Container Transport Perishables
Content
Marine Container Transport of Chilled Perishable Produce
James F. Thompson, Patrick E. Brecht, Tom Hinsch, Adel A. Kader
INTRODUCTION This publication is a guide to the proper use of refrigerated marine containers for shipping perishable produce. Shipping organization staff, transport company personnel, inspectors, surveyors, insurers, and receiving company employees will learn about important aspects of refrigerated container transport, including packaging, container and load specifications, loading, and recordkeeping. Two appendixes provide essential information: appendix A is a troubleshooting guide for diagnosing causes of poor-quality produce on arrival, and appendix B provides data on the long-term storage requirements for fresh fruits and vegetables. A companion poster that illustrates proper loading practices is also available from USDA and UC ANR (Publication 21596). Based on figure 4 in this publication, the color poster is a visual reminder of the key issues that dock personnel need to know about proper loading.
DISCUSSION POINT 1: Packages and Pallet Design
SELECTING AND USING BOXES FOR MARINE CONTAINER SHIPMENT Overview Corrugated boxes should be strong enough to withstand the effects of high humidity in storage and transport. Do not stack boxes beyond edges of pallet. Pallet loads should be unitized and secured. Boxes and inner packaging should allow vertical airflow, especially if produce is warmer than refrigeration set point temperature when stowed. Align box vents between layers of boxes. Use pallets where deck board spacing aligns with box vents. Corrugated fiber-board loses strength over time when it is supporting a load. For example, after supporting weight for 10 days, a fiberboard box will have only 65 percent of its original laboratory-determined strength. Fiberboard also absorbs moisture and weakens when it is exposed to high relative humidity, which is generated by the produce inside the box. Laboratory testing of fiberboard is based on material in moisture equilibrium with air at 50 percent relative humidity (RH) and a temperature of 23°C (73°F). Fiberboard in moisture equilibrium with air at 90 percent RH will have only 40 percent of its original stacking strength. Absorbed moisture also causes fiberboard to expand slightly, increasing box dimensions and sometimes causing warping. Wood and plastic boxes are not affected by humidity and maintain their strength in high-humidity conditions. Fiberboard boxes should be strong enough to withstand several weeks of initial storage and sea journeys of 4 weeks or more without failing from fatigue or high humidity. Because marine transport periods are usually much longer than highway transport periods, boxes for marine shipment usually need to be stronger than those used for domestic shipment. Fiberboard can be made stronger by using proprietary waxes and treatments, by adding more fiber weight to the board, or by selecting a stronger box design. Box venting also reduces box strength. Box vents should be located away from the vertical edges of the box and generally should not account for more than 5 percent of the box wall area. Boxes with greater than 5 percent venting must be specially designed to provide adequate strength. Select box dimensions and a pallet design that will enable box corners to be well supported by deck boards (fig. 1). Fiberboard boxes are strongest near the corners. If they are not well supported, the boxes will fail and allow produce to be crushed. Nominal outside box dimensions should allow for panel bulge and variation in box placement on the pallet. Do not stack boxes beyond the edge of the pallet. DISCUSSION POINT 2: Package Strength
Pallet loads should be well secured so that they do not shift in handling or transport. Stacking tabs or palletizing glue can assist in preventing boxes from sliding past each other. The pallet and load can also be unitized (tied together with net wrapping or corner braces and banding). Boxes should extend to the edge of the pallet. Free space at the periphery will allow the load to shift in transport. The packaging system shipped in marine containers should be designed to work with the container’s airflow pattern. Containers are usually equipped with a bottom-air delivery system (fig. 2). Air from the refrigeration unit flows first to the floor, up through the load, horizontally across the top of the load, and then back to the refrigeration unit air return opening. Pallets, boxes, and inner packaging should have enough venting and air spaces to allow vertical airflow through the pallet load. If air cannot flow through the packages, it will flow around the pallet load, with little opportunity for proper temperature management. This will cause greater variation in produce temperature throughout the container load. Boxes should have at least a 3 percent venting area on top and bottom panels. Most boxes also have vents on the side panels to allow initial cooling. These allow air to flow horizontally to find open vertical venting if top or bottom vents are blocked by pallet deck boards or packaging materials. Vents on horizontal box edges (sometimes called shoulder vents) are less likely to be blocked by produce or packaging materials than vents that are more toward the middle of panels. Vents for vertical and horizontal airflow should align when boxes are register-stacked or crossstacked. Interior packaging and pallet deck boards should not block airflow through box vents.
