Understanding Whisky Production · Unit 4: Fermentation

Chapter 8: Fermentation Science: Glycolysis and Congeners

The biochemical pathway of sugar to alcohol, the four yeast growth phases, temperature control, and the synthesis of esters and higher alcohols.

Fermentation is where biology takes command of the distillery. When yeast is pitched into cool, oxygenated wort, it first adjusts to its surroundings, consumes available oxygen to strengthen its cellular membranes, and then launches into rapid exponential growth.

Under the Crabtree effect, high sugar concentrations prompt the yeast to produce alcohol even in the presence of oxygen. As glucose is dismantled via the glycolysis pathway, the yeast’s amino acid metabolism (the Ehrlich pathway) throws off higher alcohols (fusel oils). These higher alcohols react with fatty acids inside the cell to forge esters, yielding a spectrum of aromas spanning green apples (ethyl hexanoate), pear drops (isoamyl acetate), and floral notes (phenethyl acetate).

The Terminology of Fermentation

There are some key words used specifically in the fermentation process, which we all need to know. Some may have been introduced to you in the raw material processing chapters. Let us now build upon that knowledge before discussing fermentation theory.

The sugar solution prior to fermentation has various names throughout the distilling world. These include wort (unfermented liquid in cereal-based production), must (unfermented grape juice), or sometimes just unfermented liquor. All of these liquids have one thing in common - they all contain various concentrations of fermentable carbohydrate. The amount of this sugar (or fermentable carbohydrate) is measured using a density meter or saccharometer (sometimes also referred to as a hydrometer).

The Physics of Sinking and Floating

Specific gravity is simply the universe’s polite way of asking whether a substance is lighter or denser than water. If you drop a block of Scots pine into a loch, it bobs on the surface; drop an anvil, and it heads immediately for the bottom with zero regard for romantic philosophy.

In distilling, we use this exact principle to measure our sugars. Dissolving barley sugars into water creates a dense, viscous liquid with a high specific gravity (typically 1.050 to 1.080). A glass hydrometer placed into this sweet wort bobs high out of the liquid like a buoy on a calm sea. As yeast devours the heavy dissolved sugars and replaces them with light, volatile ethanol (which has a specific gravity around 0.789), the liquid thins out dramatically. The hydrometer sinks progressively deeper into the wash, tracking the quiet disappearance of sugar and the glorious emergence of alcohol.

Distillers measure gravity using saccharometers calibrated to various scales: Specific Gravity (relative to water at 1.000 at 20°C), degrees Plato (°P), or degrees Brix. More modern distilleries employ digital vibrating-tube density meters that deliver the verdict down to four decimal places without anyone needing to squint through misted glass.

Let us now review some additional points and terminology relating to spirit fermentation:

• The original gravity (OG) of an unfermented liquor - this is the specific gravity before fermentation has begun. This can range from 1.045 to 1.080 in the spirits industry. It is also known as ‘declared gravity’.
• During fermentation, yeast action on the fermentable carbohydrates converts them into alcohol, biomass, other low concentration flavour compounds (congeners) and carbon dioxide.
• The decline in fermentable carbohydrate concentration, combined with an ever-increasing alcohol concentration, reduces the density of the fermenting liquor.
• Alcohol is significantly less dense than water.
• The final gravity (FG) is the specific gravity measured at the end of fermentation. Again, this will have a range of values from 1.005 to 0.996 (e.g. five above the density of water to four below the density of water), depending on fermentation efficiency and the original gravity of the unfermented liquor.

The Theory of Fermentation

Fermentation is the process in which yeast converts sugars into alcohol, biomass, carbon dioxide, key flavour components (congeners) and heat.

Efficient fermentation depends upon a carbohydrate source, a healthy viable yeast, and a suitable, clean fermentation vessel. Some form of temperature control is required if the ambient temperature is variable.

We can refer to a domestic situation to help us to understand what is required.

