Citrus at Its Best

ORANGES and grapefruit have become tremendously important in the diets of Americans. I give one fact as proof, if proof were needed: Between 1933 and 1943 our production of oranges increased from 47 million boxes to 105 million and of grapefruit from 14 million boxes to 55 million. Almost as obvious as the fact that we eat a lot of citrus fruits is the fact that they are bound to vary in quality. Let us examine, as a subject worth the attention of everyone who eats or grows oranges and grapefruit, what makes them good—specifically, the relation of maturity to quality in citrus fruits.

In the early years of citrus growing in the United States the decision as to the proper time for picking the fruit was made largely by the individual grower. The increase in production resulted in more business-like methods in handling the crop, so that eventually the industry cooperated in having State laws passed to effect more orderly marketing. Minimum standards were based on a break in the color of the rind, volume of juice, and the content of soluble solids and citric acid in the fruit, and the ratio between them.

The Department began in 1935 a comprehensive study of the factors that affect the quality of the juice of Florida oranges. Then we made similar studies of Florida grapefruit, tangerines, and Temple oranges.  The results are applicable, in the strict sense, only to Florida fruit, but probably apply generally to the same varieties grown elsewhere, although we have not determined the extent to which they may thus apply in other States. The investigations included observations and measurements of physical characteristics and analyses of the chemical constituents of the principal varieties at definite intervals, beginning with immature fruit and continuing until it had become fully mature and ripe.

Maturity refers to a stage of development of a fruit; ripening refers to the process by which a mature fruit becomes edible when held under suitable conditions. A mature fruit has attained the degree of development in which it will ripen with acceptable eating quality. Fruits with starchy reserves, like apples and pears, may be mature at harvestime, although many late varieties do not become ripe until sometime later, when they attain fully their soft, juicy, aromatic qualities. In contrast, oranges and grapefruit owe their sweetness to natural sugars occurring as such; they contain practically no starch, and do not undergo such a marked change in composition as apples and pears after being picked.

Since the ripening processes occur only while the fruits are on the tree, it can readily be understood that they should not be harvested until they are mature and therefore ripe. Instead of increasing in palatability after harvest, these fruits tend to lose quality, the rate of this loss depending on the temperature at which they are held. The higher the temperature, the more rapid the deterioration.

Quality often is associated with appearance, firmness, freedom from blemishes, and thickness and texture of rind, but actually it is determined by the texture of the flesh, juiciness, content of total solids (principally sugars), total acid, ratio of total solids to acid, aromatic constituents, and vitamin and mineral content. The age of the fruit is also important, because immature fruit is usually very acid or tart, whereas overripe fruit held on the tree too long may become insipid or develop off-flavors.

There are, of course, no hard and fast lines of demarkation between the successive stages through which fruit passes from the time it is first formed until it completes its growth and development. As fruit develops from an immature stage to full maturity, it becomes more and more pleasant to the taste, until it reaches perfection for any given variety.  Thus the greatest amounts of sugars are found in fruits that are left on the trees until they reach maturity. Conversely, fruits picked before they have become mature neither contain their potential maximum of sugars, nor do they develop any more sugars after picking. While sugars increase, acidity decreases as the fruits become more mature, and the most desirable eating quality is reached when there is such a balanced blending between the total solids (principally sugars) and the total acid (citric) as to make the fruit most palatable.

Because most consumers probably consider sweetness the most essential character in oranges, the desired condition of balanced blending may be said to occur, for all practical purposes, when the fruit contains its maximum potential sugar content. The term “tree-ripened fruit” is often used. It means simply that the fruit has remained on the tree until it is ripe enough to be relished or has attained its potential maximum content of sugars. Any inference from the use of this term that the fruit can be ripened after harvest, even to a negligible degree, is incorrect. The color of the skin can be altered by artificial means; therefore, the color of the skin may have no bearing on its stage of maturity or ripeness. A wholly green fruit may be fully ripe under certain natural conditions, and a fully yellowed fruit may be immature under other conditions. It is the composition of the fruit and not its color that determines whether it is ripe ornot. Itisasripe as it ever will be, when it is picked.

It is extremely important, therefore, to delay picking until the fruit reaches a desirable stage of maturity if it is to satisfy consumers. It is correspondingly important also to base standards for judging maturity on criteria adequate for the purpose.

When oranges and grapefruit are still immature, the rind and flesh have a greenish color; the juice vesicles appear like grains of rice and are not distended with juice as in the mature fruit. The walls of these juice cells are thick and conspicuous, the juice itself is greenish-yellow to yellow, lacks aroma, is acid to very tart, and has a raw, immature taste. As development progresses and the fruit matures, the greenish color in the rinds disappears and, in oranges, the fruit takes on its characteristic aroma and orange color, or, in the case of grapefruit, a tannish-yellow color. Progressive changes also occur within the fruit, the vesicle cell walls becoming thinner and the vesicles distended with juice.

The size and weight of fruit usually increase, but the greatest rate of increase is during the period of development before maturity. The vol- ume of juice increases until the fruit is ripe. Then it remains rather constant until the fruit starts to dry out. When this happens the fruit Joses somewhat in volume as well as in flavor.

We analyzed more than 13,000 individual fruits to determine how size affects the volume of juice, total solids, and total acid at different times before and during the harvesting period for the principal varieties of Florida oranges. We found that as the fruit ripens there is an increase in volume of juice and total solids and a decrease in acidity regardless of the size of the fruit. The smaller fruits contained more solids and acid and, on the basis of the standard packed box (1⅗ bushels), a greater volume of juice. Thus the packs of smaller sizes weigh the most.

