Beer Spoilage Organisms

Microbial contamination can originate from a variety of sources in the brewing process. Raw materials, air, brewing water, additives, and even pitching yeast can act as a constant supply of contaminants. Residues remaining in brewhouse tanks, pipelines, valves, heat exchangers, and packaging equipment harbor microorganisms too that represent a potential source of recontamination. Some of the effects of contamination range from comparatively minor changes in beer flavor and fermentation performance to gross flavor and aroma defects, turbidity problems, abnormal attenuation rates, and reduced yeast crops.

A number of microorganisms have been reported to be beer spoilage microorganisms, among which bacteria, as well as so-called wild yeast and molds.

Bacteria

Beer is a poor and rather hostile environment for most microorganisms. Its ethanol concentration and low pH is lower than most bacteria can tolerate for growth. Furthermore, the high carbon dioxide concentration and extremely low oxygen content makes beer a near to anaerobic medium. Beer also contains bitter hop compounds, which are toxic. Only a few bacteria are able to grow under such inhospitable conditions and are able to spoil beer.

• Gram Positive Bacteria

Gram-positive bacteria are generally regarded as the most threatening contaminants in the brewery because of their rapid growth rate and tolerance to high temperatures and low pH conditions. Most hazardous microorganisms are those belonging to the genera Lactobacillus and Pediococcus and are often referred to as lactic acid bacteria because of their propensity to produce lactic acid from simple sugars.

• Gram Negative Bacteria

Important Gram-negative contaminants in the context of beer brewing are acetic acid bacteria, Zymomonas spp., Pectinatus spp., and various Enterobacteriaceae. Several members of this group not only distort the fermentation process or produce undesired by-products but also have been reported to survive the fermentation process and to transfer into the finished product.

Wild Yeast

Wild yeast is any yeast other than the pitching yeast. Wild yeast can be isolated at all stages of the brewing process from raw materials, wort, pitching yeast, and fermenting beer, through to the packaged product and the dispense system. Wild yeast can produce unintended flavors because of differences in ester, fusel alcohol, and diketone production. They are particularly known for producing phenolic or medicinal notes. In the presence of air, some wild yeast can grow rapidly and form a film on the surface of the beer, which can cause haze. Other effects may include primary yeast fermentation and separation difficulties, significantly lower terminal gravities, and a higher, alcohol content in the finished beer. The lower terminal gravities are due to the ability of wild yeast to ferment sugars (such as maltotetraose and dextrins) not used by the primary yeast.

Molds

Molds are non-chlorophyll-bearing plants that range in size from a single spore to large cell aggregates. Commonly occurring genera are Mucor, Penicillium, Aspergillus, Cladosporium, Geotrichum, and Rhizopus. Most molds are able to grow well at ordinary temperatures, the optimum range being from 25 to 30°C, although some species can grow at 35°C or even higher temperatures and others at much lower temperatures. Molds are normally aerobic organisms and can grow over a wide pH range, although most species prefer an acid pH.

Microbiological Quality Assurance

All breweries need quality assurance to maintain confidence in the beer they produce. It is required for a variety of functions from checking the quality of the raw materials, through monitoring beer production and packaging operations, to checking final product quality.

There are essentially two approaches to microbiological testing, the conventional techniques that involve inoculating a solid or liquid medium with a brewery sample, and after incubation, examining for the presence of absence of growth. General-purpose media for the cultivation and identification of microorganisms can be prepared from wort or beer but commercial media provide for consistency and ease of use.