WSU Tree Fruit Research & Extension Center

Postharvest Information Network

Saturday, February 16, 2019

WSU-TFREC/Postharvest Information Network/Retaining the Firmness of Apples in Storage

Retaining the Firmness of Apples in Storage

Harvest Maturity

The cornerstone of apple quality is firmness. Apples that lack a minimum firmness are unpalatable regardless of flavor components. Some years, apple firmness unaccountably declines faster than in other years, but it is still useful to review some principles in conserving firmness. Postharvest practices only slow the rate of deterioration of the fruit–they cannot make "old" fruit "younger" or soft fruit firmer.

Aside from the inherent firmness developed within the apple during the growing season, the maturity of the fruit when it is harvested is the most important factor in determining post-storage quality. Washington's Apple Maturity Program (AMP) is viewed with interest by fruit growers in other regions. South Africa's maturity program and those of many other regions are patterned after the AMP. The AMP is an excellent synthesis of objective laboratory data and years of horticultural experience. Its meetings are an excellent place to exchange information about industry activities before and during harvest, a worthy investment of time and money. The AMP observations on fruit maturation are carefully considered and well discussed, yet they are not followed often enough. Our industry should pay more attention to this organization and its work.

Picking date should be determined after carefully balancing the internal factors of the fruit and the desired marketing period. Notice that the term is factors–no single factor can result in an accurate determination every year.

Rapid Establishment of Storage Conditions

Whether fruit is destined for regular storage or controlled atmosphere (CA), speed is of the essence. Rapid reduction of field heat is important in conserving firmness. Rapid oxygen reduction is critical for CA.

Appropriate Room Atmosphere

Scientists have found a large difference in firmness between fruit in CA and that in regular storage. Fruit placed in CA immediately after harvest for as little as one month is firmer than that held in regular storage for the same period.

There is also a difference between fruit in CA at 2+% oxygen and that in 1-1.5% oxygen. Fruit held at lower oxygen levels will be firmer. The difference in fruit firmness after storage between 0.8% and 1.2% oxygen is not as great as the difference between 1.0% and 2.0%. Fruit may be injured at very low oxygen levels; therefore, accurate computerized monitoring and control are important. Overmature, watercored fruit is also at very high risk.

Several warehouse managers have asked about ethylene levels in apple storage rooms. Limited testing in previous years has shown that some rooms run as high as 1,000 ppm ethylene. Scientific tests with chambers and commercial rooms as well as industry experience have not shown any increase in fruit firmness from scrubbing ethylene. Scientists report that both the low levels of oxygen and cold temperatures inhibit fruit response to ethylene in storage rooms. In addition, ethylene is like a toggle switch which is activated at 1 ppm. Greater concentrations have the same effect on fruit as 1 ppm. Ethylene-free storage is a whole program that starts with preclimacteric fruit, battery-operated forks and careful segregation to keep ethylene levels below 1 ppm. This may be necessary in the future to reduce storage scald but has not shown a benefit on firmness.

Storage Samples

At harvest, operators have samples of fruit taken from every grower lot in every room and hold these fruit near the door so that they can be easily obtained for warm room sampling. Twenty apples per lot per time of testing are needed. Testing should occur at intervals during the storage season. Some operators test every three weeks. Firmness loss is the most accurate method of determining fruit deterioration in these tests. It is not necessary to test for starch or soluble solids. Cut each apple to check for internal browning or decay after testing for firmness.

To do the tests remove a 20-apple sample of each grower lot from each room and separate the fruit into two groups of ten apples each. Keep each grower lot and room separate. Pressure test each of the 10 apples twice, avoiding the sun and shaded sides, and compute the average for each lot. Allow the additional 10 apples to remain at room temperature (70°F) for 1 week for domestic or 2 weeks for export shipments. Pressure test and obtain a per-lot average. Compare both the drop in pressure of the lot and the average firmness of the lot with data from other lots in the room. Compare the variability of the lots in that room with those in other rooms. This information will help in scheduling of the room openings.

Packing Temperature

Once the fruit is removed from storage the best way to retain firmness is through cold temperatures. Our studies in a number of packinghouses have shown that fruit temperature rises the most in heated dump tanks. Fruit temperature rises 20°F through even brief immersion in water at 80°F. Reducing the temperature of the dump tank water can add shelf life to the fruit.

Ventilation should be used after packing. The Washington industry has removed most of the hand-holes in the boxes, leaving no ventilation. This is an unwise policy in my opinion since it impairs cooling palletized fruit. It would be better to have boxes with vent holes and to use forced air to cool the fruit as is done in the California berry and stone fruit industries

Another alternative is to loosely stack the boxes on the pallet, using the same techniques as the pear industry. The separation of the boxes on the pallet would allow the air to circulate along the sides of the boxes rather than simply speeding past the outside.

Fruit cooling rooms should have a great deal of air circulation, and holding the temperatures below 30°F would help speed cooling. Fruit should be carefully monitored and removed when cool.

Judging Fruit Firmness

The industry sets minimum firmness standards on Red and Golden Delicious apples shipped from Washington. Shippers are not allowed to ship Red Delicious apples in which not more than 10% of the apples test below 12 lbs. Golden Delicious apples cannot have more than 10% of the apples below 10 lbs. These are not average numbers, these are minimums.

Shippers who obtain average numbers for a lot of fruit are confused about whether a lot would pass inspection under the law. We are developing data to help make that determination. The following preliminary data are for your testing program only, not legal minimums

The procedure is as follows:

  1. Test the firmness of 7 apples for the lot using two punches per fruit. The lot can be one grower's fruit or a single size apple from a number of growers.
  2. Figure out what the standard deviation is for those numbers using a calculator that gives standard deviation. Standard deviation is a number that expresses how variable the firmness in the lot is. Put the firmness values into the calculator and it computes the standard deviation. The more variability (standard deviation) in firmness, the higher the average firmness value needs to be.
  3. Using Table 1, you can determine what the average firmness value needs to be greater than in order to have not more than 10% of the fruit below 12 lbs. for Red Delicious or 10 lbs. for Golden Delicious. (Note of caution: This table needs further testing. Please use it this year with caution and get back to me with your experiences.)

Table 1. Minimum average firmness levels needed to meet current Washington standards (derived from Grant, E. L. & R. S. Leavenworth. 1972. Statiscial quality control. p. 678. McGraw-Hill. New York, NY).

Minimum Average Firmness
Standard DeviationRed Delicious1Golden Delicious2
1The value in this column is the minimum average firmness for Red Delicious so that not more than 10% of the fruit will be below 12 lbs.
2The value in this column is the minimum average firmness for Golden Delicious so that not more than 10% of the fruit will be below 10 lbs.


During the storage season, it is critically important to keep track of fruit firmness. Doing warm room tests is the best method of determining the rate of deterioration of the fruit. Temperature management is the best way to slow fruit softening during and after packing. Reducing dump tank temperature and rapid cooling of boxed fruit are critical

Dr. Eugene Kupferman, Postharvest Specialist

WSU Tree Fruit Research and Extension Center
1100 N. Western Ave., Wenatchee, WA 98801

Tree Fruit Postharvest Journal 3(4):4-6
December 1992

Tree Fruit Research & Extension Center, 1100 N Western Ave, Washington State University, Wenatchee WA 98801, 509-663-8181, Contact Us