The Role of Ethylene in Determining Apple Harvest and Storage Life
This report compiles information on the use of ethylene measurement as an indicator of apple age, and discusses recent developments in low ethylene storage. Generally, scientists are in strong agreement about the importance of ethylene in controlling plant behavior and senescence. However, there is little agreement on the actual method of sampling trees for fruit, measuring the evolution of ethylene, or interpreting the results. Ethylene monitoring and scrubbing is successfully used by a few innovative growers in the eastern United States and the United Kingdom. Scientists need to do a great deal more work with Red and Golden Delicious grown in the Pacific Northwest and stored under commercial conditions before specific recommendations can be made.
Background on Ethlyene
Ethylene is a biologically active chemical that exists as a gas under normal conditions. It regulates many aspects of plant growth and development, and is biologically active in minute amounts. The odor of ethylene is not readily detectable at physiological levels, but in large concentrations it smells a great deal like acetylene.
Ethylene is considered to be a naturally occurring plant hormone. All plant tissues are capable of producing ethylene, although the production rate is normally low. Production is known to be regulated by the plant tissue depending on the stage of development and environmental factors. As part of the normal life of the plant, ethylene production is induced during certain stages of growth, including fruit ripening, leaf abscission and flower senescence. Ethylene production increases as a result of bruising or wounding, water or heat stress, or the application of certain growth regulators. The amount of ethylene dissolved within the plant liquid may be the most active triggering mechanism. In summary, the gaseous plant hormone ethylene occurs naturally in most plant tissues. Its production can be stimulated or depressed by either internal or external factors. An increase in ethylene promotes (or inhibits) many plant functions, such as fruit ripening and leaf or fruit abscission, etc.
Fruit Age & Harvest
Measurement of the rate of ethylene evolution can be used as an indicator of apple age. One of the most dramatic changes in the chemistry and physiology of apples as they begin to ripen is the accelerated production of ethylene. Prior to initiation of the ripening process, the rate of ethylene production is so low that it is barely detectable, even with the most sophisticated instruments. For example, internal ethylene concentrations are generally below 0.15 parts per million (ppm).
As ripening begins, as defined by the change in respiration, the production of ethylene increases dramatically. This rapid increase is called the climacteric. increasing logarithmically, the rate of ethylene production can increase 100-fold in just 2 days. This tremendous burst in ethylene production provides a convenient dividing line against which to categorize apples and pears as either preclimacteric (not yet producing significant quantities of ethylene), or post-climacteric (producing ethylene). (See Figure 1.)
Fortunately, detached fruit begin the climacteric before fruit still attached to the tree. The closer preclimacteric fruit is picked to the occurrence of the climacteric on the tree, the shorter the time from harvest until ethylene is produced. Scientists are investigating how to use the ethylene climacteric to predict harvest.
The Apple Maturity Program
Dr. Max Patterson (WSU) has been measuring the ethylene evolved by fruit collected for the Apple Maturity Program (AMP) from orchards throughout Washington State. A "flow-through" measuring system is used, and data are electronically transmitted to Wenatchee, where they are discussed at weekly meetings.
The goal of the AMP is to predict the optimum harvest date for apples to be placed in long-term CA storage for 8 to 11 months. Certain AMP orchards have had fruit sent to Pullman for ethylene analysis on a weekly basis over the past 7 years. Results of these tests have been accumulated and are currently being statistically analyzed. Until analysis is completed, the correlation between the AMP selected harvest date, the fruit quality examinations and the ethylene evolution cannot be made with certainty.
After data on eight Red Delicious orchards were manually compared over 7 years, it was found that ethylene evolution preceded the AMP recommended harvest date for long-term CA by one week only 11 times (21% of the time). In 1980, significant quantities of ethylene were not detected in the test orchards until very late in the sampling period. On the other hand, the ethylene climacteric occurred after the AMP recommended harvest date 36% of the time (1980 data were excluded).
