v° "I*'//, Division of Agricultural Sciences UNIVERSITY OF CALIFORNIA '''''^f} HANDLING SWEET CHERRIES FOR THE FRESH MARKET ■> W. C. MICKE • F. G. MITCHELL CALIFORNIA AGRICULTURAL Experiment Station Extension Service CIRCULAR 560 CAECAG 560 1-20 (1972) Sweet cherries require careful management of temperature as well as maximum protection from rough handling. Once picked, they should be rapidly cooled to 32°F and kept at that temperature until they reach the consumer. THIS CIRCULAR discusses details involved in the handling methods needed to deliver cherries to the market in good condition. March, 1972 THE AUTHORS: W. C. Micke is Extension Pomology Technologist, Davis. F. G. Mitchell is Extension Pomologist, Marketing, Davis. [2] HANDLING SWEET CHERRIES FOR THE FRESH MARKET No management program can improve the quality of sweet cherries after they are harvested, but careful management can prevent loss of quality. Poor man- agement, however, can cause serious losses during the crop's journey from tree to fresh market. To complicate the prob- lems in distribution, damage resulting from poor practices at one stage of handling may not become apparent until later in the marketing sequence. Sweet cherries are among the most perishable of California's stone fruits, and therefore must be constantly protected during handling — once quality is lost, it can never be regained. Fruit deterioration can take three forms : • Pathological rotting caused by microorganisms • Physical .... damage from moisture loss, bruising, or other injury • Physiological breakdown or self-destruction of fruit which often leads to over-ripening and senescence. Pathological. Among the fungus dis- eases which can cause serious losses are brown rot (Monilinia friicticola) , grey mold (Botrytis cine re a) , and Rhizopus rot {Rhizopus stolonifer) . To reduce loss from these organisms, proper handling — especially temperature management — is essential, and correct application of fun- gicides in the orchard and packing house can be vital. (For fungicide recommen- dations, consult the current "University of California Pest and Disease Control Program for Cherries.") Physical. Physical injury is especially crucial in cherries because of the effect on fruit and stem appearance. Green, plump stems often indicate fruit fresh- ness, and damaged stems can reduce con- sumer appeal. Cherries must be protected from moisture loss at all stages of han- dling, as such loss causes fruit and stems to shrivel, thus detracting from market appearance and reducing fruit weight. Additionally, when shrivel occurs after packing the fruit may loosen enough to move about in the container and bruise in transit. Moisture loss is affected by several fac- tors, including temperature, relative hu- midity, and air velocity. To minimize this loss, a 32°F holding temperature is best. (Care must be taken to avoid freezing the cherries.) However, even at these low temperatures serious moisture loss can occur unless a relative humidity of 90 to 95 per cent is maintained. The effect of air velocity on moisture loss is dis- cussed in the section on cooling. Most physical injuries such as bruis- ing, skin punctures and cuts are caused by rough handling, which can be avoided. Injured areas on fruit — even minute ones — may serve as points of en- try for rot organisms. Physiological. Sweet cherries are alive, and the rate at which their physiological processes ("rate of living") occurs is higher than for most deciduous fruits. This physiological activity is largely tem- perature-dependent and increases 2 or 3 times with each 18°F of temperature rise. Thus, sweet cherries may deteriorate twice as fast at 50°F as at 32°F. At normal field temperatures activity is so high that fruit will begin to destroy itself soon after it is harvested. There- fore, prompt cooling of the fruit is essen- tial if the potential shelf life of the cherry is to be realized. [3] 9% •^jm Effect of moisture loss on stems. The top left fruit has a plump, bright stem while deterioration becomes progressively worse (left to right, top to bottom), with the lower right cherry showing a badly shriveled and darkened stem. Cherries for fresh market are picked with the stem attached. Minimum damage m to the cherry occurs if the stem (rather than the fruit) is held during picking. Care r must be taken to avoid injuring the spur, which bears future years' fruit. Fruit should be dumped gently to avoid brusing or damaging. [4] RESPIRATION^ RATE d L_t 14 8 12 16 TIME (DAYS) A IN MGS. COt/KG. FRUIT /HOUR Effect of temperature on the respiration rate (a measure of the "rate of living") of Bing cherry — the higher the temperature the shorter the shelf life. Respiration readings at each temperature were discontinued with the first evidence of decay. HANDLING FRUIT IN THE ORCHARD For best flavor, cherries should be har- vested as fully ripe as possible. Maturity is determined by color, and "black" varieties such as Bing are usually picked when fruit turns a solid bright red. Such fruit will turn dark-red to black during its 5 to 8 days transit to eastern markets. Fruit transported rapidly by air or to nearby markets can be harvested at a [5] TEMPERATURE °F 100 BOX IN SUN BOX IN SHADE AIR^ TOP^ -jS 1 CENTER l ^ BOTTOM ' 1 I 2 3 40 I 2 3 4 TIME IN HOURS Effect