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WSU-TFREC/Postharvest Information Network/Factors of Loss and The Role of Heat Removal for Maximum Preservation of Sweet Cherries



Factors of Loss and The Role of Heat Removal for Maximum Preservation of Sweet Cherries


Manuscript

There is no postharvest ripening in the cherry that makes it better or more edible than it was at harvest. Therefore, in order to maximize any competitive edge, the objective throughout harvesting, handling, storage, transit, and retail operations should be to preserve the physical, chemical and biological state that existed at harvest.

There are four major factors responsible for postharvest loss of perishable commodities. Sweet cherries are no exception. These are:

  1. Damage

  2. Water Loss

  3. Compositional Changes

  4. Decay

Progressive deterioration caused by loss factors is controlled by the postharvest environment. In all cases this is composed of an internal environment inside the dermal barrier of the perishable as well as an external environment outside the dermal barrier. The postharvest environment variables consist of temperature, gas composition and pressure of the atmosphere.


1986 Market Observations

Bruising and pitting were primary defects and causes of loss of grade in the 1986 sweet cherry crop market arrivals. Importance of these defects was revealed from inspections conducted by the Washington Cherry Marketing Committee and the Federal-State Inspection Service and reported at the Washington Cherry Institute.

Warm average temperatures on arrival and extended delivery periods without good refrigeration were other major observations reported. These reports focused attention on damage as the principal factor of loss, and on temperature as the principal environmental variable in the 1986 cherry arrivals in eastern United States markets.


Damage

Bruising and pitting are symptoms of fruit injury resulting from damage. The injury that develops is characterized by structural distortion, cell rupture, and cell leakage.

In 1971, joint industry, extension, and research meetings focused on possible causes of severe fruit pitting reported in terminal markets. Following these meetings, we examined packed boxes of Extra Fancy Washington dark red cherries to determine if pitting would develop in our research storages and laboratory. We first observed, upon opening the packed fruit, that stems from adjacent fruits were pressed against some of the fruit by the top of the box. We removed some of the affected fruits and placed them in a covered glass dish with the contact area up, then we watched these contact areas collapse and sink into pits conforming to the size and shape of the contact area.

We next removed apparently sound fruit and briefly depressed stem ends against the fruit flesh. Within minutes there was no visual evidence that an indentation had occurred in the fruit flesh. After 5 days at 86°F, 81% of these indentations had developed pits. Since these tests were done with mature fruit, we next selected less mature fruit and made 160 indentations with the rounded tip of a smooth glass rod (for better control and to avoid tearing the skin) and placed the fruit at 75°F. Another 160 were treated the same way but held at 32°F. Fruit developed 100% pitting at both temperatures.

Equal numbers of fruit were selected with no visible indications of damage. They were not intentionally damaged but were observed under the same conditions. Only 4% of these at 75°F and 0.6% at 32°F developed pitting.

Since these results only reflected the response to force, we broadened our base to include damage by heat and cold. We damaged 100 cherries by each method and observed the point of contact for pitting. Heat was applied by holding a glass rod in a flame before touching (not indenting) the surface of the fruit. Cold was applied by immersing the tip of the rod in liquid nitrogen and touching the fruit surface. Force was applied as before by lightly indenting the flesh with the rod tip. Each form of damage produced 100% pitting.

Over the years since our first observations, we have inflicted various degrees of impact and compression forces on thousands of cherries. However, when cherry contact is made with any given force it is made on a specific unit area. Pressure is force per unit area. Therefore, the same force over a large area will exert a small pressure, whereas when exerted on a small area it will exert a relatively large pressure. For example, a 7-gram cherry dropped on a 1-square-millimeter raised tread on a traction belt is subjected to a pressure equivalent to 10 pounds per square inch or 1440 pounds per square foot. If the 7-gram cherry drops on a flat belt contacting an area 1/8-inch in diameter (8 square millimeters), the pressure is reduced to the equivalent of 1.25 pounds per square inch, or 180 pounds per square foot. The size of the objects that can indent a cherry fruit is a critical feature in causing pitting.

In over 15 years of research with sweet cherries conducted in the Washington State University Postharvest Laboratory at Pullman, there has been no difference in the ability to create sweet cherry bruises or pits with spot applications of searing heat, sub-freezing cold, impact, compression or tension forces identical to that in commercial bruised or pitted fruit. We have not been able to produce bruising or pitting injury by subjecting fruit to cold, nonfreezing temperatures, by immersing fruit in water, or by immersing fruit in cold water. Tension on the stem which ruptures vascular connections within the fruit without causing stem detachment can also produce bruise injuries on the stem-end.

Differences between bruising and pitting are a matter of size and appearance, based on the degree and expression of injury from the initial damage.

