Papaya (Carica papaya)
CARICACEAE, Papaya Family
Papaya (Carica papaya) is the widely accepted vernacular name for one of the most popular of the sweet, tropical tree fruits. Cultivation of papaya began in Central America (no old records, unfortunately), but quickly after the discovery of the New World, seeds of papaya were carried to Old World tropical areas. One can commonly find papaya trees growing around homes in all tropical countries. Because home-grown crops are unregistered, it has been impossible to obtain reliable estimates of total world papaya production, but annual production must be several million metric tons.
The papaya tree is one of the most distinctive plants on earth. It is generally 4 to 5 meters tall and unbranched, but old plants may be 10 meters tall. The trunk looks a little like an upside-down, gray carrot. This plant has huge leaves, up to 75 centimeters across, that are palmately and then pinnately lobed. Leaves, which have very long petioles, are clustered near the top of the tree, because the lower ones are shed, leaving large, circular scars on the bark. The stem is soft and weak because it has very flimsy wood (secondary xylem) and a hollow center.
One often sees several papaya plants growing around a home in the tropics. This may be because the papaya is an important, regular fruit in the diet, but it is also often necessary because papaya is usually dioecious, so that at least one male and one female plant are required for fruits to be produced. There are, however, cultivars that are monoecious (separate male and female flowers on the same plant), and the popular cultivar Solo has perfect flowers (each flower on a plant has both stamens and a pistil). Male flowers are small and borne in long clusters, whereas one to several female flowers are produced on a short stalk in the axil of each leaf. Typically only one female flower per leaf develops into a fruit; the trunk is thereby so covered with fruits that one can barely see the stem. The oldest, mature fruit is the lowest one, and the youngest the uppermost one. Therefore, once a plant starts to produce, there is a continuous supply of fruits. All mom has to do is to send her tallest or best climbing son out to the family papaya tree to harvest the lowermost papaya fruit.
The ripe fruit has a yellow and green skin (exocarp), often with tints of red and orange, and a sweet, semi-firm, yellow-orange pulp (mesocarp). Within the central cavity (locule) of the fruit are hundreds of dark seeds that almost appear bluish when the fruit is cut open; they are about the size of small peas and are attached to the pulp (parietal placentation). Although shoppers in the United States are accustomed to seeing hand-sized papayas in the grocery store, natives tend to eat papayas that are the size of a large cantaloupe or a small watermelon, i.e., 3 to 60 kilograms in weight, and fruits 50 centimeters in length are not uncommon. Papaya fruit is most commonly eaten fresh for dessert or made into a drink, although it can be canned and now occurs in canned tropical fruit salads and designer fruit drinks. The seeds of the best fruits are saved and replanted. Unfortunately, papaya cannot be propagated by vegetative cuttings, so that one is not guaranteed that the seed planted from the "good" fruit will in turn produce fruits of the same quality.
All organs of the plant contain laticifers, and a white latex flows freely from any cut surface. Long ago natives learned that papaya latex is a very effective meat tenderizer. Tough meat was wrapped in fresh leaves for several hours to make it tender. The active tenderizing ingredient is a protein-digesting enzyme called papain, which is very similar to human stomach pepsin. Interestingly, some of the early, crude studies of plant protein structure were made by digesting the proteins into pieces with the use of papain. Papain has been commercially produced by scoring unripened fruits with longitudinal cuts and then collecting the copious latex in containers set on the ground below. The latex is sun- or oven-dried into a powder; the papain powder most commonly is marketed in the United States as Adolph's Meat Tenderizer. Much of the papain is produced in Tanzania. There are a variety of other uses for this interesting enzyme, which is fairly similar to another protolytic enzyme found in pineapple, bromelin.