Role of Calcium in Delaying Softening of Apples and Cherries
Fruit firmness is an integral part of quality and is of increasing importance in today's marketplace. This was evident during the late marketing period of the 1984 Golden Delicious apple crop. The combined effects of a large 1984 apple crop and poor fruit size resulted in a large quantity of fruit still unmarketed late in the storage period.
Some 1984 fruit entering the market was soft and of extremely poor quality. These fruits depressed market demand and resulted in losses estimated in the millions of dollars. Concerned groups began to call for restrictions on shipments of Goldens that do not meet minimum firmness requirements. Further interest in understanding and controlling apple softening has arisen due to the decline in Alar usage.
Fruit firmness is also important in sweet cherry quality. Industry leaders have recently expressed a great interest in cherry softening caused by rain or water damage. When moderate cherry cracking occurs due to heavy rainfall near harvest, the fruit is carefully sorted before it is sent to market. Experience has shown that such fruit becomes soft and has poor quality. The following report discusses some of our work on the role of calcium in fruit quality as it relates to firmness in apples and to water-induced softening in cherries.
Firmness of apple fruits is determined by a combination of factors. Figure 1 shows a cross section of an apple peel along with the underlying tissue. The outermost layer of peel is covered by a thick, waxy cuticle. Beneath the cuticle lies a multilayer of small, thick-walled cells having a large amount of cell to cell contact. These structural features are responsible for the toughness of the apple peel. Large, thin-walled parenchyma cells lie beneath the peel. These cells make up the apple cortex or flesh. Due to their large size and reduced area of cell to cell contact, large intercellular spaces exist.
To better understand the changes responsible for flesh softening, we compared the cell structure of Golden Delicious apple fruits at harvest time and 7 months later. We also looked at fruits that were treated with calcium and stored for 7 months. The results of this study are shown in Figure 2.
The flesh of fruits possessing high textural quality at harvest time had a large amount of cell to cell contact. Fruits that had been stored for 7 months became mealy, showed very little cell to cell contact, and appeared to have an increase in intercellular space. The calcium-treated fruits remained firm after 7 months of storage, had a greater amount of cell to cell contact, and did not appear to have increased intercellular space.
Transmission electron micrographs show the cell wall of calcium-treated and untreated fruits in Figure 3. Extensive degradation of the cell wall, especially the middle lamella, was observed in the untreated fruits after 7 months of storage and resulted in cell wall separation. In contrast, the cell wall structure of the calcium-treated fruits showed very little breakdown.
Commercially, pressure tests are used to measure textural quality in apples. Other tests that provide additional information on the textural properties of fruits are being used in research. We have developed a system for measuring the tensile strength of apple tissue that can be adapted to the recording laboratory penetrometer developed by A. J. Topping of the East Malling Research Station. Figure 4 shows a diagram of a tissue plug that is used to measure tensile strength.
The system proved to be useful, and the data we obtained correlated well with firmness readings recorded using a penetrometer. In addition, we were able to save the apple tissue for mineral analysis and other tests. Scanning electron micrographs of the fractured surfaces of tissue cylinders prepared from Golden Delicious and Starr apple varieties further substantiated the importance of intercellular space and cell cohesion in textural quality (Figure 5).
We measured the tensile strength of calcium-treated and untreated fruits to determine if calcium was able to maintain the tensile strength of the tissue. Our results showed a very good correlation between calcium content and tensile strength. The importance of calcium in maintaining cell to cell cohesion can be further illustrated (Figure 6). Tissue pieces of calcium-treated and untreated apples were cut with a razor blade, and observed under a scanning electron microscope. Rather than cutting through the cells of untreated fruits that had become soft and mealy, the blade tended to slide between cells. In calcium-treated fruits that had a firm and rigid cell structure, the blade cut through the cells. The cumulative information we have obtained strongly supports the importance of cell to cell cohesion in textural quality in apples and the role of calcium in maintaining cell cohesion.
Water Softening of Cherries
Cultural factors, which include crop load and harvest date, are known to influence cherry firmness. Physical factors, such as cell turgor, are also important in determining fruit firmness. Water loss, a serious problem in cherries, results in decreased cell turgor and fruit firmness. The rate of fruit water loss is largely determined by a waxy covering on the fruit, called the cuticle. Scanning electron micrographs of cherry fruit skin show a very thin (1 micrometer) cuticular layer covering the cherry surface (Figure 7). We have been investigating the problem of cherry softening as a result of water damage. Findings will be presented in three sections:
Mode of water penetration
Water induced damage
Mode of Water Penetration
As mentioned, a thin, waxy cuticle covers the cherry fruit. The cuticle is impregnated by a dense distribution of stomates, which become dysfunctional as the fruit matures, and may become permanently fixed in an open position (Figure 8). Our studies suggest that water penetration through the cuticle is more important than stomatal penetration. We prepared and sectioned water-damaged cherries. We observed very little water damage associated with the stomata! structure even when extensive cuticular damage had occurred (Figure 9).
