Views on CA Storage of Apples
In the early 1920s, England imported large quantities of produce from other parts of the world. Importers were looking for a way to transport apples so that they would arrive in better condition. Drs. Franklin Kidd and Cyril West attempted to modify the atmosphere surrounding the fruit as a replacement for refrigeration. Although modified atmosphere did not replace refrigeration, this experiment led to research on the effect of modified atmospheres on stored commodities. The work of Kidd and West is directly responsible for the development and application of modified and controlled atmosphere (CA) to extend the marketing season of apples.
Early CA Research
In the United States, Professor F.W. Allan initiated CA research in California. A CA room was built there, but CA was not adopted commercially. Dr. Robert Smock joined Allan for 2 years in the mid-1930s, before transferring to Cornell, New York.
New England orchardists were growing a number of apple varieties, particularly McIntosh. This cold sensitive apple cannot be stored at or below 32°F. At higher temperatures there is a more rapid rate of living. If that rate could be slowed, storage life could be extended, lengthening the marketing period. Smock directed his efforts to this task. Through his work CA was adopted, and with great success. The first CA room of Red Delicious was attempted in Washington in the late 1950s. A commercial quantity of fruit was first stored in a mylar tent in a Yakima area warehouse. The warehouse stored about 1,000 bushels of fruit with good results.
Dr. Archie Van Doren studied under Smock at Cornell, where he helped set up and run a number of CA experiments. Van Doren came back convinced that CA storage was beneficial. Through his efforts, Washington developed an extensive CA storage program. Van Doren also was instrumental in promoting a state law which has set standards for fruit sold under the "CA" label.
When CA was first started, the system for measuring the gases was rather crude. It involved the use of an Orsat and chemical absorption of CO2 and O2 from a known volume of gas sample. This slow procedure was not error-free. Development of newer electronic equipment made rapid and accurate sampling of CA atmospheres possible.
Current Research on Low Oxygen
The fruit industry now has the potential to adopt lower O2 levels in place of the 2 to 3% originally suggested for Red Delicious. Investigations have been conducted at USDA-ARS in Wenatchee and in Drs. Max Patterson's and Robert Kennedy's lab at Washington State University, Pullman, to determine minimum levels of oxygen adequate for apple storage. These results have been substantiated by work outside of the state by Dr. Paul Chen at Hood River, Oregon, and by Drs. Sam Lau and Michael Meheriuk in British Columbia.
It now looks as though 1% oxygen is adequate for Red Delicious grown in the Pacific Northwest, but some injury may result at close to 1/2% oxygen. I believe this is related to the stage of fruit maturity, the amount of fruit respiration at harvest, and the temperature at which the fruit is stored. These factors must be carefully considered. Researchers have looked at the effect of lower levels of oxygen and have found considerable increase in the retention of firmness and improvement in the condition of the fruit. It is important that we move in this direction. We knew from some early work that 1½% was acceptable, but we did not know how much lower we could go. Now we have a better idea. More recent work has taken CA down to 1% oxygen. This lower level of oxygen (1%) has actually been achieved in a few Red Delicious commercial rooms with very good success. The added advantage of lowering oxygen concentration (and slowing the rate of respiration of this fruit) is that changes which normally take place in stored fruit are greatly reduced. Less bitter-pit and less storage scald occur. Apples have greater retention of firmness and acidity, which are important aspects of fruit flavor.
Temperature and Oxygen Levels are Related
Research in England shows how temperature and oxygen affect the rate of fruit respiration. When oxygen is reduced at temperatures above 35°F, there is an immediate reduction in the rate of respiration. From this standpoint it seems logical that both refrigeration and reduction in O2 could be working together (Fig. 1). Reducing oxygen after the room has been sealed will trigger a reduction in the rate of respiration. While lowering the amount of oxygen lowers rate of respiration, refrigeration reduces fruit temperature. Together, these processes slow the rate of living. I recommend that you fill the room fairly quickly and start pulling the oxygen down immediately, since the level of oxygen must fall below 10% before it affects respiration at 32°F.
