Some bacteria are almost invariably present during brewing, having been transported into the brew by air, water, or the yeast culture. Certain bacteria may be present in small quantities without noticeably affecting the finished beer, but small concentrations of other bacteria can quickly ruin it. In some beer styles, bacteria in the ferment give a beer its particular character, but on the whole, bacterial contamination and growth need to be discouraged by strict sanitation. Bacteria are only tolerated in a lager brewery where they are cultured to reduce the mash acidity.
The countless types of bacteria oxidize or ferment a wide variety of organic substances. Fortunately, only a relatively few types of bacteria are encountered during brewing, and no pathogenic bacteria can survive in beer. Bacteria grow on sugar, wort, beer, protein, and hop residues, and even on the yeast. By careful control and strict sanitation, flavor and stability problems caused by bacterial contamination of beer can be kept in check. The brewer must work to eliminate any contaminant, or reduce it to a level where its growth will not appreciably affect the finished beer.
Because bacteria adapt and mutate so readily (far more readily than yeast), they can emerge as the dominant fermentation microbe from a relatively small number of cells. It is also because they mutate so readily that bacteria are so difficult to classify. Although they have a host of characteristics, they are initially categorized by whether or not they are stained by gentian violet (the Gram stain).
The gram-positive bacteria encountered during brewing are the Peptococcaceae (family) Pediococcus (genus) and the Lactobacillaceae Lactobacillus. These are grouped together as lactic acid bacteria, and were formerly referred to as “beer sarcina” (a term more specifically applied to Pediococcus). Both operate anaerobically and ferment simple sugars to lactic acid; they have little effect upon protein.
The several species of Pediococci are strictly anaerobic, globular (“cocci”) bacteria occurring singly, paired, or in cubicle groups called tetrads. They form diacetyl and inactive lactic acid from dextrins and glucose. Heterofermentive strains also ferment maltose, fructose, and sucrose, producing acetic acid as well. In general, the Pediococci produce a disagreeable taste, odor, and turbidity (cloudiness). Contamination is most often from calcified trub deposits on poorly cleaned equipment; it is Pediococcus’s ability to survive even rigorous cleaning, sheltered by “beerstone” deposits that make them the most pernicious brewery contaminants.
The Lactobacillus are the single genus of their family. These long, thin, curved rods occur singly or paired at obtuse angles. Microaerophiles, they form lactic acid by the fermentation of carbohydrates in even oxygen-poor solutions. Although they do not cause odor, they may produce a sour taste and turbidity. Several species of Lactobacillus and Pediococcus also cause a ropy or gelatinous fermentation, or a silky turbulence, formed by the excretion of an extracellular slime. It disappears briefly during stirring, but reforms as chains upon the beer surface.
Lactobacillus delbruckii (pH 5.6 to 5.8, active below 131 degrees F [55 degrees C]) is a heat-tolerant or thermophilic homofermentive species that grows on malt. It is especially well suited to the acidulation of the mash, without producing undesirable flavors or turbidity. It metabolizes glucose and yields only lactic acid. Because it is favored by anaerobic conditions, its growth is encouraged by holding a tight, saccharified mash, closely covered, at above 95 to 120 degrees F (35 to 49 degrees C). This inhibits both aerobic and nonthermophilic bacteria.
When lactic acid bacteria, and especially thermophilic heterofermentive strains, are active in the mash or its extract, they may spoil it by acidification and souring, turbidity, or the formation of off-flavors, most notably a rancid-butter taste from the diacetyl diketone. The same symptoms in cooled wort, in young beer, and sometimes in aged beer are more likely due to contamination by Pediococcus cerevisiae (pH 5.5, active over a wide temperature range, most active at 70 to 77 degrees F [21 to 25 degrees C]) and other nonthermophilic strains.
The thermophilic gram-positive bacteria are inhibited by isohumulones from the hops and usually will not survive in bitter wort or in beer. Lactic-acid bacteria are often contaminants from pitching yeast or from air. They may be the most significant infectious organism in the fermentation.
The gram-positive bacteria are rarely a problem in the aged beer because they have highly complex nutritional requirements and are inhibited by hops. During fermentation, the yeast will have absorbed many of the essential amino acids, making them unavailable to the lactic acid bacteria. Only when the proteolysis and precipitation of protein has been poor, when aging beer is not separated from deteriorating yeast sediment, or when temperature shock to the yeast causes it to autolyze will the bacteria be able to obtain enough amino acids to support reproduction.
The most significant gram-negative bacteria commonly affecting the lactic-acid mash is the coliform Clostridium butyricum (butyric acid bacteria). These are thick, principally anaerobic rods that putrefy the mash by forming rancid-smelling ethylacetic acid (butyric or butatonic acid). They are inactive above 112 degrees F (45 degrees C) and very active below 104 degrees F (40 degrees C). This spore-forming bacteria may also occur in beer.
