As brewers, we often hear about tannins and how they are bad for our beer. But what are tannins? Where do they come from and how do they affect our beer? In this article, I’ll give an overview of tannins and their role in brewing. In a second article, I’ll discuss the nitty-gritty details of how tannins enter the brewing stream, and how to influence the rate at which they do in your home brewery. In a third article, I’ll discuss a little of bit of the chemistry of specific tannins that come from barley malt, hops and oak barrels.
What are Tannins?
Tannins are a class of polyphenolic molecules. “Polyphenolic” means that they are made from many phenols. Phenol is an organic chemical comprised of six carbon molecules bonded in a ring. (And here, “organic” means containing carbon.) There are three major classes of tannins. Two of the classes — condensed and hydrolysable tannins — are found in plants. The third class is found in brown algae, and hence not relevant to brewers.
The word “tannin” comes from wood tannins that are used to tan animal hides into leather. All tannins are polyphenols, but not all polyphenols are tannins (in the sense of being capable of tanning animal hides). Still, “tannin” is often used as a blanket term encompassing all polyphenols similar to tannins. As we are homebrewers and not home tanners, I’ll use the terms “tannins” and “polyphenols” interchangeably. The tannins relevant to brewing are water soluble and the rate at which they dissolve into solution is affected by all the usual variables, including time, temperature and pH.
Where Do They Come From ?
In vascular plants, tannins are produced by organelles (called tannosomes) and sequestered in vacuoles. Tannins react strongly with proteins, so having them floating free in the plant cell’s cytoplasm would interfere with metabolism. Tannins are most often found associated with growing tissues and are thought to be produced to discourage herbivores. They also play a role in the decomposition of plants.
One place tannins are frequently found is in seeds. As brewers, this is important because barley malt is made from barley seed. Hardwoods also contain a lot of tannins, and so beer aged in new oak barrels will be very tannic. (And of course, wine makers purposely age many red wines in oak for exactly that reason. Tannins bring “structure” to wine.) Tannins are also found in the strigs and bracts of hops. The types of tannins found in barley malt, hops and wood are different and I’ll cover this in more detail later.
Effect on Brewing and Beer
Tannins are extracted from malt during mashing and hops during the boil. They react with proteins in the wort to form the hot break. Tannins also react with proteins as the wort is chilled to form the cold break. In addition, tannins in beer can react with proteins to form complexes that cause haze. Tannins are brown and can also add a small amount of color to wort or beer.
Tannins in beer can be reduced by fining with PVPP (polyvinylpolypyrrolidone), often sold in homebrew shops under as PolyClar AT. Ironically, tannic acid (a preparation of some specific tannins) is sometimes used for fining for proteins in beer. Tannic acid preferentially binds to haze-forming proteins, but does not interact as strongly with foam-positive proteins
All beer contains tannins. In fact, some level of tannins is required in order for the beer to taste acceptable. (In one experiment, beers with their tannins levels severely reduced below normal levels were judged to be less appealing that normal beers.)
However, if tannins are present in excess, they cause astringency in beer. Astringency is a drying, puckering sensation caused by tannins reacting with salivary proteins on your tongue. Tea (made from steeping the tannin-rich leaves of tea plants) is almost always astringent and astringency in beer can sometimes give it an iced-tea like character. The sensation of astringency is sometimes confused with the taste of bitterness — and this is further confused by the fact that some tannins are additionally bitter (i.e. trigger a response in our bitter taste buds in addition to reacting with salivary proteins).
Tannins are present in all beers, above the threshold of detection. When tannins are present in excess, they negatively impact beer by causing astringency. Tannins help by precipitating break material in the kettle, but these same interactions can result in haze when they occur in beer. Tomorrow, I’ll go step by step through the brewing process and tell how to influence the amount of tannins extracted from the plant materials. In general, increased temperature, pH and time lead to more tannins being extracted. As we’ll see, tannin extraction is not an all-or-nothing thing. If you heat your mash from 168 °F(76 °C) to 171 °F (78 °C), it’s not as if zero tannins are being extracted at 168 °F(76 °C) and you’ve opened the floodgates at 171 °F (78 °C). In practice, it is fairly easy to limit tannins to below the level at which they will negatively affect your beer.