Using the currently recommended CA regimes for marketing during the latter trimester of the season (June, July and August), fruit to be stored must be harvested immediately before the climacteric rise begins. Consequently, using ethylene evolution as a sole indicator to determine harvest for long-term storage can at times mislead the grower into picking fruit which is overmature for long-term CA. Late harvested fruit will have neither the excellent quality the market demands, nor shelf life.
Growers are encouraged to add to the number of tests they are using to check for maturity, but they should recognize that no one indicator of maturity has been a reliable predictor of success in long-term CA. The determination of harvest date of Red Delicious for long-term CA storage must be made on the basis of a combination of horticultural qualities. See the AMP Handbook for details.
Length of Harvest Season
Studying the rate of ethylene evolution by AMP sample orchards has allowed the group to keep track of how rapidly fruit from a particular block is maturing, and implies how long it may be possible to continue harvesting for CA. Rapid ethylene evolution heralds the rapid onset of watercore and a reduction in storage life. Fruit which has developed watercore has passed the climacteric and contains appreciable amounts of ethylene. This fruit should not be placed in CA.
Allowing fruit to hang on the tree after the ethylene climacteric has begun should only be done with the full knowledge that ripening is underway.
To analyze for ethylene, use highly sensitive instruments. It is important to detect ethylene at levels of less than 0.10 parts per million. The instrument must be able to analyze many samples quickly and to distinguish clearly between ethylene and other gases. The type of instrument used for this purpose is called a gas chromatograph and is common in most sophisticated scientific laboratories. A few manufacturers have attempted to market a simplified gas chromatograph for ethylene analysis. Only qualified researchers should undertake comparison testing of moderately priced ($3,000 to $7,500) gas chromatographs under industry conditions. Manufacturers of simplified gas chromatographs include Hach, Carle Company; Neogen Carp; AID; and Hewlett Packard. Cost should not be a major consideration in the selection of a gas chromatograph. Accuracy, speed and repeatability should be considered.
A method of sampling trees, blocks, orchards, and rooms needs to be carefully researched.
Work at WSU's postharvest laboratory has shown the importance of consistent sampling within a tree. Light penetration, age of the bearing wood, spur age, rootstock, pruning, and limb orientation all influence the ripening pattern of fruit on a single tree. This influence can be extreme, as fruit on some branches produce over 200 ppm ethylene, while others produce less than 5 ppm on the same date. Sample fruit from the same location(s) repeatedly, rather than from different locations on the tree.
Dr. Dave Blanpied of Cornell University measured ethylene in apples hanging on the tree to look at the variation which naturally exists among fruit on a tree, among trees in an orchard, and among orchards. He found small differences in the initiation of the ethylene climacteric among trees in an orchard block. However, there were differences of 10 or more days among blocks on a farm and farms in a small town. Consequently, he feels that if the ethylene climacteric is used as an indicator of harvest maturity, each block must be monitored. The interval between the earliest and latest fruit on the tree ranged between 7 to 14 days depending on cultivar and whether Alar had been used. A lesser interval occurred between trees in a single block. Dr. Blanpied worked on Empire, McIntosh and Red Delicious apples.
Several methods of testing the rate of ethylene evolution by fruit have been developed. One involves sampling apples from an orchard in advance of harvest, and monitoring the time it takes for ethylene to accumulate in a sealed container. Testing is done on freshly harvested fruits at regular intervals. With each successive harvest, the number of hours from harvest until the climacteric decreases as the fruit matures. A graph is plotted which allows the grower to predict the climacteric in the orchard.
Another strategy for predicting harvest dates for long-term CA injects ethylene gas into a jar filled with freshly picked apples. The time it takes for the fruit to respond to ethylene is determined. This technique stimulates apples to produce ethylene sooner than they would in a natural state as they become more sensitive to applied ethylene.
The method selected by Dr. Patterson to measure the rate of ethylene evolution for the AMP employs a sophisticated, automated flow-through system. This system was chosen for its accuracy and ability to test a great number of fruits at the same time. Detached fruit is placed in a chamber and supplied with ethylene-free air at a constant rate. Effluent air is monitored daily for flow rate, ethylene and carbon dioxide concentrations. The data are used to calculate both ethylene production and respiration rates. Fruits are held for 7 days. The number of days before consistent daily ethylene production above 0.05 µL/kg/hr is equal to the number of days of "green life" (days prior to the climacteric). As ripening approaches green life decreases.
