Color as an Indication of the Picking Maturity of Fruits and Vegetables
by L.C. Corbett, an excerpt from the 1916 Yearbook of the Department of Agriculture
Fruit color is a factor intimately associated with fruit maturity. The small child uninstructed in the arts is not attracted to the cherry tree until the fruits are colored, and he soon learns from experience to choose the fruits that are sweetest by his sense of color values associated with the perception of taste. While this is a simple illustration of the value of the color test for fruit maturity, it nevertheless illustrates the fact that unconsciously a value is placed on the color in fruits in order to arrive at an estimate of their palatability. This being the case, may there not be physiological activities or results directly associated with the life of the fruit which can in a measure be estimated by the color values in the fruit? That such is the fact is quite evident from the results observed in the behavior of various fruits which have been subjected to careful control conditions in repeated experimental tests conducted by the investigators of the United States Department of Agriculture.
A concrete and striking example of the value of fruit color as an index to fruit picking maturity is afforded in the results of the picking and storage investigations conducted to determine the factors contributing to the successful storage of the apple. Early in the work upon this subject, undertaken by Taylor and Powell, it became evident that some physiological activity of the fruit itself was intimately concerned with the phenomenon termed “scald” in the stored fruit. As the work progressed it became more and more evident that scalding could not be wholly attributed to storage-house conditions or management. It was possible with the perfection attained in refrigerating apparatus and in storage-house construction and insulation to maintain practically constant, uniform conditions from year to year. Not withstanding these facts, varying results as regards scald were obtained, not only from year to year, but in different lots of fruit in the same house. These results focused attention more clearly upon the fruit itself.
Comparison of Stages of Maturity
Accordingly, a series of observations was conducted to ascertain the relation of early (immature), medium (mature), and late (overripe) picking to the behavior of the fruit in what had been determined to be the most nearly ideal storage temperature. As one of the important teachings of this investigation there appeared results such as are recorded in the color reproductions presented herewith. The immature fruit illustrated in Plate A, which is characteristic of early-picked fruits of the Rome Beauty apple in the Yakima Valley of Washington, is distinguished by more or less development of the red or overlay color, but the important point for consideration in connection with fruit at this stage of maturity is the composition of the ground color. A careful study of this specimen reveals the fact that much vivid leaf green is present in the skin of the apple. Fruit picked at this stage of development, when withdrawn after a storage period of six months at a constant temperature of 32 degrees F, presented the appearance illustrated in Plate B. This shows an advanced stage of “scald,” which when present to any appreciable extent decidedly affects the food value as well as the merchandising value of the fruit. It will be noted not only that the skin shows the characteristic browning of scald, but that the discoloration has extended far below the surface of the skin; in fact, the cells of the skin as well as many layers of cells in the flesh of the fruit have ceased to function, as is indicated by the browning of the walls.
The specimen shown in Plate C represents a fruit in which, before picking, there had been a slight increase in the development of the overlay color. The area covered was a little larger and the color more intense than in the apple shown in Plate A. The significant fact, however, is observed in the ground color. There is less intensity in the green pigment, and the leaf green, while still observable, is less intense, having in a measure been replaced by white, a phenomenon familiar to every close observer of changes in the color values of fruits as they ripen. The chlorophyll, or leaf green, has grown less vivid and has certainly decreased in amount. This fruit, while still too immature for best storage behavior, responds to a six months’ period in storage in the manner shown in Plate D. This apple, while showing a considerable amount of scald, is by no means as badly affected as the fruit shown in Plate B. While the amount of skin discoloration shown in Plate D is considerably less than that shown in Plate B, and while this discoloration has not been communicated to the underlying flesh of the apple, so that for practical purposes the apple is sound and fit for eating or for culinary uses, yet its merchandising value has been greatly impaired because of the discoloration. The brown baked-apple appearance characteristic of scalded fruit is taken as an indication that the fruit has passed the limit of its commercial storage life. Any considerable amount of scald, therefore, rapidly depreciates the market value of the fruit affected and is a phenomenon to be overcome or avoided as far as practicable.
