Beer Spoilage Organisms
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. These bacteria include both Gram-positive and Gram-negative species as listed in Table 19.2.
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.
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