Tannins

A type of condensed tannins, found in particular in grape and known as proanthocyanidins, are polymers of 2 to 50 (or more) flavonoid units that are joined by carbon-carbon bonds, which are not susceptible to being cleaved by hydrolysis. Those can nevertheless undergo acid-catalyzed cleavage in the presence of a nucleophile like phloroglucinol, benzyl mercaptan or cysteamine leading to the formation of oligomers that can be further analyzed.

Proanthocyanidins from wikipedia: represent a group of condensed flavan-3-ols, such as procyanidins, prodelphinidins and propelargonidins, that can be found in many plants, most notably apples, maritime pine bark, cinnamon, cocoa, grape seed, grape skin (procyanidins and prodelphinidins),[2] and red wines of Vitis vinifera (the common grape).

However, bilberry, cranberry, black currant, green tea, black tea, and other plants also contain these flavonoids. The berries of chokeberry, specifically black chokeberry, have the highest measured concentrations of proanthocyanidin found in any plant to date.[3][4] [5]

Proanthocyanidins can also be isolated from Quercus petraea and Q. robur heartwood (wine barrel oaks).[6]

Apples contain on average per serving about eight times the amount of proanthocyanidin found in wine, with some of the highest amounts found in the Red Delicious and Granny Smith varieties.[7] All lifted from Wikipedia.

Below are two images of tannin synthesis pathways.

From Robbins et al., 1998. Genetic Manipulation of Condensed Tannins in Higher Plants. Plant Physiology. Vol. 116: 1133-1144. Green box shows the enzymes we were allowed to follow while red Xs indicate enzymes off limits.