CA Storage of Bing Cherries
The basis for any storage regime for dark sweet cherries is temperature and humidity control. The prompt removal of field heat within two hours of harvest coupled with holding flesh temperatures at 30-32 degrees F. have given excellent results. If refrigeration is used without CA, flesh temperatures should be held at 30 degrees F. If CA is used, flesh temperatures can be 32 degrees F. Be aware that fleshy stems will freeze at slightly below 30 degrees F.
The objective of controlling the atmosphere (CA) is to lower the rate of respiration of the fruit below that achieved by solely reducing the temperature, thus prolonging storage life. Work at WSU's Post Harvest Laboratory in Pullman has shown that CA can be an added benefit to the storage of cherries, especially when 30 degrees F cannot be maintained.
The normal composition of air includes (among others) approximately 21 percent oxygen, 79 percent nitrogen, and 0.03 percent carbon dioxide. Of these, only the oxygen and the carbon dioxide are metabolically active--the nitrogen is inert. Increasing the carbon dioxide or reducing the oxygen will reduce the rate of living of the fruit. This reduction is the basis for slowing fruit aging, retaining internal quality and condition, and extending postharvest life.
Some commercial controlled atmosphere (CA) storage of cherries has occurred. In 1979 it is estimated that 120,000 boxes were put in a 20 percent carbon dioxide CA atmosphere.In 1981, all export fruit contracted to Japan from Yakima were shipped in transit containers charged with carbon dioxide, allowed to equilibrate to about 20 percent carbon dioxide, and maintained near that percentage until arrival.
Although the cherry responds very favorably to various forms of atmosphere control, (low oxygen or high carbon dioxide), CA should not be contemplated until pulp temperatures are in the 32-35 degree F. range to ensure the safety of the fruit from anerobic fermentation.
Experience has demonstrated the importance of paying attention to several factors which are: 1) large, firm fruits, 2) freedom from damage, and 3) the prevention of moisture loss. Fruit should be large and firm with high soluble solids and free of rain cracking and rain softening. Picking, hauling, and packing operations should attempt to minimize physical damage since damage promotes pitting and bruising. Select only good fruit, free of damage, for CA.
Fruit moisture must be preserved from the moment of harvest in order to maintain freshness. Conditions that accentuate aging and moisture loss before CA will result in a loss of freshness and quality that become increasingly visible as the length of storage increases. Transit environment recommendations must consider delivery time plus storage and display time in order to maximize the treatment effect.
Preventing Moisture Loss
Vapor diffusion resistance values developed by Gary Apel in our laboratory show that sweet cherry has one of the poorest barriers to loss of water vapor. Comparing values with Delicious and Golden Delicious reveals a potential for moisture loss for cherries from 7-14 times greater than these apple cultivars.
Every effort must be made from the moment of detachment to provide temperature and relative humidity environments that will minimize the movement of moisture from within the fruit to the external atmosphere. We have observed freshly harvested cherries lose weight at the rate of 1 percent per hour under laboratory drying conditions. Under the same conditions, stems may lose 4 percent per hour. Field and packing house conditions may result in much greater losses.
Moisture losses will be minimal when internal and external environments equilibrate just above the freezing point of the tissue and at humidities approaching 100 percent.
Cherries are commercially packed in 12 or 20 pound fiberboard boxes within polyethylene film liners. The boxes are frequently stacked 66-99 to a pallet. Heat removal under these conditions is very slow. The boxing and palletizing system creates barriers that may result in widely different gas atmospheres in the room and in the box before equilibration occurs. Therefore, the atmospheres selected must lie between that which is the most effective for fruit longevity, and those causing toxicity. Fortunately, the cherry is quite tolerant of a wide range of gas levels and has wide limits between favorable responses and toxicity.
Experiments at WSU, Pullman
We have had Bing cherries in some form of experimental modified atmosphere treatments on an annual basis since 1971. Treatments have included low pressures, various carbon dioxide and oxygen combinations, static systems, and flowing systems.
