Apple Maturity Indices
Guide H-314
Esteban Herrera, Extension HorticulturistCollege of Agriculture and Home Economics New Mexico State University
This Publication is scheduled to be updated and reissued 1/03.Apple quality is largely determined by how mature the fruit is when harvested. If apples are picked prematurely, fruit is likely to be small, poorly colored and off-flavored. Immature fruit is subject to diseases such as bitter pit and soft scald when stored for an extended period. Overmature apples, besides having a decreased storage life, have poor storage quality and develop mealy fruit susceptible to internal break down.
Water core may develop while fruit is still on the tree or during storage, depending on cultivars. In addition, if ripe apples are left on the tree for an extended period, excessive dropping can occur. 'Red Delicious' apples develop poor dessert quality if picked immature. If fruit remains on the tree in an attempt to get high red color development, overmaturity results.
The term maturity is used interchangeably with ripeness; however, they are two distinctive phases. Apples need to complete the maturity stage before they start ripening. Immature fruit when harvested fails to progress toward a good edible fruit or to a good processing condition. Maturity is also related to the stage when full size is obtained. Ripeness is related to internal changes in the apple leading to better quality development, mainly an increase in sugar content, and an improvement in flavor and aroma.
The maturity at which apples should be harvested depends on the final use of the fruit. If an apple is to be sold in a fresh fruit market shortly after harvest, the fruit may be allowed to ripen almost entirely on the tree. If the fruit is to be held in storage for an extended period, it should be picked earlier. If the fruit is to be processed soon after harvest, then it must be picked with the proper quality.
Description of several indices of fruit maturity follows. Generally a combination of indices are used. Fruit measurements will vary among cultivars, or may vary from one tree to another in the same cultivar, or among orchards as much as or more than from one crop year to the next. For this reason, no single index has ever provided a consistently dependable guide for harvest maturity year after year.
Types of Indices
Ground color changes refers to underlying green or yellow color. This method is one of the most useful and refers to the amount of green chlorophyll showing in the skin of red apples. The exact shade of green or yellow depends on the variety and differs somewhat with seasons (less dependable when season is early). The following ratings are usually used: dark green, medium green, light green, yellowish green, greenish yellow. In general, the fruit advances to maturity as its ground color changes from green to yellow.
Ease of separation from the spur When ready to be harvested, most apple varieties can be easily separated from the spur without breaking the stem by lifting it with or without a slight rotating movement.
Some varieties like 'McIntosh' and 'Red Delicious' tend to drop early. Fruit drop may increase as a result of early frost or excess nitrogen. Varieties such as 'Jonathan', 'Stayman and 'Winesap' retain their fruit until it is overmature. In addition, seasonal conditions and culture affect the tenacity with which fruit stems cling to the spurs. Plant regulators sprayed at the end of the season, like Ethrel (Ethephon) and auxins, alter the maturity index. Ethrel advances fruit ripening and stop-drop chemicals prevent fruit drop.
Firmness of fruit Flesh becomes softer as an apple matures on the tree. Firmness of the flesh can be determined by cutting off a thin slice of skin and flesh and using a hand operated pressure tester to record the pressure, in pounds, necessary for a 7/16-inch plunger to penetrate the flesh. This process should be repeated 10 to 20 times with apples picked randomly.
Commercial harvests vary depending on the variety. 'Red Delicious' apples should be harvested at 15 to 18 pounds of pressure for optimum storage. 'Golden Delicious' apples should be harvested at 14 to 17 lbs. If long-term storage is anticipated, fruit should be harvested in the higher firmness range. If the fruit is for immediate shipping or use (roadside operations), the lower firmness range is desirable. Apples with a firmness greater than 18 pounds are usually immature. This index, even when used independently, is of value in determining when apples are too soft or ripe for storage.
It is important to remember that firmness varies from season to season and factors other than maturity can affect firmness. For example, fruit on the outside of the tree may have a higher pressure rating than fruit on the inside. Fruit size will also influence firmness readings. Large fruit is usually softer.
Soluble solids content Sugar content of the juice increases as the apple matures. Soluble solids of apples, which are highly correlated with sugar content of apples, are measured with a hand refractometer. The accuracy of this index varies from year to year and from tree to tree, depending upon the leaf-fruit ratio. Percentage of soluble solids for commercial harvest depends on variety. 'Red Delicious' apples should be harvested around 10%. If fruit is to be used immediately, soluble solids around 12% is recommended. In general, apples should be harvested in a 10 to 12% range for optimum cold storage quality. Soluble solids content for immature fruit ranges between 8 and 9%.
Starch content Starch content gradually decreases as the apple
matures. Slices of apples are dipped in a 75 ml solution of iodine crystals
(2.5 gms) plus potassium iodide (10 gms) for
1 minute. Drain off excess solution and let dry for a few minutes.
The starch will react with iodine and appear as a blue-black pattern. This
index, however, is not widely used because it is not very practical in
nature and must be performed on apples that have not been stored.
Days from full bloom The period from full bloom to actual harvest is fairly constant for any given variety. However, when temperature records are kept and heat units are calculated, a better estimate is obtained. The average number of days from full bloom to maturity in five apple varieties are as follows:
| Variety Days | To Maturity |
| Jonathan | 135-145 |
| Red Delicious | 145-155 |
| Golden Delicious | 150-160 |
| Winesap | 160-175 |
| Rome Beauty | 165-175 |
Development of red color Grower's experience may be needed for this index because a grower must know the variety or apple strain. Some varieties may be fully colored several weeks before optimum harvest time. Excess nitrogen fertilizer could cause poor fruit color. Potassium applications can improve fruit color only when this nutrient is deficient. Iron deficiency can also cause poor fruit color. Additional variation may be caused by climatic conditions, irrigation, soil type and other cultural practices.
Grade for 'Red Delicious' apples in the United States is based primarily on the percentage of red skin color. Extra fancy grade must have 70%, while fancy grade must be 40% of the surface area with good color. With a little practice, ratings for individual apples can be made to the nearest 5 to10%.
For yellow apples like 'Golden Delicious' there are charts available that contain various degrees of coloring, depending on the degree of apple maturity. Apples in the field are compared against colors on the chart to learn about apple maturity. Color of 'Golden Delicious' apples progresses from dark green to yellow. Five ratings are usually used: dark green, green, light green, yellowish green and greenish yellow.
Seed color Seeds turn brown or black as they mature inside the fruit. The time of coloration often varies from season to season. An average rating for seed color can be made as follows: seeds all white, brown color beginning to show on sharp end of seeds, 1/4 of seed with brown color, 1/2 area of seed brown, 3/4 area of seed brown and all of seed brown. Record the average for each apple.
Fruit sampling Sampling should start 5 weeks before expected earliest harvest date for best results. Sampling should be done at regular intervals, at least weekly, and on the same day each week, if possible. Apples should be taken from at least five trees randomly selected to represent the current season's crop. From each tree, samples of four apples should be picked about shoulder height, or from each side of the tree. Weekly averages of maturity measures from individual trees should be tabulated. Comparison of results indicates the pattern of changes that occur within the orchard from week to week.
The time to begin harvest is not a simple decision. Experience gained in following the fruit development and accumulation of recorded measures, along with notes on climate conditions, will make this decision easier.
Reprinted January 1998
Electronic Distribution: January 1998