of warm air and sunlight on cherry fruit temperatures at various positions within a box. Shaded cherries remain considerably cooler than exposed cherries, even though they warm up as the surrounding air temperature increases. Wind accelerates fruit warming when air tempera- ture exceeds that of the cherries. darker color, thus allowing its flavor to develop further on the tree. Fruit must always be handled gently. Special care is needed to pick cherries with stems attached and without damag- ing the fruiting spur. If too many cher- ries are grasped at one time, the fruit may be squeezed and bruised. Decayed and damaged fruit should be discarded during picking. Fruit is normally picked into buckets which are then emptied into wooden field lugs. Bruising and damage must be avoided when filling buckets and transferring fruit to field lugs. Padding the bottom of buckets reduces fruit in- jury. Overfilling of lugs can result in fruit crushing when lugs are stacked. Because of their dark color, cherries absorb heat rapidly when exposed to the sun and will soon be warmer than the Filled cherry boxes should be stacked in the shade, and the top box should be covered. Harvested fruit should be transported to pack- ing house or cooler soon after picking to avoid being warmed in orchard. ~*# : | [6] Picked cherries should be protected from expo- sure to sunlight and wind as much as possible. Un- loading inside the packing house helps protect fruit. surrounding air. Thus, cherries must be protected by stacking filled lugs in the shade and covering the top box immedi- ately. If shaded fruit remains in the orchard, it may become exposed to direct sunlight later in the day. Moisture loss and subsequent fruit shrivel will result from exposure of harvested cherries to warm, dry, field conditions. With air movement, the temperature of the shaded fruit will quickly approach that of the surrounding air. Therefore, cherries should be removed from the orchard as soon as possible after picking even if they are well protected. To minimize field delays there should be frequent deliveries of small loads of fruit. Cherries should be delivered to the packing shed on a properly suspended vehicle moving at a moderate speed and with reasonable care taken to prevent bruising, unnecessary warming, and ex- cessive moisture loss. Air suspension on vehicles is best for fruit transit, but a good spring suspension system is satis- factory especially if tire air-pressure is reduced. The extent of fruit warming and moisture loss during transit depends on air temperature and velocity and the time in transit. At normal transit speeds both fruit warming and moisture loss can be high; weight losses of 1.5 per cent have been measured in exposed por- tions of a load during a transit period of less than 1 hour. Limited observations indicate that cov- ering a load with wet canvas just prior to transit may reduce moisture loss and delay fruit warming. (Fruit near the top of a covered load responded approxi- mately like fruit in the center of an ex- posed load with regard to warming and moisture loss.) To avoid any "heat trap" effect, the covered load must not stand in the sun for prolonged periods. Covered vans may be desirable for long hauls if sufficient ventilation is provided to pre- vent accumulation of heat during transit. A shaded unloading area at the packing house also helps protect fruit. CHERRY PACKING PROCEDURES Most cherries are loose-packed (loosely random-filled) into shipping lugs with only a limited amount of face-packing (hand-placing the top two layers in rows) for special markets. In most packing lines the fruit is dumped on conveying [7] « * II M IB I ■ 1 E\W Top left: At most packing houses, cherries are dumped on conveyors and hand-sorted for de- fects and off-grade fruit. Top right: Stem cutters (rotating circular saws) divide clusters of cherries into individual fruit (with stems attached) to facilitate sizing. Bottom left: A water spray (often containing fungicide) is used to reduce friction during sizing, thus lessening incidence of injury. Bottom right: Diverging-roll sizer commonly used for sizing cherries. belts, moved past blowers which remove leaves and trash, and then passed through stem cutters (rotating circular saws or knives) which divide clusters into in- dividual fruit with stems attached to facilitate mechanical sizing. A diverging- [8 roll sizer is commonly used, and a water spray is applied to reduce friction be- tween fruit and rolls, thus lessening the incidence of injury. Hand-sorting is done at various points on the packing line to eliminate injured, immature, decayed, Above: Lidding is usually the final step in packing, after which the fruit is cooled and shipped. Left: After additional hand sorting, cherries are loose-filled into lugs. and other defective fruit. Fungicides are often applied during the packing opera- tion. Shipping containers are then filled with fruit to a uniform net weight and the containers are lidded. Cherries are adaptable to tight-fill packing, which consists of random filling of containers with fruit to a constant weight, settling this fruit by carefully controlled vibrations, applying a top pad, and firmly fastening the lid (Mitchell, et ah, 1968a) . The resulting tight pack and slight pressure from the pad