Preserving Granny Smith Quality and Condition
ManuscriptProduction and marketing of the Granny Smith apple is increasing in economic importance in Washington state. Granny Smith achieves exceptionally high quality in appearance, texture, and flavor under Washington climatic and soil conditions. It also is subject to deterioration, which can impair quality and limit the market period. This is a summary of existing information and findings from research conducted at the WSU Postharvest Laboratory, Pullman, relative to holding Granny Smith apples for the late storage season. It reports what our laboratory knows, believes, or thinks we know at this time.At its best, the Granny Smith is a hard, green, slightly acid, juicy, uniquely flavored cultivar which appeals to the palate of a select group of apple eaters. With senescence it becomes softer, loses its green color, develops yellow pigments, loses acidity and flavor, becomes mealy and loses palatability. It is also subject to a series of disorders which may be initiated on the tree, or during regular atmosphere (RA) or controlled atmosphere (CA) storage. These disorders further detract from the apple's appearance and palatability. Some of these include: light green color, sunscald, latent sunscald, watercore, bitter-pit, scald, chilling injury (core flush), senescent breakdown, fermentation and carbon dioxide accentuated chilling injury. Although the Granny Smith holds up exceptionally well under adverse environments, attention to optimum physiological age, the most effective postharvest treatments and precise control of the storage environment are all equally essential if the packers wish to maintain premium quality for late season marketing.
ColorPart of the appeal of the Granny Smith is its bright green color, which contrasts sharply with that of the red and yellow cultivars. It should have a good, uniform green color, not light and not streaked. The principal pigment of green apples is chlorophyll. Greenness is proportional to the quantity of chlorophyll present in the skin cells. Formation of chlorophyll is directly proportional to the level of nitrogen nutrition of the fruit. The degree of greenness at proper harvest is therefore primarily a response to nitrogen fertilization. Some apparent decrease in greenness occurs with final enlargement on the tree prior to harvest when static levels of chlorophyll are diluted by the increase in surface area. Dynamic destruction of chlorophyll occurs concomitantly with the climacteric and, therefore, occurs too late to be a useful harvest indicator.Genetic control of formation and destruction of chlorophyll dictates differences in degree of greenness among cultivars. Rootstocks may also affect the degree of greenness, presumably through differences in nutrient absorption. Greenspur and a Washington State University mutant have the deepest and most uniform green observed here, followed by the Standard Granny, Granspur and the Early Granny cultivar, respectively. We have noted differences in color among different rootstocks that were taken from different orchard locations. The fruit from trees on EM 26 rootstocks maintained a more uniform green color during storage and poststorage shelf life than did fruit from trees on MM 106. Fruit from MM 106 loses chlorophyll in streaks, beginning in the stem cavity. This difference may relate to location or differences in nutrient uptake. Under the long days and high light intensity common in Washington orchards, some fruit develop a pink coloration due to anthocyanin synthesis. Such fruit are very attractive, and analyses of sugar and acid content reveal that these are superior in quality to the all green fruit, although this does not fit the image of an all green apple. Do not discriminate against the pink tinged fruit; they might constitute a premium grade.
SunscaldVisible sunscald, develops on trees following hot days with high solar radiation. Affected fruit can be readily removed during sorting. A latent sunscald, which develops in storage following incipient injury becomes a storage problem. Little can be done to reduce injury except to try to protect nonacclimated fruit from direct radiant exposure.
WatercoreWatercore is a condition of apples that develops on the tree. Space between cells that is normally filled with air becomes filled with liquid. There are two forms of watercore. Hot, radiant conditions that cause sunscald, also promote a liquid logging of flesh near the surface. However, watercore is also an aging disorder that appears after the initiation of the climacteric. Whether advancement into this stage and appearance of watercore are accelerated by a series of cold nights, as is true of some cultivars, has not been determined.
Bitter-pitGranny Smith fruit are particularly prone to develop bitter-pit. Bitter-pit is the result of calcium deficiency. When the deficiency is most severe, pitting will occur on apples still on the tree. When the deficiency is less severe and pitting has not occurred on the tree, it can occur in storage unless growers supply calcium in a postharvest treatment. The fruit takes up most of the calcium it can use early in its life on the tree. It is difficult to influence calcium nutrition after this period with soil applications.
ScaldScald is a major disorder of Granny Smith. The earlier fruit is harvested before the climacteric, the greater the potential for scald and the more difficult it is to control scald with postharvest antioxidant drenches. In comparing fruit responses in some of our work, one-half of the fruit were drenched with 2,000 parts per million (ppm) diphenylamine plus Benlate (2,200 ppm diphenylamine plus Mertect the second year) immediately after harvest. Half of the drenched and undrenched fruit were infused with a calcium solution.
