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Monday, February 27, 2017

WSU-TFREC/Postharvest Information Network/Packing Red Delicious Apples in Washington State

Packing Red Delicious Apples in Washington State

Sampling Pattern

This report summarizes four years of study on the packing practices and fruit quality of Red Delicious apples.

Red Delicious apples were sampled from the packinglines from October to May each year. Red Delicious apples were sampled from the 1989 through the 1992 packing seasons. Packinghouses from each of the major growing districts were included (Table 1).

Table 1. Location of packinglines and number samples for Red Delicious included in this study.

North Central Washington4445
Total packinglines per season16261826
Total samples per season 353334335239

Fruit Firmness

The retention of harvest season firmness is essential since consumers demand a "crisp" apple, especially with Red Delicious. New regulations regarding minimum firmness levels were enacted by the industry and Washington State in 1990. During 1989, inspectors were working on verbal descriptions rather than actual pounds of firmness. In early 1990, the regulation that "not more than 10% of the fruit may be below 12 lbs." was implemented.

When viewed on a crop-year basis, the firmness levels for the fruit sampled from all years were higher than those sampled in 1989. In later years either fruit firmness was inherently higher (1990 and 1991 crops), or packers diverted to processing lots of marginal firmness prior to packing rather than risk the possibility that the fruit would not qualify for shipment (1992 crop). The average firmness level at time of sampling of the 1992 crop was below that of the 1990 and 1991 crops and shows the annual variability in firmness (Table 2).

Table 2. Yearly average firmness (lbs.) of Red Delicious at time of sampling and after 7 days at 70°F.

CA Storage

Time of
After 7 days
at 70°F
Time of
After 7 days
at 70°F
13.5 (1.6)**
13.4 (1.1)
15.4 (1.6)
14.2 (1.8)
15.7 (1.7)
15.3 (1.7)
16.2 (2.0)
15.1 (1.9)
16.1 (1.6)
14.1 (1.9)
14.46 (1.9)
13.6 (2.0)
14.6 (2.0)
14.1 (1.9)
* no warm room study for 1989 crop.
** numbers in ( ) are standard deviation.

It is useful to look at the week-to-week variability in the firmness over several years. In most years there was a drop in firmness just prior to the beginning of the opening of CA. However, both the extent and timing of this drop varies from year to year. Figure 1 plots the average firmness by week for all the samples tested immediately upon receipt by the lab. This shows the lack of firmness of fruit at packing in 1989 as compared with the other three years, as well as the dramatic drop in firmness in November. The 1990 and 1991 crops had very high firmness all season long with little decline in firmness in the 1990 crop.

When firmness of the apples was measured after 7 days at 70 °F, the 1992 samples were the softest (Figure 2). Warm room testing was not performed on the 1989 fruit. The drop in firmness of fruit stored in regular storage averaged 1.1 lb while drop in firmness of fruit from CA averaged 0.5 lb.

Shelf Life

We looked for a pattern of firmness loss during warm room tests to provide storage operators a tool to anticipate the loss of a specific amount of firmness per week following packing. It appears from the data that the amount of firmness a specific lot will lose during the warm room test is related to that lot and type of storage (CA vs regular) more strongly than which week it is taken out of storage (Figure 3).

Short-Term CA

Red Delicious fruit stored in CA for even a short period of time averaged 0.6 lb. firmer than that stored in regular storage (Table 3) when fruit sampled during the same week were compared.

Table 3. Comparison on the firmness of Red Delicious fruit stored in regular storage with that stored in "special" CA. Data reflect firmness levels from the first 90 days of storage each crop year at time of packing.

Year Regular
storage (lbs.)
CA (lbs.)

Summary of Firmness

The average firmness of Red Delicious at time of packing over the four years was 14.8 lbs. (Standard Deviation = 2.1). Regulations imposed during the 1989 marketing season modified the minimum firmness allowed. When the 1989 crop is excluded, the average firmness of Red Delicious at time of packing averaged 15.4 lbs. over the three years.

Fruit firmness varied by season, with fruit packed from the 1989 crop the least firm at 13.5 lbs. (1.6). The 1991 crop firmness was highest at 16.2 lbs. (2.0).

Firmness after warm room (shelf life) tests remained higher in fruit from CA than fruit from regular storage. Regular stored fruit lost 1.1 lbs. when held at 70°F for 1 week while fruit stored in CA lost only 0.5 lb.

Short-term (less than 90 days) CA stored fruit was 0.75 lb. firmer than fruit stored in regular storage over the same length of time.

The magnitude of firmness loss during warm room tests was erratic from week to week and no pattern, other than type of storage, was evident.

Soluble Solids

Soluble solids rise in storage as starch is converted to soluble solids (sugars). Soluble solids levels in fruit from the 1992 crop were higher in both the beginning and the end of the marketing season than in previous years (Figure 4). This is another indication that the 1992 harvest was delayed.