DISCUSSION POINT 3: Package Ventilation and Cooling Process
LOAD PLANNING Overview
Produce should have enough postharvest life for the trip and for marketing at the destination. Specify the carrying temperature in both Celsius and Fahrenheit scales. Specify the flow rate of fresh air exchange. Specify humidity, controlled or modified atmosphere treatments, or other special treatments if they are requested. Select cargo, dunnage, pallets, blocking, and bracing so that the load covers the entire floor of the container and meets highway weight regulations. Different kinds of produce shipped together in one container should have compatible – temperature needs – ethylene sensitivity – relative humidity needs – modified or controlled atmosphere requirements, if used – odors or chemical treatments – airflow characteristics. Produce items should not be included in a shipment if they do not have enough postharvest life for the trip and subsequent marketing at the destination. Appendix B lists typical produce postharvest life in air storage under optimal conditions, and table 1 lists produce with a postharvest life of less than 2 weeks. Produce that has already been stored before shipment will have a correspondingly shorter life in transport and at destination. Controlled atmosphere (CA) or modified atmosphere (MA) treatments can sometimes be used to increase produce postharvest life. If controlled atmosphere treatment has a benefit, it is typically about a 30 percent increase in storage life. The ocean carrier and the organization that purchases the transport services should agree on the desired carrying temperature for the load. The set point temperature should be clearly specified in both Celsius and Fahrenheit scales to reduce confusion between the two systems (table 2). Appendix B lists the optimal temperature for long-term storage of perishable commodities. The container thermostat is usually set by the ocean carrier based on the agreed-on carrying temperature and the company's knowledge of the specific capabilities of the container used for the trip. DISCUSSION POINT 4: Load planning
If a load has been properly cooled to transport temperature before loading and is properly loaded in a container that has a relatively new design, the thermostat can be set close to the long-term storage temperature. Newer container designs have supply-air temperature sensors and controllers that automatically control refrigeration based on supply air temperature if they are set at temperatures for chilled cargo. Because these containers can
control air temperature within 0.5°C (1°F) or better under most conditions, produce is rarely exposed to temperatures much below the set point temperature. This should prevent freezing or chilling damage if the thermostat is calibrated and set at the agreed-upon carrying temperature. Thermostats for older refrigeration systems with return-air temperature controllers should be set at least 1°C (2°F) above the long-term storage temperature to prevent freezing or chilling damage to produce. Systems that have switchselectable temperature sensing should be set to the supply-air temperature sensing for chilled cargo; in contrast, return-air temperature sensing is used primarily for frozen produce. Both types of containers automatically control temperature on the basis of return air temperature if they are set for a frozen load temperature. Container air exchange is set based on preventing elevated carbon dioxide (CO2) levels caused by produce respiration. Low oxygen levels are rarely a problem if carbon dioxide levels are properly controlled. Use the respiration rates listed in appendix B to find the air exchange rate in table 3. The air exchange rates in table 3 are calculated to keep carbon dioxide levels below 0.3 percent in a tightly sealed container. This is much below the damage threshold for many commodities. Lower air-exchange rates can be used if a commodity or a mix of commodities can withstand higher carbon dioxide levels. In fact, some commodities benefit from high carbon dioxide levels. The recommendations in table 3 assume that the produce is stowed at recommended temperatures. Ventilation should be specified based on airflow per time in cubic meters per hour (m3>/hr or cmh) or cubic feet per minute (ft3/min or cfm). Ventilation settings described as "percent vent opening" are not meaningful because performance characteristics and the wide-open vent capacity can vary greatly for different container designs. For example, a 20 percent vent opening corresponds to about 80 cmh in one particular container and less than 60 cmh in another. Airflow for a particular vent setting is also influenced by factors that influence evaporator fan output. For example, a 60 Hz electrical supply usually increases airflow by 20 percent compared with a 50 Hz electrical supply because of higher airflow from the container fan system. Also, dirty fan blades reduce fan airflow and air ventilation. Do not use ventilation rates above the recommended levels. Excess ventilation increases energy consumption. In hot, humid environments, it may also increase evaporator coil icing and reduce the consistency of temperature control. Controlled atmosphere or modified atmosphere treatments are sometimes used to increase the postharvest life of some produce items compared with storage in normal air. Properly selected treatments slow produce respiration, retard ripening, and may inhibit the development of decay. These treatments generally add expense to the cost of shipping. Appendix B lists the recommended controlled atmosphere conditions for selected commodities. DISCUSSION POINT 5: Temperature and Air-Flow Control : CostBenefit Analysis Commodities that are particularly sensitive to damage from moisture loss and wilting may benefit from increased humidity levels in containers. High humidity is usually obtained by using a spray humidification system available on some newer containers. A disadvantage of these units is that they require temperature settings slightly above 0°C to prevent the nozzle from freezing. High humidity also weakens fiberboard. Moisture loss can also be reduced by packing produce in plastic bags or box liners or by using plastic boxes that do not absorb water. The loaded container must meet highway weight regulations. In the United States, for example, the total gross vehicle weight must not exceed 36,288 kg (80,000 lb). There are
also separate weight limits for each vehicle axle to insure a well-distributed load. This results in a net produce load of 18,144 kg (40,000 lb) to 21,909 kg (48,300 lb), depending on the weights of the truck, container, chassis, and generator set (table 4). Even the amount of fuel in the generator set and truck can have a significant effect on net produce load. One gallon of diesel fuel weighs 7.1 pounds (3.2 kg), and one liter weighs 0.85 kg (1.9 lb). A container with a nose-mounted generator set (sometimes called a clip-on generator set; see back cover) will often require that less than full pallets be loaded in the first few pallet rows of the container in order to meet axle weight limits. Maximum load weight can be obtained only if the transport company informs their customer about the specific weight of equipment used for a particular load. Without this information the container will probably be loaded to a safe weight of about 18,144 kg (40,000 lb).
Loads with more than one type of produce must be set up so that different types of produce loaded together have compatible temperature ranges and ethylene sensitivities. Use table 1 to select compatible produce. table 1 groups common fruits and vegetables by temperature range and ethylene sensitivity. Produce in the same column can be safely held at the same temperature range (listed at the top of the column). If produce in different temperature ranges are mixed, produce quality will be compromised, especially with longer transit times. The greater the difference in recommended temperature between the produce, the greater the potential for quality loss. DISCUSSION POINT 6: Humidity, Weight Restriction and Container Stuffing Mix
Dried vegetables in the yellow row of table 1 should not be mixed with any other produce in the table. These vegetables should be held in a 50 percent to 70 percent relative humidity environment to prevent decay. Most of the vegetables in the lowest temperature range (0° to 2°C [32° to 36°F]) are sensitive to moisture loss and should be held at more than 90 percent relative humidity or packaged to minimize water loss. The other vegetables and fruit should be held at 85 percent to 95 percent relative humidity. Ethylene-sensitive vegetables (green row) should not be mixed with ethylene-producing fruits, luffa, and tomato (blue row). If for some reason they must be mixed, damage may
be reduced by using a fresh air exchange rate of 45 cfm (table 3). Ethylene scrubbers may also reduce damage. In some instances a controlled atmosphere will allow ethylenesensitive produce to be shipped with ethylene-producing produce, but the acceptable produce combinations and atmosphere prescriptions are not well documented. Produce that is neither ethylene sensitive nor ethylene producing (pink row) can be mixed with produce above or below them in the same temperature column.
