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WSU-TFREC/Postharvest Information Network/Effects of RetainTM on 'Delicious' Apple Maturity and Storability

Effects of RetainTM on 'Delicious' Apple Maturity and Storability


During the fall of 1995, Abbott Laboratories sought assistance from the Washington Apple Maturity Program to perform maturity tests on apple samples treated with ReTainTM (ABG 3168) under an Experimental Use Permit (EUP). Samples from six 'Delicious' orchards were tested to determine the effects of ReTainTM on harvest maturity and storability. The study was expanded in 1996 to include 25 orchards and repeated again in 1997 with 10 orchards. ReTainTM was registered for use in 1996.

Testing began approximately two weeks before the estimated harvest date and continued each week until approximately two weeks after harvest. Twenty apples from each sample were placed into CA storage for evaluation after approximately 7 months. After storage, 10 apples were tested immediately out of CA (+0) and 10 apples were tested after an additional 10-day warm room treatment at 68 F (+10).

Internal ethylene was suppressed in apple samples treated with ReTainTM during harvest testing in all three years.

During each of the three years of this study, ReTainTM treated samples from some blocks were ½ to 1 lb firmer at harvest than the untreated controls, while other blocks exhibited less dramatic treatment effects at harvest. Firmness retention during CA storage in 1995 and in 1996 appeared to be enhanced by ReTainTM, with effects increasing as sampling dates progressed. Each year treated samples harvested after September 15 were approximately 0.5 to 2.0 lbs firmer than untreated samples immediately out of storage. After the 10-day warming, treated samples were still firmer than untreated samples by 0.5 to 1.0 lb. (Figures 1 and 2).

After mid-September during each of the three years of the study, starch scores averaged approximately 0.5 to 1.0 higher in untreated samples than in treated samples. Soluble solids at harvest were also consistently higher in untreated than treated samples. After mid-September, soluble solids averaged approximately 1/2 to 1 brix higher in the untreated samples than in the treated samples. Differences in soluble solids could be somewhat related to starch content. Less starch-to-sugar conversion in treated apples could result in lower soluble solids. Soluble solids differences diminished during storage. Treated samples from the 1995 and 1996 crops generally gained more soluble solids in storage than untreated samples, making soluble solids nearly equal in treated and untreated samples.

Data collected during the past three years indicate that in many instances, harvest can be delayed for 1 to 2 weeks longer in ReTainTM treated apples than untreated apples, without compromising firmness or storage life. During 1995 and 1996, delaying harvest for 1 to 2 weeks permitted the fruit to grow 1 to 2 box sizes larger and increased color development by approximately 10%. Box size and color development did not increase appreciably after mid-September in 1997. Although development of storage scald was minimal during the 1995 and 1996 storage seasons, a 1 to 2 week harvest delay might reduce the incidence of scald if cold nighttime temperatures are lacking early in the season.

One of the most pronounced effects of ReTainTM on 'Delicious' samples was watercore suppression. Twenty-five percent of untreated apples tested on September 25, 1995 contained watercore while only 6% of treated apples were affected. By October 9, 91% of untreated apples and 45% of treated apples contained the disorder. After the 1995-1996 storage season, a greater incidence of internal browning was observed in untreated than treated samples. Incidence of internal browning in untreated samples seemed to correspond with the amount of watercore present at harvest. Ninety-five percent watercore was observed in untreated samples tested on October 16, 1995. Like-samples, harvested during the same date and tested after storage plus 10 days warming, contained 37% internal browning. Internal browning was almost nonexistent in treated samples. Even when 73% of treated samples contained watercore on October 16, no internal browning was observed in either the 0-day or 10-day post storage treatments (Figure 3).

By September 23, 1996 only 2% of treated apples contained watercore, while 11% of untreated apples had the disorder. By October 7, 15% of treated apples and 45% of untreated apples were affected (Figure 4). Incidence of internal browning was minimal in 1996 storage samples. In 1997, 5% of treated apples and 13% of untreated apples harvested on September 22 contained watercore. By October 6 more than 60% of untreated apples had the disorder while less than 20% of treated apples were affected (Figure 5).

In most instances, ReTainTM is a valuable product for increasing packouts at the warehouse by suppressing watercore, maintaining firmness before harvest, and reducing firmness losses in storage. Greater packouts also can be realized when harvest is delayed without compromising apple firmness or storage life. In many instances delaying harvest for one to two weeks can increase apple size and color and reduce the likelihood that storage scald will develop. ReTainTM also promises to be a valuable management tool for control of harvest timing, resulting in more convenient scheduling of multiple block harvests and more efficient use of harvest crews.

Since no two crop years are exactly alike, apple maturation and the response to ReTainTM applications will vary from year to year. Seasonal variation in weather and orchard conditions cause apples to mature differently and at different rates each year. Accordingly, ReTainTM-treated apples will mature differently each year. Apple maturation also can be site-specific. Differences in elevation soil type, apple cultivar, or airflow patterns can cause apples to mature differently and at different rates. Characteristics of each orchard must be considered when evaluating maturity and storability of the crop.

Joyce A. Thompson, Manager

Washington Apple Maturity Program
1104 N. Western Ave., Wenatchee, Washington 98801

14th Annual Postharvest Conference, Yakima, Washington
March 10-11,  1998

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