Bruises--are larger and become visible more rapidly. Bruised tissue is softened and generally discolored by water-soaking or browning of the tissue. Discoloration is slowed in colder fruit.

Pits--are smaller, sunken areas that frequently develop well after the damage occurred and appear concomitantly with collapse of the damaged tissue. Transpirational losses and reabsorption of liquid from damaged tissues by adjacent sound tissue hasten development of pits. The appearance of pits can be accelerated by incubating known damaged fruit in warm, relatively dry air.


Responses to Damage

There are a number of physiological responses to injury from damage. Most of these responses are interrelated and magnify each other.

Respiration--Injury produces an increase in the rate of respiration, which generates more heat and uses up energy resources faster. Higher production of heat by the fruit increases fruit temperature and accelerates the aging process. It also increases the total heat load and cooling time.

Ethylene--Following injury, the fruit suddenly produces wound ethylene. Ethylene is known as the ripening hormone in climacteric fruits and is largely responsible for senescence and abscission of plant organs, including fruits. Wound ethylene from injured fruit also promotes increased respiration rates in sound cherries, loosens the stem-cap and stimulates the aging process.

Browning--Rupture of fruit cells destroys subcealluar structural organization and physiological function. Concurrent disorganization of cell organelles responsible for specific activities permits mixing of enzymes and substrates responsible for turning injured tissues brown and stops specific reactions carried out in sound tissue.

Decay--Decay is the result of invasion by alien organisms and subsequent destruction of the host tissue. Fungi, bacteria, and yeasts abound in postharvest environments, waiting for opportunities to infect the host. When injury is sustained that penetrates the dermal barrier the penetration site instantly becomes an infection site for decay organisms. Exudate accumulation on the surface from bruises that do not penetrate provide liquid and nutrient media for spore germination and decay organism growth.


Minimizing Damage As a Factor of Loss

Identification of Source

If injury in response to damage is a major problem on arrival at terminal markets, then it is important to know when, where, and how the damage occurred, in order to eliminate or minimize future damage. Where does the responsibility for damage reside? Did it occur before the fruit was shipped, during transit or after? If before, did it occur in the orchard during harvesting and field handling (the grower's responsibility) or did it occur in the packinghouse during the sizing, sorting and packing operations (warehouse responsibility)?

In either case, what operations are responsible and how is the damage inflicted? This information can only be determined from a damage evaluation program. While it may be desirable to have an evaluation of the industry as a whole, it only becomes personally meaningful for remedial measures at the individual orchard and packinghouse. One picker, one cluster cutter, or one segment of a line may contribute most of the damage in a specific case of loss. These sources of damage may be quite different for another grower or warehouse.

Source identification may call for a sampling and inspection program where random samples, from each picker and each step in the handling, are removed and inspected--twice. Major damage will be apparent by critical inspection for gross deformation, water-soaking and softness of damaged areas. A second inspection will be necessary to reveal the tissues that were damaged but only become noticeable from the slow collapse of cells and the sinking of affected surface areas into the fruit body. Care must be exercised in taking samples and inspecting fruit so that damage is not inflicted during evaluation. If percentages of fruit that are damaged exceed tolerances allowed in the packed box then steps must be taken to eliminate those sources.

Corrective action--will differ according to the cause of damage. In one case it may call for better picker supervision or incentive programs. It may call for a reduction in the number of times fruit is poured from one container to another; a reduction in jarring during stacking; a reduction in hauling over rough terrain; a rebuilding of packing lines to decrease velocities, elimination of drops and minimizing compression in bulk or package. Imposition of impact, compression, shear or tensile forces are the paramount causes of distortion and damage. Reduction of damage that occurs after fruit leaves the warehouse will call for unified industry action to exert the necessary influence for corrective action.

Whether the problem can be corrected is a matter of assigning damage a major priority and determination to eliminate causes. Bear in mind that segments of the industry pick, pack, and ship virtually damage-free Rainier cherries, the most vulnerable commercial cultivar for displaying damage.

Heat Transfer

Heat is transferred directly from fruit to other materials by conduction, convection and radiation. All methods of heat transfer are involved to some degree in cooling after harvest, but the primary heat transfer in commercial cooling of cherries is convection, in which heat is given up to a colder liquid (water) or a colder gas (air). The transfer of cherry heat to cold water is extremely efficient. The relatively small-sized cherry sphere has a short radius and relatively large surface to volume ratio for efficient heat movement. Continuous movement of colder water past the fruit provides a constantly renewed source of cold at the interface of fruit and water. Heat transfer by utilizing colder air occurs in the same fashion. However, the task of moving colder air rapidly through bulk volumes of fruit is more difficult than moving water, and cannot be accomplished efficiently with standard room-cooling procedures. Special techniques to increase the velocity of cold air and to channel the air past all the fruit surfaces greatly increase the cooling efficiency.