Most of our treatments have been with flowing systems of premixed gases, metered to small lots of fruit in chambers in cold rooms with adequate refrigeration for rapid heat removal. Gas mixtures were prepared by adding carbon dioxide and/or nitrogen to air to provide an analyzed composition with + 5 percent of stated values. Fruit characteristics were evaluated after 0, 2, 4, 6, and 8 weeks in storage.
Our experiments have shown that dark sweet cherries can be stored for eight weeks or more without appreciable deterioration of fruit quality. Unfortunately, cherries are rated by some buyers as much on the basis of greenness of the stem as on fruit quality. This results in the necessity to arrive at a storage regime which is suitable for both a vegetative tissue (stem) as well as a fruit.
Several CA Options
There are three major options open to the warehouse manager who wishes to extend the marketing season of fresh cherries. Each option has its advantages and its disadvantages.
Air storage at 30 degrees F. is the simplest option.
The second option is high carbon dioxide (20 percent or more) at a temperature around 35 degrees F. High carbon dioxide involves the introduction of carbon dioxide into the room so that 20 percent carbon dioxide results. By adding 20 percent carbon dioxide, the oxygen in the room will naturally fall to about 17 percent (21 percent oxygen x 0.8).
The third alternative is to consider storing cherries in low oxygen. Oxygen levels for this treatment should be about 1 percent and the carbon dioxide should be kept below 1 percent by scrubbing.
CA Effect on Firmness
Figures 1 and 2 are typical averages of firmness data obtained at 2, 4, 5, and 8 weeks in storage using a Durometer hardness tester modified to prevent puncturing the fruit. In this system the larger the number, the firmer the cherry.
Cherries will increase in firmness above harvest values after refrigeration and removal of field heat. This increased firmness is present even after the fruit is returned to room temperatures. Loss of firmness occurs with time after harvest and is more rapid at progressively higher temperatures above 32 degrees F. Although high carbon dioxide allows more stem deterioration than low oxygen, high carbon dioxide retained fruit firmness better over the long storage period.
CA Effect on Soluble Solids
At 32 degrees F. there is little change in soluble solids after 8 weeks of storage (Figure 3). However, at 35 degrees F. there are losses in air which can be retarded by low oxygen or even more so by high carbon dioxide. High carbon dioxide (20 percent) is slightly more effective in reducing the loss of soluble solids than other atmospheres when temperatures are above 32 degrees F.
CA Effect on Decay
High carbon dioxide has consistently reduced the amount of decayed fruit as well as the amount of decay per fruit (Figure 4). This suppression is much greater at 32 degrees F than at higher temperatures.
CA Effect on Color
Low temperatures, low oxygen, and high carbon dioxide have been observed to suppress further red coloration (anthocyanins) in cherries. Color ratings of random samples seldom reveal the suppression due to variability. A better indication is given in Figure 5 showing pigment differences after 5 weeks in samples selected for uniform color before storage. After storage to O°C. the pigment synthesis is inhibited most by 20 percent carbon dioxide, and lower oxygen levels, than in air.
Improved Shelf Life
Limited data has indicated that fruit respiration is reduced during the post storage shelf-life period by high carbon dioxide CA. Reductions of nearly 10 percent below air or low oxygen stored fruit have been observed over a 3-day period at 68 degrees F following storage.
Controlled atmospheres are additional tools, beyond low temperature and humidity, that can be used to arrest aging, maintain quality and condition and further extend postharvest life of sweet cherries. The sweet cherry is tolerant of a reasonably wide range of concentrations of oxygen and carbon dioxide levels in the storage atmosphere. Either raising the carbon dioxide component to 20 percent or decreasing the oxygen to 1 percent have significantly extended cherry life.
Max E. Patterson(1) and Eugene Kupferman(2)
(1)Department of Horticulture and Landscape Architecture, Washington State University (2)WSU Tree Fruit Research and Extension Center
1100 N. Western Ave., Wenatchee, WA 98801
Post Harvest Pomology Newsletter, May, 1983, Volume 1, No. 2.