Chilling InjuryIt has been known for a long time in the Southern Hemisphere that Granny Smith apples are sensitive to chilling. Injury develops, followed by cell death and browning at temperatures well above freezing. When storage temperatures are maintained below 38°F (3.3°C), the fruit is subject to chilling injury. This shows up over time as browning of the core tissue between the vascular bundles and the seed cavity. Susceptibility is modified by phosphorus nutrition, growing temperature, physiological stage at harvest, and postharvest oxygen and carbon dioxide levels. Within the chilling range the threshold temperature may differ among orchards and between years due to differences in the modifying variables. Chilling injury goes under a series of descriptive terms, such as core flush, low temperature breakdown (LTB), brown core, and carbon dioxide enhanced brown core. In some cultivars soft scald is a form of chilling injury. We have attempted to lower the chilling threshold temperature by postharvest treatments and CA storage regimes. We are trying to store at 33.8° F (1 °C).
Senescence is the prelude to death. In apples it begins with the start of ripening. Ethylene generated by the fruit triggers senescence naturally. This ethylene is responsible for terminating maturation and bringing the fruit to maturity. Maturity does not necessarily occur at the same time or the same firmness, soluble solids, starch, or acidity. Ethylene also initiates the respiratory climacteric.
Preclimacteric detached fruit produce ethylene slightly sooner than identical fruit left on the tree. These fruit also commence ripening sooner. Preclimacteric detached fruit will start ripening from external sources of ethylene in proportion to the ethylene concentration. The metabolism associated with the climacteric uses a large proportion of the energy needed to maintain life.
Preservation depends primarily on how well the energy stored in the fruit can be conserved. This in turn depends upon the physiological stage at harvest (whether pre or postclimacteric), and upon the postharvest temperatures, atmospheric gas com position, and the durations the environments are imposed. Cold temperatures, low oxygen and elevated carbon dioxide each inhibit ethylene production. Carbon dioxide inhibits action. These factors can delay the climacteric of preclimacteric fruit and suppress fruit consumption of energy. The optimum procedure is to harvest just prior to the climacteric (due to natural ethylene production) to develop the maximum energy inputs and quality potential, to optimize the reduction of scald and watercore, and to delay senescent breakdowns. We have found that a postharvest fungicide/antioxidant treatment, in addition to its primary tasks, also lowers the respiration rate. Adding a calcium treatment further reduces respiration. These effects are additive to proper harvest timing in preserving quality and reducing disorders. To obtain maximum conservation of fruit energy, immediately follow postharvest treatments with the most effective temperature and controlled atmosphere environment.
FermentationFermentation is the production of acetaldehyde and ethanol by fruit, in response to inadequate oxygen in CA. It denotes a switch from aerobic respiration to anaerobic respiration. In addition to causing flavor taints, excess levels of the fermentation products result in death of fruit tissues. Apples require higher levels of oxygen for respiration at high temperatures than at low temperatures.
Carbon Dioxide Accentuated Core FlushCarbon dioxide is present in air at about .03% or 300 ppm. Because this amount is so small, people often fail to realize that an increase to 1.0% in a CA room is an increase of 3,333 times the amount in air. Possible benefits from elevated CA room carbon dioxide above levels in air have often been missed because of excessive increases of carbon dioxide. In CA, oxygen may be reduced to 1/10 or 1/20 of the amount in air, while carbon dioxide may be increased 3,000 to 10,000 times. In this case the compatible ratio of these two atmospheric gases has been further increased, by as much as 200,000%. Therefore, it is not surprising that carbon dioxide can become toxic, or enhance disorders at low oxygen levels. This is true in Granny Smith fruit where excessive carbon dioxide to oxygen ratios at chilling temperatures accentuate core flush. Whereas, appropriate ratios of carbon dioxide may suppress respiration, ethylene production and ethylene action.
The Postharvest Laboratory at Pullman, through the generosity of the Auvil Fruit Company, was able to renovate existing RA storages to state of the art computer operated CA facilities recently. The facilities were designed to minimize pressure fluctuations, provide maximum flexibility for generating mixtures of up to three gases, provide a spectrum of CA gas composition mixtures, provide minimum fluctuations in gas composition during adjustment, eliminate additional heat load and atmosphere pollution and perform all control and monitoring routinely with precision and accuracy. This facility provides a capability to exercise any of the existing strategies for CA operation, such as high CO2 pretreatment, rapid CA, standard CA, low O2, ultra-low O2, or other strategies yet to be developed. It has permitted a much greater breadth and depth of testing for optimum Granny Smith storage environments than would have otherwise been possible. Physically the facility includes four CA rooms and 12 chambers with automated control and monitoring and 12 chambers with manual control and automated monitoring. Besides the CA, a dedicated RA room provides a check environment.
