PITCHING AND PROPAGATION

Yeast is a living organism (a single cell fungi) capable of converting sugars to alcohol. Yeast colonises or “infects” the fermentation, using nutrients to produce new yeast and to build up energy until all the available nutrients are used.

When all the nutrients are depleted, the yeast settles out and goes into “hibernation”, or stationary and settlement phase until they are presented with more nutrients, allowing them to grow and multiply again.

The science or craft of the brewer is to manage this process to produce an effective fermentation. The brewer has to have a system to introduce the yeast into the wort to start the fermentation and remove the yeast again once its work is complete.

Yeast management in the brewery is part of a continuous cycle of cropping followed by serial re-pitching, with excess yeast being removed and sold as a brewery co-product and new yeast being introduced as a brewery culture.

Yeast Pitching

Fermentations can be pitched with yeast recovered from a previous brew or with a new batch of yeast grown up from a culture in the brewery or commercial dried or stabilised liquid culture. The yeast cells must be thoroughly mixed with the cooled and aerated wort from the brew house to ensure an efficient fermentation.

The most effective yeast mixing is by pitching in line during the transfer of the cold wort from the wort cooler to the fermenting vessel. It is essential that the pitching yeast used is in a healthy condition.

The excess yeast is either sent to waste or goes forward into the maturation tank along with the green beer.

Before pitching, the yeast should be checked for microbial contamination (contaminants should be absent), yeast density (number of cells per ml), yeast viability (% live cells - >95%), yeast vitality (physiological status of yeast - healthy), and temperature of storage (yeast normally stored cool 2 – 4oC).

During fermentation the number of yeast cells increase by between three and five times, but the initial pitching rate must be sufficient to ensure that there are an adequate number of cells present to fully utilise all the available sugars. Pitching rate required depends on: u Original gravity u Yeast strain u Type of beer (ale or lager) u Fermentation temperature u Fermentation design

A typical Ale fermentation requires a pitching rate of around 8 to 10 million cells per millilitre or 0.5 litres of yeast slurry per hectolitre of wort for at 1045 (11.25%) gravity.

A typical Lager fermentation, which is generally carried out at a slightly lower temperature, is pitched at a slightly higher rate around 15 million cells per millilitre or 0.75 litres of yeast slurry per hectolitre of wort at 1045 (11.25%) gravity.

It is important that most of the yeast cells pitch are alive. Usually, yeast used for re-pitching will have over 95% living cells. The number of living cells are reduced by poor storage conditions or excessive acid washing. Under pitching the wort will result in a slower fermentation which can lead to:

1. Longer lag phase before fermentation starts (this may also give bacteria an opportunity to thrive and grow before true fermentation gets going)

2. Inadequate attenuation leading to incomplete utilisation of wort sugars and higher than specification final gravity.

3. Low yeast count at the end of fermentation resulting in longer diacetyl stand and potential flavour problems.

4. Because of the longer fermentation cycle yeast health may suffer resulting in a poorer fermentation when the yeast is used again.

But how do we control yeast count at pitching? There are several techniques which can be used to calculate pitching rate. The four most common ones are:

Yeast Count Using A Microscope

The slurry is carefully diluted and transferred to a hemacytometer cell where the number of yeast are counted. This is the standard reference method and by knowing the dilution is possible to calculate the total yeast in the slurry. The pros of this method are that it is simple to use, requires a low investment and results can be obtained quickly. The respective cons here are that it is open to human error, difficult to interpret and the process measures all cells including dead cells. Results need correcting for viable cells only.

To correct for yeast viability the sample is stained with methylene blue where dead cells stain blue under the microscope and live active cells are colourless.

Fluorescent Staincellometer

The NucleoCounter® YC-100™ is an automated cell counter which uses a camera to count the total number of yeast cells and a fluorescent dye, propidium iodide (PI) which is a nuclear staining (nucleic acid binding) dye that enters dead cells with compromised membranes and identifies the dead cells give an accurate measure of live yeast cells.

While this is simple, quick and reliable, it also requires the use of an expensive instrument and also requires calibration.

Yeast Cell Culture

A slower method involves plating out a sample and growing and counting the number of yeast colonies. Living cells with grow showing viable colonies only. Using slide culture results are available after 18 hours while using plate culture it takes three days. While it delivers accurate results, it can be affected by growth conditions while flocculant strains can aggregate giving a false count.