Over the four years of testing the average soluble solids level was 13.3% (0.8 St. Dev.). There was no statistically significant difference in soluble solids levels in CA or regular stored Red Delicious (12.9% [0.8 St. Dev.] and 13.4% [0.7 St. Dev.], respectively). Fruit from the 1992 crop had the highest soluble solids level.


Red Delicious apples harvested in 1992 had significantly higher acidity levels than those harvested over the last four years, except after March 1 (Figure 5). Acid levels in more mature fruit are normally low, so this is an indication of how different the fruit characteristics were in 1992.

Over the four years the average acidity of Red Delicious was 0.257% (0.059 St. Dev.). There was no statistically significant difference in acid levels in CA or regular stored Red Delicious (0.258% [0.062 St. Dev.] vs 0.255% [0.057 St. Dev.], respectively). Again, fruit from the 1992 crop had significantly higher acid levels than fruit from other years.


The greatest rise in fruit temperatures occurs in the dump tank. This is due to the ability of water to heat fruit more rapidly than air. Tabulated below are the season averages for fruit, water, and air temperatures at various points in the packingline (Table 4).

Table 4. Average and high temperatures of fruit, water, and air in packing lines.

Location 1989 Avg/High* 1990 Avg/High 1991 Avg/High 1992 Avg/High
Fruit before dumping45/5141/4541/4437/60
Dump tank water81/10075/10380/11878/123
Fruit before waxing62/68na**58/6949/78
Dryer air122/142115/139116/145115/165
Fruit at try filler61/7260/7062/7355/79

*Avg. is average value of all packinghouses samples. High is the largest value encountered.
**Not available for comparison

The industry should find methods to reduce the temperature of fruit going into the box since warm temperatures speed deterioration of fruit. Methods of reducing fruit temperatures would be to lower dump tank temperatures or forced air cool boxed apples.

Dump tank water is heated in order to raise the temperature of the skin of the fruit to clean fruit to obtain a high shine. A method needs to be found of obtaining a good shine without raising the fruit temperature. Chemical suppliers should work with the industry to develop alternative methods of cleaning and waxing fruit without heating it.

An alternative strategy would be to improve the cooling of boxed fruit through improved box design to include vent spaces which would allow for rapid forced air cooling.

Firmness Loss

This survey study has shown that the average temperature of Red Delicious apples at time of packing was 60°F. Since the refrigeration system of a truck is unable to remove any heat above that of respiration, it is unlikely that apples will be cooled in transit. The temperature of the flesh will equilibrate to something between the 40°F core and the 60°F temperature 0.5" below the skin.

A series of experiments was undertaken to determine how rapidly Red Delicious firmness would decline at different temperatures. Packed fruit from the 1990-1993 crops were purchased from commercial packers. Over the four years of this study, ten lots of Red Delicious were tested. Nine of the lots were from CA and the tenth was from regular storage. Five boxes of size 113 apples were purchased at the same time from the same orchard.

Boxes were obtained from commercial packers and placed into chambers held at 32°, 40°, 50°, 60° or 70°F. One tray from each box was tested after fruit arrived at the appropriate temperature. Additional samples were removed each week over a 4-week period. Half of these apples were tested immediately and the other half was tested after being held at 70°F for 1 week to get an indication of shelf life. Tests were run for firmness, soluble solids, and acidity. Fruit were also inspected for external and internal disorders.

Apples held at 60°F lost 3 lbs. firmness as compared with 1 lb. at 32°F after 21 days (Table 5 and Figure 6). Note the rapid loss of firmness at 60° or 70°F as compared with fruit cooled to 40° or 32°F. Figure 6 shows this trend in fruit directly out of the holding rooms. There appears to be a very significant acceleration in firmness loss after 14 days at temperatures above 40°F.

Table 5. Firmness loss in Red Delicious from date of sampling to examination date.

temp (°F)
Days at temperature
714 2128
32-0.3 lb-0.6-0.8-1.2



Red Delicious apples were packed at an average flesh temperature (at 0.5 inch below the skin) of 60°F. The highest flesh temperatures averaged 74°F. Boxed fruit will not cool rapidly, and truck refrigeration units are not capable of cooling fruit. Studies described here show that when Red Delicious are held at 60°F for 14 days they lose 2.3 lbs. Even if one assumes that the internal temperature stabilizes at 50°F, then the loss in firmness in 14 days averaged 1.1 lbs. This loss increases significantly following warm room testing.

This study proved that the largest rise in fruit flesh temperature occurs in the heated dump tank at packing. The industry should either reduce the temperature of the water in the dump tank, which averaged 79°F (high average of 111°F), or create a method of cooling packed fruit. Forced-air or pressure cooling is an inviting, cost-effective alternative used by other apple and fruit industries around the world and should be explored for use in Washington.

Dr. Eugene Kupferman, Postharvest Specialist

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

Tree Fruit Postharvest Journal 5(2):5-11
August 1994

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