Some produce items can exchange odors with other selected produce items. See the notes at the bottom of table 1 for precautions. Certain produce items have a short postharvest life and are not suited for container shipments. This is particularly true if short-lived items are held at non-optimal temperatures. For example, asparagus has a maximum postharvest life of three weeks if held at 2.5°C (36°F). If it is shipped at 0°C (32°F) in a load of broccoli, it will be subject to chilling injury after only 10 days. Modified atmosphere packaging can sometimes be used to increase produce life and allow produce to be shipped to destinations that require several weeks transport time. If a controlled atmosphere environment is used it should, at a minimum, not reduce the postharvest life of any of the mixed commodities. DISCUSSION POINT 6: Container Stuffing Mix
PRODUCE TEMPERATURE AT LOADING Overview Perishable produce should temperature before loading. be cooled to its desired carrying
Most produce cools very slowly in a container, and its storage life is reduced if it is warm when loaded. If produce must be loaded warm because there are no cooling facilities in the production area, proper packaging and loading are essential to achieve the fastest possible cooling rates. Under the very best conditions a container can cool produce in 2 to 4 days. Under poor conditions produce will never cool to the desired temperature and will be of poor quality upon arrival.
Produce should arrive at the loading dock at the proper temperature for shipment because it is often difficult to send produce back to the cooler if it is found to be too warm for transport at the dock. Cooler managers should be entrusted with the responsibility of ensuring that produce is properly cooled before it reaches the shipping dock. Loading docks and load assembly areas should be refrigerated. Dock supervisors should ensure that produce does not warm on the dock and should conduct a final produce temperature check before loading. CONTAINER OPERATING CONDITION Overview Container refrigeration and generator units should be inspected, pretripped, and repaired, if necessary, before each trip. Containers should be cooled to near carrying temperature before loading. Turn off refrigeration unit before doors are opened for loading. Containers should look and smell clean, floors should be free of refuse, and should be in good repair before loading. Container should be free of toxic materials and sources of bacterial contamination. Container operators should track past shipments to verify container is acceptable for food transport. The container company should inspect and clean each container before each trip. The diagnostic system for the refrigeration equipment should be checked and needed repairs made, and the temperature control system should be regularly calibrated.
The container should arrive at dock at near the desired carrying temperature to prevent produce loaded near container walls from warming under hot ambient conditions or cooling too much under cold ambient conditions. In temperate climates with low humidity, the container should be cooled to the carrying temperature. If loading from an open dock in a humid environment, the container should be cooled to the dew point temperature of the outside air. Temperatures below the dew point cause water to condense on walls, which may damage fiberboard packages. In all cases the refrigeration system should be turned off when the container doors are open. This prevents moisture from condensing on the evaporator coil. Use an infrared thermometer set to inside wall temperature to determine conventional thermometer can also against an inside wall and covering it come to a constant temperature. the emissivity level of the wall to check the if the container has been adequately cooled. A be used by placing the temperature probe with a dry cloth. In a short time the probe will
The container should be clean on arrival at the loading dock. It should smell clean and the floors and drains should be free of debris. The container should be free of serious damage. Doors should seal tightly when closed and interior wall surfaces should be in good repair. Refrigerated containers can be used for transporting a wide variety of commodities. Some of these commodities may not be compatible with food because their residues may contaminate produce loads. Even previous loads of food may be a source of bacterial contamination. Container operators should be able to guarantee that the vehicle is free of residues from sources of bacterial contamination or toxic materials. Some container operators have policies that restrict shipping toxic materials in containers that are designated for shipping perishable produce. However, most container operators have few restrictions for shipping dry cargoes in refrigerated containers and cargo is sometimes mis-declared. Consequently, the container should be adequately cleaned during pre-trip maintenance to avoid the possibility of crosscontamination between perishable foods, non–food-grade cargo, and foodborne bacterial pathogens. The container operator should monitor and record the types of cargo previously moved in a container.
