Infection is the introduction or presence of undesirable microorganisms in beer or its raw materials. The severity of infections may range from imperceptible to severe. In the extreme, infections can cause hazes, acidity, or off-flavors and may make the beer appear unsightly or become undrinkable. Although beer infections are not dangerous to human health, allowing infected beer to reach the consumer is quite harmful to any brewery’s reputation and business.
Life essentially runs on sweet liquids and many organisms can spoil beer. The term “wort spoilers” is sometimes applied to spoilage organisms that tolerate oxygen well and grow best before fermentation has lowered the wort pH and produced ethanol. Others are referred to as “beer spoilers”—these tend to prefer anaerobic conditions and survive well in lower pH environments and those containing alcohol. The most common organisms considered beer spoilers by most brewers are Lactobacillus, Pediococcus, and wild yeasts including Brettanomyces. Each of these has its own preferred set of nutrients, temperature range, pH range, and growth rate.
Mankind has been brewing for at least 6,000 years and probably much longer, but consciously managing beer microbes has been practiced only in the past century and a half. Previously, beer had been simply consumed fresh and local before the inevitable spoilage would occur, and although procedures were known that would make less-spoiled beers, the brewers were largely ignorant of the microbiology behind these procedures. The existence of yeast as a microbe was only discovered in 1674 by Antonie van Leeuwenhoek, the inventor of the modern microscope. And it was only in 1859 that Louis Pasteur put an end to the theory of “spontaneous generation,” when he demonstrated unassailably and for the first time that living microorganisms are the sole cause of food and beer spoilage, as well as the agents of fermentation.
After the kettle boil, wort should be perfectly sterile. Thereafter, infection can occur during wort cooling, fermentation, cold transfer, or packaging. The brewer’s best weapons against undesirable microbes in the finished beer are proper sanitation in the brewery’s cellar and packaging area, as well as the use of sterile bottles and kegs. Many breweries, especially large industrial ones, also sterile filter their beer on the way to the bottling line and/or pasteurize it after packaging. These steps prolong a beer’s shelf life by removing or killing potential infectious organisms and thereby make it more probable that a drinkable product reaches the consumer’s table. But pasteurization, if improperly performed, can induce stale or “cooked” flavors and aromas. Sterile filtration can remove bacteria but can also strip away flavor, aroma, body, and even color.