Tannins are often used in the wine world to help classify dry reds or oak-aged whites, which produce an "astringent, mouth-coating feeling," according to Wine Enthusiast.
Tannins are polyphenols found in certain plants, like grape leaves. They "bind readily with proteins, cellulose, starches, and minerals," protecting substances from decomposition.
To make the brine, mix one tablespoon of salt with ½ cup of white distilled vinegar, ½ cup of water, 6 tablespoons of sugar, and a teaspoon of "mildly flavored" tea leaves.