DISCUSSION POINT 7: Pre-requisites for Container Stuffing
CONTAINER LOADING Overview Cover the entire floor with pallets, boxes, or other materials. Load produce below the red height-limit line. Stabilize load to prevent shifting in transit. A uniform temperature can be maintained in the load only if the container floor is completely covered with pallets, boxes, or solid material from the front bulkhead to the end of the floor rails. When the floor is covered, refrigerated air is forced up through and around the packages (fig. 2). If the produce is palletized, the pallet openings for forklift tines (sometimes called pallet pockets) should also be covered to prevent air from traveling horizontally through the openings and escaping into an open vertical channel between pallet loads. The floor area between pallets can be covered with sections of fiberboard. The fiberboard should cover the floor and any pallet openings and should be held down with boxes of produce or attached to pallets. Placing a few boxes of produce at the rear of the load is an effective way to cover the space at the rear of the container. If the floor and pallet openings are not completely covered, refrigerated air will short-cycle back to the refrigeration unit and bypass most of the load. Under hot ambient conditions, produce with poor airflow around it will tend to arrive too warm, especially if it had not been thoroughly cooled prior to transport or if it has a high respiration rate. An open floor in the front of the container allows refrigerated air to flow through these open areas, causing produce in the rear of the container to be warm because very little refrigerated air reaches it. Under cold ambient conditions, produce in areas with low airflow is in danger of freezing or chilling injury. The load can be stabilized in several ways. A 20-pallet (48 in by 40 in [approximately 1.2 m by 1 m]outside pallet dimensions) load, with 9 pallets loaded straight in and 11 pallets turned 90 degrees before loading, completely fills a container width and produces good stability (fig. 3). Air bags or other load-restraining devices in the rear of the load prevent produce from moving back against the doors. Some shippers may need to reduce the number of box layers on each pallet to allow heavy produce to use a 20-pallet load and still meet highway weight limitations. If the pallet load weight is too great to allow a 20-pallet configuration, an 18-pallet loading pattern can be used (fig. 3). All open floor spaces and exposed pallet openings must be blocked to prevent refrigerated air from bypassing the produce and short-cycling back to the refrigeration unit. The two patterns with each pallet touching a sidewall are very stable. A specially designed plastic airbag that fills the entire center gap between pallets can be used to stabilize the sidewall load and prevent air from passing through the center gap. figure 4 shows the general procedures for proper container loading. Hand-stowed loads should also cover the entire floor (fig. 5). Boxes should be stacked tightly against each other and tightly against the walls. This type of stacking also provides support to help minimize box failure. Boxes should be oriented to align box vents so that air can flow through the load. Never leave open vertical or horizontal channels between boxes. Boxes should be stacked to a constant height to foster uniform airflow through the load. DISCUSSION POINT 8: Container Stuffing Plan
TEMPERATURE RECORDING DEVICES Overview Install a calibrated temperature recording device. Mark the date, time, and placement on the recorder. Refrigerated containers usually have equipment that automatically records refrigeration system functions and the air temperature inside the container. This information provides a detailed record of refrigeration system performance during a trip. Newer containers also have probes that can be used to monitor produce temperature. However, this data is usually available only to the ocean carrier unless cargo owners make a special arrangement with the carrier before the trip. Shippers should install their own temperature recording equipment to generate data for their own records. Temperature recorders should be rugged enough to withstand vibration and impact and still maintain calibration. The internal clock and temperature sensors should be calibrated. Recorders measure the temperature of the environment immediately surrounding them. Actual load temperature can be determined only by packing the recorders in boxes or inserting temperature probes into produce.
For convenience, temperature recorders are often installed on the top of pallets in front, middle, or rear locations. However, interpreting the data may be difficult because the recorders will be exposed to air that is influenced by the heat generated by the produce. For example, if produce is warm at loading, the recorders may indicate warm conditions even when the refrigeration unit is producing adequately cooled air. The temperature at the recorder may also be influenced by improper loading practices that allow conditioned air to bypass the rear of the load. Recorders should never be in contact with the walls or roof of the container because the temperature they record may be affected by heat transmitted through these surfaces. To keep a record of refrigeration system performance, a recorder should be placed at the floor next to the front bulkhead. Recorders should always be marked with the date and time they were started and also with their location in the load. If this information is missing, it is very difficult to interpret the temperature data. A note on the recorder should specify whether Greenwich mean time or local time is being used. Integral recorders usually use Greenwich mean time.
DISCUSSION POINT 9: Condition Monitoring
RECORDKEEPING Table 5 is a sample checklist that can be used to ensure that a container is suited for the refrigerated load, that it is set correctly, and that the produce has been stowed correctly. The checklist is also a valuable tool in diagnosing the causes for poorquality produce on delivery. Table 5. Container Loading Check List. o o o o o o o o o o o o o o o o o o o o o o o o Container interior is clean. Container is odor free. Container is not damaged. Door seals are in good repair and floor drains are open. Refrigeration unit is operational. Container is cooled to desired loading temperature. Container identification is documented. Correct cargo is loaded into container. Cargo is at specified pulp temperature. Cargo is properly stowed Package count is correct. Color photographs are taken of loading pattern, cargo and container Partlow paper chart installed, if Partlow recorder is available Portable temperature recorder charts are marked with load identification, start time, and date. Portable temperature recorders are started and marked with their location Portable temperature recorders are placed at correct locations in the load. Portable temperature recorders' make and identification numbers are recorded. Generator is operating and has sufficient fuel. Thermostat is set to correct temperature, check for Celsius or Fahrenheit specification. Fresh air exchange rate is set properly. Vehicle meets highway weight regulations. Security seal is properly attached to rear door of container. Security seal number is recorded. Mark position of recorders, air bags and other details of stowage on diagram below:
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James F. Thompson, Patrick E. Brecht, Tom Hinsch, Adel A. Kader
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James F. Thompson, Patrick E. Brecht, Tom Hinsch, Adel A. Kader
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James F. Thompson, Patrick E. Brecht, Tom Hinsch, Adel A. Kader
Partlow Process/Temperature Controller - Chart Recorder Overview Principle of Operation. . Partlow offers a wide variety of digital and mechanical temperature controller and circle chart recorder models and accessories designed for temperature, process and environmental control applications. Controllers and recorders overall control functionality is very similar with the primary difference, between recorders and controllers, being that recorders also offer a paper hardcopy of process results for validation purposes.