The extent to which scald can be overcome or avoided through picking by color, even in some of the varieties known to be unusually susceptible, is brought out in Plates E and F. The apple illustrated in Plate E brings out clearly the color values which the grower and picker must learn to recognize in order to be able to determine the stage of maturity to which the fruit must attain before it may be placed in cold storage at the temperatures recognized as satisfactory for the storage of apples and come out in a sound, attractive condition and possessing a high merchandising value.
A critical study of the color values of the apple shown in Plate E reveals the fact that there is only a trace of true leaf or chlorophyll green remaining in that part of the skin of the fruit least highly colored. This will invariably be found to be the side most shaded by the foliage of the tree and is therefore the least mature portion of the fruit. It is here that the skin pigments are least stable and most likely to be affected by the temperature. The leaf green, as has been noted, is small in amount, and its intensity is much less than in the fruits illustrated in Plates A and C. The ground color has been modified also, being, instead of greenish white, either white or yellowish white. Here we find the suggestion of the development of the true ground color of the fruit. The ground color or underlay in this variety is a shade of yellow, and as soon as there is a suggestion of the development of this characteristic color the fruit has reached a stage in its life when it can with safety be removed from the tree and placed under temperature conditions which slow down or inhibit the normal ripening processes to such an extent that instead of ripening to edible maturity in a period of a few weeks the ripening process is extended over a much greater period, which we term the normal storage period for the variety.
Conditions Determining the Storage Period
The storage period of any variety of apple is long or short for that variety according to the degree of care exercised in picking it at the proper stage of maturity indicated by its growth and coloring and upon the treatment to which it is subsequently subjected. If it is harvested too green, even though the other factors contributing to successful handling and storage are carried out, the results shown in Plates B and D may be expected; but if, in addition to picking it too green, the fruit is allowed to stand in a warm packing room for several days previous to placing it in storage, the storage period will be shortened and the amount and seriousness of the scald augmented, provided the varieties to be stored behave normally in other respects in storage. The closer the condition of the fruit approaches that shown in Plate E the better it will carry in cold storage, assuming that it is carefully handled from the tree to the storage house, that it is not allowed to remain for more than a few hours at a high temperature after it is removed from the tree, and that whether placed in common or cold storage it is reduced to a temperature of 32 degrees F as soon as possible, the fewer hours the better, and then maintained without undue fluctuation at as nearly this temperature as is possible throughout the storage period.
Although the grower may observe and guard against all of the physical conditions which are known to affect the life of fruit in storage, such as careful handling, quick transfer from tree to storage room, and the maintenance of a suitable storage temperature, if the fruit itself has not reached a proper stage of maturity (Pl. E) the observance of these precautions will avail little. The physiological behavior of the fruit, even if stored under an ideal environment, will be that shown in Plate B if it is exceedingly immature, that shown in Plate D if it is approaching a satisfactory stage of maturity, and that shown in Plate F if it has reached a condition of maturity suitable for storage. Color, then, is the important factor by which the grower can determine the condition of ripeness of fruit for the best behavior in cold storage.
The illustrations here presented represent only one variety, and the color values shown are characteristic only of this variety. The general principle, however, holds for all varieties of apples and pears thus far subjected to cold-storage tests. In order to apply the lesson taught by the different stages of ripeness in the Rome Beauty apple here illustrated, it will be necessary merely to observe closely the true color values shown by each sort in the process of ripening and then to attempt to harvest the fruits at the time when these signs are most developed. In order to do this, the grower and picker must carefully train his sense of color values and observe closely the changes which take place in the make-up of the general color scheme of each variety. The points to be borne in mind are the decrease of the amount and intensity of leaf green in the skin of the fruit and the replacing of the green by white and gradually by yellow, if yellow is the normal ground color of the fruit, as is the case with the Rome Beauty apple.
The development to a slight extent of the normal ground color is desirable, but this process should not be allowed to go too far before it is checked by picking and placing the fruit in storage. This caution should be heeded, for it is found that overripe fruits are quite as unsatisfactory for long keeping in storage as are those picked too green. The ability to judge the stage of ripeness so as not to pick the fruit underripe or to allow it to remain on the tree until overripe is the end desired. Careful attention to the color scheme of varieties is one essential, and another is to evaluate color changes in the developing fruit so as to pick each individual fruit at a time when it will keep longest and hold its color best. A careful study of the plates here presented will serve as a foundation, but the picker must carry with him to the orchard an accurate mental picture based on close observation for each of the varieties in his collection. The more perfect the mastery of the color problem for each variety the greater will be the success with the variety in storage, other things being equal.
