r I S c i II A ■r ^ in 1 [^ w STRAWBERRIES by F.G MITCHELL E. C MAXIE AS GREATHEAD FOR - J pertinent Station Did you know that . . . • An estimated $5,000,000 worth of California strawberries are dis- carded between the field and the table each year? • Each handler of strawberries, from the grower to retailer, may do damage which he does not see? • Deterioration can come from three sources — injuries, self-destruc- tion and decay? • Any program aimed at reducing market losses of strawberries must start with careful picker training and supervision? • Berries held at 50° F will last only one fourth to one half as long as fruits held at 32° F? • Berries exposed to the sun after picking quickly reach temperatures considerably above air temperature? • Short delays at field temperatures before cooling will reduce the amount of marketable fruit? Properly harvested crate of high quality strawberries. MAY, 1964 The Authors: • F. G. Mitchell is Extension Po- mologist, Marketing, Agricultural Extension Service, Davis. • E. C. Maxie is Associate Pomol- ogist, Agricultural Experiment Sta- tion, Davis. • A. S. Greathead is Farm Advisor, Monterey County, Agricultural Ex- tension Service. F@U 1 STRAWBERRIES FRESH F.G.MITCHELL • E. C. MAXIE • A. S. GREATHEAD This circular is intended as a guide for growers, shippers, and other hand- lers of fresh strawberries. It tells how to prepare and handle California straw- berries for fresh market. Advice given will pertain to berries going into any fresh outlet but certain steps in the operation may not be as critical when the berries are marketed locally (and quickly) . High fruit quality in the market is only possible if fruit of high quality is produced in the field. This places re- sponsibility on the grower to use good cultural practices, such as irrigation, fertilization, weed control, pest and disease control, and to carefully super- vise field operations to correct prob- lems before they may affect fruit quality. (Alderman et al., 1962, and Holland et ah, 1962). Strawberries are one of the most per- ishable of all fruits. They are essen- tially full-ripe at harvest as opposed to many other fruits that are picked "green" and ripened later. They have a high rate of metabolism (they "live fast") and will destroy themselves in a relatively short time, even without the presence of decay organisms. Strawberries are living organisms. As all living things they must stay healthy, for their health determines the quality of the fruit the consumer receives. The structure of the strawberry makes it vulnerable to spoilage. It has a thin, tender skin that is easily broken. The seeds are easily torn away, and the flesh of the fruit is soft, allow- ing it to be easily crushed and bruised. Any injury may invite an attack by decay organisms to which this fruit is very susceptible. Thus, the strawberry fruit, having a high rate of metabolism, being suscep- tible to injury, and vulnerable to decay organisms in the field, is ready prey to many postharvest ills. Losses of Cali- fornia strawberries destined for eastern markets are staggering. During recent years it has been estimated that the equivalent of the yield of 2,000 acres become inedible annually before reaching the consumer. This means that $5,000,000 worth of our straw- berries are discarded between field and table each year. In addition, a very important, but undetermined loss is the reduction in quality of fruits that are consumed, and its effect on sales volume and price. Strawberry fruits begin to lose qual- ity before they are harvested. When the sun rises on a field of strawberries [3] the ripe fruits start to deteriorate by decay or internal breakdown, and this deterioration continues until the fruit is eaten. The quality of the fruit when the consumer receives it, or even whether the fruit ever reaches him, depends on how good a job is done by the various handlers. Each handler of strawberries, from the grower to the retailer, may do damage he does not see. Since the dam- age does not appear while the fruit is under his control he may not be held immediately responsible for it. How- ever, any damage which results in loss of fruit or quality will affect ultimately the returns of all handlers of straw- berries. Importance of Temperature The most important way to slow down spoilage of strawberries is to re- move field heat and maintain the fruit at a low temperature. For maximum life, keep strawberries as close to 32° F as is practical. Why is temperature so important? As stated before, a strawberry fruit will destroy itself even if it is free of disease organisms. When the temperature of a strawberry is raised from 32° to 50° F its rate of deterioration will increase two to four fold. This means that fruits at 50° F have a life expectancy of only one fourth to one half that of fruits at 32° F. If the temperature of the fruit is around 85-90° F, as may happen in