Reference 1. http://postharvest.ucdavis.edu/Pubs/marine_transport/Marine_Transport.shtml 2. http://www.partlow.com/content.aspx?id=99
James F. Thompson, Patrick E. Brecht, Tom Hinsch, Adel A. Kader
INTRODUCTION This publication is a guide to the proper use of refrigerated marine containers for shipping perishable produce. Shipping organization staff, transport company personnel, inspectors, surveyors, insurers, and receiving company employees will learn about important aspects of refrigerated container transport, including packaging, container and load specifications, loading, and recordkeeping. Two appendixes provide essential information: appendix A is a troubleshooting guide for diagnosing causes of poor-quality produce on arrival, and appendix B provides data on the long-term storage requirements for fresh fruits and vegetables. A companion poster that illustrates proper loading practices is also available from USDA and UC ANR (Publication 21596). Based on figure 4 in this publication, the color poster is a visual reminder of the key issues that dock personnel need to know about proper loading.
DISCUSSION POINT 1: Packages and Pallet Design
SELECTING AND USING BOXES FOR MARINE CONTAINER SHIPMENT Overview Corrugated boxes should be strong enough to withstand the effects of high humidity in storage and transport. Do not stack boxes beyond edges of pallet. Pallet loads should be unitized and secured. Boxes and inner packaging should allow vertical airflow, especially if produce is warmer than refrigeration set point temperature when stowed. Align box vents between layers of boxes. Use pallets where deck board spacing aligns with box vents. Corrugated fiber-board loses strength over time when it is supporting a load. For example, after supporting weight for 10 days, a fiberboard box will have only 65 percent of its original laboratory-determined strength. Fiberboard also absorbs moisture and weakens when it is exposed to high relative humidity, which is generated by the produce inside the box. Laboratory testing of fiberboard is based on material in moisture equilibrium with air at 50 percent relative humidity (RH) and a temperature of 23°C (73°F). Fiberboard in moisture equilibrium with air at 90 percent RH will have only 40 percent of its original stacking strength. Absorbed moisture also causes fiberboard to expand slightly, increasing box dimensions and sometimes causing warping. Wood and plastic boxes are not affected by humidity and maintain their strength in high-humidity conditions. Fiberboard boxes should be strong enough to withstand several weeks of initial storage and sea journeys of 4 weeks or more without failing from fatigue or high humidity. Because marine transport periods are usually much longer than highway transport periods, boxes for marine shipment usually need to be stronger than those used for domestic shipment. Fiberboard can be made stronger by using proprietary waxes and treatments, by adding more fiber weight to the board, or by selecting a stronger box design. Box venting also reduces box strength. Box vents should be located away from the vertical edges of the box and generally should not account for more than 5 percent of the box wall area. Boxes with greater than 5 percent venting must be specially designed to provide adequate strength. Select box dimensions and a pallet design that will enable box corners to be well supported by deck boards (fig. 1). Fiberboard boxes are strongest near the corners. If they are not well supported, the boxes will fail and allow produce to be crushed. Nominal outside box dimensions should allow for panel bulge and variation in box placement on the pallet. Do not stack boxes beyond the edge of the pallet. DISCUSSION POINT 2: Package Strength
Pallet loads should be well secured so that they do not shift in handling or transport. Stacking tabs or palletizing glue can assist in preventing boxes from sliding past each other. The pallet and load can also be unitized (tied together with net wrapping or corner braces and banding). Boxes should extend to the edge of the pallet. Free space at the periphery will allow the load to shift in transport. The packaging system shipped in marine containers should be designed to work with the container’s airflow pattern. Containers are usually equipped with a bottom-air delivery system (fig. 2). Air from the refrigeration unit flows first to the floor, up through the load, horizontally across the top of the load, and then back to the refrigeration unit air return opening. Pallets, boxes, and inner packaging should have enough venting and air spaces to allow vertical airflow through the pallet load. If air cannot flow through the packages, it will flow around the pallet load, with little opportunity for proper temperature management. This will cause greater variation in produce temperature throughout the container load. Boxes should have at least a 3 percent venting area on top and bottom panels. Most boxes also have vents on the side panels to allow initial cooling. These allow air to flow horizontally to find open vertical venting if top or bottom vents are blocked by pallet deck boards or packaging materials. Vents on horizontal box edges (sometimes called shoulder vents) are less likely to be blocked by produce or packaging materials than vents that are more toward the middle of panels. Vents for vertical and horizontal airflow should align when boxes are register-stacked or crossstacked. Interior packaging and pallet deck boards should not block airflow through box vents.