Responses to Temperature

All perishables are responsive to temperature. Temperature governs the rates of reactions of pathogens as well as living hosts. Temperature also governs the expansion and contraction of atmospheric gases and the vaporization, condensation and quantity of water vapor in air. Prompt removal of heat and maintenance of cold temperature arrests development of all the factors responsible for loss.

Respiration--Within the physiological temperature range of approximately 32°F to 95°F, perishables generally increase their rate of respiration (rate of living) 2 to 2½ times for each 18°F rise in temperature. Cherries are no exception, and in comparison with other perishables they have neither the highest nor the lowest respiration rates. Perishables having the highest respiration rates have the shortest postharvest life. The inherent differences in metabolism form the principal basis for differences in perishability of all perishable commodities. The relative change in rate of living with change in temperature is shown in Table 1. Rate changes are conservatively based on a 2-fold increase in respiration for each 18°F increase in temperature.

Table 1. Change in the relative rate of living with each 18 ºF change of temperature.

Temperature (ºF)
Relative Rate
32
1
50
2
68
4
86
8

This shows that if the respiration rate doubles for each 18°F, which is reasonable for cherries, respiration is 2 times as great at 50°F, 4 times as great at 68°F, and 8 times as great at 86°F as at 32°F. The relative importance of these rate changes in conjunction with temperature and exposure time to cherry lifespan is shown in Table 2.

Table 2. Postharvest storage life relative to given temperatures.

Temperature (ºF) Storage life (days) Storage life (%)
32
24
100
50
12
50
68
6
25
86
3
12.5

The storage life remaining upon cooling to 32°F at harvest is 100%. If 100% of the storage life at 32°F was 24 days, then the fruit would only last 12.5% of its life, or 3 days at 86° F. If harvested fruit were in the orchard, the warehouse, in transit, in the terminal market, in the retail store and in the home for the equivalent of 16 hours at 86°F instead of at 32°F all the time, then 22% or 5.3 days of the expected postharvest life would have been lost from the total life expected. This fruit would only last 18.7 days instead of the expected 24 days after cooling.

Based only on the effect of temperature on compositional changes linked to respiration, storage for 28 days at 32°F is reasonable. However, respiration is not the only loss factor affected by temperature. Water loss and decay are also influenced by temperature. Development of symptoms of injury from damage is accelerated at higher temperatures. Total deterioration is the sum of the effect of temperature on all factors responsible for loss.

Judicious use of temperature is the best weapon available in the fight against perishable loss. Heat removal from cherries reduces the impact of each of the major factors responsible for loss. Unless a perishable is chilling-sensitive, and sweet cherries are not, the most effective temperature for maximum retention of the harvest freshness, cell structure, chemical composition and overall quality and condition is just above the freezing point of the tissue. In individual cherries, this could be as low as 25°F, and is in large part dependent on the percent soluble solids present in the fruit. The soluble solids tend to serve as a natural antifreeze. Stems freeze at a higher temperature because they are lower in soluble solids. In order to strike a compromise with the differences in freezing temperatures of fruit, low soluble solids in stems, and lack of uniformity and precision of temperature control in storages, and still cool for maximum preservation, cherry fruit should be reduced to 30°F or to 32°F as quickly as possible. Fruit should be kept at that temperature until ready to be consumed. Holding at 30°F requires a full understanding of the capabilities and limitations of individual refrigeration systems and storage rooms. Operators should also know that sensitivity to injury by lowered oxygen and elevated carbon dioxide to oxygen ratios, increases at the lowest temperatures.

Some confusion lingers concerning adoption of rapid cooling because cold fruit responds differently to different types of damage. Cold cherries are more resistant to compression damage or squeezing, but are more susceptible to impact damage or dropping. This is because cold cells resist deformation better, but at the same time are more brittle. By delaying cooling to reduce impact damage, one increases the losses in quality caused by accelerated moisture loss, higher rate of living and increased decay. A much more logical and desirable avenue is to rapidly cool the fruit, and eliminate or minimize the causes of impact damage. Rapid cooling on arrival at the packinghouse has no effect in altering incidence of damage incurred during harvest operations.

Cooled fruit regains varying amounts of heat during the sorting and packing process. Once rewarmed fruit is packed in plastic liners in fiberboard cartons and stacked on adjacent pallets, it is extremely difficult to remove heat and re-lower fruit temperatures. Fiberboard is an excellent insulation, while plastic liners stop convection. A further drawback to packing warm fruit is that it transpires copiously. Attempts to cool packed fruit primarily cool the plastic liner and cause the transpired water vapor to condense inside the liner much as water condenses on the outside of an ice-cold glass in a warm, humid room. Water that collects in the bottom of the liner, in contact with moderately warm fruit, causes fruit cracking and promotes germination and growth of decay organisms. Fruit respiration also speeds up and generates increasing amounts of heat, which in turn increase respiration further with even higher heat production. This does not occur with cold fruit in the box.

