As the time in storage increases, however, browning progresses from lightly discolored patches to pitted and sunken areas on the skin surface wherein several layers of cells have become affected. Therefore, scald becomes the visible symptoms resulting from damage to the first few layers of cells. It is a complex problem that develops over a relatively long time and has a number of complicating factors. For this reason, it remains one of the most persistent physiological disorders of apples in the world.
BRIEF HISTORY OF SCALD PROTECTION
In the early part of this century, scald was the major cause of postharvest loss of apples in the United States. Before the discovery of the antioxidants DPA and ethoxyquin in the 1950s, scald was prevented by wrapping fruit individually in oil-impregnated wraps. It was later discovered that the oil in the wrap actually absorbed the purported volatile scald initiator alpha-Farnesene from the apple surface. Unfortunately, this method was costly and slow.
In the early 1960s, DPA and ethoxyquin were just coming on the market, and after a few trials it was evident that the use of oil wraps did not compare in cost or effectiveness to the newly developed antioxidants. Even though research continued into the cause and control of scald, it was evident the most economical solution to scald was the topical application of DPA on apples and of ethoxyquin on pears.
BIOLOGY OF SUPERFICIAL SCALD
Scald can best be described in developmental stages. The first stage occurs during the first one to two months, during which time an endogenous compound in the epithelial tissue called alpha-farnesene begins to increase. Alpha-farnesene is a compound whose exact functions are not completely understood but which is a precursor in the synthesis of a number of molecules, including squalene, which is a membrane component and a relatively good natural antioxidant.
The second stage begins as the accumulated levels of alpha-farnesene begin to be oxidized to a class of compounds known as conjugated triene hydroperoxides. The formation of these hydroperoxides takes place through short-lived intermediary molecules known as peroxide radicals (free radicals) which are extremely reactive in the cell and capable of inactivating proteins, oxidizing cell membranes, forming unusable polymers, and generally causing organelle dysfunction within the cell.
Normally, an optimally functioning cell is capable of defusing such destructive compounds very quickly and diverting the energy to other cellular functions. After the fruit has been stored at near-freezing temperatures for 80 to 100 days, it is not inconceivable that some of the biochemical pathways could either stop functioning or carry out their duties very sluggishly.
In addition, not all pathways would be slowed at comparable rates. So, theoretically, whereas one biochemical pathway might function at 60% efficiency at 32°F, another might only function at 10%, and the system as a whole would become unbalanced.
This research is in progress in Eric Curry's Wenatchee, Washington, laboratory. It is during stage II in which treatments such as diphenylamine (DPA) and ethoxyquin, both commercial antioxidants, are effective, because they inhibit oxidation of alpha-farnesene and reduce the build-up of the toxic metabolites. These processes continue for one to two months, often without visible symptoms. The damage to the cell, however, is cumulative and irreversible.
Stage III begins when damage to the tissue becomes sufficiently great to cause browning. This gets progressively worse and may culminate in cell death. Symptoms at this stage may vary in severity depending on the cultivar, the length of storage, and conditions under which it was stored. There are many occasions when fruit removed from storage and ripened for several days at 20 to 25°C exhibit a rapid increase in scald development. This can be quite disastrous to retailers preparing to display fruit that has been out of storage, in transit, and sitting in a retailer's stock area for a week or so.
RESEARCH DIRECTIONS
Scald has been a problem worldwide wherever apples are stored for more than a few months, and even though there has been protection in the form of DPA and ethoxyquin drenches, the problem still exists and has not been solved. A number of researchers around the world have continued to work on this problem--some for a good portion of their research careers. In March 1992, an International Scald Symposium was held in Wenatchee, Washington, the purpose of which was to define what was known about the disorder, and determine in which areas to strengthen research efforts.
Since that time, a number of additional research projects have begun, both to understand the cause of the disorder as well as to find alternate methods of reducing its presence.
These research areas can be categorized into three broad areas: 1) prestorage prediction of scald potential in storage, including the search for biochemical markers; 2) protecting fruit already harvested, by either modifying the storage environment or inhibiting its development with the use of chemicals; and 3) preventing the disorder from occurring by either breeding in a protective gene, or breeding out the cause.
It is beyond the scope of this brief research update to review the many projects and approaches in progress in each of these categories--all are important. Instead, We will focus on scald prediction.
PREDICTION
It would be valuable to be able to predict the degree to which scald would develop in storage, especially if a pre-treatment such as drenching with an antioxidant could be reduced or even eliminated. Several researchers are attempting to understand the relationship between scald development, fruit maturity, and environmental factors such as temperature, relative humidity, and light to develop such predictive tools.
The ambient temperature before harvest appears to play an important role in the susceptibility of fruit to scald. Generally, cool temperatures tend to reduce scald potential, and high temperatures worsen it. Studies which attempt to correlate reduced scald potential with temperature are often complicated because two factors occur simultaneously: 1) the occurrence of cooler days and nights; and 2) the advancement of fruit maturity.