packed berries left in an open field, the market life of the fruit is reduced to only a few hours. Relative Humidity The relative humidity at which strawberries are held will affect their weight loss and appearance. While relative humidities approaching satur- ation (100 per cent) may promote the development of decay organisms, too low a relative humidity for any ex- tended period will increase weight loss and may quickly make the fruit look old and shriveled. To avoid this, hold the relative humidity at approximately 90 per cent. Fruit RotS (Wilhelm, 1961). Another important source of straw- berry spoilage is fruit-rotting organ- isms. Fruit-rotting fungi can grow rapidly in a basket of berries and des- troy the fruit. The rate and amount of decay in fruit may be greatly increased by injuries during picking or handling, and by delayed cooling. Gray mold rot, caused by the fungus Botrytis cinerea Pers., is the most com- mon of the fruit rots. Field infection by this organism is favored by cool, wet conditions. It is most serious during rainy periods and during cool, wet, or foggy weather. The organism causing the disease is present in every straw- berry field at all times. It is the most common organism found in older leaf tissue. The disease causes severe losses in the field before harvest and also in storage and transit of the harvested fruit. Since the organism is able to penetrate the unbroken skin of the berry, one decayed fruit can lead to the decay of surrounding fruit, producing a "nest" of decayed berries. An infected fruit first shows soft, dull colored spots. Later the fungus coats the fruit with its gray growth. During picking, spores from moldy fruit may contaminate healthy berries. Rotting may continue during transit and marketing, and any injury in- flicted during picking or handling will favor the disease. Usually, gray-mold problems have their origin in the field. A second disease of stored fruit is caused by the fungus Rhizopus stoloni- fer (Ehr. ex Fr.) Vuill. (=7^. nigricans Ehr.) This disease is favored by high temperatures and humidity. Like gray [4 mold, it causes severe losses in the field as well as in storage. Infected fruit is soft and watery, giving rise to the name "leak." Under conditions of high hu- midity, the organism will produce a long whiskery type growth on infected fruits, and like gray mold the organ- ism can move from infected fruit into adjacent healthy fruit, producing the nesting effect. While there are no commercial means of completely eliminating fruit rots, growers, shippers, and handlers can do much to minimize losses. For instance: • Remove from the plant all fruit (green or ripe) showing signs of de- cay at each picking. Do not put any fruit showing decay into the crate. Fully instruct pickers accordingly. Apply fungicides in the field to re- duce decay in field and storage. Con- sult the current University pest and disease control recommendations for detailed information. Cool the fruit as soon after harvest as possible and keep it cool. The lower the temperature, the slower the advance of these decay organ- isms. For best control, hold fruit as close to 32° F as possible. mi Proper supervision of the entire har- vest operation is essential. This must include supervision of all aspects of the harvest — picking, field handling and local transport to the cooler. Field supervision may be handled by the owner or manager of a small operation or by hired foremen of larger operations. Whatever the ar- rangements, these field bosses must be Picking crew at work in strawberry field. Picking from only the half of the bed nearest to picker eliminates excessive reaching and simplifies selection of ripe berries. ,*iujr &m $523kWk m I. I >*i* .af> "»-5S JEr S*#« -'IFWii |JR _.#5^Ki*dP#? Field picking of strawberries, showing desir- able crate and basket arrangement. Note picking stand to keep crate out of dirt. The picker is picking, sorting, and packing in one operation, avoiding unnecessary handling. Picking from only the half of the bed nearest to picker eliminates excessive reaching and simplifies selection of ripe berries. under careful control of management since their activities will have pro- found influence on the shipping qual- ity of the fruit. Picking Strawberries are harvested at differ- ent stages of ripeness, depending on the time and distance to market (Hol- land et al. } 19(32). Fruit for local mar- ket should be fully ripe and firm while fruit for long distance shipment should be picked at the pink or three- fourths colored stage. Strawberries in California are al- most universally shipped in fiber crates holding 12 one-pint baskets. Three main types of baskets are used — molded plastic, paper, and wood ve- neer. The plastic baskets are open for air movement and for visual inspection by the consumer. Paper baskets do not allow visual inspection, but provide some cushioning protection