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The avocado (Persea americana), also known as palta or aguacate (Spanish), butter pear or alligator pear, is a tree native to the Caribbean, Mexico, South America and Central America, classified in the flowering plant family Lauraceae. The name “avocado” also refers to the fruit (technically a large berry) of the kupa shell that contains a pit (hard seed casing) which may be egg-shaped or spherical.
Avocados are a commercially valuable crop whose trees and fruit are cultivated in tropical climates throughout the world (and some temperate ones, such as California), producing a green-skinned, pear-shaped fruit that ripens after harvesting. Trees are partially self-pollinating and often are propagated through grafting to maintain a predictable quality and quantity of the fruit.
History
P. americana, or the avocado, has a long history of cultivation in Central and South America; a water jar shaped like an avocado, dating to A.D. 900, was discovered in the pre-Incan city of Chan Chan1 The earliest known written account of the avocado in Europe is that of Martín Fernández de Enciso (c. 1470–c. 1528) in 1518 or 1519 in his book, Suma de Geografía que Trata de Todas las Partidas y Provincias del Mundo.23 The first written record in English of the use of the word ‘avocado’ was by Hans Sloane in a 1696 index of Jamaican plants. The plant was introduced to Indonesia in 1750, Brazil in 1809, the Levant in 1908, and South Africa and Australia in the late 19th century.
Health benefits
High avocado intake has been shown to have an effect on blood serum cholesterol levels. Specifically, after a seven day diet rich in avocados, hypercholesterolemia patients showed a 17% decrease in total serum cholesterol levels. These subjects also showed a 22% decrease in both LDL (bad cholesterol) and triglyceride levels and 11% increase in HDL (good cholesterol) levels.18
Uses
The fruit of horticultural cultivars ranges 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. The fruit has a markedly higher fat content than most other fruit, mostly monounsaturated fat, and as such serves as an important staple in the diet of various groups where access to other fatty foods (high-fat meats and fish, dairy, etc) is limited. A ripe avocado will yield to a gentle pressure when held in the palm of the hand and squeezed. The flesh is typically greenish yellow to golden yellow when ripe. The flesh is prone to enzymatic browning and turns brown quickly after exposure to air. To prevent this, lime or lemon juice can be added to avocados after they are peeled.
The avocado is very popular in vegetarian cuisine, making an excellent substitute for meats in sandwiches and salads because of its high fat content. The fruit is not sweet, but fatty, distinctly yet subtly flavored, 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. Avocado is popular in chicken dishes and as a spread on toast, served with salt and pepper. In Brazil and Vietnam, avocados are frequently used for milk-shakes and occasionally added to ice cream and other desserts. In Brazil, Vietnam, the Philippines and Indonesia, a dessert drink is made with sugar, milk or water, and pureed avocado. Chocolate syrup is sometimes added. In Australia it is commonly served in sandwiches, often with chicken. In Ghana, it’s often eaten alone in sliced bread as a sandwich.
In Mexico and Central America, avocados are served mixed with white rice, in soups, salads, or on the side of chicken and meat. In Peru avocados are consumed with tequeños as mayonnaise, served as a side dish with parrillas, used in salads and sandwiches, or as a whole dish when filled with tuna, shrimps, or chicken. In Chile is used as a puree in chicken, hamburgers and hot dogs, and in slices for celery or lettuce salads. The Chilean version of caesar salad contains large slices of mature avocado. In Kenya, the avocado is often eaten as a fruit, and is eaten alone, or mixed with other fruits in a fruit salad, or as part of a vegetable salad.
Nutritional value
About 75% of an avocado’s calories come from fat, most of which is monounsaturated fat. Avocados also have 60% more potassium than bananas. They are rich in B vitamins, as well as vitamin E and vitamin K.19 They have the highest fiber content of any fruit – including 75% insoluble and 25% soluble fiber.20
A fatty triol (fatty alcohol) with one double bond, avocadene (16-heptadecene-1,2,4-triol), is found in avocado.21
Cultivation
The tree grows to 20 metres (66 ft), with alternately arranged leaves 12 centimetres (4.7 in) – 25 centimetres (9.8 in) long. The flowers are inconspicuous, greenish-yellow, 5 millimetres (0.2 in) – 10 millimetres (0.4 in) wide. The pear-shaped vegetable is 7 centimetres (2.8 in) – 20 centimetres (7.9 in) long, weighs between 100 grams (3.5 oz) – 1,000 grams (35 oz) grams, and has a large central seed, 5 centimetres (2.0 in) – 6.4 centimetres (2.5 in) long.5 It is considered by many to be a drupe, but is botanically classified as a berry.67
The subtropical species needs a climate without frost and with little wind. High winds reduce the humidity, dehydrate the flowers, and affect pollination. In particular, the West Indian type requires humidity and a tropical climate which is important for flowering. When even a mild frost occurs, premature fruit drop may occur, although the Hass cultivar can tolerate temperatures down to ?1°C. The trees also need well aerated soils, ideally more than 1 m deep. Yield is reduced when the irrigation water is highly saline. These soil and climate conditions are available only in a few areas of the world, particularly in southern Spain, the Levant, South Africa, Peru, parts of central and northern Chile, Vietnam, Indonesia, Sri Lanka, Australia, New Zealand, the United States, the Philippines, Malaysia, Central America, the Caribbean and Mexico, the center of origin and diversity of this species. Each region has different types of cultivars. Mexico is the largest producer of the Hass variety, with over 1 million tonnes produced annually.