Measuring ethylene in fruit hanging on the tree can avoid some of the problems in "bulking" samples in a sealed bucket or jar. These problems include the difficulty of determining if one apple in the bucket caused the others to give off ethylene before they naturally would do so. A single apple may have moldy core (a common problem in Washington in 1985), or be stressed by bruising or moisture conditions. The timing of ethylene evolution in fruits on the tree will not correspond with that of fruit from the same tree in a bucket, since removing the fruit from the tree promotes earlier ripening.
There is little agreement among scientists as to the best method of testing fruit. Warehouses wishing to measure ethylene should choose one method and use it consistently, since it is difficult to compare the results of one method with those of another.
The application of Alar (Daminozide) to apples delays the appearance and suppresses the subsequent rate of the production of ethylene. Combining Alar with low-oxygen CA storage has been shown to maintain the firmness of Cox more effectively than CA storage alone. In limited trials with Red Delicious, low-oxygen storage has not had a strongly demonstrated effect on Alar treated Red Delicious.
Strategies to Avoid Ethylene
One interesting aspect of ethylene is that fruit producing ethylene can stimulate the production of ethylene by other fruits. This is used to advantage by placing ripe apples in a container with unripe pears or tomatoes to ripen them. However, the fruit industry needs to protect fruit from ethylene.
Carefully select and isolate fruit for long-term CA from fruit which may be producing ethylene. Have the orchardist avoid mixing fruit from weak, sick or stressed trees with the stronger CA quality fruit. Since ethylene is produced by internal combustion engines, propane fork lifts and trucks should be carefully tuned, and exhaust gases vented away from the fruit. The exhaust from a fossil fuel engine often contains approximately 500 parts per million (ppm) ethylene.
The amount of ethylene produced by propane burners varies from zero to greater than 10 ppm, depending upon the condition of the catalyst, temperature of the catalyst, and the amount of oxygen in the intake atmosphere. In other words, the amount of ethylene generated is proportional to the completeness of combustion.
Low Ethylene CA Storage
Ethylene can be scrubbed from storage rooms. Commercial machinery designed to remove ethylene from CA storage facilities has been developed in England, Poland, Italy and New York.
The cornerstone for successful ethylene control in storage is to start by harvesting preclimacteric fruit; isolating it from climacteric fruit and other sources of ethylene; and using a scrubber to remove ethylene as it is generated by the fruit. It is currently impossible to maintain the extremely low level of ethylene needed after the fruit begins to generate ethylene in large quantities.
Drs. Frank Liu, Jim Bartsch and Blanpied of Cornell University have been investigating the response of apples to low ethylene CA storage. They found that McIntosh and Empire apples stored in a low ethylene CA had more acid, more aroma and higher firmness scores compared with apples stored under the same conditions but without the removal of ethylene. Both senescent breakdown of McIntosh and flesh breakdown of Empire were reduced. The low ethylene effects were far more pronounced on preclimacteric fruit.Dr. Liu did not find appreciable differences in fruit quality after low ethylene storage of Delicious, Golden Delicious or Idared apples even after 7 months in CA. However, Delicious apples stored for 7 months in low ethylene CA (without DPA) did not develop scald. Apples in CA with ethylene developed severe scald. Fruit stored in 500 ppm ethylene had more scald than fruit stored with less ethylene (10 ppm).
Researchers at East Malling Research Station, England, have used low ethylene CA storage on Cox, Bramley and Golden Delicious over the past few years with varying success. Bramley apples stored for 33 weeks in low ethylene had no scald, while 68% of the control fruit were scalded. There is a limit to the length of time scald can be controlled in low ethylene storage, since some scald appeared late in the season on the low ethylene treatment. This scald was of lighter color and less extensive than the scald on the control fruit. Harvest maturity is thought to play a role on the ability of the low ethylene treatment to resist scald. Half as many fruits were affected by Bitter Pit in the low ethylene treatment as compared with the control. Ethylene scrubbed fruit were approximately 2.2 pounds firmer than the control.