The additional benefit from packing cold fruit is that cold fruit is more resistant to compression damage. It seems reasonable that most of the damage forces received after fruit is in the packed carton would be compression forces due to weight and squeezing. Therefore, pitting and bruising from compression in transit and during marketing are reduced on cold fruit. Consequently, there is a dual need to cool cherries rapidly after harvest and to cool the cherries again before cartons are filled.

One of the least understood roles of temperature is its dominant effect on fruit moisture loss. This is because temperature is a major factor controlling water vapor pressure in perishable tissues. Water vapor is lost from inside the fruit. Water vapor pressure of air, either inside or outside a fruit, is that part of the air pressure due to the water content in the air. A surprising amount of air is present inside the fruit. Water vapor pressures increase with increasing temperature and with increasing water vapor content in the air. Warm fruit has a higher water vapor pressure than cold fruit does. Whenever the water vapor pressure inside the fruit is greater than the water vapor pressure outside the fruit, water vapor is forced out through the skin. Movement of water vapor out of the fruit is governed by the strength of the pressure and the resistance of the cherry skin to vapor diffusion. When the vapor diffusion resistance of the sweet cherry is compared with that of other perishables, the sweet cherry has one of the poorest surfaces protecting against water loss of those measured (Table 3).

Table 3. Comparison of cultivar vapor diffusion resistance (VDR).

Perishable Water VDR (seconds/meter)
Onion
71,000
Potato
54,000
Pear
30,000
Apple (RD)
34,000
Apple (MAC)
23,500
Apple (GD)
17,000
Banana
3,700
Sweet cherry
2,500
Radish
150
Taken from G.W. Apel, 1983. unpublished data, Washington State University

Lack of freshness can be equated with moisture loss. Each perishable is capable of losing a given percent of the original weight as water vapor before it becomes visibly apparent. If a cherry can lose 5% of its harvest weight before the loss becomes visible, then a cherry that has lost 4.5% is still attractive. If 4.5% moisture was lost before the fruit was shipped it would be impossible for the fruit to be attractive for very long on retail shelves. Retail practices which feature unrefrigerated mass displays directly oppose laws of good preservation. Cherries can lose 1 to 1.5% of their original weight in an hour on a hot dry day.

Warm temperatures directly affect fruit softening. This relates more to fruit resiliency than to a breakdown of cell structure. With aging, there is loss of structure adjacent to the stony endocarp surrounding the seed. When fruit is cooled to near 30°F, there is an irreversible firming action which causes fruit to be firmer after several weeks storage and rewarming, than it was when first cooled. There is also a proportional increase in red pigment synthesis with increasing temperatures; but, at 30°F, there is almost no color change over 6 to 10 weeks of storage. High temperatures also hasten an intangible loss of energy from the fruit, making it more susceptible to invasion by decay.

Cherry decay organisms are parasitic lower plants. Like higher plants, they have optimum temperatures for growth, minimums which are too cold for growth, and maximums at which decay organisms are killed. They also must have a source of moisture and nutrition. Free water is required for some species to germinate and grow. Condensation water, juice exuded from fruit, fresh cracks and punctures, all are sources of free moisture. Nutrition is derived from the fruit. Juice exudate and wounds provide instant access to nutrition. Other decay organism species are capable of penetrating the dermal barrier for nutrients.

Organism germination and growth occur slowly at low temperatures. Decay organism growth, fruit infection and decay are proportionally greater at progressively warmer temperatures. The cold temperatures best for keeping fruit are also the best temperatures for controlling infection by fungi, bacteria and yeasts.


Summary

Physical damage is the cause of sweet cherry injury, resulting in bruises and pits. Differences in area of contact and pressure are responsible for the form which the injury takes. Practically all bruising and pitting found in terminal markets is caused by postharvest damage. Determination of sources of damage and their elimination is essential for delivery of injury free fruit. There is no substitute for the immediate and continued use of cold for maximum preservation of sweet cherries. This is best attained at the lowest temperature possible without freezing fruit or stems. With precise temperature control, 30°F is better than 32°F. Common fruit temperatures in the 39°F to 45°F range allow rapid fruit darkening, and are a poor substitute for good cooling. At the best temperatures, all the factors responsible for loss in quality, condition, appearance and grade are arrested, and the fruit achieves maximum retention of its quality at harvest.

Dr. Max E. Patterson, Professor and Horticulturist

Department of Horticulture and Landscape Architecture, Washington State University

Post Harvest Pomology Newsletter, 5(1): 3-9
May 1987

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