prevents fruit movement and reduces vibration injury during transit. Most cherry pack- ing lines can be easily adapted to this method. Because this system must be carefully applied to avoid introducing other injury problems, shippers should explore tight-fill packing cautiously be- fore making any major conversion. The standard shipping container for California cherries is the Calex lug — a wooden box 3% inches deep, 13% inches wide, and 16% inches long, used for the 18- to 20-pound cherry loose-pack. Face-packing is generally done in a Campbell lug, which is 4 inches deep, 11% inches wide and 14% inches long. Corrugated containers are in limited use, especially for air shipments since their lighter weight is an advantage. The wax coating used on some corrugated con- tainers can help protect fruit from mois- ture loss. The wax coating can also slow the penetration of moisture into the corru- gated board, and thus aid in maintaining [9] if#«iia CKMi i z 7 * <* 10 ii i: Two wooden shipping containers used for cherries. The Calex lug (left) is the common shipping container and the Campbell lug (right) is for face packing. the rigidity of the container. Corrugated containers must be constructed to pro- vide sufficient stacking strength to pro- tect the fruit during a normal marketing period under high humidity conditions. Although most cherries are packed to depths of 4 inches or less, recent studies indicate that a 6-inch depth could safely be used. Although there is considerable latitude in the depth for cherry con- tainers, fruit damage does increase at depths greater than 6 inches. Therefore, a container could be designed with a greater depth and smaller horizontal di- mensions to hold the same amount of fruit as the present lug — the smaller top surface would reduce top bulge and sub- sequent fruit loosening and damage. In some states, cherries are packed in sealed polyethylene liners within the ship- ping containers. These liners have been reported to reduce decay and preserve stem freshness and fruit brightness (Gerhardt, 1956), but in California lim- ited observations suggest that such liners may intensify fruit rotting. If liners are used they must be opened before the fruit is warmed to avoid oxygen starva- tion and fermentation of the fruit. They have not been commercially accepted in California and are not recommended. PROPER COOLING IS ESSENTIAL Fruit should be cooled as soon after har- vest as possible to minimize deteriora- tion. Delays of more than 24 hours with- out cooling have been observed in some commercial operations. Even holding noncooled cherries overnight in the pack- ing house can cause serious deterioration, including fruit softening, stem darken- ing, decay, and increased moisture loss. Studies have shown that water loss [10] •/•SHRIVELED FRUIT 60 8 16 HOURS DELAY BEFORE COOLING Delays of more than 4 hours between har- vest and cooling caused increased fruit shrivel. Following initial delays, cherries were held for 8 days at transit and marketing temperatures before shrivel evaluations were made. occurs most rapidly during the first 8 hours following harvest, and then slows somewhat. The amount of shriveled fruit increased from 8 per cent after 4 hours delay to 33 per cent after 8 hours delay between picking and cooling. These re- sults demonstrate the need for rapid fruit handling and cooling. In California, sweet cherries are nor- mally packed before cooling; however, if excessive packing delays are anticipated, fruit should be cooled prior to packing. Some warming will occur during pack- ing, but it can be minimized by packing rapidly and immediately returning packed fruit to the cooler. Air condi- tioned packing facilities can lessen fruit warming. Cherries should be cooled rapidly and thoroughly. While 40 °F is often the goal for commercial coolers, studies show that physiological activity is slowest just above the freezing point of the fruit and that growth of rot organisms is slowed and moisture loss reduced at tempera- tures near 32°F. Sweet cherries have been reported to freeze at approximately 29°F (Whiteman, 1957), although the freezing point can vary with variety, growing conditions and maturity. While 32°F is considered safe for sweet cher- ries, the thermostat setting for any cool- ing room will depend upon the lowest temperature reached during cycling of the refrigeration system. High air velocities are best for effec- tive cooling, but will cause rapid and con- tinuous water loss from fruit if main- tained during subsequent holding pe- riods. Once fruit is cold, air velocity need be only high enough to remove heat pro- duced by fruit respiration and outside air leakage into the room. Thus, if cherries are to be held after initial cooling, fan controls are needed to reduce air velocity or the fruit must be moved to a holding area having a low air locity. COOLING METHODS Cooling methods are a detailed subject and are only briefly covered in this cir- cular. More information on the subject can be found in a publication by Guillou (1960). Room cooling. This is accomplished by exposing fruit (usually after packing) to circulating cold air. This is the most commonly used method for cooling cher- ries and allows fruit to be cooled and held without rehandling. Room cooling has several limitations. Fruit