Microbial contamination can arise from various sources during the brewing process. Raw materials, air, brewing water, additives, and even pitching yeast can serve as constant suppliers of contaminants. Residues left in brewhouse tanks, pipelines, valves, heat exchangers, and packaging equipment can also harbor microorganisms, posing a potential risk of recontamination. The effects of contamination range from minor changes in beer flavor and fermentation performance to significant defects in flavor and aroma, turbidity issues, abnormal attenuation rates, and reduced yeast yields.
A number of microorganisms have been identified as spoilage agents in beer, including bacteria, wild yeast, and molds.
Bacteria
Beer is generally an inhospitable environment for most microorganisms. Its ethanol concentration and low pH levels are usually too low for bacterial growth. Additionally, the high carbon dioxide concentration and extremely low oxygen content make beer almost anaerobic. Beer also contains bitter hop compounds, which are toxic. Only a few bacteria can survive under such conditions and have the ability to spoil beer.
Gram-Positive Bacteria
Gram-positive bacteria are considered the most concerning contaminants in breweries due to their rapid growth rate and tolerance to high temperatures and low pH conditions. The most problematic microorganisms belong to the Lactobacillus and Pediococcus genera, commonly known as lactic acid bacteria because they produce lactic acid from simple sugars.
Gram-Negative Bacteria
Acetic acid bacteria, Zymomonas spp., Pectinatus spp., and various Enterobacteriaceae are important Gram-negative contaminants in beer brewing. Some members of this group not only disrupt the fermentation process or produce unwanted by-products but can also survive fermentation and transfer into the final product.
Wild Yeast
Wild yeast refers to any yeast other than the pitching yeast. Wild yeast can be present in all stages of brewing, from raw materials and wort to pitching yeast, fermenting beer, packaged product, and dispense systems. It can result in unintended flavors due to variations in ester, fusel alcohol, and diketone production. Wild yeast is 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 beer’s surface, leading to haze. Other effects may include difficulties in primary yeast fermentation and separation, significantly lower terminal gravities, and higher alcohol content in the finished beer. The lower terminal gravities are attributed to the wild yeast’s ability to ferment sugars not utilized by the primary yeast, such as maltotetraose and dextrins.
Molds
Molds are non-chlorophyll-bearing plants that vary in size from single spores to large cell aggregates. Common genera include Mucor, Penicillium, Aspergillus, Cladosporium, Geotrichum, and Rhizopus. Most molds thrive at ordinary temperatures, with the optimal range being 25 to 30°C. Some species can grow at 35°C or even higher temperatures, while others can grow at much lower temperatures. Molds are typically aerobic organisms and can grow over a wide pH range, although most species prefer an acidic pH.
Microbiological Quality Assurance
All breweries require quality assurance to ensure confidence in their beer production. It encompasses various functions, including assessing the quality of raw materials, monitoring beer production and packaging operations, and evaluating the final product’s quality.
There are two main approaches to microbiological testing: conventional techniques involving inoculating a solid or liquid medium with a brewery sample, and after incubation, examining for the presence or absence of growth. While general-purpose media can be prepared from wort or beer for cultivating and identifying microorganisms, commercial media provide consistency and ease of use.
