are microorganisms that can change the flavor, aroma, or appearance of beer in a manner deemed undesirable by the brewer. Although there is some overlap, brewers seek to divide spoilage organisms into the categories of “beer spoilers” and “wort spoilers,” with the latter largely inhibited by alcohol, pH, and/or the anaerobic environment created by yeast fermentation. Spoilage organisms are mostly limited to about 40 species of yeast and about 50 of bacteria.

A purist might argue that any organism beside a pure yeast culture would be a spoilage organism. Another perspective, possibly more lambic-tinged, could easily find over a dozen other bacteria and yeasts that would give desirable qualities when grown in some beers. Many brewers, especially craft brewers in the United States, are now welcoming a larger range of microflora. Therefore, the concept of “spoilage” is very much based on the wishes and expectations of the brewer and consumer. See lambic beers, sour beers. By making sweet wort, the brewer is purposefully creating the ideal food source for the brewery’s yeast. It is an excellent meal for many other creatures as well. Although perhaps 80% of all yeast and bacteria would eat the sugars in wort, brewers limit the potential spoilers in many ways, starting with the pH, temperature, and nutrition of the wort. Boiling the wort sterilizes it and allows the brewer to introduce the desired inoculation; the brewer starts with a blank slate from a microbiological standpoint. This is a contrast to traditional winemaking, where any organism on the skin of the grape would remain alive and eventually ferment the wine, with the winemaker seeking to encourage some organisms and discourage others. In both beverages, fermentation lowers the pH and creates alcohol, two developments that some spoilage organisms will not survive.

The organisms that do survive can wreak considerable havoc in finished beer. The simplest symptoms of spoilage contamination are haze in the beer or muddled, “unclean” flavors. Furthermore, off-flavors such as buttery (diacetyl), vegetable (dimethyl sulfide), sulfurous, medicinal (phenols), and even putrid (butyric acid) can be formed. Beers may become soured by lactic acid bacteria consuming starches or sugars and creating lactic acid or by acetic acid bacteria combining oxygen with alcohol to produce acetic acid.

See also bacteria, diacetyl, off-flavors, quality control, assurance, and wild yeast.