Tannins

Tannins are naturally occurring compounds found in the leaves of grapevines, oak, tea, and other plants, known for a slightly astringent, mouth-puckering quality. In fermentation, tannins are best known for their traditional role in helping vegetables, particularly cucumbers, stay firmer during a longer ferment, which is why older recipes often call for adding a grape leaf, oak leaf, or a small amount of black tea to a jar of pickles.

The enzymes naturally present in cucumbers and other vegetables can soften them over time as they sit in brine, and tannins are believed to help counteract that softening enzyme activity at the surface of the vegetable. Alongside tannins, using fresh produce, trimming the blossom end of cucumbers, and keeping ferments at a cooler temperature all contribute to a crisper final texture.

A common point of confusion is expecting tannins alone to fix a texture problem that actually stems from over-ripe produce, too little salt, or too warm a fermenting spot, when in reality crisp texture usually comes from several factors working together rather than any single ingredient. Small amounts are generally used, since too much can make a ferment taste noticeably bitter or overly astringent.