Botany
Oaks have spirally arranged leaves, often with lobed edges, and bear a nut called an acorn. The genus Quercus is native to the Northern Hemisphere and includes deciduous and evergreen species. The leaves of pedunculate oak (Quercus robur) are dark green above and paler below. This species is native to Europe and western Asia. The leaves have a cordate base and 3-6 rounded lobes, divided no further than halfway to the midrib. By autumn they are often covered in small disks of spangle gall. In the Turkey oak section (Cerris) the leaves typically have sharp lobe tips with bristles, and in the Ilex oak section they are evergreen, with bristle-like extensions on the teeth. North America has the largest number of oak species, with approximately 160 in Mexico and about 90 in the United States, and China ranks second with approximately 100.
Tannins
The acorns and leaves of oaks contain tannic acid, which helps to guard against fungi and insects. Tannins are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins. Their astringency causes the dry and puckery feeling in the mouth that follows the consumption of unripened fruit, red wine or tea. Quercus tannins are a mixture of condensed and hydrolysable tannins. In Quercus rubra, the tannin composition is dominated by condensed tannins. Tannins are broadly classified into condensed tannins and hydrolysable tannins, and dicots can produce either type alone or as mixtures. In the oak family, Fagaceae, 73% of the species tested contain tannin. Tannins in senescent tissues also retain their ability to complex with proteins and inactivate soil enzymes. Climate changes the tannins of red oak leaves: under dry and ambient warm conditions the trees produced higher quantities of tannins that were less polymerized, while under wet conditions they produced lower quantities, with more polymerized condensed tannins. Gallic acid, ellagic acid and quercitrin are among the phenolic compounds found in Quercus robur.
Tanning
Vegetable tanning uses tannins, which occur naturally in the bark and leaves of many plants. Oak is one of the primary barks processed in bark mills and used in modern times. Tannins bind to the collagen proteins in the hide and coat them, which makes the hide less water-soluble and more resistant to bacterial attack.
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