Tannins during a decoction

This is a very good question that inevitably arises when knowledgeable all-grain brewers begin thinking about doing a decoction mash. The conventional rule is to mash-out at around 170 ºF (77 ºC) and not to exceed this temperature during wort collection for the reason you mention. In a traditional triple decoction mash, the mash begins at around 104 ºF (40 ºC) and a portion of the thick mash is removed, boiled and returned to increase the temperature to about 122 ºF (50 ºC). This cycle is repeated to heat the mash to 140 ºF (60 ºC) and then up to 158 ºF (70 ºC). So there are three times where a portion of the mash (always the thick mash) is removed, boiled and returned to the resting mash to provide heat. My view on the conventional rule about keeping the temperature of infusion and step mashes below 170 ºF (77 ºC) makes sense when you consider what happens to the mash during wort separation. As the wort gravity drops, the pH of the wort flowing from a mash bed increases, and with the increase in pH, the solubility of polyphenols increase. With this, you run the risk of getting a grainy flavor if you have high pH and low gravity runnings combined with high temperature. A decoction mash is different. Thick portions of mash are removed and boiled. The wort in mash is very concentrated, usually about 18–22 ºPlato. This means that the concentration of sugar is high. It also implies that the wort protein content is high as well since there have been no steps taken to remove protein from the wort, for example wort boiling and trub separation. During the mash boil in a decoction mash, protein from the malt reacts with tannins and precipitate. The pH is also “normal” (~5.2) at this point in time and the solubility of tannins is still relatively low compared to that seen in the last runnings from the lauter tun. I admit that the following statement is an educated guess but I would venture to bet that the reaction between protein and polyphenols is significant and explains why decocted beers are not overly astringent. I was in attendance at a National Homebrew Conference in Baltimore 11 years ago and Dr. Klaus Zastrow, a well-known retired Brewmaster from Anheuser-Busch, was speaking about the history of lagers. After his talk I asked Dr. Zastrow the same question you asked me and he gave me a slightly different answer. He actually began by disagreeing with the premise that decocted lagers are no more astringent than other lagers (which was my premise). I wish I spoke German, at least enough to hack by in brewing terms, because Dr. Zastrow had a specific word to describe the astringency of decocted beers and explained that this certain flavor attribute was one of the desirable hallmarks of decocted lagers. He did not imply the flavor was unpleasant but explained in English that this attribute gives the beer a certain briskness. Brisk is a tea term and is the opposite of flat or soft and I interpreted his statement to mean that decocted beers had more, uh how do you say, cojones. I hope my brief answer helps you in your quest and that this has given you the confidence to brew up a batch of that Pils with cojones you’ve read so much about!