The Color Factor in Tomatoes
As a further contribution to information concerning the relation of color to picking maturity in plant products other than apples, it is desirable to mention the relation which the color at picking time bears to the color and general character of the canned product, as well as catsup made from tomatoes. Tomatoes, like apples, present a wide diversity of colors, but since red is the desired color in canned tomatoes, catsups, and pulp, only those varieties possessing red or scarlet pigments will be considered.
The tomato during the process of ripening passes from the leaf-green color which characterizes it during the early period of its development to a whitish green before the normal fruit color begins to develop. The green tints give way to the white, and this in turn is replaced in the desirable canning sorts by red, which gains in intensity as the process of ripening progresses, until at full maturity the pigmentation is complete and permanent. When this stage has been reached there is no further increase in the intensity of the color, and when tomatoes have reached this stage of ripeness are subjected to the temperatures required for proper sterilization in the operation of processing the canned product there is no loss in color and the product possesses the deep red so desirable in high-grade goods. At this stage of maturity the pulp can be concentrated by boiling to the consistency required for catsup without loss of color. If, on the other hand, the tomatoes are gathered before pigmentation is complete and are subjected to the processing required for canning or catsup making, the resulting product will not be of a deep red color, but will vary from a reddish straw color to light red, according to the degree of pigmentation attained by the fruits when harvested.
Before the passage of the food and drugs act, which requires that no food may be colored in a manner whereby inferiority is concealed, the question of the stage of maturity at which fruits or vegetables were picked or processed gave manufacturers little concern, for lack of coloring could be made up by sweating or by the use of dyes.
Not that such practices are discouraged by the regulations of the act, and the trade still requires high-colored and, fortunately, high-flavored products as well, the stage of maturity and the degree of pigmentation play an important part. Fortunately for the industry, nature has provided a method by which the demands of the trade can be met in a legitimate way; that way is via the ripe-fruit route.
Ripened tomatoes of desirable color will, when properly handled, produce a high-colored canned product or a high colored catsup. Half-ripened red tomatoes will not produce a canned product or a catsup of a bright red color. The pigment in such fruits is not stable and fades slightly when the pulp is subjected to the temperatures required for sterilization or concentration. In the case of the tomato the heat of processing reduces the pigment and leaves a product varying from straw color through the shades of red, depending upon the stage of color development attained by the fruit when picked.
Disadvantages of Harvesting Immature Products
Immature apples when placed in storage develop various degrees of scald, depending upon the pigmentation or color development attained by the fruit up to the time of storage. As the apple matures it loses the leaf green which characterizes the young, immature fruit; this is replaced by white, and this in turn by the ground color and the normal pigmentation of the variety. Under commonstorage or cold-storage conditions, immature fruits in which the chlorophyll, or leaf green, has not disappeared do not retain this color in storage, but assume the baked-apple appearance characteristic of scalded fruit, the intensity or severity of the scalding depending upon the stage of maturity attained by the fruit, as illustrated in Plates A, C, and E.
The practical result from placing immature apples in storage is loss of color and that from processing immature tomatoes is loss of color. In both cases deterioration is due to unstable pigments in the product. The apples lose color when fruits with imperfect pigmentation are subjected to storage temperatures, whether or not the conditions to which they are subjected are low enough to check normal ripening processes. It would appear, on the other hand, that ripe fruits have developed stable pigments which are not affected in the apple by cold storage temperatures nor in the tomato by the heat required for sterilization or concentration.
The practical lessons are: (1) Loss from scald will be lessened by harvesting apples after the leaf green has disappeared and the normal ground color for the variety has begun to develop; (2) high-colored and high-quality canned tomatoes and high-colored and high-quality catsup can be made only from ripe tomatoes of varieties possessing a red pigment. The time for picking fruit for special purposes must be determined by the behavior of the fruit. Put the fruit to the test; it will give the answer.