Reduction of Surface Pitting in Sweet Cherries
Surface pitting of sweet cherries has long been a problem in the fresh market cherry industry. It has now been well established that the majority of pitting is the result of physical damage.
Surface pitting of sweet cherries is the development of depressions on the surface of the cherry that occurs after the fruit has been mechanically damaged.
In dark cherries the anthocyanin pigment, which accounts for the red color, masks the browning of the damaged tissue. Injuries therefore go unnoticed until the tissue collapses and a pit (sunken area) develops. Pitting can occur within a day under conditions in which water loss and metabolic rate are rapid, or it may take weeks if cherries have been kept at low temperature in plastic liners.
With normal bruise damage, the bruising, softening and browning are visible more immediately. Both type of injuries are important to the industry.
Types of Damage
Pitting and/or bruising occurs when enough pressure is applied either gradually (compression) or instantaneously (impact), to cause shearing and breakage of cells in the flesh of the cherry. Compression damage results from the pressure on fruit by such things as the fingers of the pickers, the weight of cherries in a bucket or bin, and the rollers on a sizer. Impact damage results from the dropping of a cherry into a metal bucket or drops of as little as one inch as the cherry moves through the packing line.
Firmness and Pitting
The sensitivity of cherries to pitting is largely a function of fruit firmness. Firm cherries better resist impact and compression pressures than do soft cherries. Unfortunately, all of the factors governing cherry fruit firmness are not completely understood. Nevertheless, some general statements can be made.
All commercially important cultivars of sweet cherries are extremely susceptible to mechanical damage. Fortunately for the industry, Bing is the least sensitive. Van, on the other hand, is usually so covered with pits by the time it reaches the terminal market that it is often unsaleable.
For example, when Van cherries harvested at commercial maturity are dropped onto a rough surface from a height of as little as two inches, 50 to 80 percent of the cherries will become pitted. An equivalent drop of Bings will result in 20 to 40 percent pitting. Mixing of Vans with Bings can lead to whole carloads being downgraded if boxes of cherries with a high percentage of Vans are inspected. A firm Van cherry is still more susceptible to pitting than a soft Bing.
The greater the accumulation of photosynthates into a fruit, the firmer that fruit will be at any given maturity. Thus, cherries that are the largest in size and have the highest soluble solids are usually the firmest. The amount of photosynthates that are accumulated in a given fruit depends on the supporting leaf area and amount of light available to those leaves. Firm fruit are therefore produced from trees with adequate vigor, good light penetration throughout the canopy (yearly pruning) and a high leaf-to-fruit ratio. Further, fruit usually become progressively firmer from the inside to the outside and from the bottom to the top of the canopy. Crop load plays a very significant role in determining fruit quality.
Another critical aspect of fruit firmness is the temperature of the fruit. Firmness doubles when the fruit temperatures are reduced from 90 to 32 degrees F. Fruit picked during the heat of the day are softer than those picked in the morning.
The temperature of the fruit when it is being roughly handled alters the extent of pitting. Although cold fruit are much firmer than warm fruit, they are more brittle. This makes cold fruit more susceptible to impact damage than to compression damage. The converse is also true--that warm fruit is more susceptible to compression damage than it is to impact damage.
Fruit Water Status
Excessive orchard irrigation or prolonged water uptake by the fruit during heavy rains promotes softening. However, when cherries are hydrocooled there is little water uptake due to the cold water temperature and the short time span.
The highest quality cherries, those that are large, firm and high in soluble solids, are the most resistant to pitting. Cultural practices that enhance quality decrease susceptibility to pitting. Unless the economics of an extra early harvest take precedence, Alar sprays, which cause softer and smaller cherries, should be avoided. A series of calcium sprays (below) has helped counteract the softening effect of Alar in experiments.
The larger, firmer cherries that result from the application of gibberellic acid (GA) will be less susceptible to pitting and of higher quality.
Multiple preharvest calcium sprays are another approach to achieving firmer fruit less subject to pitting. Weekly applications, starting 3-4 weeks before harvest, of 4.5 pounds of 77 percent flake calcium chloride per 100 gallons of water, plus non-ionic surfactant, applied to drench have been successful in firming fruit and decreasing surface damage. Calcium sprays, however, may slightly decrease fruit size.
Calcium sprays, in addition to decreasing pitting, can also offer some protection from rain cracking. In preliminary studies, a single spray of GA plus 3 to 4 weekly preharvest calcium sprays were found to maintain normal fruit size and give additional protection against cracking.
Harvesting cherries at the correct maturity is paramount to achieving high quality. No improvement in quality occurs after picking. Appropriate harvest maturity should be selected on the basis of the combined quality factors of size, color, soluble solids, and firmness, not on any individual component itself.
Pitting at Picking
Surveys from British Columbia have indicated that at least 15 to 30 percent of all surface damage occurs during the harvest and subsequent handling in the orchard. Since compression bruising is more serious when the fruit is warm, picking early in the morning can reduce finger bruises. The depth of cherries in a bin should be reduced to minimize compression damage. Picking buckets and bins should be padded. Pickers should be closely supervised to minimize the damage caused by dropping cherries into buckets. Full buckets should be kept in the shade. Dark cherries left in the full sun can reach temperatures over 100 degrees F. Bins should be covered with wet canvas or foam and delivered to the packinghouse frequently.
The fruit should be cooled rapidly after harvest. For optimum quality and shelf life sweet cherries should be kept at 30 to 32 degrees F. However, at this temperature the fruit is brittle and thus is very susceptible to impact damage. (Impact damage occurs whenever the fruit falls or is dropped). In view of this, an immediate hydrocooling of cherries to 40 to 45 degrees F before packing, followed by a second hydrocooling of cherries to 32 degrees F at the end of the line (just prior to boxing), would be ideal. Rapid reductions in fruit temperature are imperative.
Packing lines should be re-examined with the extreme sensitivity of cherries to mechanical damage in mind. Factors contributing to damage such as belt speed, drop height, and the number of drops, should be minimized. Drop height should be reduced and rough surfaces, such as steel grates or rough pointed traction belts eliminated. Figure 1 illustrates the damage that can be expected when cherries are dropped from various heights similar to drops found in packing lines.
Since it is difficult to identify sources of injury until the pit develops, Drs. Patterson, Loescher and Rhee have recently developed a technique for rapid bruise visualization. This technique, based on unmasking brown injured areas by rapidly bleaching the red color, will be field tested during the next few seasons. For a less rapid evaluation procedure, cherries from the same lot can be pulled from different locations on the line, held uncovered at room temperature for 1 to 2 days and observed for pitting.
The first step in the reduction of pitting is an awareness that mechanical damage is the cause. Second, pitting can be reduced by growing a large, firm, sweet cherry which is high in soluble solids, and is harvested at the dark red to light mahogany stage. These cherries are more resistant to the damaging pressures found during cherry harvesting and handling. Greater economic incentives should he established by packinghouses for growers producing such high quality cherries with increased resistance to pitting. Third, a conscientious effort must be made to identify and eliminate or minimize sources of damage throughout the pick, pack (store), and shipment process. It is only by awareness to details and application of corrective measures that pitting can be minimized.
Kim D. Patten and Max E. Patterson(1) and Eugene Kupferman(2)
(1)Department of Horticulture and Landscape Architecture, Washington State University (2)WSU Tree Fruit Research and Extension Center
1100 N. Western Ave., Wenatchee, WA 98801
Post Harvest Pomology Newsletter, May 1983, Vol. 1, No. 2.