DISCUSSION POINT 3: Package Ventilation and Cooling Process
LOAD PLANNING Overview
Produce should have enough postharvest life for the trip and for marketing at the destination. Specify the carrying temperature in both Celsius and Fahrenheit scales. Specify the flow rate of fresh air exchange. Specify humidity, controlled or modified atmosphere treatments, or other special treatments if they are requested. Select cargo, dunnage, pallets, blocking, and bracing so that the load covers the entire floor of the container and meets highway weight regulations. Different kinds of produce shipped together in one container should have compatible – temperature needs – ethylene sensitivity – relative humidity needs – modified or controlled atmosphere requirements, if used – odors or chemical treatments – airflow characteristics. Produce items should not be included in a shipment if they do not have enough postharvest life for the trip and subsequent marketing at the destination. Appendix B lists typical produce postharvest life in air storage under optimal conditions, and table 1 lists produce with a postharvest life of less than 2 weeks. Produce that has already been stored before shipment will have a correspondingly shorter life in transport and at destination. Controlled atmosphere (CA) or modified atmosphere (MA) treatments can sometimes be used to increase produce postharvest life. If controlled atmosphere treatment has a benefit, it is typically about a 30 percent increase in storage life. The ocean carrier and the organization that purchases the transport services should agree on the desired carrying temperature for the load. The set point temperature should be clearly specified in both Celsius and Fahrenheit scales to reduce confusion between the two systems (table 2). Appendix B lists the optimal temperature for long-term storage of perishable commodities. The container thermostat is usually set by the ocean carrier based on the agreed-on carrying temperature and the company's knowledge of the specific capabilities of the container used for the trip. DISCUSSION POINT 4: Load planning
If a load has been properly cooled to transport temperature before loading and is properly loaded in a container that has a relatively new design, the thermostat can be set close to the long-term storage temperature. Newer container designs have supply-air temperature sensors and controllers that automatically control refrigeration based on supply air temperature if they are set at temperatures for chilled cargo. Because these containers can
control air temperature within 0.5°C (1°F) or better under most conditions, produce is rarely exposed to temperatures much below the set point temperature. This should prevent freezing or chilling damage if the thermostat is calibrated and set at the agreed-upon carrying temperature. Thermostats for older refrigeration systems with return-air temperature controllers should be set at least 1°C (2°F) above the long-term storage temperature to prevent freezing or chilling damage to produce. Systems that have switchselectable temperature sensing should be set to the supply-air temperature sensing for chilled cargo; in contrast, return-air temperature sensing is used primarily for frozen produce. Both types of containers automatically control temperature on the basis of return air temperature if they are set for a frozen load temperature. Container air exchange is set based on preventing elevated carbon dioxide (CO2) levels caused by produce respiration. Low oxygen levels are rarely a problem if carbon dioxide levels are properly controlled. Use the respiration rates listed in appendix B to find the air exchange rate in table 3. The air exchange rates in table 3 are calculated to keep carbon dioxide levels below 0.3 percent in a tightly sealed container. This is much below the damage threshold for many commodities. Lower air-exchange rates can be used if a commodity or a mix of commodities can withstand higher carbon dioxide levels. In fact, some commodities benefit from high carbon dioxide levels. The recommendations in table 3 assume that the produce is stowed at recommended temperatures. Ventilation should be specified based on airflow per time in cubic meters per hour (m3>/hr or cmh) or cubic feet per minute (ft3/min or cfm). Ventilation settings described as "percent vent opening" are not meaningful because performance characteristics and the wide-open vent capacity can vary greatly for different container designs. For example, a 20 percent vent opening corresponds to about 80 cmh in one particular container and less than 60 cmh in another. Airflow for a particular vent setting is also influenced by factors that influence evaporator fan output. For example, a 60 Hz electrical supply usually increases airflow by 20 percent compared with a 50 Hz electrical supply because of higher airflow from the container fan system. Also, dirty fan blades reduce fan airflow and air ventilation. Do not use ventilation rates above the recommended levels. Excess ventilation increases energy consumption. In hot, humid environments, it may also increase evaporator coil icing and reduce the consistency of temperature control. Controlled atmosphere or modified atmosphere treatments are sometimes used to increase the postharvest life of some produce items compared with storage in normal air. Properly selected treatments slow produce respiration, retard ripening, and may inhibit the development of decay. These treatments generally add expense to the cost of shipping. Appendix B lists the recommended controlled atmosphere conditions for selected commodities. DISCUSSION POINT 5: Temperature and Air-Flow Control : CostBenefit Analysis Commodities that are particularly sensitive to damage from moisture loss and wilting may benefit from increased humidity levels in containers. High humidity is usually obtained by using a spray humidification system available on some newer containers. A disadvantage of these units is that they require temperature settings slightly above 0°C to prevent the nozzle from freezing. High humidity also weakens fiberboard. Moisture loss can also be reduced by packing produce in plastic bags or box liners or by using plastic boxes that do not absorb water. The loaded container must meet highway weight regulations. In the United States, for example, the total gross vehicle weight must not exceed 36,288 kg (80,000 lb). There are
also separate weight limits for each vehicle axle to insure a well-distributed load. This results in a net produce load of 18,144 kg (40,000 lb) to 21,909 kg (48,300 lb), depending on the weights of the truck, container, chassis, and generator set (table 4). Even the amount of fuel in the generator set and truck can have a significant effect on net produce load. One gallon of diesel fuel weighs 7.1 pounds (3.2 kg), and one liter weighs 0.85 kg (1.9 lb). A container with a nose-mounted generator set (sometimes called a clip-on generator set; see back cover) will often require that less than full pallets be loaded in the first few pallet rows of the container in order to meet axle weight limits. Maximum load weight can be obtained only if the transport company informs their customer about the specific weight of equipment used for a particular load. Without this information the container will probably be loaded to a safe weight of about 18,144 kg (40,000 lb).
Loads with more than one type of produce must be set up so that different types of produce loaded together have compatible temperature ranges and ethylene sensitivities. Use table 1 to select compatible produce. table 1 groups common fruits and vegetables by temperature range and ethylene sensitivity. Produce in the same column can be safely held at the same temperature range (listed at the top of the column). If produce in different temperature ranges are mixed, produce quality will be compromised, especially with longer transit times. The greater the difference in recommended temperature between the produce, the greater the potential for quality loss. DISCUSSION POINT 6: Humidity, Weight Restriction and Container Stuffing Mix
Dried vegetables in the yellow row of table 1 should not be mixed with any other produce in the table. These vegetables should be held in a 50 percent to 70 percent relative humidity environment to prevent decay. Most of the vegetables in the lowest temperature range (0° to 2°C [32° to 36°F]) are sensitive to moisture loss and should be held at more than 90 percent relative humidity or packaged to minimize water loss. The other vegetables and fruit should be held at 85 percent to 95 percent relative humidity. Ethylene-sensitive vegetables (green row) should not be mixed with ethylene-producing fruits, luffa, and tomato (blue row). If for some reason they must be mixed, damage may
be reduced by using a fresh air exchange rate of 45 cfm (table 3). Ethylene scrubbers may also reduce damage. In some instances a controlled atmosphere will allow ethylenesensitive produce to be shipped with ethylene-producing produce, but the acceptable produce combinations and atmosphere prescriptions are not well documented. Produce that is neither ethylene sensitive nor ethylene producing (pink row) can be mixed with produce above or below them in the same temperature column.