Successful production of your own cakes, tarts, pies (or any other cooked product) relies on a number of factors. A recipe requires the correct amount of ingredients, good quality raw materials, clean baking dishes, a calibrated temperature oven to cook the food, and someone who can carefully follow instructions. We can now look at the fermentation process through this lens.

There are two key inputs to the fermentation process: Sugar-rich liquor produced from the raw materials of the distillers’ choice. A suitable quantity of yeast of the required strain. This provides the driving force in the conversion of this sugar solution to alcohol and other desired flavour compounds.

Pre-fermentation, the temperature of the sugar solution will often be higher than the yeast will tolerate. Therefore, the solution will be cooled to around 15-25°C prior to yeast addition. The yeast is added to the fermenter during the early stages of filling. Fermentation will proceed until all the available carbohydrates have been converted to alcohol.

The following stoichiometric equation shows the action of yeast on glucose to produce alcohol:

The above graph shows the X-axis as the timeline from start to end of fermentation. The dual attribute Y-axis shows specific gravity and percentage alcohol of the fermenting liquor. We can see the decrease in gravity during fermentation, with an almost mirror-like increase in alcohol concentration.

Several other analytes can also be measured during the fermentation process. These include pH, total acidity and temperature. Modern fermentation processes generally have automatic temperature measurement and logging throughout fermentation. More on the significance of these later in this chapter.

The sugar rich liquor required for an efficient fermentation contains a number of different sugars, depending on the raw material source:

• The primary sugars in cereal-based liquor will primarily contain glucose, maltose (two glucose units), maltotriose (three glucose units) and dextrins. Dextrins are larger molecules containing four or more linked glucose units. They may eventually be broken down to the smaller sugar units during fermentation through continued action of starch-degrading enzymes such as limit-dextrinase.
• For grape and molasses fermentations the primary sugars will be glucose and fructose and sucrose, respectively,
• The primary sugar in agave is fructose.

These will be covered in more detail in later fermentation chapters.

Fermentation Process

We can now look at the various phases of fermentation in some detail.

This simple schematic gives a great representation of the various phases of fermentation. This diagram shows each of these as discrete phases for simplicity but in reality this is much more complex. Phases 2 and 3 in particular take place simultaneously. Let us discuss each phase in more detail.

Phase 1

Addition of Yeast to the Unfermented, Sugar-rich Liquor

• The yeast added is called an inoculum. This process can be described as ‘yeasting the mash’ or ‘pitching the yeast’. Both descriptions refer to the same process.
• The yeast should be added at the earliest filling of the fermenter, when the liquor is at the correct temperature. Failure to add the yeast at the earliest opportunity can increase microbial contamination risk from any competing microbial activity present.
• The quantity of yeast added varies greatly between distillers throughout the world. The processes after inoculation are very similar for all fermentation, but the rate at which they take place can differ markedly.
• Yeast cells also need the provision of amino acids, minerals and vitamins to sustain them during multiplication and fermentation. These are provided by the raw materials themselves in cereals-based fermentation but can also be added as supplements where regulation and legislation allow it.
• Lower inoculation rates are used in a winery, where fermentation can take around 14 days. Higher inoculation rates are likely to be used in whisky fermentations, where fermentation time may last only around 48 hours. Generally, higher rates of inoculation lead to faster fermentation.
• Phase 1 is often called the ’lag phase’ because there appears to be little, or no, action while the yeast is adapting to the new conditions. During this time, it is preparing to start growth and cell division processes.

Phase 2

The Growth Phase

Phase 2 starts a few hours after inoculation, anywhere between 3 and 24 hours, depending on conditions and the quantity of yeast added. Along with the fermentable sugars, the unfermented liquor contains dissolved oxygen and other nutrients. In this condition, the yeast cells multiply quickly and also start to produce alcohol. This is sometimes described as the aerobic fermentation phase, where the yeast cell count multiplies in the presence of this limited oxygen source.

