Understanding Whisky Production Unit 4: Fermentation
Chapter 9: Cereal Fermentation in Practice: Washbacks, Times, and Bacterial Action
Wooden larch washbacks versus stainless steel, short versus long fermentations, and how lactic acid bacteria develop fruity ester complexity.
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Distillery fermentation vessels (washbacks) reflect an ongoing dialogue between tradition and sanitisation. Stainless steel washbacks look undeniably clinical and can be cleaned automatically with caustic sprays; traditional wooden washbacks constructed from Douglas fir or larch look picturesque and harbour a semi-permanent population of friendly bacteria tucked into the grain of the wood.
The clock plays a decisive role. In a short fermentation of around fifty hours, the wash is pumped to the still as soon as the yeast finishes eating, resulting in a clean, bready, cereal-forward wash. If fermentation is allowed to run for seventy to one hundred hours, surviving lactic acid bacteria flourish in the declining alcohol wash, producing acidity that subsequently reacts in the pot still to form fragrant, butterscotch-like esters.
The Scope of Cereals Fermentation
Cereal grains are the raw material of choice for many spirits producers throughout the world.
The word cereal is derived from Ceres, the Roman goddess of harvest and agriculture.
They are used in many whisky products (Scotch, Irish, American, Canadian, Indian and Japanese) to name but a few. They can also form the basis of neutral and fuel alcohol production. Cereals are an expensive raw material; all distillers need to maximise production efficiency to produce a quality product.
Key Operating Insight
Welcome back. So, what were your thoughts? Hopefully, your list will have included cereals such as maize (corn), wheat, barley, rice and rye.
You will remember from the previous cereal’s lessons, that the main cereal component used in spirits production is found in the grain’s endosperm. This area contains the main carbohydrate source (starch), which we will process and convert into a fermentable sugar liquid (wort). This is the primary raw material feed for this type of fermentation.
Cereals are used in spirits production for several reasons. These include: They are the main source of starch (and subsequently fermentable carbohydrate) during fermentation. They contribute to the flavour of the distilled product (e.g. peated and non-peated dried malt in malt whisky production). In the case of barley, it provides a source of endogenous (derived and produced within the cereal) enzymes for converting starch to fermentable carbohydrate during mashing.
The Regulatory Impact
Specific regulations govern the use of raw materials in the production of distilled spirits. Some of these regulations are very specific, while others are less so.
We will now take a little time to review some of these regulations in relation to whisky. We will explain some of the restrictions in four of the largest spirits producing countries. This is not the full legislative framework, but it will certainly give you a deeper understanding of the impact it may have on cereals-based fermentation. The same principles apply to those spirits producers not covered by these specific pieces of legislation.
Did you know that there is no naming convention for whisky or whiskey? It is instead very much country specific.
Scotch Whisky
The 2009 Scotch whisky regulations define the raw materials that can be used to make Scotch whisky. These specify that “Scotch whisky” means a whisky produced in Scotland, that has been
- Distilled in Scotland from water and malted barley (to which only whole grains of other cereals may be added) all of which have been processed at that distillery into a mash.
- Converted at that distillery into a fermentable substrate only by endogenous enzyme systems.
- Fermented at that distillery by the addition of yeast.
- Distilled at an alcoholic strength of less than 94.8% abv (alcohol by volume) so that the distillate has an aroma and taste derived from the raw materials used in, and the method of, production.
Irish Whiskey
The 1980 Irish whiskey act defines the raw materials which can be processed to make Irish whiskey. According to these regulations, to be called Irish whiskey, the spirits shall have been distilled in the State or in Northern Ireland from a mash of cereals which has been saccharified by the diastase (diastase is the collective term for the mixture of malt enzymes that break down starch during mashing) of malt contained therein, with or without other natural diastases. It has been:
- Fermented by the action of yeast.
- Distilled at an alcoholic strength of less than 94.8% abv in such a way that the distillate has an aroma and flavour derived from the raw material used.
Canadian Whisky
Current Food and Drugs Regulations C.R.C. (Consolidated Regulations of Canada) define the raw materials that can be processed to make Canadian whisky, Canadian rye whisky or rye whisky. They shall:
- Be a potable alcoholic distillate, or a mixture of potable alcoholic distillates, obtained from a mash of cereal grain or cereal grain products saccharified by the diastase of malt or by other enzymes and fermented by the action of yeast and other micro-organisms.
- Possess the aroma, taste and character attributed to Canadian whisky.
- Be mashed, distilled and aged in Canada.
American Whisky
There are both federal and state codes which define the raw materials used in the production of American whiskey. The regulations vary depending on spirit type. However, American whiskey is a distilled beverage produced in the United States from a fermented mash of cereal grain. The US Code of Federal Regulations governs the production of American whiskey, with some key points including:
- Rye whiskey is made from a mash that consists of at least 51% rye.
