Maximizing Postharvest Quality of Gala and Fuji Apples
Apple production around the world is at record levels nearly everywhere apples are grown. Markets have not expanded to keep pace with the increasing supply, and economic instability in various markets has resulted in significant changes in the apple business in the past couple of years. Many of the forces driving the world apple market are beyond the control of growers, warehouse managers and storage operators. Fruit quality management, however, is something that can and must be controlled to remain competitive with all that other fruit. Changes in cultivars in the northwest apple industry in the past 10 years have brought good and bad days to the industry. For Gala and Fuji, the good days have been very good, but the bad days usually result from different factors. These cultivars represent extremes in ripening characteristics leading to some different considerations for postharvest management. When properly managed, Gala and Fuji hold their great quality for long periods. Under less than optimal conditions, quality loss can occur rapidly for both cultivars.
As with all apples, Gala fruit quality after storage is determined by appearance, texture and flavor. Its postharvest appearance characteristics are determined primarily in the orchard as Gala is not highly susceptible to scald, CO2 injury or other peel disorders. Occasional problems with lenticel spot induced by poor nutrition or postharvest chemical injury have occurred but in general storage of this cultivar is not limited by cosmetic disorders. Gala has a fine firm texture that can be lost rapidly after harvest under poor storage conditions. A loss of several pounds firmness is not uncommon even under CA conditions; therefore, harvest at relatively high pressures, 17 to 18 lbs., is advisable for fruit to be stored any length of time. Much of the firmness loss detectable using a penetrometer is lost in the first 30-60 days after storage. After that period penetrometer readings decrease at a slower rate although textural changes continue that render the fruit spongy or rubbery. CA storage is very effective at slowing Gala firmness loss provided the atmosphere is imposed soon after harvest.
Like quality, flavor has several components. Aroma compounds released by intact fruit or by cutting or chewing are perceived in the nose. Basic tastes--sweet, sour, astringent and bitter--are detected on the tongue. Mouthfeel also is a component of flavor and is the combined sensations in the mouth during eating. The components of flavor are impacted by harvest maturity, storage conditions and storage duration. Sugar content increases as starch breaks down after harvest. Over an extended storage period, soluble solids content increases from harvest then does not change much. CA is effective for reducing the rate of acid loss during storage. This is critical to flavor because apples taste flat when the acidity is too low. Aroma production is negatively impacted by CA conditions; however, aroma recovery after CA occurs pretty well through 60 -90 days storage. After that, flavor will be composed primarily of basic tastes: sweet, sour, and mouthfeel. Reducing acid loss is therefore very important for long term storage of Gala because the basic tastes become a larger part of flavor when aroma is reduced after CA.
Although most of the ripening characteristics of Gala have been found to respond well to CA conditions, other factors have resulted in poor quality fruit after storage. Poor quality in some cases has been the result of "market-based storage management." Apples harvested to be sold in the early season may not find buyers due to weak market conditions. If several weeks pass and the fruit remains in regular cold storage, imposition of CA is not very effective. Fruit harvested late and not sold will present an even bigger problem. The ripening characteristics of Gala require storage conditions to be made at harvest, using "fruit quality-based storage management." This type of management considers maturity and storage conditions first rather than the chances of early sales. Using this approach, if the market stays bad, fruit quality will be much better after longer storage compared to waiting at harvest to gauge the market.
Fuji apples are prone to several physiological disorders during storage. Among these are the cosmetic disorder stain and internal breakdown. The risk of both of these problems increases with later harvest dates, and environmental and physiological factors play roles in both disorders.
Stain is a peel disorder that typically affects the blush side of the fruit. It often appears on the margin of sunburn and has a well defined, rather than diffuse border. Although stain can appear prior to harvest, typically the symptoms develop after harvest during storage. The discoloration generally appears 1 to 3 months after harvest, although earlier or later onset can also occur. Storage in CA slows the onset and reduces the severity of the visual symptoms but CA does not prevent stain. Use of diphenylamine (DPA) is ineffective at preventing the symptoms.
Stain appears to be induced pre-harvest during the later part of fruit development. Experiments where apples were bagged for two week periods from July through October indicate light exclusion in late September and October significantly reduced the incidence of stain. During this period, ripening processes begin including changes in peel chemistry and physiology. Chloroplasts begin to breakdown and other pigments are produced resulting in a change in the ground color from green to yellow. Red pigments also are produced in increasing amounts. Environmental changes are also occurring, cooler temperatures and shorter days mark the entry to fall. Although less frequent compared to summer in the NW, clear sunny days occur well into October and early November. It may be a combination of the changes in apple peel physiology combined with the late summer sun that induces the injury that subsequently develops into stain.