Current ResearchGranny storage experiments were designed to compare physiological age at harvest, postharvest antioxidant and calcium effects, and a series of controlled atmosphere gas combinations. Fruit were observed at harvest and after periods of approximately 3½, 7 and 10½ months. At each removal fruit from all treatments were measured for color, starch, soluble solids, pH, acidity, firmness, ethanol and acetaldehyde. At the same times, the incidence of internal and external disorders was recorded. Another set at each removal was held at 68°F (10°C) and then observed for the same parameters plus respiration and ethylene production. Treatments consisted of two harvests x 4 postharvest treatments x 5 environments. Each treatment replicate was removed three times for immediate, and again for 7-day shelf life measures of the above quality and condition parameters. Twenty fruits, four from each of five boxes, were measured for each parameter of each treatment at each observation for 2 years. From these and other experiments, a considerable volume of data has been generated which permits certain conclusions at this time.
There is no practical difference in quality parameters at harvest between fruit picked just prior to the climacteric and fruit picked at the climacteric, but there is an appreciable difference in storage life. The preclimacteric fruit maintain harvest quality parameters better and longer than the later harvested fruit.
The DPA-fungicide drench gives good scald and pathogen control at the optimum CA regime. The addition of an infusion of calcium slightly enhances control of disorders and quality retention. Starch disappears under the best of treatments, but soluble solids remain relatively constant. The largest changes affecting quality are acid loss and decrease in firmness.
In the absence of oxygen, fermentation products are produced sooner by the physiologically older fruit. The threshold oxygen level for accumulation of fermentation products is not consistent between years but resides between 0.4 and 0.8% under our conditions. Accumulations of ethanol and acetaldehyde occur considerably before they can be detected by taste. However, concentrations below the taste threshold are not toxic to the fruit. Upon being sealed in an anaerobic environment, fruit requires more than a week of complete absence of oxygen to accumulate levels that can be tasted.
Preclimacteric fruit, a postharvest drench and calcium infusion followed by a rapid CA environment of 33.8°F (1 °C) and 1% oxygen, plus carbon dioxide not allowed to exceed 1%, have consistently given freedom from bitter-pit, scald, watercore and core flush, an absence of fermentation and retention of near harvest quality and condition.
I wish to acknowledge the considerable contribution to the Washington State University postharvest program and the knowledge of pre- and postharvest fruit behavior developed by the former graduate students whose original research work is listed in the following references.
Apel, G.W. 1983. Ethylene production and conversion of 1 -amino-cyclopropane-1-carboxylic acid to ethylene as measures of physiological age of apples and quantification of ethylene in solution. PhD Thesis, Washington State University, Pullman.
Chavez, C.H. 1983. Evaluation of horticultural quality, harvest maturity, and storage environments in Granny Smith apples. MS Thesis, Washington State University, Pullman.
Chu, C.L. 1980. Study of current maturity indices in relation to the position of 'Red Delicious' apples on the tree. PhD Thesis, Washington State University, Pullman.
Kvaale, A. 1968. Changes in pigmentation of Golden Delicious apple fruit as related to maturation and ripening processes. MS Thesis, Washington State University, Pullman.
Lizana, L.A. 1965. Enhancement of anthocyanin pigment in Malus. MS Thesis, Washington State University, Pullman.
Looney, N.E. 1966. Some biochemical and physiological aspects of maturation and ripening of climacteric fruits. PhD Thesis, Washington State University, Pullman.
Mansour, M.F. 1985. Relation of fruit position to ripening, quality, composition and storability of Golden Delicious apples. PhD Thesis, Washington State University, Pullman.
Marlow, G.C. 1982. Sorbitol metabolism and watercore in apples. MS Thesis, Washington State University, Pullman.
Nichols, W.C. 1986. Ethanol and acetaldebyde accumulation during low-O2 storage of apples. MS Thesis, Washington State University, Pullman.
Saikia, B.N. 1969. A study of ultrastructural changes during maturation and ripening of the apple fruit. PhD Thesis, Washington State University, Pullman.
Dr. Max E. Patterson, WSU Horticulturist
Department of Horticulture and Landscape Architecture, Washington State University
Post Harvest Pomology Newsletter, Vol. 4, No. 2