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