- Rye malt whiskey is made from a mash that consists of at least 51% malted rye.
- Malt whiskey is made from a mash that consists of at least 51% malted barley.
- Wheat whiskey is made from a mash that contains at least 51% wheat.
- Bourbon whiskey is made from a mash that consists of at least 51% corn (maize).
- Corn whiskey is made from a mash that contains at least 80% corn maize.
We have included the above information, not to make you an expert of regulations, but to show you the huge variety of different mash bills and process conditions employed to produce the desired spirit. Some of these process conditions can directly affect the fermentation of cereals-based wort.
These regulations relating to cereals-based wort fermentation allow you to:
- Use non-endogenous enzymes in most other spirits products other than Scotch whisky.
- Use yeast and other micro-organisms in some circumstances.
- Only use endogenous enzymes in Scotch whisky production.
Use the internet to research, establish and understand the framework of regulation and legislation relating to your particular product(s) and country/region of production.
Yeast Inoculation Rate
Here is a worked example of how to calculate the amount of yeast required for fermentation. You have a mash bill of 20 t of cereal. Your inoculation rate is 1% wt/wt on a dry weight basis of yeast to cereal ‘as-is’. Let us calculate the quantities of dried, compressed and cream yeast that you will need to add to this fermentation. Note, this is an abnormally high rate of inclusion, but it will serve the purpose in this calculation.
We can start with the 20 t ‘as-is’. This is the weight of the mash bill at the actual moisture of the raw materials.
1% of this mash bill is 20 x (1/100) = 0.2 t or 200 kg. So, we need to add 200 kg dry weight of yeast. These forms of yeast have solids of 95%, 25% and 18% (refer back to the Yeast as a Raw Material lesson).
This means that for every 100 kg of each form, we will have 95 kg, 25 kg and 18 kg of dry material, respectively.
Three quick calculations [(200/95X100), (200/25X100), (200/18X100)], show us that we need 211 kg, 800 kg and 1,111kg of dry, compressed and cream yeast respectively to ensure that we deliver a 1% inclusion on a dry weight basis.
The actual yeast inoculation rate in cereals-based fermentation (as an absolute number) varies. It depends on whether the distiller uses yeast from a commercial supplier, or whether the distillery employs a yeast optimisation or growth process. For example, in Scotland an inoculation rate of live cells per litre of wort would be circa 2 x 10 10 cells per L. Whether this is cream, dried yeast, or yeast rich wort if yeast optimisation is used is irrelevant - it’s the live cell count that is important.
This is an ideal opportunity for you to establish the current inoculation rates within your own distilling business.
Cereal Wort Fermentation Process
We do not need to cover the basics of the four phases of fermentation again, These principles are the same for a cereal wort fermentation. The image below can serve as a quick reminder showing these as four discrete phases for simplicity. In reality this is more complex process with phases 2 and 3 in particular taking place simultaneously.
The final wort or sugar solution at the end of cooking and mashing, which is fed forward to fermentation, may or may not still contain all the residual cereals solids (husk and insoluble solids). Where these are all fed into fermentation, and then onto distillation, they will include all the yeast debris produced during fermentation.
Please note:
- These solids can sometimes concentrate at the top of the fermenter. This can cause pressure build-up in the wash below the solid layer, which is eventually released.
Wash is the term used in whisky/whiskey distilleries for the product of fermentation, i.e. the charge for the wash still. This can also be known as beer in some whiskey-producing countries.
- This rapid release of pressure can cause spillage from the fermenter, loss of raw material, and obviously efficiency loss.
- If the plant is not effectively sanitised, it becomes a potential source of microbial contamination.
- Some spirits producers do not feed these solids forward, as they are removed prior to fermentation at the end of mashing. The resulting wort is thus free of these heavy solids, and contains only soluble and fine insoluble solids. These will eventually contribute to very clear or cloudy wort.
- This is quite an important difference in the fermentation feed. It is vitally important to remove all the residual solids from the fermenters at the end of fermentation, as they will become a key location for microbial contamination.
As discussed in the previous fermentation lesson, there are two main inputs to the fermentation process: a suitable supply of wort (sugar solution) and a suitable quantity of yeast of good viability and low bacteria content.
For efficient fermentation to take place, it must occur in a clean fermentation vessel, with suitable wort addition and fermented wash discharge arrangements. The wort produced from cereals in the cooking and mashing processes is a very complex medium, consisting of:
- Simple fermentable sugars
- Amino acids
- Proteins
- Small and large dextrins (carbohydrates generally containing four or more glucose units)
- Minerals and many other compounds
These are important component parts of wort as they provide the nutrients to maintain a healthy and active yeast population for cell multiplication and fermentation.
Generally, this wort will have been processed at the optimum temperature for conversion of starch to fermentable sugars (around 65.0 ° C) through the action of endogenous enzymes or the addition of commercially produced enzymes (non-endogenous enzymes).