against rough handling. Wood-veneer baskets, no longer the primary container, are now used mainly for local-market berries. The fruit is picked directly into the crate in which it is to be shipped. The following steps are important for pickers (Mitchell and Greathead, 1959): • Always use a picking cart or stand, to keep the crate off the ground. • Handle the full crate very gently. The tender fruit is very easily bruised. • Be careful that feet and knees do not injure any green fruit hanging in the furrow. This can reduce the quality of later harvests. • Pick berries with the caps (or calyx) on. Take hold of the fruit gently be- tween the thumb and first two fingers. With an upward or forward twist of the wrist, snap the stem off. Pick special stem grade fruit by pinching the stem one to two inches from the calyx. • Hold the fruit loosely in the hand without squeezing. Remember that any time you squeeze the fruit it is bruised and will discolor, so handle it gently. Place the fruit in the crate — do not drop it. Do not hold too many fruit in the hand at one time. • Throw away any berries showing rot, sunburn, insect injury, severe misshape, and also soft and overripe berries. Remove all of these berries from the plant and place them in the bottom of the furrow. If the cap has been removed accidentally, dis- card the berry. • Do not fill the crates so full that berries will be crushed when crates are stacked. [6] .Sim* t " o 0k W? 1 ; SB* Selection of berries by a careful picker resulted in 14.4 per cent unmarketable fruit (left) while a careless picker caused 33.7 per cent of the berries to be unsalable. Picker damage can nullify all other attempts to maintain fruit quality. To check importance of picker damage, a test was set up to compare the level of marketable fruit from pickers with different abilities at picking, sorting and packing. Two pickers, selected by the grower, were set side by side in the field but were not told that their fruit was part of a test. Five crates were taken from each picker, identified, and then held for eight days at 41° F be- fore sorting. The more careful picker's The experienced picker will snap fruit with- out bruising the berry or breaking the cap. lot contained 14.4 per cent unmarket- able fruit compared to 33.7 per cent for the less careful picker. This illus- trates a wide variability in the care exercised by pickers. Any program aimed at reducing market losses of strawberries must start with thorough picker training and supervision. Field Handling and Hauling Problems in field handling of straw- berries can include physical damage and inadequate protection against high temperature or delayed cooling. Most growers make a real effort to avoid rough handling of their fruit during picking, stacking, and loading. However, they may overlook good temperature management because the end results are not so immediately ap- parent. Nevertheless, temperature management in the field can strongly influence the ultimate quality and shelf life of the berries (Maxie et al., 1959a, 19596). To determine the importance of temperature in strawberry manage- ment, it is necessary to understand the effect of temperatures on strawberry deterioration. Temperatures actually attained in the field The upper left graph on page 9 shows the temperatures of strawberries in different positions in the crate and in shaded and sunny locations. Note the definite advantage of shading. Strawberries exposed directly to the sun quickly reach temperatures con- siderably above air temperature be- cause their dark color readily absorbs heat from the sun. During long expos- ure, even berries in shaded positions ultimately reach temperatures ap- proaching that of the air. You will face this situation under normal field con- ditions where temperatures are fairly high. Effect of high temperatures on the berry The upper right graph on page 9 shows the respiration rate of Shasta strawberries held at temperatures rang- Pitkup and loading is done close to the pickers and with as little delay as possible. Note the inverted crates used for shading stacks on the ground. ing from 32° to 100° F. As respiration rate roughly equals the rate of living of the berries it obviously is to your advantage to hold fruit at lower tem- peratures. Note the rapid drop of the respiration rate at higher tempera- tures, due to destruction and death of the tissue. It is important, therefore, to reduce the temperature of the fruit as soon as possible after harvesting. Some growers hold their fruit in the field for prolonged periods. Obviously, at extremely high temperatures this will lead to rapid destruction of the fruit. However, if you pick the berries when it is fairly cool and if you shade them in the field, you can slow down their destruction to some extent. But they may warm up rapidly as a result of a warm summer breeze or during their transport on an open flat bed