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Combating beer pests

Different types of microorganisms cope more or less well with the conditions in beer and can affect the appearance, smell and taste of the beer. Dr. Christian Dekant gives an overview of the most common beer pests and recommendations for action in case of microbiological problems.

Two hands with disposable gloves hold a petri dish with pink liquid The brewery laboratory tracks down beer pests (Picture: Drew Hays, Unsplash)

What to do in case of microbiological problems?

Harmful organisms do not find it easy to survive and multiply in beer. The so-called self-protection of beer is related to the low pH value around 4.5, anaerobic atmosphere (CO2), the alcohol content of around 5 percent by volume, hop bitters with antibiotic effect and the lack of nutrients such as sugar and amino acids. Pathogenic germs and heat-resistant germs hardly play a role in the brewery environment and in the beer because of these factors. Therefore, the brewer can achieve microbiological safety with relatively low temperatures during pasteurisation.

A petri dish in which various microorganisms have grownThe microflora in a brewery should be kept under close observation and checked regularly to detect beer pests as early as possible (Picture: Michael Schiffer, Unsplash)

Hazard due to biofilms


However, biofilms are a potential problem for the quality of the beer. Pioneers such as slime-forming acetic acid bacteria nest in damp places in the production area.

In their wake, other beer pests take advantage of the environmental conditions that are favourable for them and often oxygen-free inside the biofilms.

After all, these germs adapt to the beer environment and can cause problems during production. While germs floating freely in the beer can be killed relatively easily by pasteurisation, this is much more difficult with biofilms.

The complex society of different species in a biofilm creates an effective protection against heat and conventional cleaning agents by secreting a slimy matrix of polysaccharides. An already established biofilm can often only be controlled by scrubbing. 

Causes and recommendations for action in the case of sensory problems in beer


First of all, you should get an overview of possible causes and recommendations for action in case of microbiological problems, because this is the only way to know which beer pest you are dealing with. Or whether you should pay a little more attention to your yeast [2]. 

Both beer pests and neglected yeast can be behind sensory problems. 

The causes of diacetyl in beer can be, for example, poor vitality of the yeast, a yeast that is too old or has been pitched too often, or yeasts that have been stored too warm or for too long. The countermeasure is quite simple: use a young, vital yeast for pitching.

Contamination with pediococci or carry-over through filters also leads to diacetyl in the beer. You can remedy this by renewing the yeast and analysing the weak spots in the unfiltered beer. 

Contamination with lactobacilli, secondary contamination at the filler or contaminated dosing yeast in the case of Hefeweißbier can be remedied by cleaning the filler or capper or by replacing the dosing yeast.

Gram-negative Enterobacteriaceae in the operational yeast, too warm or long storage of the yeast or aeration of the yeast are also possible causes of diacetyl in the beer. Here, too, it helps to renew the yeast, store it in a cool place and aerate it only when brewing. 

A slow diacetyl reduction can also have its cause in a poor vitality of the yeast, a yeast that is too old, run too often, stored too warm or too long. Here, too, it is recommended to use young, vital yeasts and to store the harvest yeast in a cool place at 0-3 °C. 

DMS in beer is usually due to contamination of the wort or yeast with enterobacteriaceae. Particular weak points for this are the wort filter and the flotation tank. Here it helps to check the wort path, the wort aeration and other installations, and to clean the dome and level indicator of the flotation tank. 

In the case of a broad after-bitterness and empty beers, it is advisable to eliminate weak points at the filler or in the beer path from the filter. The cause is often contamination with over-fermenting yeast such as S. distaticus.


Causes and recommendations for action in case of problems with fermentation


The cause of poor fermentation can be zinc deficiency, the zinc content of the wort should be above 0.15 mg/l. If this is not the case, measures such as biological mash acidification should be taken.

If the desired Es is not reached, it is usually because the yeast is aged or stressed or has poor vitality (ICP>6). To remedy the problem, improve the yeast technology (dead yeast cells with methylene blue staining below 4 %).

If the fermentation runs smoothly, but the fermentation activity drops significantly afterwards, the cause is usually diauxia, i.e. a lack of zinc, amino acids, biotin and easily usable sugars (glucose-maltose ratio). Here, too, one should check the zinc content of the wort and raise the FAN content, for example by a mash rest at 48 °C.