Further complications develop because each year has a slightly different weather pattern which could alter the way in which the fruit matures. The influence of these two factors is very difficult to separate, and it becomes almost impossible to construct a model based solely on one or the other with any measure of accuracy. Therefore, scald prediction models are likely to be best developed with both a weather component and one related to fruit maturity.
In 1995, the Washington Tree Fruit Research Commission funded a cooperative project between Curry at the USDA/ ARS Tree Fruit Research Laboratory, and Drs. William Bramlage and Sarah Weis in the Department of Plant and Soil Sciences at the University of Massachusetts in Amherst, to develop such a predictive model.
Among numerous other projects, Bramlage (presently chair of the Department of Plant and Soil Science) has been working in the area of scald research for several decades and is recognized as a leader in this field. Several years ago, Bramlage and Weis solicited data from at least ten apple growing regions around the world to determine which, if any, components were common to the development of scald.
Analyzed were 41 seasons of measurements and records related to scald on Delicious apple. They worked at great length to explore similar relationships and extract meaning from these data. Included in this study were five years of data from Curry's laboratory.
In addition to determining that scald was correlated with preharvest temperature, rainfall, sunlight, and fruit maturity, three important results came from the Massachusetts scientists: 1) that the relative importance of the above factors varies among geographic locations, as well as within years in a location; 2) that any successful scald-prediction equation will have to be generated from data gathered within a specific geographic region; and 3) that the development of a prediction model would best be used to classify the fruit into broad scald categories such as very susceptible, moderately susceptible, and not susceptible. Of course, as with any forecasting, there is a measure of error and therefore some risk.
Another potential benefit for the prediction models is that of modifying pretreatment based on a prediction of lessened scald potential. For example, if fruit is harvested early while the weather is still very warm, it is likely that scald levels will be high. On the other hand, if two weeks later, the maturity level has advanced and the fruit has experienced several cool nights, the scald potential might be predictably lower. If appropriate, less DPA might then be required to control scald; or perhaps a modified atmosphere would assist or even control scald to an acceptable level. These ideas are being tested both in Washington and Massachusetts, and preliminary results suggest this may be the case.
In the initial comparison of data from orchards in the Wenatchee area with data from orchards in Massachusetts we found an interesting development. First, if there were no other outside influences, scald would have a natural tendency to decrease as the length of time on the tree increased. This is the component that deals with fruit maturity and is likely to be true wherever the fruit is grown.
When environmental components such as temperature, humidity, or solar radiation are added to the model, the natural tendency for scald to decrease is influenced in either a positive or negative way. If the temperature were monitored in Massachusetts from August 1 until harvest, it would be more common for an orchard to experience many hours below 50°F than it would hours above 85°F. This accumulation of cool weather reduces scald susceptibility and therefore is one of the principle components in their model.
On the other hand, similar monitoring in eastern Washington would likely show a trend toward the opposite. There would be more days with higher temperatures and fewer with cooler temperature. In this case, the amount of heat increases scald susceptibility which appears to be the case for the Washington model. Thus, in the cooler more humid environment of the east, it appears we can use cool temperatures to predict scald, whereas in the hot arid climate of the west we may have to use hot temperature to predict it. In both cases, the most common temperature pattern predominates.
PROTECTION AND PREVENTION
As mentioned earlier, there are a number of other scientists around the world researching different aspects of apple scald and its control, including studies related to mineral nutrition, growth regulators, natural antioxidants, heat treatments, and controlled atmosphere. In addition, a limited number of scientists are working on eliminating the problem altogether.
This kind of research would revolve around a cultivar immune (or nearly so) to the disorder. If we truly had such a cultivar, then we could ask why it is immune. Is it due to the cultivar lacking the capability of producing alpha-farnesene? Are the gas diffusion properties of the cuticles different? Are the cells better able to detoxify the deleterious free radicals? What protects the cells? Can we engineer cells with higher levels of antioxidants?
When some of these questions are answered, scientists can begin modifying genomic composition to confer a significant measure of protection to the fruit. Getting to that point, however, will require more studies into the nature of the disorder and the environmental influences that affect it.
Until we have learned to prevent the development of scald intrinsically, however, we must use all the means available to control it as best we can. An integrated approach to scald control would be to use a combination of predictive models and control methods to reduce the disorder economically. Certainly, a number of useful technologies will develop from the combined efforts of the many workers involved.
ERIC A. CURRY
Plant Physiologist
USDA-Agricultural Research Service, Tree Fruit Research Laboratory
Wenatchee, Washington
SARAH A. WEIS
Research Associate, Department of Plant and Soil Sciences
University of Massachusetts, Amherst, Massachusetts
Copyright © 1996, Good Fruit Grower
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