Apples ripen at a slower rate on the tree than off the tree. Once we harvest apples, we must slow the rate of ripening. We are harvesting the fruit for long-term storage before it is ready to ripen, and before it starts on the climacteric rise and increases ethylene production. This fruit, if stopped from further change in metabolic pattern, will have good edible quality late in the season.
CO2 Reduces Respiration
The respiration process produces CO2. The rate of respiration can be slowed by increasing the CO2 around the fruit. As the CO2 builds up, it produces a back pressure and reduces the fruit's rate of respiration or rate of living. Work from England indicates that 5% CO2 reduces the rate of respiration, but that 10% CO2 has no further effect (Fig. 2). However, this is not true for all varieties. Early work on Golden Delicious indicated that at higher CO2 levels there is an additional reduction in the rate of respiration. Using high CO2 (levels of CO2 at 5, 10, 15 and near 20%) we found that the CO2 had no immediate effect on the fruit firmness. After a 10-day treatment period, we put fruit into regular commercial CA storage and examined it in February, March, and May. Retention of fruit firmness after storage related directly to higher concentrations of CO2.
However, problems have occurred at high CO2 levels. If researchers maintain the CO2 level at 20% or higher for an extended period (such as 10 days or longer), internal fruit quality suffers. In addition, Goldens may develop a surface burn as a direct result of moisture on the surface of the apple in the presence of high CO2. Fruit picked earlier, or fruit with less natural wax is most sensitive to this condition.
Even 3% CO2 can cause superficial burn on Golden Delicious during pull down where surface condensation has developed during the defrost cycle. However, in a flow-through system where there was no chance for condensation on the fruit, Pat Patterson has successfully held Golden Delicious at 8% and at 12% CO2. Actually the 8% CO2 fruit in that particular year was the best fruit in the lot. This experiment indicates how much response to CO2 can vary, and shows the potential damage from higher CO2 levels.
In some of our early work on Red Delicious we found that 3% or higher CO2 gave good firmness right out of storage. But treated apples placed in the ripening room for a few days softened rapidly. This fruit developed internal discoloration, core flush or flesh browning, and the associated off-flavor. More work needs to be done with CO2 levels relative to the recommended oxygen levels. We have not seen problems at 2% CO2 on Red Delicious. Since we have seen problems at 3% it seems logical to keep the CO2 level a little lower, at 1% or 1½%.
Rapid Oxygen Reduction
Dr. Lau of British Columbia reduced oxygen rapidly, immediately following harvest of Golden Delicious. The effect on firmness and acid level at 32°F was compared for fruit placed immediately in 1½% O2, and fruit placed in 1½% O2 after a delay of 5 and 9 days, respectively (Fig. 3).
Fruit put rapidly into low O2 CA storage retained more firmness and acids than fruit which experienced a delay in the reduction of oxygen. In other words, rapid reduction of oxygen reduces the amount of respiration while retaining firmness and acidity. This gives the fruit added flavor in the spring or summer when it is put on the market.
It is important to work out ideal gas levels for each individual variety, because all fruit does not respond the same. We need to establish the oxygen concentration for varieties other than Reds and Goldens. Consider the stage of development when fruit is placed in low oxygen. Fruit advanced in maturity and having a rapid rate of respiration needs more oxygen than fruit that is picked prior to the preclimacteric minimum. We must then be careful with rapid O2 reduction. A good practice is to fill a CA room in 3 days and use another 3 days to pull the oxygen down to 2 to 3%. In other words, 6 to 7 days from the time the first fruit is harvested to the point when the O2 has reached 2 to 3%. At that point, the fruit is still warm and using considerable oxygen, which will continue to lower the CA room oxygen level.