truck to the shipper's facility (lower left graph on page 9). An air velocity of just five miles an hour, a very mild wind, warms the fruit nearly to air temperature in 20 to 30 minutes. This rapid warming results from an unlim- ited supply of warm air passing directly over the fruit. As a result of cooling by the evaporation of water from the fruit, berries in the center of the basket, somewhat protected from air, are not warmed to the air temper- ature. This water loss, however, causes further reduction of fruit quality. A substantial temperature rise during a very short period of open transit will take hours to overcome in the normal pre-cooling facility, where both refrig- eration capacity, air flow are limited. To check the effect of delays in cool- ing upon marketability, berries were held lor different periods of time fol- lowing harvest at 85° F and then rap- idly cooled to 40° F. The lower right graph on page 9 shows the results of grading the berries after a week at 40° F. A very rapid reduction in the amount of marketable fruit resulted from holding the fruit at 85° F before cooling. Tests with Shasta Strawberries advise: hp «■» w en mm n Shade crates in the field 1 2 / s*"^*\ "^ ■ //' / X > / / / / / / 3 \ / / / / "\ / // 4_ - . // /'' X" y // / / ''/ y ->^_^^ if ' V High temperatures ivill "kill" berries ioe 104 u. 100 5> 96 0) O i 92 3 88 0) a 1 84 H 1 80 u. 76 72 9AM 10 II 12 N IPM. 2 3 4 5 Time 1 — Fruit, top of basket in sun; 2 — Fruit, bottom of basket in sun; 3 — Fruit, top of covered basket; 4 — Fruit, bottom of covered basket; 5 — Air, in sun. Simulated tests show that berries crated in the field should not remain uncovered in the sun. Berries warm up after harvest W0 tOO 120 J«0 Time in Minutes • — . Top of basket, 5 mile per hour wind • — • Center of basket, 5 mile per hour wind .__. Top of basket, no wind ••"■* Center of basket, no wind Berries transferred from 41° to 85° F will warm rapidly (regardless of location in basket) in a warm summer breeze. Berries on top of basket will warm rapidly even with no breeze. SHASTA Time in Days The influence of temperature (32° to 100° F) on respiration rate — which is a measure of the rate of living. The higher the tem- perature the shorter the life. Don't delay cooling 80 ^x^ 70 \ *: 60 N. O) >< f * N. >> / >y^ -Q c »40 >v 0) u f »' \^ &. 30 N 20 / • • MARKETABLE FRUIT ■» 10 - *♦• — • n • • FRUIT SHOWING DECAY 1 2 3 4 5 6 7 Hours at 85° F. Very short delays before cooling to 40° F resulted in sizable losses in marketable berries. [9] Loading strawberries in field. 5T®»M re- The shipper and the carrier are sponsible for good handling practices as soon at the fruit is under their con- trol. They, too, have to watch out for physical handling and temperature management. Cooling and Holding As indicated before, berries should move as quickly as possible from pick- ing to cooling. Rapid cooling will also further decrease the time during which the berries are exposed to higher tem- peratures. Forced-air cooling, a recent development now in commercial use, appears promising in greatly reducing the time for cooling (Guillou, 1960). Forced-air cooling is a method whereby the air is forced to pass through the container and around the fruit. By comparison, standard cool- ing is accomplished by passing the air past the crates or pallets with little air passing through the container. With forced-air cooling, the time required to lower the temperature of the fruit for transit is reduced to one-sixth to one-eighth of the time required under standard cooling conditions. As the upper graph on page 11 shows, in one test a cooling period of 7 to 9 hours was required under standard cooling conditions, while forced-air cooling re- quired less than 1 hour. To assess the advantages of the han- dling and cooling procedures of straw- berries, a series of trial shipments were conducted. Rapid handling and thor- ough forced-air cooling were combined as one treatment and compared to fruit handled in the normal manner. In all cases, the fast cooled lots reached 40° F within approximately 1 hour after picking. In contrast, the normally handled fruit required from 8 to 14 hours between picking and the time the fruit reached 40° F. Fruit tempera- tures at the time of harvest for the various trials ranged from 68° to 84° F. In each case, marketability was in- creased by the rapid handling of the fruit. The increase ranged from 8 to 21 per cent. The lower graph on page 11 shows the over-all results of grading follow- ing arrival of the fruit in terminal markets. The fast cooled lots had ap- proximately 1 additional day of shelf life at time of arrival, and their rate of deterioration following arrival was slightly slower than the rate of deteri- oration of the normally cooled lots (Mitchell et al., 1961). This shows a real advantage in terms of increased marketable fruit resulting from good management during