Despite the low pH, high ethanol content, and hop antimicrobials ordinarily present in beer, microorganisms are responsible for many (but certainly not all) of the defects that occur in beer. Chemical and physical defects are also common and can cause significant problems for brewers. However, preventing or minimizing entry and growth of microbial contaminants throughout the beer-making process is absolutely essential for consistent manufacture of high quality beer. This is no simple matter, because fungi, wild yeasts, and bacteria are naturally present as part of the normal microflora of the raw ingredients, the brewery environment, and the brewing equipment. Moreover, even if a heat or filtration step is included at the end of the beer-making process, the damage may have already been done.
Although the water activity of stored barley is too low to support the growth of fungi, yeasts, and bacteria, those organisms begin to grow as soon as the barley is steeped, with increases of about 1 to 3 logs. Kilning eventually inactivates some microorganisms; however, the finished malt may still contain as many as 106 bacteria, 103 fungi, and 104 yeast per gram.Various species of lactic acid bacteria that account for the majority of bacteria found in malt and that may cause several serious defects in the finished beer are particularly important (discussed below). However, even fungi that are present at relatively low levels can cause problems. For example, some strains of Fusarium can infect barley in the field and produce the toxin deoxynivalenol. Aside from its possible toxicity to humans, deoxyni-valenol is correlated with the "gushing" defect in packaged beer. Gushing is characterized by excessive gassing or over-foaming when packaged beer is opened. Although the exact cause of gushing is not clear, it appears that deoxyni-valenol or another mold-produced product (usually from Fusarium, but other fungi may also be involved) serves as a seed or nucleation site, at which carbon dioxide microbubbles are formed.
Even though the beer fermentation requires yeast, it is also true that wild yeasts can cause several different types of product defects. Wild yeasts, including species of Zygosaccharomyces, Kluyveromyces, and other Saccharomyces, may compete with brewing yeast during the fermentation, and, in some cases, reach high levels. Since these yeasts typically do not flocculate as do brewing yeasts,they remain in the beer, creating cloudiness and filtration problems. Some of these yeasts also produce killer toxins that can potentially inhibit the brewing yeast culture and dominate the fermentation. Other yeasts, referred to as diastatic yeasts (of which Saccha-romyces cerevisiae var. diastaticus is the most well-known), can hydrolyze and ferment dextrins, and are particular post-fermentation problem contaminants for two main reasons. First, they often produce phenolic off-flavors in the beer, often characterized as medicinal. Second, by fermenting the body-forming dextrins, they make the beer "thinner." Finally, aerobic yeasts, which include members of the genera Pichia, Debaryomyces, and Brettanomyces, can produce acetic acid, volatile esters and alcohols, and other oxidized compounds, and cause turbidity problems.
Although high mashing temperatures (along with low pH) restrict growth of most bacteria, some heat-tolerant bacteria can survive and grow during the mashing step. However, far more serious are those bacteria that contaminate the wort after the kettle boil step and grow in the beer during the fermentation and post-fermentation steps. Lactic acid bacteria represent the most bothersome organisms in the brewing operation and are the cause of the most common defects. Of particular concern are species of Lactobacillus, including Lactobacillus brevis, Lactobacillus casei, Lactobacillus plantarum, and Lactobacillus del-brueckii.
During growth in wort, these bacteria ferment wort sugars and produce several undesirable end-products, especially lactic acid. Of these bacteria, L. brevis is heterofermenta-tive, meaning that it produces acetic acid, CO2, and ethanol, in addition to lactic acid. Some lactobacilli produce 3-hydroxypropi-onaldehyde, which serves as a precursor for synthesis of acrolein, a compound that imparts a bitter flavor. Importantly, species and strains of Lactobacillus, as well as other lactic acid bacteria—most notably Pediococcus acidilactici, Pediococcus inopinatus, and Pe-diococcus damnosus—are responsible for perhaps the most objectionable defect in beer, namely, the production of diacetyl.
Diacetyl is a 4-carbon molecule that imparts a buttery flavor and aroma that, while pleasant in cultured dairy products, is highly offensive in beer. Pediococci (and P. damnosus, in particular) produce appreciably more diacetyl than other lactic acid bacteria and are perhaps the most serious microbial contaminants that affect beer quality. They are capable of growth over a wide range of temperatures, including those used during both the ale and lager fermenta-tions.These bacteria are also tolerant of low pH, ethanol, and, depending on strain, hop a-acids.
