Beer Spoilage Organisms
Spoilage by 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.
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. In traditional brewing practices Lactobacillus and Pediococcus cause 90 percent of the beer-spoiling incidents, and they can be found throughout the brewing process.
Lactobacillus has been found in late fermentation, conditioning beer, pitching yeast, and packaged beer. Priest believes that Lactobacillus spp. is more dangerous during conditioning and packaging (Priest, 1987). Lactobacillus spp. is found in dried yeast deposits in fermenters and in dried wort droplets on equipment that has not been cleaned thoroughly. Malt husks are another source of lactic acid bacteria.
L. pastorianus. L. pastorianus is generally considered to be the most typical of the beer spoilage Lactobacillus spp. Like other rod-shaped bacteria, L. pastorianus can cause a silky appearance to contaminated beer.
L. delbrueckii. L. delbrueckii grows very quickly in unhopped wort and causes rapid spoilage of mash tun worts. It metabolizes glucose and yields only lactic acid.
Pediococcus is one of the most well-known beer-spoiling bacteria. They are found at many stages in the brewing process from wort through packaged beer. Beer spoilage caused by Pediococcus spp. is characterized by acid production and more notably the formation of high concentrations of diacetyl, accompanied by a reduction in yeast growth and low fermentation rates (Briggs et al., 1982).
P. damnosus. P. damnosus is one of the most common species found in beer. Due to its ability to grow even at low temperatures, spoilage can occur in the fermentation and maturation stage of beer production as well as in bottled products. Pediococcus contamination is also a problem in pitching yeast, as the bacteria is thought to bind to the yeast cell and is difficult to remove.
Important Gram-negative beer spoilage bacteria include acetic acid bacteria (Acetobacter, Gluconobacter) certain members of the family Enterobacteriaceae (Escherichia, Aerobacter, Klebsiella, Citrobacter, Obesumbacterium) as well as Zymomonas, Pectinatus, and Megasphaera. 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.
Acetic Acid Bacteria
The relevant acetic acid bacteria are rod shaped aerobic bacteria from the genera Acetobacter and Glucanobacter, which are both are capable of converting ethanol into acetic acid. Acetobacter is responsible for ropiness of beer whereas Glucanobacter causes a cider-like aroma and prefers high gravity environments.
Acetobacter spp. Acetobacter oxidizes ethanol into acetic acid, otherwise known as vinegar, the primary off-flavor caused by infection. Acetobacter can also oxidize acetate to carbon dioxide and water. Acetobacter is hop-insensitive, non-sporing, and very acid-tolerant. The surface contamination they cause is often apparent as an oily or moldy film.
Gluconobacter spp. Gluconobacter are Gram-negative obligate aerobes and develop best in worts exposed to oxygen including pitching yeast, balance lines, draft lines, cask beers, and barrels. They also oxidize ethanol into acetic acid, but unlike Acetobacter, they do not produce carbon dioxide through oxidation of acetate.
Enterobacteriaceae are a large family of Gram-negative, often rod-shaped, facultative anaerobes including the genera Obesumbacterium, Citrobacter, Enterobacter, Hafnia, Klebsiella, Rahnella, Salmonella, Serratia, and Shigella. They are often referred to as coliforms. They can grow in the presence or absence of oxygen, which in the context of brewing increases the number of locations where spoilage can occur.
Obesumbacterium proteus. Obesumbacterium proteus is a very common Gram-negative beer spoilage bacterium and poses a big risk in breweries. It is generally considered a wort spoiler by producing diacetyl and dimethyl sulfide, and it may impart a very noticeable parsnip odor to the wort. O. proteus develops only during the early stages of fermentation. O. proteus is known to stall fermentations, leading to long, slow fermentations and eventual under attenuation. O. proteus can reduce nitrate to nitrite, which results in a decrease in fermentation rate, a reduction in yeast crop, a slowing in the rate of pH fall, and an increase in both beer color and vicinal diketone content. O. proteus is tolerant up to 6 percent ABV (Jorgenson, 2017)
Pectinatus species are Gram-negative, obligate anaerobes, which means that they are able to thrive in both aerobic and anaerobic environments, with contamination indicated by turbidity and high amounts of acetic acid, propionic acids, and acetoin, resulting in a sour rotten egg aroma and flavor.
Megasphaera is found in wort and lower-alcohol packaged beer. Beer spoilage caused by this organism results in a similar extreme turbidity as Pectinatus and the production of considerable quantities of butyric acid together with smaller amounts of acetic, isovaleric, valeric, and caproic acids.
Zymomonas spp. is a major anaerobic beer-spoiling bacterium that is characterized by strong turbidity, acetaldehyde, hydrogen sulfide, and over-carbonation.
Z. anaerobia. Zymomonas anaerobia is described as the most dangerous of Zymomonas spp. since it can cause spoilage within a relatively short time.
Z. mobilis. Z. mobilis grows under micro-aerophilic conditions. Spoilage in beer can occur within a few hours; however, the organism is comparatively rare.
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