An Introduction to Cherry Quality and Handling
The overall quality of the cherry crop shipped from the Pacific Northwest has varied greatly over the last few years. Frost, rain and other acts of nature have taken their toll. In attempting just to get the crop in the warehouse, it is sometimes easy to forget the complexity of the cherry fruit. This article describes the physical and physiological nature of a cherry. Preharvest and postharvest factors of cherry handling are important to cherry quality and are interrelated. None should be overlooked if delivering quality cherries is the goal.
Description of a Cherry
It is important for growers and warehouse personnel to understand what a cherry is made of and how it grows, in order to be able to foster and retain high quality. Dr. Patterson, WSU, has offered several definitions of a cherry fruit:
- A bag of water, air, and organic
- A series of compartments within
compartments, individually packaged in a larger master
- A fragile, living entity at the cellular level as complex as human life; while at attached to the tree, nourished by the tree; but when detached, must live off of itself at the mercy of the environment.
This final definition implies the fragile nature of a cherry and the challenge facing the industry.
The "skin" is the wrapper which keeps out dirt and disease and holds the fruit together. It is a barrier to gas diffusion. The "skin" is made up of the cuticle, the epidermis and the hypodermis. It is not a great wrapper, since it lets water vapor escape and can take up water readily at harvest and crack.
There are different types of cells in the flesh and the "skin" of the cherry. In the skin, the epidermis is covered by a layer of cuticle which reduces water loss. The cuticle is a coating made up of wax and polyesters. The epidermal layer of cells is the largest of the stone fruits. Within the epidermal layer are the lenticels which allow for gas exchange. Under the epidermis is the hypodermic layer of cells, which has very strong, small, thick walled cells.
Texture in cherries is not completely understood; however, the composition of the cell wall and its thickness plays an important role. Within the rigid cell wall, the greatest portion of the cell is the vacuole, where many different organic compounds pounds are stored (Figure 1). Within the vacuole are sugars, red pigments (anthocyanin) and acids. Cytoplasm surrounds the vacuole. In the cytoplasm are plastics which are green (chlorophyll) or yellow (carotenoids), and the mitochondria responsible for respiration.
The cytoplasm is surrounded by a living membrane which allows specific minerals, organic compounds and gases to move in and out of the cell. Within the cell, each organ is packaged in a membrane; life depends upon this compartmentalization. Therefore, maintenance of the membranes is critical to life itself. The concept of turgor pressure is important to an understanding of cherry quality. Turgor is the swollen condition of a cell caused by outward pressure against the cell membrane and wall. A loss of turgor means a collapse of the cell's rigidity and danger to the cell's contents. Wilted cherries have insufficient cell turgor.
Scientists divide the growth rate of cherry fruit tissue into a three-stage pattern (Figure 2). The first (I) and third (III) are phases of rapid fruit growth, and the second (II) is a relatively quiet stage. The second stage correlates with pit hardening. Stage I is the period of cell division and the following growth (II & III) is a result of cell en enlargement.
The ultimate size of the cherry is the result of a combination of cell division and cell enlargement. The length of the cell division phase is important in determining cherry size. "Cool" seasons extend the cell division period (Stage I) and larger fruit results. "Hot" temperatures during this period shorten this phase, resulting in smaller fruit. Temperature is also important during stage III. During periods of normal temperatures growth continues. However, if temperatures are too hot, growth stops and ripening is accelerated. Fruit size is also determined by crop load. Heavier crop loads result in smaller fruit (Figure 3).
Although cherry fruits are basically made up of water, sugar and air, they do contain other compounds. There is no reserve carbohydrate (starch) in cherry fruits, in contrast to apples and pears, which have a great deal of starch. The organic acids (malic and chlorogenic) and astringent materials (tanners) decrease as harvest approaches.
As harvest approaches, the fruit changes color from green to yellow to red to dark red (mahogany). The green pigment is chlorophyll, which is necessary for photosynthesis and the accumulation of sugars within the fruits. These chlorophyll molecules are contained within the plastics which are in the cytoplasm. As the plastics are destroyed toward harvest, chlorophyll is also destroyed. The yellow pigmentation is probably due to carotenoids. The red pigmentation is anthocyanin, which increases in quantity as the fruit begins to mature and then senesce.
One of the best methods of rating fruit color is to use the cherry color comparators developed in Canada and available from Okanagan Federated Shippers, (1476 Water St., Kelowna, B.C. V1Y 1J5, telephone 604-762-2236). I find the comparators numbered 3 and 33 the most useful.
Respiration is a measure of the rate of living- somewhat like breathing in animals. Oxygen is taken in and carbon dioxide is given out. Sugars and other compounds are oxidized to release stored energy. Heat and water are also produced. Some fruits (apples, pears) have a marked rise in respiration (called the climacteric) as senescence approaches, while others do not. There is quite a controversy as to whether cherries have a climacteric. Dr. Patterson believes that while cherries have a climacteric, it is relatively insignificant in comparison with the climacteric of other fruits.
Respiration and the climacteric can be affected in a number of ways. If enough carbon dioxide is accumulated, respiration will be slowed. If oxygen is insufficient for the oxidation of these food molecules, respiration will also be slowed.
Temperature strongly affects respiration. The warmer the temperature, the faster the respiration. In fact, respiration doubles for each 18°F rise in temperature. Low temperatures retard color change, softening, loss of soluble solids, and decay. Low temperatures also lower vapor pressure deficits, decrease respiration, and increase firmness. The effect of low temperature is proportional to the temperature itself with 30°F being far better than 32°F and above.
Preharvest Factors Affect Cherry Quality
Some of the preharvest practices which affect fruit quality include: crop load, growth regulators, temperatures, harvest maturity, water status, light and mineral nutrition.