handling and cooling of strawberries. Because they are extremely perish- able, strawberries should be marketed as rapidly as possible after harvest. This is particularly important for fruit destined for eastern markets where the [10 Forced-air speeds cooling 60 B 45 80 70 60 50 40 30 20 10 Forced-air cooling brings berries down to holding temperature (about 40° F) within 1 hour, room cooling takes 7 to 9 hours. Data: Rene Guillou. Cartons on pallets cooled in 36° F air. Fast cooling prolongs life of berries At any point during marketing, the fast- cooled berries had approximately 1 addi- tional day of shelf life. FAST COOLING NORMAL COOLING ARRIVAL I 2 3 Days Following Arrival [11] On arrival at the cooling plant pallets are used to transfer fruit from the truck to the cooler. This reduces handling delay and damage. transit period may take as long as one week. If holding is necessary, keep the time as short as orderly marketing al- lows. During such holding periods, main- tain the temperature of the fruit at 32° F. It may be difficult to maintain the desired low temperature of storage fruit if warm fruit is introduced into the same room. If you use forced-air cooling, you can pass the cold air across the storage fruit before it passes through the warm fruit. If constant in-and-out traffic is necessary in a room in which cold fruit is being held, you may consider the use of air "curtains." This directed flow of air across the door opening reduces the mixing of the cold and warm air on either side. Loading and Transportation The loading of berries for transport to market must receive the same care as all other steps in the marketing process. The personnel has to be ade- quately trained and supervised to avoid rough handling of the berries. Warming of the fruit during loading must be prevented since it may cause fruit sweating and will necessitate ad- ditional cooling to remove any ab- sorbed heat. The carrier vehicle should be pre-iced and cooled before loading starts. The cold room should be lo- cated close to the loading docks and fruit moved directly from storage to rail car or truck. If possible, protective loading curtains or tunnels between [12 these points should be provided to reduce heat leakage. While the main cooling is accom- plished by water ice, it is common practice to place approximately 1,000 pounds of dry ice in a paper-lined crib atop the center brace of the car. This is done just before closing the doors on a rail car after loading /Allen et aL, 1944; Ryall, 1956a and b). Studies have indicated that the closest point of the crib should be at least 8 inches from the berries to avoid freezing of the fruit. The paper wraps on most of the cakes of dry ice are left intact to lengthen the effectiveness of the treatment. Sometimes, an additional 200 pounds are broken up and "salted" into the ice bunkers. The primary purpose of the dry-ice treatment is to build up the carbon dioxide concentration of the atmos- phere within the car. Several studies have shown that a carbon dioxide con- centration of 10 to 20 per cent is bene- ficial for the strawberries — it slows their respiration and the growth of decay organisms so that the fruit may arrive in market firmer, brighter and with less decay than nontreated ber- ries. Studies on the effectiveness of this dry-ice treatment have indicated that an initial concentration of 10 to 20 per cent carbon dioxide is reached within three to four hours, and that the possibility of maintaining a satis- factory concentration depends on the tightness of the equipment. The dry ice which is salted into the ice bunkers helps to build the initial concentra- tion. As soon as the fruit is properly loaded and the dry ice is in place, the car doors should be closed and remain closed until arrival. The car fans should be started and run continu- ously until rail transit begins. In me- chanical refrigerator cars, trucks, or cars equipped with a supplemental Correct method of loading strawberries in refrigerated car. Crates are stacked so that wire support (seen sticking out on top of the load) reinforces the stacks. The strong ends of the containers parallel the direction of car movement, thus reducing possible damage to the containers. ' Dry ice is used to increase carbon dioxide in the car atmosphere thus reducing spoilage and delaying ripening during transit. Paper is wrapped around most of the cakes to make the carbon dioxide last longer. Note the covers on the first three tiers next to the brace to help protect fruit against freezing. power supply the fans should be run thermostatically until unloading. The standard procedure in shipping strawberries by rail is to fill the ice bunkers with ice and add 2 or 3 per cent salt prior to loading, to replenish ice and salt after loading, and to re-ice enroute at designated icing stations (Ryall, 1956a). The requirements for icing and salting will vary with