Although the exact mechanism of stain induction remains unknown, irradiating Fuji apples with UV-b light can induce stain-like symptoms. Fruit exposed to UV-b in the laboratory develop symptoms that are indistinguishable from stain. Symptoms only develop after apples are irradiated and held in cold storage. Fruit harvested at two week intervals throughout the season then treated with UV-b only develop stain if harvested in late September or October. Apple peel responds to UV-b light by altering pigment synthesis to protect against the high energy of the irradiation. The ability to respond to UV-b changes as fruit development progresses late in the season and may become less efficient. This may be associated with subsequent development of stain.
Postharvest temperature also appears to play a role in stain development. Fruit harvested without symptoms stored at warm temperatures do not develop as much stain as fruit stored cold after harvest. Apples exposed to UV-b show the same response, fruit stored warm does not develop stain while the incidence increases with decreasing storage temperature. However, the temperature where stain incidence decreases is too high to prolong storage life. Short term warm temperature storage has as yet proven unsuccessful at reducing stain incidence although immersion in 120°F water for 1 minute reduced stain incidence. More research is planned to further investigate these responses to postharvest temperature.
Mineral analysis of stained peel has sometimes indicated a high magnesium to calcium ratio. Not necessarily low calcium, just lots more magnesium. This characteristic is not always found in stained peel, however it raises the possibility that fruit nutrition may also play a role in stain susceptibility. Postharvest dips in calcium chloride solutions or calcium infusion by submerging apples in a calcium chloride solution reduced stain incidence but further confirmatory work is necessary. Considering the other fruit quality benefits from assuring adequate fruit calcium nutrition, field and postharvest calcium programs may be a means of reducing stain. Other management practices, such as regular cropping, avoiding excessive vigor and efficient use of nitrogen fertilizer, also contribute to maximizing fruit calcium content.
Fuji apples are also prone to internal breakdown if CA conditions are unsuitable. Maturity at harvest, watercore, storage temperature and storage atmosphere are factors contributing to the occurrence of internal breakdown. Injury can vary from small, light brown areas associated with watercore to browning of most of the cortex. Cavities can also develop in the injured areas. Avoiding late harvest with advanced maturity and severe watercore, keeping storage temperatures above 32°F, and maintaining CO2 at 1% or less in CA can reduce the risk of internal breakdown.
Harvest maturity and watercore are the least controllable variables because market forces also play a role in determining when to pick. In some years, watercore develops before fruit are ready for harvest based on color and internal maturity. By the time fruit development has progressed to an acceptable maturity, watercore may be severe. Research with other cultivars such as Delicious and Braeburn has shown internal breakdown that develops after harvest can be reduced in fruit treated with DPA at harvest. A series of experiments with Fuji has shown the same result. Fuji apples exposed to high CO2 in either air or CA at warm or cold temperatures have significantly less internal breakdown if the fruit are treated with DPA prior to storage. This response enhances the value of DPA used for scald control during long term storage. It also provides more flexibility to storage operators by allowing fruit to be stored in CA after DPA treatment that would not otherwise be suitable because of the risk of internal breakdown. There are some risks associated with drenching with a DPA solution, particularly the potential for inoculation of fruit with fungi that cause decay. Proper use of DPA solutions as well as avoiding drenching of overmature Fuji fruit can reduce the risk while maximizing scald control and avoiding development of internal breakdown.
High quality apples always have a market. The returns may not always be the best, but they are guaranteed to be better than what low quality fruit brings. The potential for rapid loss of Gala fruit quality after harvest and development of Fuji storage disorders provide points to ponder for growers, warehouse managers and storage operators. Fruit storage is a system based on fruit maturity and technology inputs. Nowhere is the phrase garbage in garbage out more applicable. The technology to maintain quality exists, the challenge is to use it to maximize what arrives at the warehouse.
Acknowledgements: The excellent research and technical support of Luiz Argenta, David Buchanan, Xuetong Fan, and Janie Gausman is gratefully acknowledged, as is funding received from the Washington Tree Fruit Research Commission.
Dr. Jim Mattheis, Plant Physiologist
USDA, ARS Tree Fruit Research Laboratory
1104 N. Wenatchee Ave.
Wenatchee, WA 98801
14th Annual Postharvest Conference,
March 10-11, 1998