Lesson 6 - Microorganism Detection and Control

The various stages of wort and beer production offer both opportunities and challenges for microorganisms. In this final section we consider their survival and proliferation in terms of the physical processing environment and products produced in brewing.

Depending on the stage, microorganism can be affected differently by carbon or nitrogen source, oxygen, hop substances, temperature, pH or alcohol.

Mash

The mashing process produces a nutrient-rich mash in terms of carbohydrate and nitrogen, as their breakdown forms sugars and free amino nitrogen (FAN). Fortunately, the temperature in mashing is sufficiently high and the mashing duration sufficiently short to prevent the proliferation of beer-spoiling microorganisms.

In the event of a plant breakdown or system failure causing temperature reduction in the mash, lactic acid bacteria can survive and grow. This results in acid production, which is encouraged in traditional Reinheitsgebot (‘purity law’) German beer brewing as a means of correcting higher mash pH where high residual alkalinity exists.

In cases of extended mashing, Bacillus can cause acidification and conversion of nitrate to nitrite, a precursor of the carcinogenic nitrosamines. Clostridium can grow in these conditions, producing butyric acid.

Bacterial growth in mash can affect subsequent wort separation due to polysaccharide production.

Wort

After mashing and wort separation, the wort is boiled to effectively sterilise it. At this stage, with a relatively high pH (around 5.3) and an environment rich in sugar, nitrogen, and micronutrients, the wort is an ideal growth medium for most Gram-negative bacteria and the hop-tolerant Gram-positive bacteria, Lactobacillus and Pediococcus. Wort should never be stored at temperatures below 65 °C for any length of time before pitching; even at 50-65 °C, thermophilic lactobacilli (e.g. L. debrueckii) and aerobic, spore-forming bacilli can grow, resulting in prompt acidification.

If enterobacteria such as Klebsiella, Citrobacter, Enterobacter, Obesumbacterium, and Escherichia are allowed to develop in wort, they have several detrimental effects on the flavour of the finished beer. They are Gram-negative, hop resistant, and aerobic, so can thrive in this environment, producing DMS, organic acids, and 2,3-butanediol which result in an unpleasant parsnip or vegetal aroma.

Enterobacteria are also often associated with phenolic off-flavours arising from the decarboxylation of substituted 4-hydroxycinnamic acids from malt. These compounds are reduced by the yeast to yield 4-ethylphenol and 4-ethylguaiacol and corresponding ‘smoky’ taints.

Fermentation

In order to minimise the chance of spoilage, wort is cooled as quickly as possible and pitched with yeast.

The wort is oxygenated and is also a rich source of carbon and nitrogen, as well as micronutrients such as vitamins and minerals. At the start of fermentation, the pH is weakly acidic and able to support both bacteria and wild yeast growth.

Unless the yeast is a pure culture, there is further risk of contamination at this stage. The most common bacterium in British top-fermenting yeast is Obesumbacterium proteus. This organism originates from water, but it is partly adapted to brewery conditions and can be found amongst pitching yeasts after two or three consecutive fermentations. It grows during the early stages of the fermentation but is inhibited by the low pH and rising ethanol concentration 24-36 hours after pitching. However, O. proteus tends to associate with the yeast and rise to the surface of the fermentation with it, consequently being skimmed (harvested) and repitched into the subsequent fermentation.

In low numbers O. proteus has little effect, but as its population increases with successive repitchings it has several detrimental effects, including slowing of the fermentation rate and an increase in the final pH of the beer. Flavour impairment is principally due to the production of DMS and other sulphur compounds.

O. proteus is not prevalent in lager pitching yeasts but Enterobacter agglomerans has been found and has similar effects.

Enterobacteria and other Gram-negative bacteria are rapidly inhibited by the fermentation conditions of alcohol and lower pH, and generally have little effect on the beer. These organisms do not typically associate with the yeast and are not, therefore, returned to subsequent fermentations.

Acetic acid bacteria can sometimes be isolated from the fermentation but the anaerobic conditions that occur within 12 hours of pitching with yeast preclude their growth.

Lactic acid bacteria, principally L. brevis and P. damnosus, are sometimes present in the fermentation. They do not constitute a serious threat at this stage because competition with the yeast limits their growth. Diacetyl, the most potent flavour compound they produce, is reduced by the yeast to the flavourless butane 2,3-diol.

Brewing yeast may mutate, but any derivatives must have a selective advantage over the parent, otherwise they will be rapidly lost. Such mutant strains can be a problem if they are non-flocculent or altered in some fermentative capacity, for example deficient in maltotriose metabolism.

Wild yeasts originating from raw materials, air, or brewery plant may compete with and outgrow the culture yeast. Often these yeasts are non-flocculent and give rise to troublesome hazes, even when present in very low numbers. If the contaminating yeast is very small it can complicate filtration, again causing hazes. S. diastaticus secretes glucoamylase, an exo-amylase that removes glucose residues from dextrins and can lead to over attenuation of beers.

Several yeasts, including Brettanomyces and Candida species, can produce estery and phenolic off-flavours.

Although fermentation may seem to present an easy entry point for brewery contaminants, modern hygiene and quality assurance practices have minimised these threats, and incidences of abnormal fermentations and spoilage are low in modern breweries.

Maturation and packaging

The number of bacteria that can grow in fermented beer is limited due to unfavourable conditions created by brewing yeast: low pH of 4.0-4.5, high ethanol content, absence of readily fermentable carbohydrate, lower nitrogen concentrations, cold temperatures, hop antimicrobial compounds, and anaerobic conditions. Nevertheless, the lactic acid bacteria, principally L. brevis and P. damnosus, can grow in this environment, albeit slowly. If this happens, organic acids and diacetyl are produced and, since there is no yeast in suspension, the diacetyl is not reduced but instead accumulates, imparting a sweet ‘buttery’ flavour to the beer. Some strains of L. brevis and P. damnosus can produce rope in badly contaminated beers (as covered in the previous lesson).

Although enterobacteria do not generally grow in beer, Z. mobilis is a relatively rare contaminant of cask-conditioned ales that has become less frequent with the introduction of metal containers (rather than wood). It grows readily in beer, particularly if added fermentable carbohydrate is present (traditional cask ale primings), and produces sulphurous, estery taints due to the release of acetaldehyde and hydrogen sulphide. Again, in the absence of yeast, these compounds accumulate, seriously impairing the organoleptic qualities of the beer.

Acetobacter and Gluconobacter strains that gain access to the beer during conditioning or storage are unlikely to cause problems so long as anaerobic conditions are maintained.

Should wild yeasts, particularly Saccharomyces species, gain access to maturing beer, this is likely to result in films and hazes, often with estery off-flavours.