Because of the importance of the vitamin content of oranges and grapefruit in determining their dietetic value, data regarding the ascorbic acid (vitamin C) content of the juice of exposed and shaded oranges are especially interesting. Fruits grown on the outside of the tree and well exposed to sunshine contain from 14 to 48 percent more ascorbic acid than those grown on the shaded inside branches. From outside to inside fruits, the concentration of ascorbic acid, when calculated as milligrams per milliliter of juice, gradually becomes less. The concentration of ascorbic acid in the juice goes down as the fruit matures, but the total content per fruit remains about the same, the vitamin being dispersed in the increased amount of juice found in the more mature fruit. As the fruit becomes overmature and begins to dry out, its ascorbic acid content diminishes.

Orange and grapefruit trees grown on rough-lemon rootstock usually become larger and come into bearing earlier, and consequently produce larger crops than those grown on sour-orange rootstock. Also, on rough- lemon rootstock the fruits average slightly larger and have a little coarser texture and thicker rinds, but contain somewhat less juice on the basis of weight; eating quality is not quite so high as that of fruits grown on sour-orange rootstock. The latter usually contain slightly higher amounts of total solids and total acid than those grown on rough lemon.

Since the “big freeze” of 1894-95 in Florida, most of the varieties of oranges have been budded or grafted on rootstocks particularly adapted to the soil of the locality. But there are still in Florida many old groves of seedling orange trees on their own roots, which were grown from the seeds of the sweet orange. The fruits from these are commonly referred to as Seedlings.

Generally the kind of rootstock used is the one best adapted to the type of the soil. Certain rootstocks are better suited to light sandy soils, others to heavier soils. For example, rough lemon is a very thrifty grower, has an extensive root system, and is used most frequently in soils in which the organic matter is low and the land is rolling. Sour-orange rootstock is used mostly in level soils having more than the average amount of organic matter.

More varieties are grown commercially in Florida than in any other citrus section. The harvest season normally extends from about October to June. Several varieties of oranges, Parson Brown, Hamlin, Conner, and so on, make up the bulk of the early crop; Seedlings, Pineapple, Jaffa, and Homosassa compose the midseason crop harvested in December, January, and February, and Valencia and Lue Gim Gong constitute the bulk of the late crop.

The varieties differ in quality, but the mature fruits—in their prime eating condition—are good generally in practically all the commercial varieties. Consumers often find that midseason oranges are better than the early oranges, and assume that it is because the same early oranges have been left on the tree longer. This is only partly true. The oranges marketed in midseason are generally of different varieties. Since they follow the early varieties instead of arriving at a market bare of Florida citrus, they are generally permitted to remain on the tree to maturity before harvesting. Thus they are more likely to attain excellent quality than the early varieties.

Some of the varieties that are recognized as having better-than-average quality are Seedlings, Pineapple, Temple, and Valencia. Among these the Temple is unique in containing aromatic qualities that give the juice a desirable bouquet and flavor.

Of the two leading grapefruit varieties, the Duncan rates slightly superior to the Marsh. The differences are not great, however, and the Marsh is more popular because it is practically seedless and is easier to prepare for eating.

Other factors that affect quality include kinds and amounts of fertilizers, drought, drainage, irrigation, insects, pests, diseases, and “freezes,” which affect the vigor and physiology of both trees and fruit.

Commercial fertilizers are used in large quantities in Florida, principally because much of the grove land is a light sandy soil comparatively low in natural fertility and readily leached by rains. Under proper care and management, the trees consistently yield large crops and much work has been done to improve tree vigor and fruit quality. Noteworthy research has paved the way toward correcting deficiency diseases by supplementing the common fertilizers with minor elements. A. F. Camp, of the Citrus Experiment Station, Lake Alfred, Fla., and others have investigated the symptoms of citrus malnutrition and have demonstrated that mineral deficiencies can readily be corrected by the use of proper fertilizers. The use of copper, zinc, manganese, magnesium, and, to a lesser extent, iron, is now an integral part of commercial practice, and helps to improve the quality of the fruit.

If a drought occurs in early spring, blooming may be delayed until rains finally come. Droughts later in the season cause smaller fruits. In severe droughts so much moisture is withdrawn from the fruits as to make them soft. On the other hand, too much water in the soil may cause fruit to split.

In the final analysis, the taste of the consumer determines the demand for citrus fruits. The more vitamins and juice the fruit contains, provided there is a proper blend of sugars and acid, the more valuable it is.  Harvesting at the proper time and use of adequate standards of maturity will help to win approval by placing the fruit on the market when it is most palatable, and has the highest nutritional value. That is to say, the fruits must be left on the trees long enough to mature properly.

THE AUTHOR
Paul L. Harding, a horticulturist in the Bureau of Plant Industry, Soils, and Agricultural Engincering, has been assigned to the investigation of seasonal changes in citrus fruits at the Orlando, Fla., station since 1935. On the basis of this work, the Department and the Florida Citrus Commission adopted the internal quality grade A for Florida oranges. Dr. Harding is author or coauthor of more than 30 papers of practical importance and benefit to shippers and growers of fruits and vegetables.  He is a graduate of Iowa State College.

FOR FURTHER READING
Harding, Paul L., and Fisher, D. F.: Seasonal Changes in Florida Grapefruit, U. S. D. A. Technical Bulletin 886, 1945.