• The specific gravity starts to fall as the fermentable carbohydrate is consumed, carbon dioxide is released, and heat starts to evolve.
• Once the last of the oxygen is consumed by yeast growth, and a layer of carbon dioxide gas has formed on the top of the fermenting liquor, yeast growth becomes limited and will eventually cease.
• Both Phase 1 and Phase 2 are important stages in the fermentation process. This is because other flavour compounds such as higher alcohols like amyl alcohol are produced as by-products of yeast growth.

Phase 3

Fermentation Phase

Phase 3 starts when all of the oxygen has been consumed. It is the most vigorous phase of fermentation.

• It is the main fermentation phase, often described as anaerobic fermentation (meaning fermentation in the absence of oxygen).
• The specific gravity of the fermenting liquor falls more rapidly as the fermentable carbohydrate is converted to alcohol and carbon dioxide. It is the phase of greatest heat production increase.
• This phase is also important for flavour development of esters and aldehydes (other organic flavour compounds), which are formed at very low concentrations but can significantly affect the flavour of the final distilled spirit.
• Fermentations that have reached Phase 3 with little or no presence of microbial contaminants are unlikely to become heavily contaminated by competing wild yeasts or unwanted bacteria.
• The presence of alcohol at relatively high concentrations inhibits many bacteria. Those that are not inhibited have difficulty in competing with the vigorous active yeast for the sugar feed.

Phase 4

Post-Fermentation Phase

Phase 4 occurs after all the fermentable carbohydrate has been consumed. Fermentation slows down and eventually stops. Distilling yeasts are selected to be non-sedimentary (or non-flocculant), so the cells stay in suspension but are inactive due to the shortage of food.

• The temperature increase rate falls rapidly as fermentation stops. The temperature can sometimes even fall during this phase if allowed to stand too long.
• No more alcohol is produced; the fermented liquor can now be transferred to the feed system for the distillation process.
• This vessel has various names throughout the world including beer well, still-charging vessel, wash charger, still feed tank and so on. They all relate to the same process, which begins with the fermented liquor feed and ends with distillation.
• Feed to the distillation process should take place as quickly as process operations allow. Even this fully fermented liquor is susceptible to further microbiological contamination, given the opportunity.
• Phase 4 is often called the post-fermentation phase.

Several additional factors can be monitored during fermentation, either by sampling and laboratory analysis, or by continuous automatic logging during fermentation. These might include:

• The temperature profile throughout fermentation
• The pH profile during fermentation
• The specific gravity profile during fermentation
• The (ethyl) alcohol and other higher alcohol profiles during fermentation
• Some distillers may even measure the sugar concentrations and changes in sugar concentrations during fermentation

All of these have a common theme, which is to allow the distiller to understand fully the phases of fermentation that are relevant to spirits production processes throughout the world. A full understanding of what an efficient fermentation process looks like will allow a distiller to more fully understand and amend process conditions should the efficiency fall markedly.

Generally, distillers express the individual fermentation yield (conversion of raw material into alcohol) in terms of litres of alcohol per tonne of raw material (Lpa/t = litres of pure alcohol per tonne). This is achieved by accounting for the total mass of raw material in any given fermenter and the total alcohol produced in that same fermenter at the end of the fermentation process. Other units may also be used; these will be covered in future chapters.

Fermentation Vessel Technology

It will not surprise you to know that there are many different designs of fermentation vessels throughout the world of distilling.

In this section, we will explore the key features of fermentation vessel design. These features give the raw material feedstock the best chance to complete an efficient fermentation.

The Philosophy of Fermentation Vessels

Fermentation vessels across the distilling world range from rustic wooden vats bound with iron hoops that look as though Asterix might brew magic potion in them, to towering stainless steel silos that look as though NASA built them to store cryogenic propellant.

Despite their radically different aesthetics, all fermentation vessels share the exact same brief: to provide a secure, hospitable, and scrupulously clean arena in which yeast can consume its own weight in sugars, generate ethanol, and throw off desirable aromatic congeners before expiring. They must accommodate the violent expansion of carbon dioxide foam, survive aggressive cleaning regimes, and protect the wash from unwanted environmental opportunists.

