Tannins occur naturally in the bark of trees and bushes, in the husk of grains, in hops, and in many fruits, such as grapes. They are secondary phenolic metabolites and come in two basic groups: hydrolyzable and condensed. Hydrolysis can turn the former into glucose and phenolic acids. The latter, by contrast, are stable compounds made up of several flavonoid phenols that are polymerized with each other. Tannins are capable of binding and precipitating proteins, and as a result they have been used for thousands of years as organic preservatives, most notably for the tanning of leather. One of the key natural functions of tannins is to protect the fruits and seeds of plants through the fall and winter so that they remain healthy and capable of sprouting new shoots in the spring. In the interim, the bitter, astringent sensation caused by tannins can ward off plant-eating creatures and protect against microbial pests like molds and mildew.
Chemically, tannins are complex polyhydroxy phenols that are soluble in both water and ethanol. They contain phenolic orthodihydroxy groups, which have the capacity to form cross-linkages with proteins and peptides, which are high-molecular-weight nitrogen-based compounds. This linkage capacity is the chemical basis for the ability of tannins—and similar polyphenols present in the brewing process—to promote the precipitation of trub during the kettle boil of the wort.
The amount of tannin in the average finished beer is usually no more than 150 to 330 mg/l. About two-thirds of this is derived from the husk material of barley and about one-third from hops. The exact proportions vary, of course, with the beer’s original gravity and the quantity of hops used in the beer. Beers made with a substantial portion of wheat malt, however, tend to be much less astringent than comparable beers made entirely from a barley mash because wheat kernels are essentially free of husks. If the tannin concentration is excessive, it can cause a mouth-puckering astringency, which, depending on a person’s subjective sensitivity, could be considered an off-character. Astringency in the mouth is the not an actual flavor, but a tactile sensation. Tannins, when consumed, react with proteins in human saliva, just as they do in wort. When the saliva proteins coagulate, the saliva ceases to be a lubricant in the mouth. As a result, the inside of the mouth feels as if it is contracting and becoming leathery—a condition that we perceive as extreme dryness. This quality, although prized in moderation in red wines, tends to clash with hop bitterness in beer and is therefore generally avoided.
Just as tannins react with proteins and peptides in wort, they also do so in finished beer, where high levels can cause colloidal hazes. Hazes are more likely if the proteins in the grain, from which the beer was made, were not properly modified in the malt house or were not properly degraded in the mash tun.
Brewers will sometimes make use of the ability of tannins to link up with proteins to help stabilize finished beer against haze formation. The addition of as much as 3 g tannin/hl to the boiling wort can help precipitate unwanted proteins and thereby have a positive effect on both the clarity and the chemical–physical stability of the finished beer. The added tannin joins the precipitate. Such tannin additions are generally considered the simplest and most cost- effective method of beer stabilization. In Germany, however, such additions are a violation of the country’s Beer Purity Law (Reinheitsgebot) because some of these “kettle tannins” might reach the finished beer. A more expensive method of finished-beer stabilization is the addition of insoluble polyvinylpolypyrrolidone (PVPP) in conjunction with beer filtration. PVPP adsorbs and thus removes tannin, but because PVPP is filtered out before the beer is packaged, it conforms to the German Beer Purity Law.
MEBAK, Brautechnische Analysenmethoden Bd. IV, 2. Auflage, para. 1.4. Narziss, Abriss der Bierbrauerei, 6. Auflage, para. 2.5.3 + 7.6.4.