A poor yeast harvest, usually linked to a longer fermentation time, is due to poor yeast vitality or because bottom-fermenting yeast remains in suspension for a long time with moderate CO2 production. Here, the brewery should improve its yeast management.

Poor filterability may be due to poor yeast quality, autolysed cells as well as an unsuitable yeast pump or centrifuge. As countermeasures, the yeast vitality should be checked and the repumping process controlled during assimilation or propagation. The correct dimensioning of the pump is important here. 

Poor flavour and foam stability, lingering protein bitterness and a sulphur taste on the palate in fresh beer can also be caused by poor yeast quality. A release of proteins, fatty acids and proteinases through autolysis is also suspected. Here, too, an improvement of the yeast quality is advisable.

Gas formation and solvent-like taste in the soursop can be caused by Candida kefyr and C.magnolidae. They form CO2 and ethyl acetate ("glue" smell). Raising the temperature in the sour product to 50 °C over several passages and CO2 fumigation can remedy the situation. 

Classification of beer pests

If we take a closer look at beer pests [2], we find that the obligate beer pests are the most dangerous for beer quality. They do not mind the selective properties of the beer, they can usually multiply in the beer without adaptation.
Potential beer pests can cause problems in beers where self-protection is less pronounced - for example, at lower final fermentation levels, lower alcohol content or higher pH values. Or they are able to adapt to the conditions in the beer after some time.
Indirect beer pests, on the other hand, cannot multiply in the bottled beer, but they can already cause problems during production and infest the unfermented wort or the yeast.
In addition to the microorganisms that directly damage the beer, a number of indicator organisms are also important in the microbiological operational control of the brewery, as they are much easier and less complicated to cultivate than the "real" beer pests, which are often only present in traces.
Indicator organisms do not directly affect the quality of the beer, but if they can be detected, microbiological problems can be identified at a very early stage and nipped in the bud.

Primary and secondary contamination


In order to determine the measures to be taken to combat any microbiological problems that may arise, it has proven helpful to further divide harmful organisms into primary and secondary contaminants.

Primary contaminants already occur in the production area. Under certain circumstances, the germs can reach the bottled beer via the filter. Examples are Pediococcus damnosus and Lactobacillus lindneri. A problem for the detection is the basically very large thinning out of these germs through the filter. This is why the test results are often inconclusive, and the incubation period in the bottled container can be several weeks. By the time the brewery is made aware of a possible problem through the first complaints, the germs may have already established themselves in the entire production area.

Secondary contamination, on the other hand, only occurs in the filling area, preferably around the filler. In the wet environment (with beer contact), biofilms can easily form there. Parts of these biofilms can be thrown into open bottles by rotating machines, for example. Examples of such secondary contaminations are Lactobacillus brevis, L. casei, Pectinatus sp. or Megasphaera sp.


Extraneous yeasts


The topics of harmful yeasts, wild yeasts, foreign yeasts would go beyond the scope of this article, so we refer here to an article by Dr. Mathias Hutzler and Prof. Fritz Jacob from the Weihenstephan Research Centre, TU Munich, published in BRAUWELT No. 10, 2020, pp. 251-255.


Weakness analysis


In order to track down beer pests, it is advisable to regularly analyse the weak points in the farm. Such a weak point analysis in the bottle cellar can be carried out in an uncomplicated manner in sampling and cultivation with sterile swabs [1]. For this purpose, production staff take swabs from about 30 direct and indirect contact surfaces with beer (stars, valves, cavities, etc.).

A positive result is obtained when the colour of the nutrient solution changes after three days of incubation. A microscopic check of the results is not necessary at first, because a positive result can be generally interpreted as the formation of a relevant biofilm. Then it's time to scrub ...



1. Dekant, C.: „Was tun bei mikrobiologischen Problemen?“, GradPlato, 21.10.2020, URL: was-tun-bei-mikrobiologischen-problemen?
2. Back, W.: Ausgewählte Kapitel der Brauereitechnologie, Kapitel „Mikrobiologie“, Fachverlag Hans Carl, Nürnberg, 2008.
3. Hutzler, M.; Jacob, F.: „Schadhefen, wilde Hefen, Fremdhefen“, BRAUWELT Nr. 10, 2020, S. 251–255, URL: Schadhefen, wilde Hefen, Fremdhefen