Avocado (Persea americana)

LAURACEAE, Laurel Family

One must conclude that our nation ridicules and secretly envies California not because its citizens are carefree beach and surf bums with windblown hair, not because we tolerate a huge diversity of life styles and ethnicities, not because we embrace anything new with wild enthusiasm, but because we serve guacamole dip with wine cooler chasers at parties! Anything as green and slimy as guacamole dip cannot be good for you! True that the fruit, a 1-seeded berry, contains more calories per gram than about any other food (up to 2800 calories per pound)--the dieter's nightmare--but at least from the standpoint of triglycerides, the composition of the oil is quite healthy. Then again, who ever eats a pound of avocado flesh. Over 70% of the oil is polyunsaturated, usually oleic and linoleic acid, and oleic acid is monounsaturated, which is reputed to be best for reducing cholesterol buildup on arterial walls. Avocados are rich in B vitamins and some C and E, not A or K, and contain 60% more potassium than the banana (Musa).

The scientific name, Persea americana, tells us that its origin was in the Americas, specifically in Central America and southern Mexico. Records of its usage occur from 7000 B.C. in Mexico, of its cultivation from 6000 B.C. in Oaxaca, and of its continuous use at all levels in all of the famous archaeological sites in Mexico, including Tehuacán in Puebla, where the earliest maize fossils appeared. In these old habitations, the evidence is a seed, the structure from the center of the fruit. The seeds were small for thousands of years before there was selection for the large fruits that we find today in the stores and market places. Some reports cast doubt on these old claims and note the origin of the avocado in 291 B.C. in Mexico.

The word avocado comes from the Aztec ahuacatl, translated by the Spaniards as ahuacate and aguacate. "Alligator pear" is probably a reference to the scaly, dark green peel, although some say that the colonists were too dumb to pronounce the Aztec word. "Midshipman's butter" refers, we think, to its early uses by sailors, who took American avocados with them to sea. Other regions of the Americas had different names; e.g., the Incas called it palta.

Three types of avocado fruits are easily recognized. Sometimes these are called varieties, but generally are considered to be horticultural races, three extreme fruit forms in a continuum of design. The Mexican type, also called "drymifolia," has relatively small fruits, 75-300 grams in weight, a thin, smooth black skin, and a loose seed. This type has the highest oil content (sometimes 30%) and grows in the driest and coolest locations. The Guatemalan type is a large highland avocado, 500-600 grams in weight, has a thick, warty skin, a large, tight seed, and moderately high oil content. The West Indian type, which is large and has smooth green to reddish skin when ripe, tends to have the lowest oil content (as little as 7%). The West Indian type prefers Caribbean climates. Experts hypothesize that the three types of avocados were domesticated independently within Central America, and this is why the forms are so distinctive.

This berry is truly unusual, not only because it is oily, not sweet, but also because it never softens while still on the tree, where it remains hard and continues to grow. Avocados keep fairly well while hard but can turn soft overnight. To ripen an avocado, place the fruit into a small paper bag, including an apple or banana if you are in a hurry. The exact mechanism of softening is not entirely understood but involves the softening of the middle lamella (pectin) cementing the mesocarp cells, and appears to involve ethylene. Anyone who has prepared guacamole dip knows that the green flesh blackens fairly rapidly, a process called a polyphenolic response, and this can be retarded by immediately mixing with lemon (acidic) juice. To keep the green flesh of a cut avocado from turning black, you can immediately wash the cut surface with cold tapwater before wrapping in a plastic bag.

The peel or rind (exocarp) consists of an epidermis with a cuticle, but in the warty fruits, the epidermis is replaced with cork from a cork cambium. The warts or bumps are airy zones in the cork called lenticels. Beneath this are several layers of cells, the innermost ones being sclerenchyma. The thick, green mesocarp is composed of millions of small parenchyma cells, some that are specialized for oil storage and others that have smaller amounts of oils. The endocarp consists of several layers of thin-walled cells. In the center, the seed has a double seed coat, two cotyledons rich in starch, and a relatively small embryo. Anyone who has tried to grow avocado from a seed knows firsthand that avocado has hypogeal germination, i.e., no hypocotyl is formed, and the epicotyl grows directly into a shoot from beneath the soil.

