Science Fair Project Encyclopedia
Avocado (Persea americana) is a tree and the fruit of that tree, classified in the flowering plant family, Lauraceae. It is native to Central America and Mexico. The tree grows to 20 m (60 ft), with alternately arranged, evergreen leaves, 12-25 cm long. The flowers are inconspicuous, greenish-yellow, 5-10 mm wide. The pear-shaped fruit is botanically a berry, from 7 to 20 cm long, and weighs between 100-1000 g. It has a large central seed, 3-5 cm in diameter. An average avocado tree produces about 120 avocados annually. The fruit is sometimes called an avocado pear or alligator pear, from its shape and green skin. The avocado tree does not tolerate freezing temperatures, and so can be grown only in subtropical and tropical climates.
Barlow & Martin (2002) identify the avocado as a fruit adapted for ecological relationship with large mammals, now extinct (as for example the South American herbivorous giant ground sloths or Gomphotheres). This fruit with its mildly toxic pit, co-evolved with those extinct mammals to be swallowed whole and excreted in dung, ready to sprout. The ecological partners have disappeared, and the avocado plant has not had time to evolve an alternative seed dispersal technique.
The avocado fruit does not ripen on the tree, but will fall off (and must be picked up) in a hard, "green" state, then it will ripen quickly on the ground, but depending of the amount of oil that it has the taste may be very different. Generally, the fruit is picked once it reaches a mature size, and will then ripen in a few days (faster if stored with other fruit such as bananas, because of the influence of ethylene (plant hormone) gas). The fruit can be left on the tree until required, rather than picked and stored, but for commercial reasons it must be picked up as soon as possible.
While dozens of cultivars exist, two are particularly commonly available, 'Hass' (commonly misspelled 'Haas') and 'Florida'. The former is the most common cultivar, with a dark rippled skin, and rich, creamy flesh, accounting for more than 80% of the crop grown in California. There are several other cultivars related to 'Hass', including 'Bacon', 'Fuerte' (pictured), 'Gwen', 'Pinkerton', 'Reed', and 'Zutano'. The cultivar 'Florida', grown mostly outside of California, is larger and rounder, with a smooth, medium-green skin, and a less fatty, firmer and fibrous flesh. These are occasionally marketed as low-calorie avocados.
Certain cultivars, such as the Hass, have a tendency to bear well only in alternate years. After a season with a low yield, due to factors such as cold (which the avocado does not tolerate well), the trees tend to produce abundantly the next season. This heavy crop depletes stored carbohydrates, causing a reduced yield the following season, and thus the alternate bearing pattern becomes established.
It was introduced to the U.S. state of California in the 19th century, where it has become an extremely successful cash crop. In the United States 95% of the avocados grown are grown in California, with 80% in San Diego County. Fallbrook, California claims the title of "Avocado Capital of the World", and hosts an annual Avocado Festival . There are about 24,000 ha of avocado plantations in California.
Avocado can be grown as a houseplant from seed, although it will not normally bear fruit indoors; people enjoy it for its greenery. It can be geminated in normal soil in a large pot, or in a glass of water with a piece of charcoal for deodourising, with the top half held up by toothpicks.
The fruit of horticultural cultivars range from more or less round to egg or pear-shaped, typically the size of a temperate zone pear or larger, on the outside bright green to green-brown (or almost black) in color, and high in fat. Though the fruit does have a markedly higher fat content than most other fruit, most of the fat in avocados is monounsaturated fat, which is considered healthy in the human diet. A whole medium Avocado contains approximately 25% of the Daily Value of saturated fat. Avocados also have 60% more potassium than bananas.
The flesh is typically greenish yellow to golden yellow when ripe. The flesh oxidizes and turns brown quickly after exposure to air. To prevent this, a highly acidic juice like lime or lemon juice can be added to avocados after they are peeled. The avocado is very popular in vegetarian cuisine, making a good substitute for meats and cheeses in sandwiches and salads because of the high fat content. The fruit is not sweet, but fatty, flavorful, and of smooth, almost creamy texture. It is used as the base for the Mexican dip known as guacamole, as well as a filling for several kinds of sushi, including California rolls. In Brazil, avocados are added to ice cream and in the Philippines, a dessert drink is made with sugar, milk, and pureed avocado. Avocados may also be substituted for cheese in many sandwiches.
The name "avocado" is from its Nahuatl name 'ahuacatl' which also meant testicles, with influence from the irrelevant but much more familiar Spanish avocado an obsolete form of 'abogado' (lawyer). In some countries of South America the avocado fruit is known as 'palta', which is a name that comes from the Quechua language. The Nahuatl ahuacatl could be compounded with others, as in ahuacamolli, meaning "avocado soup or sauce", from which the Mexican Spanish word guacamole derives. The plural of avocado is avocados or avocadoes.
- Barlow, Connie and Paul Martin. (2002) The Ghosts of Evolution: nonsensical fruit, missing partners and other ecological anachronisms.
- California Rare Fruit Growers Notes on the botany and cultivation of the avocado tree.
- California Avocado Commission "Fun facts".
- California Avocado Commission Hass and related cultivars.
- University of California Agriculture Explanation of the avocado fruit's classification as a berry (and not a drupe).
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