Acetic-acid bacteria sometimes taint mashes that come in contact with the air below 122 degrees F (50 degrees C) (they are quite active below 95 degrees F [35 degrees C]), but they present a far greater danger to fermenting beer. The acetic-acid bacteria are active over the entire pH range of the brewing cycle and are not inhibited by isohumulone from the hops. They are strong oxidizers and are usually responsible for any overwhelming sour-fruit or vinegary taste and odor, and oftentimes turbidity. The surface contamination they cause is often apparent as an oily or moldy (pellicle) film. They are usually introduced to the ferment during racking; aeration of the beer by rousing or splashing provides them with sufficient oxygen for respiration. Active yeast in kraeusen beer, when added during racking, can consume the dissolved oxygen quickly enough to prevent their growth, but in racking “quiet” beer, it is imperative that all aeration be avoided. Dispensing equipment should be given frequent and complete sterilization, as it is also a point of contamination. Sour-tasting or -smelling draft beer has almost certainly been contaminated by these bacteria. The lactophilic Achromobacteraceae Acetobacter (significantly A. aceti and A. suboxydans) oxidize ethanol to acetic acid. They are short, chain-forming, ellipsoidal-to-rod-shaped aerobic bacteria. The glucophilic Pseudomonodaceae Acetomonas (Gluconobacter) excrete vinegar and gluconic acid. They are short, rod-shaped-to-ovoid, polarly flaggelated aerobic bacteria occurring singly, paired, or in chains. Achromobacter and Pseudomonas are infrequently encountered, and then only in sweet wort, because they are acid intolerant and inhibited by alcohol.
The coliform bacteria (termobacteria) commonly taint the wort by adding a dimethyl-suphide-related cooked- or spoiled-vegetable odor, caused by very rapid metabolism of wort sugars. They are waterborne, nonsporulating aerobes and faculative anaerobes most active at 98.6 degrees F (37 degrees C). Enterobacteriaceae Escherechia are straight rods occurring singly or in pairs. Enterobacteriaceae Klebsiella (Aerobacter) are nonmotile, encapsulated rods occurring singly, paired, or in chains. The cooled wort is an ideal medium for their growth. Aerobacter aerogenes is a commonly encountered source of pungent, vegetable, or sulfuric taste and ropy fermentation in both wort and green beer. They adapt and reproduce far more quickly than culture yeast. It is essential that the starter or kraeusen beer used to inoculate the wort be strongly fermenting and of similar temperature and composition to the wort into which it will be pitched. Otherwise coliform bacteria may become strongly established in the lag phase.
The source of these coliform bacteria is most commonly rinsing water. Although they are active over a wide temperature range and are unaffected by hop resins, most are inhibited below pH 4.4 and are not commonly encountered during the later stages of brewing.
| Brewing Contaminants | |||
|---|---|---|---|
|
Brewing Stage |
Symptoms |
Bacteria Responsible |
Solution |
|
Mash, at below 140°F (60°C) |
Acidity, sourness, turbidity |
Thermophilic lactic acid bacteria |
Raise temp. to above 131°F (55°C) |
|
Rancid odor |
Butyric acid bacteria |
Raise temp. to above 112°F (45°C) |
|
|
Sour, vinegar taste and odor |
Acetic acid bacteria |
Raise temp. to above 122°F (50°C) |
|
|
Cooled Wort |
Fruity or vegetable odor |
Coliform bacteria |
Pitch quickly |
|
Primary Fermentation |
Celery odor |
Hafnia protea |
Go to new yeast culture |
|
Secondary Fermentation |
Sour taste, silky turbidity |
Lactic acid bacteria |
Lower temp. |
|
Sour taste, odor and turbidity |
Lactic acid bacteria |
Lower temp. |
|
|
Aging/conditioning |
As above |
Acetic acid bacteria |
None |
|
As above |
Lactic acid bacteria |
None |
|
|
Stench |
Zymomonas |
None |
|
Enterobacteriaceae Hafnia (Obesumbacterium) are short, nonmotile, straight rods of variable shape that commonly taint the early stages of fermentation. Hafnia protea (O. proteus) are fat rods (pH optimum 6.0) that, when present, are almost always in the yeast culture and only rarely in the wort (other Hafnia strains, as well as acetic and lactic-acid bacteria, may also contaminate pitching yeast). They produce sourness, diacetyl, and a dimethyl-sulphide smell like parsnips or celery. Like others of their family, they are intolerant of very acidic solutions and do not usually affect the aging beer.
Acetic and especially lactic-acid bacteria are the most prevalent contaminants of aging and bottled or kegged beer, but Pseudomonaceae Aeromonas is also encountered. The plump rods of Pseudomonaceae Zymomonas (Achromobacter anaerobium, pH 3.5 to 7.5, temperature optimum 86 degrees F [30 degrees C], active as low as 40 degrees F [5 degrees C]) are relatively uncommon, but when they are present, they produce an objectionable acetaldehyde and rotten-egg stench in a very short time. Zymomonas anaerobia or Z. mobilis are then the usual contaminants, fermenting fructose and glucose to ethanol, hydrogen sulfide, and acetaldehyde. Ground water or soiled equipment are the usual inoculants.
Bacterial contamination can be assessed both by perceptory analysis of the wort or beer (accentuated by “forcing” closed samples at 85 degrees F [30 degrees C]) and by culturing the wort or beer on a staining or yeast-inhibiting nutrient agar in a petri dish and estimating the microbial population after several days.