Settlement Of Yeast Pellets

Commonly used in conjunction with a centrifuge it gives a measure of the wet yeast solids as a sediment and can be calibrated to give a reasonable approximation of yeast cells as shown below with results from Wyeast Labs USA. While this method is rapid and simple, it also requires calibration.

Biomass Probe

Yeast Capacitance (Kell Cell) Live cells (with complete plasma membranes) produce electrical capacitance when placed in an electric field. The total number of live cells are counted as they pass the probe. It works by putting an electrostatic charge onto each living yeast cell and then recoding the number of pulses as it passes a sensor.

This option is very accurate for living cells (intact plasma membranes). It also gives a total number of viable cells pitched while being rapid and simple. However, it requires calibration and there is a cost involved.

Image copy from Ray Anderson Brew Hist 121 pp 6 2005

Yeast Propagation

Up until the end of 19th Century, Brewers either relied on the natural floral in the Brewery or back slopping (re-pitching) as the source of yeast in fermentation.

Carlsberg Brewery in Copenhagen had sourced their yeast from Spaten Brewery in Munich but in 1883 it developed “yeast sickness” and investigations showed it was due to a major infection by a wild ellipsoid shaped yeast.

The chief microbiologist Emil Christen Hansen was able to separate the two yeasts and developed industrial equipment to grow up (propagate) the pure yeast culture which quick spread and adopted by brewers around the world.

Repeated cropping and re-pitching of yeast can lead to poor fermentations and pick up of yeast and bacterial contamination. To reduce this risk a new yeast culture should be grown. The reason for yeast propagation include:

1. Eliminating mutant yeast cells

2. Eliminating bacterial infection which can produce ATNC (nitrosamine control).

3. Replacing ageing yeast cells which cause slowing down of rate of reproduction.

4. Replacing ageing yeast cells which have changes to cell surface effecting flocculant behaviour.

5. Replacing ageing yeast cells which often show different metabolic behaviour and flavour of beer.

6. Removing dead and dying cells which contribute proteases which attack foam proteins & produce unwanted flavour notes.

7. There is a reduction in viability & vitality over time - re-establish strain proportions.

The yeast cultures used for propagation are usually stored in a central location under liquid nitrogen at -1960C to reduce any deterioration to the yeast cells. This culture is used to produce yeast slopes which can be used by the Brewery Laboratory to prepare yeast for propagation. These slopes have up to 6 months shelf life un-opened in a standard fridge and a scrape of yeast cells are removed to produce a start culture.

When planning yeast pitching or propagation system it is necessary to grow the yeast up incrementally. Usually the first stages are under aerobic conditions which encourages yeast growth and yields higher concentrations of yeast cells per ml when compared to fermentation conditions.

Yeast from long term storage is replated onto an agar slope for immediate use and stored for up to two weeks in a fridge.

A scrape of yeast from the agar slope is transferred to sterile wort and shaken to allow them to grow.

The first stages are carried out in the laboratory starting from a slope, and after around 20 litres of culture has been produced it is transferred to the Brewery propagation plant.

The last stage of yeast propagator is usually run near pitching temperature (to avoid thermal shock) and as a fermentation to retain the flavour profile of the beer.

It is usual to transfer the whole content of each vessel to the next vessel which should contain sterile wort and the transfer should be made while the yeast is still actively budding to eliminate the lag phase.

Yeast growth is carried out in normal sterile brewer’s wort with the addition of zinc salts for yeast growth and antifoam to prevent excess fobbing during aeration.

Yeast should be transferred during active growth (budding) to maximise growth and to avoid it progressing to stationary phase.

Fermentations can be pitched with yeast recovered from a previous brew or with a new batch of yeast grown up from a culture in the brewery or commercial dried or stabilised liquid culture. The yeast cells must be thoroughly mixed with the cooled and aerated wort from the brewhouse.

Typical pitching rates: u Ale yeast generations 8 - 20 or even 100’s u Lager yeast generations 4 - 16, average 4 – 8 u Dried yeast is generally only used once and sent to waste.

Further Reading

1. Hough, Briggs and Stephen “Malting and Brewing Science”

2. Lewis and Young “Brewing”

3. Ray Anderson The Transformation of Brewing: An Overview of Three Centuries of Science and Practice Brew Hist Soc 121 pp 5 – 24 2005

4. Dr Hilary - Yeast Propagation Brewers Guardian Articles Yeast Series October 1997

5. Tim O’Rourke – Yeast Fermentation –Brewers Journal B2N7 2022

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