In summary, the benefits to certain varieties from low ethylene CA storage as compared with normal CA, are chiefly in the retention of fruit firmness, the retention of acids and a reduction in the amount of scald. However, very limited tests with (New York grown) Red Delicious have not shown improved firmness or acid levels, perhaps because of the use of Alar. Research has shown an appreciable reduction in scald (without DPA).
Improved firmness and acidity have been demonstrated on varieties which are susceptible to chilling injury and must, therefore, be stored at 36°F. Studies done many years ago concluded that ethylene evolution is inhibited at temperatures close to 32°F. The rate of ethylene production by fruit is reduced by lowering the temperature and reducing the oxygen level. These same factors also reduce fruit response to ethylene. At 32°F and 1.0% oxygen, response to ethylene is usually nil.
Apples respire slowly at the low oxygen levels of Washington CA rooms. Therefore, they produce less ethylene and respond more slowly to ethylene than fruit held at higher levels of oxygen and temperatures.
Equipment to Remove Ethylene
In order for an ethylene scrubber to be effective, it must remove all of the ethylene from the storage environment. Equipment which can reduce ethylene from 100 ppm or even 1,000 ppm down to 10 ppm is worthless! Ethylene must be kept at 1 ppm or below.
There are currently two types of ethylene scrubbers on the market--a catalytic burner type and an adsorption type. A heated catalytic scrubber is manufactured by a British firm, Johnson Matthey, under agreement with East Malling Research Station where it was developed. It has been used in industry trials in the United Kingdom. Another ethylene scrubber using a different type of heated catalyst is manufactured by a Polish firm and sold in the United States by Neogen Food Tech. Corporation.
The second type of ethylene scrubber uses potassium permanganate, which oxidizes ethylene very effectively. The potassium permanganate is incorporated onto small spheres of aluminum silicate (alumina). The beads are available commercially. Drs. Blanpied and Bartsch have developed an ethylene scrubber which utilizes these beads. Scrubbers have been placed in several commercial CA rooms in New York State.
Activated carbon scrubbers remove some ethylene from CA rooms, but cannot be relied upon to remove sufficient ethylene for low ethylene storage.
Although ethylene scrubbing has been used on a few commercial rooms, there is not enough experience to determine the long range benefits of these devices- particularly under Washington conditions with Delicious.
In laboratory studies, chemical inhibitors of ethylene synthesis (AVG) have shown promising effects. Delayed ripening was seen with d'Anjou pears treated with a postharvest dip of AVG. When AVG was sprayed on apples before harvest, delayed ripening and reduced preharvest drop occurred. Other classes of chemical inhibitors of ethylene biosynthesis include: inorganic ions CO2+, N2+, membrane disruptive agents, polyamines, and antioxidants such as sodium benzoate. These inhibitors have played an important role in our understanding of the biochemical mechanisms responsible for ethylene production.
Shipping Containers CA storage reduces the generation of ethylene. Some manufacturers have attempted to incorporate CA into commercial shipping containers or to gas a pallet with high CO2 to provide a modified atmosphere container. Ethylene scrubbing may have application within the package itself by incorporating potassium permanganate into the package, either as a sachet or separate package in the container.
There is not enough information available under Washington conditions to recommended the use of ethylene scrubbing devices in packages or containers at the present time me.
Ethylene is a plant growth regulator which postharvest physiologists have studied in an attempt to understand ripening and senescence. I am convinced that the measurement and control of ethylene can become an extremely valuable tool in the apple industry's attempts to continue to improve edible fruit quality. It is only a mat matter of time before ethylene measurement and control become commonplace in the industry. Research efforts on ethylene need to be directed towards studying the commercial application of this technology. It is too early to recommend equipment and methodology to the industry at the current time.
Dr. Eugene Kupferman, Postharvest Specialist
WSU Tree Fruit Research and Extension Center
1100 N. Western Ave., Wenatchee, WA 98801
Post Harvest Pomology Newsletter, Vol. 4, No. 1