Let us explore the core engineering considerations and materials of construction.

Materials of Construction

Any given fermenter must be made from construction materials that:

• Are strong enough to hold the required volume of the fermenting liquor
• Are easy to clean and repair

The fermenter’s design must allow several process operations to take place safely. These processes include filling and emptying, carbon dioxide extraction and removal, and the possible agitation of the fermenting liquor.

Commonly used materials include:

1. Stainless steel

A) Very durable and long lasting. Highly resistant to acid corrosion and harsh caustic cleaning.

B) Can be readily manufactured to any size and shape specification.

C) Easy to install automated process controls, such as cleaning-in-place (CIP) spray balls.

D) Susceptible to pitting corrosion if exposed to high chloride levels.

E) Higher initial capital cost, repaid across decades of maintenance-free operation.

2. Mild steel

A) Cheaper to install when compared to stainless steel.

B) Readily fabricated to standard specifications.

C) Can accommodate automated CIP systems.

D) Extremely susceptible to corrosion, requiring epoxy or glass-flake linings that must be periodically inspected and renewed.

3. Wood (normally European larch or Oregon pine)

Wood, if you believe in romance; steel, if you believe in cleaning. Wooden washbacks (traditionally crafted from Canadian Douglas fir/Oregon pine, European larch, or pitch pine) are magnificent, towering artifacts that look as though they belong in an 18th-century shipyard. They possess micro-porous grain and microscopic crevices that house resident microbial communities: dynasties of lactic acid bacteria that have survived generations of hot-water flushes. These friendly squatters kick in during late fermentation to generate lactic acid and fruit-forward esters. The downside is that wood demands constant devotion: if allowed to dry out, staves shrink, seams gap, and the vessel leaks like a sieve, and cleaning requires energetic scrubbing and steaming rather than a quick squirt of caustic.

4. Concrete

A) Can be built to hold huge capacity - fuel alcohol businesses.

B) Difficult installation of peripheral process plant, unlike welding to metal vessels.

Having reviewed the construction materials and their pros and cons, we can now explore the functionality required for fermenter vessels. As already stated, these vessels come in many shapes and sizes, as shown in the above schematic. They are designed to optimise both the yeast being used and the type of spirit being produced.

Materials of Construction

Washback Materials and Fermentation Kinetics

In Scottish distilleries, washback vessels range in working volume from 15,000 litres in boutique craft operations to over 65,000 litres in commercial malt distilleries. The choice of vessel material exerts a direct influence on heat retention, microbial ecology, and spirit congeners:

Fermentation Kinetic Stages & Wash Composition

A standard Scottish single malt fermentation runs between 48 hours (short, cereal-forward) and 72 to 110 hours (long, ester-rich and complex). The biochemical conversion progresses through four distinct kinetic phases:

When wash is discharged to the wash charger awaiting distillation, it forms an alcoholic wash typically between 6.5% and 8.5% ABV, carrying suspended yeast cells, dead biomass, higher alcohols, and organic acids into the wash still. This multi-phase kinetic succession and the specific contribution of washback microflora to spirit congeners is documented in peer-reviewed research, notably Van Beek & Priest (2000, Evolution of the Lactic Acid Bacterial Community During Malt Whisky Fermentation, Applied and Environmental Microbiology) and Walker & Hill (2016, Saccharomyces cerevisiae in the Production of Whiskies, Beverages).