the outside temperature enroute — a higher percentage of salt and more frequent re-icings are to be used dur- ing hottest weather. The dry ice adds some refrigeration, and the amount of salt required to maintain the desired temperature may be reduced with dry ice. While it is desirable to keep straw- berries near 32° F to hold down decay growth and fruit respiration, care must be taken to prevent freezing the fruit. The freezing point for strawberries has been reported as approximately 30° F. This will vary with variety, growing conditions, and ripeness of the fruit. Because of constantly chang- ing climatic conditions during the sea- son and differences in available equipment, the shipper must become experienced in designating the icing procedure for strawberries. The many variables make it difficult to make a standard recommendation regarding the percentage of salt or re-icing sched- ule. [14] The problems of handling strawber- ries during wholesale and retail are similar to those during other stages of marketing (Mitchell and Maxie, 1962). Here, too, deterioration can come from three sources — injuries, self-destruc- tion, and decay. Terminal market handling must try to keep these three sources down. Rough handling cannot be tolerated at any step in the market process, and becomes even more critical at the mar- keting phase. As the berries move through the distribution channels they become older, riper, softer, more deli- cate, and more subject to handling damage. Thus, good training and su- pervision are needed to prevent rough handling. Temperature management remains important at this stage of marketing. This is the only practical means of holding self-destruction and decay losses to a minimum. Under commer- cial conditions, berries are often ex- posed to relatively high temperatures during unloading and handling in the warehouse and during transit to the retail store. The effect of such condi- tions on the warming of the fruit was shown earlier in the left bottom graph on page 9. This warming has a similar result as that of delays in the field before cooling. A test was made to determine the importance of temperature at wholesale and retail levels. No one questioned that the nonrefrigerated berries would deteriorate more rap- idly, but the difference in rate was startling. As shown in the graph on this page, about 50 per cent of the re- Refrigeration during marketing savqs fruit 70 60 50 1 40 o § 30 0) a. 20 10 Berries not refrigerated after transit become almost completely unmarketable after three days. ARRIVAL Days Following Arrival [15] [derated fruit was still marketable • Avoid unnecessary delays which alter three days while the nonrefrig- keep fruit out of refrigeration, erated fruit was almost completely • Provide refrigeration in transit from destroyed during this period. receiving warehouse to retail store. For proper temperature manage- • Use refrigerated display if possible, ment in the terminal market: if this is not possible avoid display- ing more than a few hours supply • Refrigerate at all times at 40° F or at any time. below; if fruit must be held longer • If the berries become warmed, cool than one day, reduce the tempera- them as rapidly as possible by good ture to 32° F. positioning in the cold room. Publications mentioned in this circular: Alderman, D. C, A. S. Greathead, and E.C.Koch 1962. Strawberry production in Central California. U.C. Agr. Ext. Ser. AXT— 80. Allen, F. W., W. T. Pentzer, and C. O. Bratley. 1944. Carbon dioxide investigations: Dry ice as a supplement to refrigeration of plums in transit. Proc. Amer. Soc. Hort. Sci. 44: 141-47. GUILLOU, R. ct.11 nn<* 1960. Coolers for fruits and vegetables. U.C. Agr. Exp. Sta. Bull. 7 15. Holland, A. H., Hunter Johnson, Jr., and Bernarr J. Hall. 1962. Strawberry production in Southern California. U.C. Agr. Ext. Ser. AXl— ml Maxie, E. C, F. G. Mitchell, and A. S. Greathead. 1959a. Quality study on strawberries. Calif. Agriculture 13(1), pp. 6, 15. 1959fr. Studies on strawberry quality. Calif. Agriculture 13(2), pp. 11, 16. Mitchell, F. G., and A. S. Greathead. 1959. How to pick strawberries. U.C. Agr. Ext. Leaflet, unnumbered. Mitchell, F. G., H. L. Hall, A. H. Holland, and A. S. Greathead. 1961 . Strawberry marketability. Calif. Agriculture 14(2), pp. 13-14. Mitchell, F. G., and E.G. Maxie. . 1962. Maintaining strawberry quality in the market. Produce Marketing 5(5), pp. 41-42. RV 1956a The effect of bunker salting, dry ice and air circulation on temperatures in express 'shipments of fresh strawberries. Third Conference Proceedings, Transportation ot Perishables Conf. pp. 73-76. 1956k. Precooling and transit temperatures of strawberries. Third Conference Proceedings, Transportation of Perishables Conf. pp. 151-153. Wilhelm, Stephen. 1961. Diseases of strawberry. A guide for the commercial grower. Calit. Agr. Exp. Ma.-txt. Ser. Cir. 191. 15m-5,'64(E31 L3)J.F.