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The acorn is the nut of the oak tree (genera Quercus, Lithocarpus and Cyclobalanopsis, in the family Fagaceae). It is a nut, containing a single seed (rarely two seeds), enclosed in a tough, leathery shell, and borne in a cup-shaped cupule. Acorns vary from 1 – 6 cm long and 0.8 – 4 cm broad. Acorns take between about 6 or 24 months (depending on the species) to mature; see List of Quercus species for details of oak classification, in which acorn morphology and phenology are important factors.
Acorns are one of the most important human grown foods in areas where oaks occur. Creatures that make acorns an important part of their diet include birds, such as jays, pigeons, some ducks and several species of woodpeckers. Small mammals that feed on acorns include mice, squirrels and several other rodents. Such large mammals as pigs, bears, and deer also consume large amounts of acorns: they may constitute up to 25% of the diet of deer in the autumn. In some of the large oak forests in southwest Europe, traditionally called “dehesas”, pigs are still turned loose in oak groves in the autumn, to fill and fatten themselves on acorns. However, acorns are toxic to some other animals, such as horses.
In some human cultures, acorns once constituted a dietary staple, though they are now generally only a very minor food.
The larvae of some moths and weevils also live in young acorns, consuming the kernels as they develop.
Acorns are attractive to animals because they are large and thus efficiently consumed or cached. Acorns are also rich in nutrients. Percentages vary from species to species, but all acorns contain large amounts of protein, carbohydrates and fats, as well as the minerals calcium, phosphorus and potassium, and the vitamin niacin. Total food energy in an acorn also varies by species, but all compare well with other wild foods and with other nuts.
Acorns also contain bitter tannins, the amount varying with the species. Since tannins, which are plant polyphenols, interfere with an animal’s ability to metabolize protein, creatures must adapt in different ways to utilize the nutritional value that acorns contain. Animals may preferentially select acorns that contain fewer tannins. Creatures that cache acorns, such as jays and squirrels, may wait to consume some of these acorns until sufficient groundwater has percolated through them to leach the tannins out. Other animals buffer their acorn diet with other foods. Many insects, birds, and mammals metabolize tannins with fewer ill-effects than humans. Several indigenous human cultures have devised traditional acorn-leaching methods that involved tools and that were traditionally passed on to their children by word of mouth.
Species of acorn that contain large amounts of tannins are very bitter, astringent, and potentially irritating if eaten raw. This is particularly true of the acorns of red oaks. The acorns of white oaks, being much lower in tannins, are nutty in flavor, which is enhanced if the acorns are given a light roast before grinding. Tannins can be removed by soaking chopped acorns in several changes of water, until water no longer turns brown. (Boiling unleached acorns may actually cause the tannins to be unleachable.) Being rich in fat, acorn flour can spoil or get moldy easily and must be carefully stored. Acorns are also sometimes prepared as a massage oil.