Harvest Maturity and Storage of Granny Smith Apples For Late CA and Uniform Market Quality
Granny Smith is a relatively new apple for growers in Washington State. Plantings were stimulated by favorable prices and sales appeal based on desirable color, texture, and taste quality. Production is the result of calculated risk by innovative growers.
Growers expressed to these research workers that for this apple to be a marketable success, it must deliver uniform high quality internally and externally, month after month.
We interpreted these opinions to mean that fruit should be allowed to reach its maximum quality potential before harvest, prior to initiation of ripening. Treatment for protection from diseases and disorders and a postharvest environment that would arrest metabolism at its lowest level also would be required. Handling and storage recommendations from other parts of the world were not all in agreement or appeared not to address Washington grown Granny Smith apples.
Harvest Maturity Quality Parameters
At maturity, the physiological switch-over from a development stage to a ripening stage can be likened to a command to die.
Fortunately for Washington grown Granny Smiths, this is a relatively late event in the fruit's development. This timing generally permits energy captured by the tree to be converted into a maximum potential for fruit quality before the death signal is given. There are two ways to know when this command has been given. One is the sudden rise in respiration rate following a long seasonal decline. This is known as the respiratory climacteric. The other is a concurrent, rapid rise in the synthesis of ethylene that starts from a seasonal zero baseline.
In addition to measuring respiration and ethylene on our research fruit throughout the harvest season, we also determine the time these events occur in industry orchards for the Apple Maturity Program (AMP). This information is available through the AMP office.
In summary, the fundamental strategy for an extended storage season is to let fruit develop on the tree as long as possible, but to be sure that harvesting is complete before the climacteric rise begins. Quality parameters for preclimacteric fruit at maximum development at harvest vary markedly from year to year. Most of the responsibility for these differences rests in the ability of the tree to capture energy and allocate storage materials between the tree and the fruit.
Variation and quality parameters over the past 5 years in fruit from the same orchard picked just before the climacteric rise are given in Table 1.
These data show that apples below 16.9 pounds firmness would no longer be preclimacteric in any of the 5 years. In the 1985 season, fruit would have passed from the preclimacteric into the postclimacteric stage when the firmness dropped below 20.5 pounds. Titratable acids did not go below 0.80% in 1987, but did fall to 0.59% in 1984 at the climacteric. Fruit entered the climacteric stage at starch ratings (New Zealand rating system 0 to 6) above 2.2 one season, and above 3.8 another. The soluble solids ranged between 10.7 and 12.2% in fruit entering the climacteric.
These values emphasize that quality parameters can vary markedly in different years using the same stage of maturity as determined by ethylene evolution and increased respiration.
The proper harvest maturity of Granny Smith apples depends upon the physiological maturity (preclimacteric state). However, quality parameters are independent of the physiological status. Proper physiological harvest maturity is a key factor in determining market quality after storage.
Table 2 summarizes the results of our research with postharvest protective treatments and individual components of the storage environment that extend life of the fruit. These data emphasize main effects from a series of treatments. They are combined to compare the effectiveness of low oxygen, diphenylamine (DPA), ethylene scrubbing and temperature on Granny Smith quality after storage. The lowest oxygen level listed in this series gave the best retention of fruit quality and freedom from disorders. Some of our other research has shown this is a practical safe level, provided the gas monitoring is accurate and the room CA is maintained with precision and accuracy. The DPA is not only effective in reducing disorders, but also has a physiological effect in maintaining fruit firmness. Ethylene scrubbing had little practical value for Granny Smith fruit within the treatment levels tested. Disorders, including those associated with chilling injury, were reduced the most at the lowest temperature. This temperature also gave the best retention of green color and the firmest fruit. The data do not show the best poststorage quality that was obtained from a single treatment that combined the best level of each factor.
The best combination of practices for Granny Smith harvest timing, protection and storage environment is summarized in Table 3. This combination has given excellent retention of harvest quality and storage life up to 1 year in our research.
Max E Patterson and William C. Nichols
WSU Postharvest Biology and Technology Laboratory, Department of Horticulture and Landscape Architecture
Post Harvest Pomology Newsletter, 6(2): 8-10