The diacetyl defect caused by these bacteria (referred to as "sarcina sickness" due to the original classification of pediococci as belonging to the genus, Sarcina) is detectable at concentrations as low as 0.2 ppm (or 0.00002%). In addition to flavor defects, some pediococci and lactobacilli are also capable of producing extracellular polysaccharides that cause a ropi-ness defect. This ropy or "slime" material increases beer viscosity and gives the beer an undesirable mouth feel.
Although the lactic acid bacteria are arguably the most serious spoilage organisms in beer, Gram negative bacteria also can contaminate beer and cause spoilage problems. In fact, the spoilage organisms identified by Pasteur were probably species of the genus Acetobacter, Gram negative bacteria that oxidize ethanol to acetic acid. Strains of Acetobacter are used in the manufacture of vinegar, thus, they give beer a highly objectionable vinegary flavor. It is no wonder that the French beer industry had enlisted the technical advice of Pasteur to solve this problem. Acetobacter and the related genus, Gluconobacter, consist of aerobic rods that grow well in the presence of high ethanol concentrations and are resistant to hop a-acids. Because these bacteria grow poorly, if at all, under the anaerobic conditions established during the beer fermentation, they are more likely to be a problem only when the beer is exposed to oxygen, such as during storage of beer in casks.
Other Gram negative bacteria may also occasionally contaminate and spoil beer. Included are Zymomonas, ethanol-tolerant, facultative, or obligate anaerobes associated with ale spoilage, and Citrobacter, Klebsiella, and other bacteria belonging to the family Enter-obacteriaceae. The former are prolific ethanol producers that also make lesser amounts of lactic and acetic acid, acetaldehyde, and glyc-
erol. They may also produce sulfur-containing compounds, such as dimethyl sulfide and dimethyl disulfide. As spoilage organisms, they cause a fruity flavor defect that occurs most frequently in casked beer.
Among the Enterobacteriaceae that are found in beer, the most common is Obesum-bacterium proteus. However, its role in beer spoilage is not clear. It reportedly interferes with normal growth of the yeasts, leading to poor attenuation. The anaerobic Gram negative rods include species from the genera Pectina-tus, Selenomonas, and Zymophilus. They are capable of producing organic acids, H2S, and turbidity in packaged beer.They are easily killed by heat, thus they probably gain entrance into beer during post-pasteurization packaging.
Beer spoilage does not occur only by microorganisms; a number of chemical and physical defects are also common in beer. One of the more frequent defects (although less so in recent years) is referred to as "skunky" beer flavor or light-struck flavor. It is caused by sunlight-induced photooxidation. This defect, which can range from slight to nauseous, is most noticeable in beer bottled in clear glass. The most likely mechanism by which this defect occurs is via ultraviolet or sunlight-induced cleavage of side chains on hop-derived iso-a-acids. These reactive side chains then react with sulfur-containing compounds to form products with a skunky aroma. One thiol in particular, 3-methyl-2-butene-1-thiol or MBT, has a low flavor threshold (just a few ppb) and is characteristic of this defect. Given the potential for beer to become skunky, how is it that there are still many popular beers packaged in clear glass? In general, these beers are "skunk-proofed" to prevent the light-induced reactions described above. Specifically, hop extracts (obtained via either liquid or supercritical CO2 extraction) are used that are processed such that the iso-merized iso-a-acids are converted to a reduced form. Depending on the extent of the reduction (i.e., how many hydrogen atoms are incorporated), dihydro-, tetrahydro-, or hexa-hydro-iso-a-acids are formed. Importantly, these modified iso-a-acids are very light-stable and non-reactive.
Another flavor defect is referred to as stale or oxidized. Stale beer is variously described as having cardboard-like, rotten apple, cooked, or toffee-like flavors. It is invariably caused by autooxidation reactions, not unlike the staling reactions that occur in other foods. Staling is generally time-dependent and increases during storage. Therefore, it is often the main factor that determines shelf-life of beer.The best way to control staling is simply to keep oxygen out of the finished beer.
Among the physical defects in beer, perhaps the most common is haze, which, as discussed above, occurs during cooling, and is caused primarily by proteins that form complexes with polyphenols and carbohydrates. Treatment with proteinases can reduce this problem. In addition, p-glucan-containing polysac-charides can also cause haze, gel, and filtration problems, as well as increasing viscosity. Bacterial p-glucanases can be added to the mash or wort to degrade these materials. Foam instability problems are also common and result in loss of foaming or head. Adding proteins, hop components, and gums increases viscosity and surface tension, and can therefore stabilize foaming.
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