About 74,000 acres of California farmland (6000 growers) were devoted to avocado production in the 1980s, and more now (95% of the domestic harvest), and 60% of that crop comes from San Diego County. California harvests are by hand with special avocado shears called clippers and using ladders up to 9 m in length and with long poles. This state supplies most of the domestic need and has some of the best fruits in the world. Chief among the cultivars here are 'Hass', possibly a Guatemalan form that produces from April until August, and 'Fuerte', a Guatemalan-Mexican hybrid that produces from October until March. The 'Hass' cultivar came from a plant in the backyard of Mr. Rudolph Hass in La Habra, so if you see a sign advertising 'Haas' avocados, politely tell the produce manager of the store to get smart. Mr. Hass was a postal worker, who grew trees from purchased seedlings presumably of 'Lyon' type, and in 1935 he patented one plant having fruits that his children especially enjoyed. 'Hass', propagated by H. H. Brokaw of Whittier, then outcompeted the then-popular 'Fuente' cultivar, which was available during a different season. In Florida, where the first avocados were grown in 1833 and commercially produced in 1893, there are about 18,000 acres of avocado trees; the common forms there are West Indian types or hybrids that involved West Indian avocados. Seven varieties are grown in California.

Mexico is the world's leading producer of avocados, and in this, an ancient user of the berry, about 315,000 acres are devoted to its cultivation, mainly the Mexican and Guatemalan types. Israel and South Africa boast well-developed avocado industries, and Spain is up and coming. California farmers have fears of imported fruits, mostly because they fear infestation risks to the industry, especially from Mexico, and the USDA now requires imported fruits to each bear a country of origin sticker.

Avocado is commercially successful only in those areas in which the climate is just right for plant growth, flowering, and fruit development, and, frankly, this is a tall order. This tree with evergreen leaves does not like climate that is too wet, too dry, too hot, or too cold. Freezing can kill the plants, and either hot or very cool temperatures cause the embryo to abort. Young fruits drop whenever the temperature is hot, and cold wind is considered an ill wind, if it comes during pollination, when the sun should be bright and the temperature must be moderate. Windbreaks are planted to check winds.

The story of pollination in avocado is a remarkable one, and it makes one appreciate how hard it is to get fruit to set. Avocados have flowers that open twice, on two successive days. The first day the pistil is receptive and protrudes from the flower; the second day the pistil is no longer receptive, and the stamens shed their pollen. Here is an example of protogyny, which means that the female is mature before the male ("early female"). The consequence of this is that a flower cannot self pollinate, but requires transfer of pollen, typically by bees, from another flower, often another tree. Within avocado there happen to be cultivars that open with a receptive pistil in the morning of Day I and then reopen in the afternoon with stamens of Day 2 (A Group). Other avocados open with a receptive pistil in the afternoon of Day I and then reopen in the morning with stamens on Day 2 (B Group). Some growers take advantage of these differences and plant the two near each other, so that pollen from B (AM) can be transferred to Day 1 (AM) pistils, and Day 2 (PM) pollen can be transferred to Day 1 pistils (PM). Fortunately, flowers do not always behave so regularly, so some fruits may form on trees if some of its flowers are out of synchrony. An avocado tree typically produces a million flowers per year but only sets dozens or up to 400 fruits.

Avocados are placed into nylon bags, hanging around the necks of pickers and which hold 40 pounds of fruits. Then the oblong fruits are transferred to picking bins of 600 to 800 pounds and placed in cold storage at packing houses. After precooling for 24 hours, avocados are graded on a conveyor belt, packed into lugs of 48 avocados, stacked onto pallets with 60 lugs of fruit, and then trucked to market, typically in refrigerated containers if shipped long distances or to foreign countries.

Avocado oil is also isolated for many unusual products. It has a flash point of about 600 degree F and makes an excellent cooking oil (remember also its fatty acid composition!). A folk use of avocado is to rub the peel of the fruit into the scalp, thereby moisturizing the skin, and one account notes that Koreans add these to milk for facial or body massages. Latin Americans have wrapped and given avocados as wedding gifts. In Southeast Asia, avocados are often prepared or eaten with milk and sugar, and avocados are popular in Japanese sushi rolls. Of course, in California, undoubtedly elsewhere, the avocado is also served with the hope that on an outside chance it may be an aphrodisiac! California harvests more than 300 million pounds per year--quite a load of the sexual stimulant.

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