Some produce items can exchange odors with other selected produce items. See the notes at the bottom of table 1 for precautions. Certain produce items have a short postharvest life and are not suited for container shipments. This is particularly true if short-lived items are held at non-optimal temperatures. For example, asparagus has a maximum postharvest life of three weeks if held at 2.5°C (36°F). If it is shipped at 0°C (32°F) in a load of broccoli, it will be subject to chilling injury after only 10 days. Modified atmosphere packaging can sometimes be used to increase produce life and allow produce to be shipped to destinations that require several weeks transport time. If a controlled atmosphere environment is used it should, at a minimum, not reduce the postharvest life of any of the mixed commodities. DISCUSSION POINT 6: Container Stuffing Mix
PRODUCE TEMPERATURE AT LOADING Overview Perishable produce should temperature before loading. be cooled to its desired carrying
Most produce cools very slowly in a container, and its storage life is reduced if it is warm when loaded. If produce must be loaded warm because there are no cooling facilities in the production area, proper packaging and loading are essential to achieve the fastest possible cooling rates. Under the very best conditions a container can cool produce in 2 to 4 days. Under poor conditions produce will never cool to the desired temperature and will be of poor quality upon arrival.
Produce should arrive at the loading dock at the proper temperature for shipment because it is often difficult to send produce back to the cooler if it is found to be too warm for transport at the dock. Cooler managers should be entrusted with the responsibility of ensuring that produce is properly cooled before it reaches the shipping dock. Loading docks and load assembly areas should be refrigerated. Dock supervisors should ensure that produce does not warm on the dock and should conduct a final produce temperature check before loading. CONTAINER OPERATING CONDITION Overview Container refrigeration and generator units should be inspected, pretripped, and repaired, if necessary, before each trip. Containers should be cooled to near carrying temperature before loading. Turn off refrigeration unit before doors are opened for loading. Containers should look and smell clean, floors should be free of refuse, and should be in good repair before loading. Container should be free of toxic materials and sources of bacterial contamination. Container operators should track past shipments to verify container is acceptable for food transport. The container company should inspect and clean each container before each trip. The diagnostic system for the refrigeration equipment should be checked and needed repairs made, and the temperature control system should be regularly calibrated.
The container should arrive at dock at near the desired carrying temperature to prevent produce loaded near container walls from warming under hot ambient conditions or cooling too much under cold ambient conditions. In temperate climates with low humidity, the container should be cooled to the carrying temperature. If loading from an open dock in a humid environment, the container should be cooled to the dew point temperature of the outside air. Temperatures below the dew point cause water to condense on walls, which may damage fiberboard packages. In all cases the refrigeration system should be turned off when the container doors are open. This prevents moisture from condensing on the evaporator coil. Use an infrared thermometer set to inside wall temperature to determine conventional thermometer can also against an inside wall and covering it come to a constant temperature. the emissivity level of the wall to check the if the container has been adequately cooled. A be used by placing the temperature probe with a dry cloth. In a short time the probe will
The container should be clean on arrival at the loading dock. It should smell clean and the floors and drains should be free of debris. The container should be free of serious damage. Doors should seal tightly when closed and interior wall surfaces should be in good repair. Refrigerated containers can be used for transporting a wide variety of commodities. Some of these commodities may not be compatible with food because their residues may contaminate produce loads. Even previous loads of food may be a source of bacterial contamination. Container operators should be able to guarantee that the vehicle is free of residues from sources of bacterial contamination or toxic materials. Some container operators have policies that restrict shipping toxic materials in containers that are designated for shipping perishable produce. However, most container operators have few restrictions for shipping dry cargoes in refrigerated containers and cargo is sometimes mis-declared. Consequently, the container should be adequately cleaned during pre-trip maintenance to avoid the possibility of crosscontamination between perishable foods, non–food-grade cargo, and foodborne bacterial pathogens. The container operator should monitor and record the types of cargo previously moved in a container.