When considering fermenter vessels, we must note the following:

• Their design should allow full discharge of fermented liquor either by a sloping discharge arrangement to one side of the vessel, or by central discharge through a dished or conical base.
• Cleaning involves either fully automatic sequence driven cleaning in place procedures, or manual operation.
• Steam sanitisation is sometimes preferred to chemical sanitisation. Where solids are retained as part of the fermenting liquor, it is imperative to remove any residual solids. Otherwise, they can encourage unwanted microbiological contamination.
• These vessels will either be filled through top or bottom filling arrangements. This occurs under either fully automatic control, by manually operated valves, or a combination of both.
• Some plants prefer to install top filling arrangements, as this allows aeration during the filling process. This provides much-needed oxygen to the inoculated yeast at the very start of the process (where cream or pressed yeast is used).
• Where solids form part of the fermenting liquor, as is the case with ‘grains-in’ fermentation, agitation is needed to allow the yeast to act throughout the fermenter and prevent sedimentation of the solids. Mechanical mixers or stirrers can achieve this. Alternatively, the system can recycle carbon dioxide back into the fermenting liquor.

Generally, most modern fermenters have fully secured lids and collection and discharge arrangements to vent the evolved carbon dioxide to a safe place throughout fermentation. Fully closed lids ensure that the anaerobic phase is quickly reached, when the entrained oxygen is either consumed by yeast growth or vented along with carbon dioxide during the early stages of fermentation. Once a layer of carbon dioxide has been established across the top of the fermenting liquor, anaerobic fermentation is quickly established. Fully secured lids also ensure there is little or no contamination from insects, rodents, birds or any other possible contaminant.

The fermenter will not be filled to the top, as this could cause the fermenting liquor to overflow during the vigorous phase 3, when alcohol production and carbon dioxide release is at a maximum. There must be a free board / empty void above the fermenting liquor, to allow for any potential frothing and/or excessive liquor movement. This is called the vessel headspace. Fermenter switchers (rotating wooden or metal paddle arrangements) are sometimes installed to control frothing. Other distillers may add antifoam.

In some warmer parts of the world, it may be necessary to provide fermenter cooling arrangements by designing and introducing cooling jackets to the fermenter. Alternatively, distillers may use external cooling arrangements, where the fermenting liquor is pumped from the vessel, through the cooler, and back into the vessel. There are two methods of ensuring the maximum fermentation temperature does not exceed 35°C:

1. Controlling the fermentation temperature through the cooling processes described above.

2. Cooling the fermenting liquor to such a temperature so that the maximum temperature does not exceed 35°C without the provision of any cooling. It is important not to exceed 35°C, as yeast activity is severely impaired and deactivated above this temperature.

These temperature control processes can be used during fermentation when the temperature increases naturally with the increasing rate of fermentation. Alternatively, they can be used to maintain a constant temperature throughout. Both options are used throughout the distilling industry.

We also need to consider whether these vessels should be:

• Housed within a building
• Installed in the open with some form of roof protection
• Installed in the open with no protection

They can also be designed and built as part of a fully automated distillery, under full sequence control. They can alternatively be built as stand-alone operations with full manual control. All of these options depend on the distillery’s location and the types of services available. They may also form part of a strategy where manual labour is removed by installing fully automated process control systems.

Having considered all of the above, the distiller will optimise process conditions by:

• Fixing an optimal gravity at the start of fermentation
• Agreeing the inoculation rate of a particular yeast strain for the product
• Following all cleaning and temperature control procedures
• Filling and emptying repeatedly, to ensure the delivery of fast, efficient fermentation and providing the ideal feedstock for the distilled product

Role of Other Micro-organisms

Bacteria are much smaller than yeast cells. Some are less than 1 x 10 -6 m in length. They can be rod-like, spherical, or even spiral in shape. They can also be seen through a microscope, albeit at increased magnification.

Many countries have legislation and regulation that prohibits the addition of antibacterial agents, including antibiotics, at any stage in the production of distilled spirits. It is thus almost impossible to prevent some contamination by a variety of micro-organisms. In some cases, this is welcome as it can positively affect quality. In other cases, it is very much unwanted as it can significantly affect fermentation efficiency and final product quality. The fermentation temperature of between 20-35°C, and the readily available food in the form of sugar, provides an ideal medium for bacterial growth.

The main bacterial contaminants of distillery fermentation are lactic acid bacteria. The source of lactic acid bacteria may derive from either the raw material or the yeast supply. These bacteria proliferate primarily during the latter stages of fermentation. They can reduce yield and raise acidity (lower pH). They can also produce various off-flavours.

