If you’re an extract brewer, steeping specialty grains is a simple, yet effective, way to give your homebrew a flavor boost. But you need to be aware that you can introduce off-flavors into your beer by steeping grains incorrectly. Specifically, you can get an astringent flavor, which can vary from a slightly bitter flavor to a strong sour flavor akin to sucking on a tea bag. The astringency comes from tannins, which are a polyphenol extracted from grain husks.
Since all beers are made with grains, some tannins exist in every beer. In most cases, the tannins remain below the flavor threshold. However, you can get an excess of tannins by steeping specialty grains at too high a temperature or with too much water.
Temperature
To avoid excess tannins, you need to maintain the temperature of your steeped grains below 168°F (76°C). Going higher will leach tannins into your wort. In addition, it’s a good idea to use a muslin or nylon mesh bag to contain the specialty grains as you steep them. This will keep the crushed husks contained and out of the boil. (If husks are boiled, they leach tannins.)