Very Low Oxygen
Now what if O2 does go lower than 1%? There is no particular problem if it is down for a few days, but if it is down for an extended period, permanent injury may result. In the future, in using low oxygen, you will need a quality gas chromatograph to detect gases. For example, in anaerobic respiration the gas, ethanol, accumulates rapidly. Use a gas chromatograph to analyze quickly whether the O2 level has gone too low. This safeguard technique helps if you are interested in pushing the oxygen level down to get maximum benefits.
In the work we have been doing in the lab at Wenatchee, there has not been a distinct advantage between 1% and 0.6% O2. At 1% O2 it looks as though we are getting close to the maximum benefit that we can hope to obtain. In some tests at this oxygen level, we have had very good results in controlling storage scald. This has been another reason for researching the lower oxygen level-in case we were to lose diphenylamine (DPA). Scald control for fruit from this area and Oregon looks promising at very low O2. In British Columbia, researchers have not always obtained complete control, although they have reduced storage scald. So again, the area in which the fruit is grown is going to have some influence; these things need to be considered. If you still store Reds and Goldens at 2½% O2, I think you are making a big mistake. There are absolutely no problems at 1½% oxygen. Those of you that want to go lower than 1½% O2 must be aware of the potential problems. Where possible, involve people who have access to gas chromatographs. They can help you track what is happening in your room as the season progresses.
CA Generating Equipment
The first equipment to reduce oxygen in CA was a flushing system known as "Tectrol." It burned natural or propane gas with outside air, which lowered the O2 and increased CO2. Most of the CO2 was removed before the atmosphere was fed back into the CA room. Another vent on the room had to be opened to relieve the buildup of pressure. Thus, a flushing system was established. This equipment became commercially available in the 1962 season. The operator was able to generate the CA atmosphere more rapidly, the room did not need to be as tight, and if repairs were necessary during the storage season, the room could be opened and the atmosphere could be reestablished quickly. This system added to the cost, however.
Another system, which recirculated the air from the room through the burner, used natural gas or propane. This was called the "Arcat," which became available in 1965. The atmosphere produced was returned to the room. A separate system scrubbed out the CO2 produced.
Old Tectrol units are now being replaced by units built in Italy called "Gen-O-Fresh." The "Arcagen" systems have been replaced by the "COB" burner and the "MSA" CO2 absorber which are built in Washington state. Utilization of the mechanical generating systems has become standard practice in the Pacific Northwest, particularly with the adoption of rapid CA (RCA).
Removal of CO2 has always been a problem in CA rooms. The fruit is continually producing this gas through respiration, and some of the atmosphere generators produce CO2. The necessary O2 can be maintained by adding outside air periodically, but the CO2 must be removed, either chemically or mechanically. Early use of caustic soda (NaOH solution) was plagued by several problems. The solution was so caustic it was difficult to handle; it was hard on equipment, and it created a disposal problem. Water scrubbers were developed in the eastern United States for use on apple varieties with a higher rate of respiration, and for fruit stored at temperatures above 32°F. Scrubbers put some O2 back into the room. The water picks up the O2 as it is aerated to release the CO2.
Activated carbon scrubbers are also used to remove CO2, but they have not found widespread usage on varieties with low respiratory rates. Fresh air reactivation of the carbon returns O2 to the room each time the cylinder is placed back into the scrubbing mode. A similar system using a molecular sieve (Arcosorb) became available in 1963. Fewer changes are needed for reactivation from one column to another, but more energy is required to drive off the CO2 and water. Probably the simplest system makes use of bags of hydrated lime placed in the CA rooms. The disadvantage is the space occupied by the lime and the inefficiency of CO2 absorption. Less than 20% of lime in the bag is used under normal stacking procedures.
In more recent developments a different type of atmosphere generator has been introduced from the Netherlands. Called an "Oxydrain," it utilizes NH3 rather than propane. The ammonia is cracked, forming N2 and H2. The H2 is burned with the O2 from the room, and no CO2 is produced. This generator was introduced in Europe quite a few years ago; the first units were installed in Washington in 1982.