Crop load helps to determine not only the size of the crop in terms of the number of fruits, but also the size of each individual fruit. It influences the ability of fruit to withstand handling without bruising.
The variation in crop size and individual fruit size on a single tree is immense, and is one of the industry's greatest problems. Within a specific spur or branch, the leaf-to-fruit ratio determines how much photosynthate goes into a particular fruit. Increasing leaf size will increase the size of specific fruits. Fruit size is directly related to fruit quality; larger fruits are of higher quality.
Growth regulators influence fruit quality. Gillerellic acid (GA) delays maturity and retards anthocyanin synthesis, thus delaying the development of red color. GA makes cherries firmer, but delays harvest. GA cherries have a snap to them which is preferred by consumers.
Alar on the other hand, advances maturity by accelerating anthocyanin synthesis. The price one pays for advanced maturity, however, is in softer cherries. Alar cherries should never be stored or sent to far away markets since they rapidly break down.
Selecting harvest date is important, because growers need to harvest cherries at optimum maturity in order to have the large, firm fruit that can be shipped.
The best time to harvest is at the mahogany stage of development, when firmness, fruit weight, and soluble solids are highest. Not only are mahogany colored fruits of superior quality and sweeter than red colored fruit, but they are also larger.
Fruit size is in part determined by the maturity of the fruit at harvest. There are major differences in fruit size based on harvest maturity (Figure 3). This has implications for warehouse operations. A row size 13 package contains about 1800 cherries per 20-pound box, while a row size 10 package contains half as many cherries per box. Consequently, the handling and packing of larger cherries is more economical.
Water relations influence cherry quality. Prior to harvest, water moves in and out of the fruit based on the needs of the surrounding leaves. There should be enough water in the soil to allow the fruit to quickly regain its turgor, especially during the rapid growth of Stage III.
Water is also lost through the fruit skin. It is important to understand the physics behind water loss, as cherries can lose great amounts of water, and thus lose turgor and firmness.
Water is lost from fruits or leaves as water vapor, rather than as a liquid. Pressure exerted by water vapor is termed vapor pressure. Water vapor flows from one area of higher vapor pressure to an area of lower vapor pressure. Both humidity and temperature influence vapor pressure.
Since cherries are very largely air and water, the air inside the cherry is saturated with water (100% relative humidity). If the vapor pressure of the water in the cherry is higher than the vapor pressure of the water in the air, then the water will move out of the cherry into the surrounding air. Warmer cherries lose water vapor much more rapidly than cooler cherries. On a weight basis, larger cherries will lose less water than smaller cherries, since small fruits have a larger amount of surface area per volume.
It is critical to harvest during the cooler periods of the day, when the cherries are cooler, and to keep them in the shade once they are harvested. Transport them to the warehouse rapidly and cool them down in humidified air to minimize the vapor pressure difference between the fruit and the external air. This will minimize fruit moisture loss. Conversely, if the air has a larger vapor pressure than the cherry, the cherry can begin to take up water. Fortunately, cracking has not been linked to hydrocooling or high moisture levels after harvest.
The commercially sold fresh market cherry is actually a combination of a fruit and a vegetative part (stem). Most wholesale buyers consider the turgor and color of the stem upon delivery. The optimum temperatures to conserve fruit and stem quality differ. Fortunately, both plant parts respond equally well to high levels of humidity. It is important to keep this product cold and moist.
Cherry Handling in the Warehouse
The success that a warehouse has with marketing quality dark sweet cherries depends upon the quality of the cherry that is brought into the warehouse and subsequent handling and packaging. Cherry quality cannot be improved after harvest. Cherries do not have a reserve carbohydrate (starch) which changes to sugars in storage. Cherries deteriorate from the moment they are picked.
Limited commercial trials, as well as experiments, have demonstrated that cherries can be stored for several months. However, it is critical to start out with extremely high-quality cherries and to pay attention to every minute detail in the handling, packing, and storage.
Time is of the essence in each one of the steps that it takes to handle and market sweet cherries. It is critical to cool the cherries as rapidly as possible immediately after harvest to remove excessive heat and to reduce the vapor pressure deficit. Most warehouses use Hydrocooling to rapidly reduce temperature and increase humidity. Warehouse personnel should pay attention to the warming of the cherries as they go over the cluster cutter, the belts and the sorting lines, to minimize time and increase in temperature. Progressive warehouses use a second hydrocooler right before the cherries are packed. This is a very effective way to reduce the temperature. It is more difficult to bring the temperature down after cherries are boxed, since the polyliner and the carton provide a barrier to cooling. Boxes should be imprinted with instructions to keep cherries cold as they travel to the consumer. Forced air cooling is an excellent way to reduce the temperature of boxed fruit. It is a simple technique used by many sheds in California to rapidly reduce the temperature of large volumes of stone fruits.
Fruit Weight Loss
Weight loss in cherries can be equated to the amount of water lost as water vapor, resulting in softening. Cherries have weak cuticle barriers to prevent water loss. The stems have even poorer barriers to water loss. Larger fruits are higher in sugars (soluble solids) and will be slower to lose water than fruits which are smaller and have less sugar.
The difference between the vapor pressure of the surrounding air and the vapor pressure of the cherry is a major variable in water loss. Two major factors influencing water loss in sweet cherries are relative humidity and temperature. The colder the fruit, the slower the loss of water from that fruit (Figure 4). Realization of the importance of humidity and temperature will enable the industry to do a better job handling dark sweet cherries.
Dr. Eugene Kupferman, Postharvest Specialist
I appreciate the information provided to me by Dr. Max Patterson, Professor and Horticulturist, Washington State University
WSU Tree Fruit Research and Extension Center
1100 N. Western Ave., Wenatchee, WA 98801
Post Harvest Pomology Newsletter, Vol. 4, No. 1