The styler scar may also be an important site of water entry. Our studies show that the waxy cuticular covering of the cherry does not cover the styler scar. Histochemical studies indicate that relatively little water repelling material is present at the styler scar to inhibit water penetration. This information, coupled with the fact that cherry cracking is commonly observed near the styler scar, suggests its importance as a site of water entry.
Our studies of the cherry surface also led to discovery of preharvest fractures of the cuticle. The fractures traverse the cuticle and expose the cell wall of the epidermal cells. We compared the rate of water uptake of fruits with or without preharvest cuticular fractures. Preharvest fractures more than doubled the rate of water uptake.
We observed changes occurring in the cherry fruit during water treatments. Figure 10 shows a typical example of the surface of a normal cherry fruit before being immersed in water. Following water immersion, the first change observed in the cherry surface was an increase in the turgor of the epidermal cells (Figure 11 a). As the fruit continued to swell, fractures occurred that traversed the cuticle. The cuticular fractures, difficult to detect with the naked eye, commonly occurred before fruit cracking. In some cases, severe cuticular fracturing, accompanied by a rupturing of the epidermal cells, appeared before fruit cracking and resulted in a bronzing of the fruit surface (Figure 11 b). Studies of water damaged cherries that had been prepared for microscopy showed that the initial damage involved solubilization and swelling of materials that bonded the cuticle to the epidermal cell wall. As the bonding components dissolved, the cuticle separated from the epidermal cell wall. Stress induced by the swelling process resulted in cuticular fracturing, as seen in Figures 11c and 12. Severe water damage of fruits resulted in cell rupture and loss of cell cohesion in the epidermal cell layer. Cherry fruits having water-induced cuticular fractures softened rapidly due to water loss. We concluded that loss of fruit moisture and cell turgor due to cuticular fracturing or cell rupture are two important factors responsible for water-induced softening in cherries.
The best prevention of water-induced damage is to avoid prolonged exposure of cherry fruit to water. We have lime control over water damage incurred from rainfall occurring at harvest. However, water damage to fruit can also take place postharvest. For instance, water condensation occurs in plastic-lined cherry cartons that are removed from cold storage and placed at room temperature. We have observed water damage in cherries that had been exposed to this condition for several hours.
The importance of preharvest cuticular fracturing in water uptake has already been discussed. We observed that fruits harvested from different sites had different degrees of cuticular fracturing. This would suggest that environmental and cultural factors may be important in the development of preharvest fractures. More work is needed to isolate those factors so that preventive measures can be taken.
The problem of fruit cracking due to untimely rains near harvest remains a serious problem. It has been clearly shown in immersion studies that calcium reduces the rate of cracking in sweet cherries. We have conducted cracking studies by immersing freshly harvested, turgid fruits in solutions containing distilled water or low concentrations of calcium. After 1.5 hours of immersion, significant levels of cracking were observed in fruits immersed in distilled water while fruits immersed in calcium solutions failed to crack. These studies showed that calcium does delay cracking, and that the effect of calcium is rapid.
In a third experiment, cherry fruits were immersed in a solution containing radioactive calcium for 1.5 hours. The fruits were then immediately frozen, sectioned and freeze dried for the calcium localization procedure. X-ray film exposed to the thin cherry sections for several days before being developed is shown in Figure 13. The dark areas indicate the location of the calcium.
It has been suggested that calcium affects the solubility of cell wall materials and reduces the rate of water uptake in cells. Our studies show that an increase in calcium at the cherry peel is responsible for the observed delay in cracking. Other researchers have shown that field sprays of calcium hydroxide reduce the rate of cracking. Residual spray material of calcium hydroxide on the fruit surface slowly becomes solubilized during rains, thus protecting the fruit. Calcium hydroxide has not found wide use in the industry because of the objectionable residue it leaves on the fruit. Other more soluble forms of calcium have not protected fruit from rainfall as well. New methods are being developed and tested to provide a continuous source of calcium during rainfall.
In summary, fruit firmness in apples and cherries depends upon cell wall structure and composition, cell to cell cohesion, the amount of intercellular space and cell turgor. In cherries, loss of moisture and cell turgor due to cuticular fracturing enhance fruit softening.
Gregory M. Glenn and B. W. Poovaiah
WSU Department of Horticulture and Landscape Architecture
Post Harvest Pomology Newsletter, 5(1): 10-19