An Italian "Isolcell" unit was installed in Washington for the first time in 1984. It burns natural gas or propane in an open flame. The effluent passes through a catalyst which ensures complete oxidation of any partially oxidized compounds to CO2. Activated carbon scrubs out most of the CO2 before the generated atmosphere enters the room. Carbon canisters are used during the storage period after the CA atmosphere has been established to remove CO2 generated by the fruit. The carbon is specially prepared in Germany from coal and absorbs a small volume of O2. This feature is used when switching from one column to the other to reduce the amount of O returning to the room by reactivation of the carbon.
Two other mechanical units are ready for testing. One is the pressure swing absorption (PSA) system which separates O2 from N2 in the air using a special activated carbon. The second is a membrane system which permits passage of water, O2, CO2 and some ethylene, but retains N2. The membrane has been engineered into a modular system by Dow Chemical Co. It can be designed for whatever size is needed. Monsanto has developed air scrubbing equipment under similar lines.
Still another system has been under study for the past few years and in commercial operations. This system uses liquid N2 for initial O2 pull down and for purging the CO2 during storage. Desired O2 and CO2 levels can be automatically controlled. Humidification is adjusted as necessary.
The newer systems do not produce additional CO2 in the generation of reduced O2 atmosphere. The "Oxydrain" uses agricultural grade ammonia as a fuel. The PSA and membrane systems require a compressor but no fuel; both have the potential of removing CO2 as well as O2. Both of these systems require a capital investment, but this can be amortized over several years. These units will be tested under commercial conditions this season.
The liquid nitrogen system has been tested on commercial storages both for generating the initial low O2 atmosphere and for removal of CO2 generated by fruit. A desirable system will require some capital investment for the automated controls. As long as the nitrogen can be delivered at a competitive cost, this system will be simple to operate. The only mechanical parts will involve an air pump to inject outside air into the room as needed and a sampling pump to monitor O2 and CO2 levels in the CA room.
Chen, P., K. Olsen, and M. Meheriuk. 1985. Effect of low-oxygen atmosphere on storage scald and quality preservation of 'Delicious' apples. J. Amer. Soc. Hort. Sci. 110(12): 16-20.
Couey, H. Melvin, and Kenneth L. Olsen. 1975. Storage response of 'Golden Delicious' apples after high-carbon dioxide. J. Amer. Soc. Hort. Sci. 100 (2):148-150.
Lau, O.L. 1982. The use of rapid CA to maximize storage life of apples. Controlled Atmosphere Symposium. Oregon State Univ. Symp. Series No. 1. p. 201-210.
Lau, O.L., and N.E. Looney. 1978. Factors influencing CO2 induced peel injury of 'Golden Delicious' apples. J. Amer. Soc. Hort. Sci. 103(6):836-838.
Lau, O.L., and N.E. Looney.1982. Improvement of fruit firmness and acidity in controlled atmosphere stored 'Golden Delicious' apples by a rapid O2 reduction procedure. J. Amer. Soc. Hort. Sci. 107(4):531-534.
Meheriuk, M. 1977. Treatment of 'Golden Delicious' apples with CO2 prior to CA storage. Can. J. Plant Sci. 57:467-471.
Meheriuk, M., S.W. Porritt, and P.D. Lidster. 1976. Effects of carbon dioxide treatment on controlled atmosphere storage behavior of'Mcintosh' apples. Can. J. Plant Sci . 57:457-460.
Olsen, Kenneth L.1978. Storage response of 'Golden Delicious' after short periods of low oxygen or high carbon dioxide followed by standard CA storage. Hort. Sci. 13(3):351. Abstract 98.
Dr. Ken Olsen, USDA-ARS Plant Physiologist,
Post Harvest Pomology Newsletter, Vol. 4, No. 2