in standard room coolers may not reach 40°F even after 12 hours, and cherries, being highly perishable, may deteriorate appreciably during such slow cooling particularly if they are initially very warm. This relatively slow cooling may cause delays in loading or result in warm fruit being loaded for transit. Pe- riodic introduction of warm cherries into a room containing cool fruit can cause fluctuations in temperature and relative humidity which can further slow cooling and add to deterioration problems. Uni- form room cooling is dependent upon [in HBP i8Hllt; ? ~ illlli ft as ^^^0^^ Cherries usually are cooled in large rooms. The cooling rate of fruit in such rooms is rela- tively slow and so appreciable deterioration can occur during cooling. adequate air circulation to remove warm air from around the fruit; therefore, containers must be spaced to allow for such circulation. Some fans should be operated even when people are working in cooling rooms. The venting built into containers can have an important effect on cooling rates by influencing contact between fruit and cold air. A comparison between corru- gated containers with 5 per cent side venting and the Calex lug showed cool- ing patterns to be similar. Thus, room cooling can be equally effective for both containers provided each is adequately vented and spaced for air circulation. Cooled cherries should be protected from rewarming. The cooling room should be managed so that cold air passes over cooled fruit before contacting any warm fruit that has been introduced. Ex- cessive traffic in and out of the room should be avoided, and when doors are open the room should be protected from heat leakage through the use of air bar- riers, such as swinging doors or "air curtains." Air curtains (which consist of an air flow directed across the door open- ing) can be useful in reducing heat leak- age provided they are protected from wind. Forced-air cooling. This method also circulates cold air, but in addition util- izes differential air pressure to direct air through containers and around the fruit, rather than around the outside of con- tainers. The speed of forced-air cooling can be regulated by varying the volume and static pressure of air used in the system. Forced-air cooling can be accomp- lished in one-sixth to one-eighth the time required for room cooling. Because forced-air cooling is rapid, there is greater opportunity for complete and uni- form fruit cooling; this is especially im- portant for highly perishable commodi- ties like cherries, which usually are shipped immediately after cooling. If the system is properly designed, forced-air cooling should not hinder other packing or handling practices. (Existing room coolers can often be converted to forced- air cooling with a small additional in- vestment.) Forced-air cooling has some limita- tions. Additional handling of containers may be needed if the fruit is to be held after cooling. Containers must be vented, and air flow through the pack must not be impeded. As the number of tiers of containers (distance of air flow) is in- creased, the pressure differential needed to force air through the containers be- comes greater and cooling becomes less efficient. A comparison of forced-air cooling and room cooling — forced-air cooling within 4 to 8 hours, and room cooling within 14 12] Diagram of air pattern for a forced-air cooler. Forced-air cooling utilizes differential air pres- sure to direct cold air through vented containers and around fruit. Such cooling can be done in only one-sixth to one-eighth of the time required for room cooling. to 20 hours of harvest — showed an 85 per cent greater weight loss for room- cooled cherries; shrivel also was less se- vere in forced-air-cooled fruit. These re- sults indicate a considerable advantage for forced-air cooling of cherries. Hydrocooling. This method utilizes moving cold water to cool fruit rapidly. Sufficient movement of cold water over cherries so as to contact as much fruit surface as possible is necessary for rapid uniform hydrocooling. Potential benefits include speed of heat removal and re- duced weight loss, but these benefits may be offset by the danger of spreading decay organisms as fungal spores accu- mulate in the water. Also, it is often diffi- cult to maintain water temperature low enough for adequate cooling, especially during peak harvest periods (although insulating the hydrocooler can improve its efficiency) . Fruit pitting has also been reported to be associated with hydro- cooled cherries. Because of these prob- lems hydrocooling of cherries should be undertaken with utmost caution. RAPID FRUIT MARKETING The importance of cooling cherries be- fore the normal 5- to 8-day surface tran- sit to eastern markets is generally rec- ognized, but some fruit handlers have questioned the value of cooling when the time from harvest to market is only 2 or 3 days (as, for example, in air transit or when fruit goes to local markets). With these shorter times, facilities for main- taining constant low temperatures are often lacking, and some fruit handlers believe that cherries will deteriorate more rapidly if cooled and subsequently re- warmed than if not cooled at all. [13 TEMP •F FRUIT CONDITION TREATMENT (IN PERCENT) WT LOSS 72HRS SOUND SHRIVEL DECAY 68 33 1.9 90 10