Keeping your brewery clean and sanitized is one of the most important aspects of being a good brewer. Brewing equipment should always be clean, and anything that touches wort or beer should also be sanitized. Contamination can occur at any stage in the brewing process; however, it is more likely at some times than others. In this article, I’ll explain when wort or beer is at its most vulnerable and what you can do about it. The upshot of this article is not that there are times when you can take shortcuts with cleaning and sanitation. Rather, I will argue that there are certain stages where you should take extra precautions.
Factors Affecting Contamination
Wort or beer can become contaminated at any time in the brewing process. However, there are some points at which a contaminant is more likely to be able to take hold. Several variables affect the ability of contaminating wild yeast or bacteria to live or grow. The key variables are temperature, alcohol content, pH, oxygen, nutrient availability, and competition.
Most microorganisms living near the surface of the earth grow best at temperatures near the high end of the range they usually encounter environmentally, or slightly above. Extremely cold temperatures kill many microorganisms, or at least inhibit their ability to grow. This is the primary reason that we refrigerate food.
Conversely, sufficiently high temperatures can eventually kill any microorganism. Most microorganisms cannot tolerate temperatures above 140°F. Heating food (or wort) to around 160°F will kill off almost any potential bacteria in less than a minute. [Heating milk to 161 °F/72 °C and holding it there for 15 seconds is called high temperature short time (HTST) Pasteurization.] Still, some bacteria — especially those that produce spores, such as the bacteria that causes botulism and some Baccillus strains — can survive even higher temperatures. (This is why milk is now heated to 275 °F/135 °C for up to 2 seconds. This process is called ultra high temperature (UHT) processing, or more colloquially, a botulism cook.) In brewing, spore-forming bacteria cannot survive the later stage in brewing, so they aren’t a problem.
Toxins (Alcohol and Alpha Acids)
Alcohol is toxic to most microorganisms. In fact, just a few percent alcohol will kill the vast majority of yeast and bacteria. Brewers yeast — and common wort or beer contaminants — are exceptions to the rule. These organisms have evolved the ability to tolerate high levels of alcohol.
The alpha acids in hops show some anti-microbial properties towards Gram-positive bacteria.
Most microorganisms thrive at pH levels somewhere in the vicinity of neutral (6.5–7.5 ). Extremely high or low pH values will kill most microorganisms. On the acidic side of the scale – where work and beer lie — few organisms can thrive below a pH of 4. The pH of is generally 4.0–4.4, although wine, mead, and sour beers are less (into the low 3 pH range).
Most microorganisms require oxygen in order to metabolize their food. In an anaerobic environment (lacking oxygen), only anaerobic bacteria microorganisms can live.
In order to survive in a liquid environment, organisms require a source of carbon (such as sugar or other carbohydrates). Most also require relatively high levels of nitrogen (from proteins or amino acids) to thrive. In addition, there maybe other nutritional requirements. Wort is a nutrient-rich environment. Beer is not.
The ability of a contaminant to take hold in a solution is limited by competition from other microorganisms.
In the brewing process, the susceptibility of your beer-to-be varies over time. During the boil, the only microorganisms that can survive are the spores of spore-forming bacteria. The heat of the boil kills most bacteria and the hops that are infused then will make the wort less hospitable towards Gram-positive bacteria. After the boil, when the wort is chilled and aerated, it is a nutritionally-rich environment at a moderate temperature and is highly susceptible to contamination. Quickly, however, fermenting wort becomes less and less hospitable to the majority of microorganisms in our environment. The environment in a fermenter quickly becomes anaerobic, the pH drops, and alcohol levels quickly exceed a few percentage points. Most “everyday” bacteria and wild yeast die at this point. So do any bacteria that bloom from a spore. As brewers, we usually don’t even consider – or perhaps even know the names of – the vast majority of microorganisms that are destroyed early in fermentation. These have been studied in lambic fermentations, as the wort is initially inoculated by microorganisms from the air. However, as they never become a problem in finished (non-sour) beer, they are largely ignored by brewing scientists.
Near the peak of fermentation, there is still a relatively high concentration of nutrients in wort. However, the competition for them is very stiff as brewers yeast is well adapted to beer wort. As fermentation completes, the resulting beer is low in nutrient availability, pH, and oxygen. Conversely it is high in alcohol content. In addition, if the beer is cold-conditioned, the temperature is less conducive to the growth of contaminants.
In brewing, the vast majority of our efforts in cleaning and sanitation are to kill the very small percentage of microorganisms that are anaerobic, insensitive to alcohol, insensitive to alpha acids, tolerant of low pH, and grow relatively strongly in either wort or beer. These include lactic acid bacteria such as Lactobacillus and Pediococcus, as well as the wild yeast Brettanomyces. If the beer is exposed to oxygen, then a few other organisms, including Acetobacter can present a problem. In general, your planned batch of beer is most susceptible to contamination right after the wort is chilled and aerated but before the yeast is pitched. This is when cleaning and sanitation matters the utmost. This is doubly true when making a yeast starter. If your starter is contaminated, the contaminant will simply multiply in your main batch of beer.
Unfortunately, although the general susceptibility of your beer to most contaminants decreases as fermentation progresses, you still can never abandon clean, sanitary practices. The presence of microorganisms highly adapted to wort and beer means that you must always be diligent about cleaning and sanitation. It doesn’t matter if you kill most of the bacteria in your environment if a wort-spoiling or beer-spoiling microorganism enters your brewing stream. And, in most brewing environments, these contaminants are somewhat enriched. So they pose an ever-present threat. So, although you must pay the closest attention to sanitation prior to fermentation, you can never ignore it at later stages.