The two main differences between distiller’s and brewer’s wort are: Distillers’ wort is not boiled after mashing, unlike within the brewing industry (to achieve several hop and wort reactions). The cereals-based spirits process does not have this boiling stage. As a result, some microbial contamination is inevitable. Furthermore, any residual enzymatic activity (limit-dextrinase) is not deactivated. These enzymes will play an important role in secondary conversion, where they will break down the dextrins throughout fermentation. Spirits producers do not generally recover yeast after fermentation for re-use in subsequent fermentations. Distillers only use yeast for one fermentation cycle, where it is recovered as a co-product later in the process - more of this in later lessons.
Secondary Conversion
The role of secondary conversion in cereals-based fermentations is very important.
- Only around 80% of the available starch is converted to fermentable sugars (glucose, maltose and maltotriose) along with both small and larger dextrin units during the various mashing processes.
- Those temperature and pH tolerant enzymes that have survived mashing (and remain within the wort) facilitate the breakdown of these small and larger dextrins during the course of fermentation.
- During fermentation, the active yeast initially consumes the glucose, followed by the maltose and maltotriose. As these sugars are consumed by the yeast other sugars are produced until the yeast and enzymes have converted all available carbohydrates to alcohol, carbon dioxide, other flavour compounds, biomass and energy in the form of heat.
This simple diagram gives a great pictorial representation of the totality of the fermentation process.
Fermentation Technology
As you learned in the previous lesson, there are many different fermentation vessel designs in the world of distilling. These include those that process cereals.
The fermentation process is the longest process step in the production of cereals-based, new-make spirit. Its duration can range anywhere from 45 hours, to sometimes well over 100 hours.
The total number of fermenters will exceed all the other process equipment. It is the largest piece of plant in new-make spirit production.
The total capacity will be balanced from the required weekly distillery output and the rate and capacity of the distillation process. This means that there will probably be a different number and size of fermenters for each distilling operation, based on individual production requirements.
As with most distillery plants, no one size or shape will meet all the different requirements. Fermenters are designed to suit the yeast being used and the type of product being produced. They must be designed and manufactured to ensure all internal surfaces are smooth preventing the build-up of any solid materials.
Analytical Parameters
There are several analyses which can be carried out on both wort at start of fermentation and wash either during or at the end of fermentation. They can give a great insight into what is happening in the fermenter.
Each distilling operation will have their own list of analysis required to ensure the process is operating efficiently. As already stated, the scale of distilling operations varies hugely from the craft distilling operations to the huge grain and neutral alcohol distilleries.
It is likely the larger operations will have on site laboratory facilities while the smaller operations will rely on less sophisticated methods of analysis. Some of these analyses can be carried out on fresh samples taken by operators during normal hours of production or can be retained for more sophisticated methods of analysis in the distillery laboratory.
A representative sample of the wash is required for analysis. This is an important point as analysis of non-representative samples could lead to incorrect conclusions relating to the efficiency of the fermentation process.
These analyses may include:
- Original gravity. This is a measure of specific gravity at the start of fermentation. It is an indicator of the efficiency of the mashing process. This can be measured using a hydrometer, hand held densitometer or laboratory densitometer.
- Final gravity. This is a measure of the specific gravity at the end of fermentation. It is a good indicator of fermentation performance. It can be measured using a hydrometer, hand held densitometer or laboratory densitometer.
- Alcohol concentration (% abv). This can be measured either during fermentation or at the end of fermentation. It can be used to calculate the total alcohol produced in the fermenter. Generally, this measurement requires laboratory analysis.
- PH. This is the measure of acidity or alkalinity of any solution. A pH of 7.0 is neutral, values between 0 and 6.9 are acidic, while values between 7.1 to 14 are a measure of alkalinity. A typical wash pH at end of cereal fermentation is in the range 3.9-4.1. Values below 3.9 indicate potential higher levels of microbial contamination with subsequent loss of yield.
- Glucose, maltose, maltotriose. Samples taken from the fermenter can be analysed for these individual sugars. Any residual sugar found in wash at end of fermentation is an indicator of poor yeast performance and subsequent loss of yield.
- Lactic acid. Samples can be analysed to determine the concentration of lactic acid at the end of fermentation. Abnormally high levels will indicate higher than normal microbial contamination.
There are other more sophisticated analyses which can be carried out in non-routine circumstances during production trials of new or alternative raw materials or times of general poor efficiency.
You should take some time to establish the analytical procedures implemented at your own distilling organisations and also what the expected levels of these analytes are.
Key Takeaways
Fermentation length is one of the most effective stylistic levers in malt distilling. Extending fermentation past sixty hours allows secondary bacterial conversions that add complexity, acidity, and rich fruit congeners to the resulting wash.
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