Wild yeasts can be found almost anywhere. They multiply quickly during the early aerobic stages of fermentation, often providing real competition for the cultured yeast. This reduces yield and produces unwanted ‘off-flavours’. The distiller must apply rigorous cleaning and sanitisation processes throughout all parts of the plant, thus minimising the potential impact of the above contaminants. Realistically, the source of microbial contamination can be limited to three main areas:

1. The original unfermented liquor

2. The yeast supply

3. Plant and equipment that has not been cleaned properly

Concerning distillation practices globally, bacteria can be divided into two groups:

1. Bacteria whose presence in the process is useful

2. Bacteria whose presence will cause serious issues

In the first part of this section, we explored examples of bacteria that detrimentally affect fermentation, reducing efficiency and yield. They also produce a variety of unwanted flavour compounds, including sulphur compounds, acrolein, diacetyl and fusel oils. Conversely, some types of bacteria can be useful to the distiller:

• Specific types of bacteria are deliberately added to the process, along with culture yeast, where legislation or regulation permits.
• Bacteria are used to generate sour mash in whisky production.
• Bacteria can be used to create secondary fermentation during particularly long fermentations.
• A late lactic fermentation in whisky production can take place, giving fruity and estery notes.

How to Ensure a Successful Fermentation

As we have regularly stated, the distiller’s main aim is to deliver a fast, efficient fermentation. They aim to produce it in a clean environment, producing alcohol and other important flavour compounds.

Unfortunately, things can and sometimes do go wrong. This can result in poor performing fermentations with abnormal measurements of yield efficiency, final gravity, pH and maximum temperature as examples. There are a number of things that you can do to ensure good fermentations:

• Always add the correct volume of unfermented worts. This allows for accurate accounting of process raw materials.
• Always ensure your fermenter is filled to the correct level, with a suitable headspace.
• Always add the correct quantity and type of yeast at the right time.
• Always use in-specification yeast.
• Ensure that your fermentation temperature control processes are effective for your specific climatic conditions. Do not exceed 35.0°C.
• Always clean your plant, pipework and valve systems.
• Always add your yeast as early as possible during wort filling.

The importance of the following should not be underestimated:

• Well-maintained, clean plant
• Well-defined and routinely followed process procedures and conditions
• Good quality raw materials

This is particularly relevant for distillers using wooden fermenters, as these are more likely to develop higher levels of bacterial contamination.

There is also a higher likelihood of microbial contamination during fermentation, due to the lack of a wort boiling stage in spirits production.

Every distiller will endeavour to produce a quality product at optimum efficiency and cost. However, we must accept that things can, and do, invariably go wrong. As long as we are mindful of, and prepared for, these issues, we should be well equipped to deal with them should they occur.

Forensic Fermentation: When Things Go Sideways

When a fermentation veers off course, it rarely does so quietly. Because distilling relies on living biological cultures rather than predictable inorganic reagents, an off-specification fermentation announces itself across every measurable parameter:

A sluggish gravity drop indicates that yeast has stalled (often poisoned by sudden temperature spikes above 35°C or starved of free amino nitrogen). A premature crash in pH below 4.0 coupled with surging volatile acidity signals that competing lactic or acetic acid bacteria have staged a coup, gobbling up sugars that should have become ethanol and generating sharp vinegar or cheesy aromas. Distillers monitor gravity, pH, temperature curves, and residual unfermented sugars so they can catch an unruly fermentation before the stillman charges a wash that smells like sour milk.

Key Takeaways

Fermentation generates virtually all of the volatile organic congeners present in new-make spirit. Controlling temperature, pitch rate, and wort nutrition ensures a clean, vigorous fermentation that produces the intended flavour spectrum.

About the author

John Fegan · Editor

Hailing from the land of haggis and deep-fried Mars bars, John was always going to find his way to whisky. He has run Uisge Beatha since 2015 and Whiskipedia.com since 2020.