Harvest and post-harvest
An average avocado tree produces about 120 avocados annually. Commercial orchards produce an average of 7 tonnes per hectare each year, with some orchards achieving 20 tonnes per hectare.8 Biennial bearing can be a problem, with heavy crops in one year being followed by poor yields the next. The avocado tree does not tolerate freezing temperatures, and can be grown only in subtropical or tropical climates.
The avocado is a climacteric fruit (the banana is another), which means that it matures on the tree but ripens off the tree. Avocados used in commerce are picked hard and green and kept in coolers at 38 to 42°F (3.3 to 5.6°C) until they reach their final destination. Avocados must be mature to ripen properly. Avocados that fall off the tree ripen on the ground, and depending on the amount of oil they contain, their taste and texture may vary greatly. Generally, the fruit is picked once it reaches maturity; Mexican growers pick Hass-variety avocados when they have more than 23% dry matter and other producing countries have similar standards. Once picked, avocados ripen in a few days at room temperature (faster if stored with other fruits such as apples or bananas, because of the influence of ethylene gas). Premium supermarkets sell pre-ripened avocados treated with synthetic ethylene to hasten the ripening process.9 In some cases, avocados can be left on the tree for several months, which is an advantage to commercial growers who seek the greatest return for their crop; if the fruit remains unpicked for too long, however, it will fall to the ground.
Breeding
The species is only partially able to self-pollinate, because of dichogamy in its flowering. This limitation, added to the long juvenile period, makes the species difficult to breed. Most cultivars are propagated via grafting, having originated from random seedling plants or minor mutations derived from cultivars. Modern breeding programs tend to use isolation plots where the chances of cross-pollination are reduced. That is the case for programs at the University of California, Riverside, as well as the Volcani Centre and the Instituto de Investigaciones Agropecuarias in Chile.
The avocado is unusual in that the timing of the male and female flower phases differs among cultivars. There are two flowering types, “A” and “B”. “A” cultivar flowers open as female on the morning of the first day and close in late morning or early afternoon. Then they open as male in the afternoon of the second day. “B” varieties open as female on the afternoon of the first day, close in late afternoon and reopen as male the following morning.
Propagation and rootstocks
While an avocado propagated by seed can bear fruit, it takes roughly 4–6 years to do so, and the offspring is unlikely to resemble the parent cultivar in fruit quality. Thus, commercial orchards are planted using grafted trees and rootstocks. Rootstocks are propagated by seed (seedling rootstocks) and also layering (clonal rootstocks). After about a year of growing the young plants in a greenhouse, they are ready to be grafted. Terminal and lateral grafting is normally used. The scion cultivar will then grow for another 6–12 months before the tree is ready to be sold. Clonal rootstocks have been selected for specific soil and disease conditions, such as poor soil aeration or resistance to the soil-borne disease caused by phytophthora (root rot).
Cultivation in California
The avocado was introduced from Mexico to the U.S. state of California in the 19th century, and has become an extremely successful cash crop. Ninety-five percent of United States avocado production is located in southern California, with 60% in San Diego County.1314 Approximately 59,000 acres (approximately 24,000 hectares) of avocados are grown in California. Fallbrook, California, claims the title of “Avocado Capital of the World”, and both Fallbrook and Carpinteria, California, host annual avocado festivals.
Hass cultivar
While dozens of cultivars are grown in California, the Hass avocado is today the most common. It produces fruit year-round and accounts for the majority of cultivated avocados in the US.315 All Hass avocado trees are descended from a single “mother tree” that was raised by a mail carrier named Rudolph Hass, of La Habra Heights, California.215 Hass patented the productive tree in 1935. The “mother tree”, of uncertain subspecies, died of root rot and was cut down in September, 2002.315
Other cultivars
Other avocado cultivars include Bacon, Fuerte, Gwen, Pinkerton, Reed, Spinks and Zutano. The fruit of the cultivar Florida, grown mostly outside 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.
“A” cultivars: Hass, Gwen, Lamb Hass, Pinkerton, Reed.
“B” cultivars: Fuerte, Sharwil, Zutano, Bacon, Ettinger, Sir Prize, Walter Hole.1011
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, resulting in a reduced yield the following season, and thus the alternate bearing pattern becomes established.