DISCUSSION POINT 7: Pre-requisites for Container Stuffing
CONTAINER LOADING Overview Cover the entire floor with pallets, boxes, or other materials. Load produce below the red height-limit line. Stabilize load to prevent shifting in transit. A uniform temperature can be maintained in the load only if the container floor is completely covered with pallets, boxes, or solid material from the front bulkhead to the end of the floor rails. When the floor is covered, refrigerated air is forced up through and around the packages (fig. 2). If the produce is palletized, the pallet openings for forklift tines (sometimes called pallet pockets) should also be covered to prevent air from traveling horizontally through the openings and escaping into an open vertical channel between pallet loads. The floor area between pallets can be covered with sections of fiberboard. The fiberboard should cover the floor and any pallet openings and should be held down with boxes of produce or attached to pallets. Placing a few boxes of produce at the rear of the load is an effective way to cover the space at the rear of the container. If the floor and pallet openings are not completely covered, refrigerated air will short-cycle back to the refrigeration unit and bypass most of the load. Under hot ambient conditions, produce with poor airflow around it will tend to arrive too warm, especially if it had not been thoroughly cooled prior to transport or if it has a high respiration rate. An open floor in the front of the container allows refrigerated air to flow through these open areas, causing produce in the rear of the container to be warm because very little refrigerated air reaches it. Under cold ambient conditions, produce in areas with low airflow is in danger of freezing or chilling injury. The load can be stabilized in several ways. A 20-pallet (48 in by 40 in [approximately 1.2 m by 1 m]outside pallet dimensions) load, with 9 pallets loaded straight in and 11 pallets turned 90 degrees before loading, completely fills a container width and produces good stability (fig. 3). Air bags or other load-restraining devices in the rear of the load prevent produce from moving back against the doors. Some shippers may need to reduce the number of box layers on each pallet to allow heavy produce to use a 20-pallet load and still meet highway weight limitations. If the pallet load weight is too great to allow a 20-pallet configuration, an 18-pallet loading pattern can be used (fig. 3). All open floor spaces and exposed pallet openings must be blocked to prevent refrigerated air from bypassing the produce and short-cycling back to the refrigeration unit. The two patterns with each pallet touching a sidewall are very stable. A specially designed plastic airbag that fills the entire center gap between pallets can be used to stabilize the sidewall load and prevent air from passing through the center gap. figure 4 shows the general procedures for proper container loading. Hand-stowed loads should also cover the entire floor (fig. 5). Boxes should be stacked tightly against each other and tightly against the walls. This type of stacking also provides support to help minimize box failure. Boxes should be oriented to align box vents so that air can flow through the load. Never leave open vertical or horizontal channels between boxes. Boxes should be stacked to a constant height to foster uniform airflow through the load. DISCUSSION POINT 8: Container Stuffing Plan
TEMPERATURE RECORDING DEVICES Overview Install a calibrated temperature recording device. Mark the date, time, and placement on the recorder. Refrigerated containers usually have equipment that automatically records refrigeration system functions and the air temperature inside the container. This information provides a detailed record of refrigeration system performance during a trip. Newer containers also have probes that can be used to monitor produce temperature. However, this data is usually available only to the ocean carrier unless cargo owners make a special arrangement with the carrier before the trip. Shippers should install their own temperature recording equipment to generate data for their own records. Temperature recorders should be rugged enough to withstand vibration and impact and still maintain calibration. The internal clock and temperature sensors should be calibrated. Recorders measure the temperature of the environment immediately surrounding them. Actual load temperature can be determined only by packing the recorders in boxes or inserting temperature probes into produce.
For convenience, temperature recorders are often installed on the top of pallets in front, middle, or rear locations. However, interpreting the data may be difficult because the recorders will be exposed to air that is influenced by the heat generated by the produce. For example, if produce is warm at loading, the recorders may indicate warm conditions even when the refrigeration unit is producing adequately cooled air. The temperature at the recorder may also be influenced by improper loading practices that allow conditioned air to bypass the rear of the load. Recorders should never be in contact with the walls or roof of the container because the temperature they record may be affected by heat transmitted through these surfaces. To keep a record of refrigeration system performance, a recorder should be placed at the floor next to the front bulkhead. Recorders should always be marked with the date and time they were started and also with their location in the load. If this information is missing, it is very difficult to interpret the temperature data. A note on the recorder should specify whether Greenwich mean time or local time is being used. Integral recorders usually use Greenwich mean time.
DISCUSSION POINT 9: Condition Monitoring
RECORDKEEPING Table 5 is a sample checklist that can be used to ensure that a container is suited for the refrigerated load, that it is set correctly, and that the produce has been stowed correctly. The checklist is also a valuable tool in diagnosing the causes for poorquality produce on delivery. Table 5. Container Loading Check List. o o o o o o o o o o o o o o o o o o o o o o o o Container interior is clean. Container is odor free. Container is not damaged. Door seals are in good repair and floor drains are open. Refrigeration unit is operational. Container is cooled to desired loading temperature. Container identification is documented. Correct cargo is loaded into container. Cargo is at specified pulp temperature. Cargo is properly stowed Package count is correct. Color photographs are taken of loading pattern, cargo and container Partlow paper chart installed, if Partlow recorder is available Portable temperature recorder charts are marked with load identification, start time, and date. Portable temperature recorders are started and marked with their location Portable temperature recorders are placed at correct locations in the load. Portable temperature recorders' make and identification numbers are recorded. Generator is operating and has sufficient fuel. Thermostat is set to correct temperature, check for Celsius or Fahrenheit specification. Fresh air exchange rate is set properly. Vehicle meets highway weight regulations. Security seal is properly attached to rear door of container. Security seal number is recorded. Mark position of recorders, air bags and other details of stowage on diagram below:
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James F. Thompson, Patrick E. Brecht, Tom Hinsch, Adel A. Kader
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James F. Thompson, Patrick E. Brecht, Tom Hinsch, Adel A. Kader
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James F. Thompson, Patrick E. Brecht, Tom Hinsch, Adel A. Kader
Partlow Process/Temperature Controller - Chart Recorder Overview Principle of Operation. . Partlow offers a wide variety of digital and mechanical temperature controller and circle chart recorder models and accessories designed for temperature, process and environmental control applications. Controllers and recorders overall control functionality is very similar with the primary difference, between recorders and controllers, being that recorders also offer a paper hardcopy of process results for validation purposes.
Reference 1. http://postharvest.ucdavis.edu/Pubs/marine_transport/Marine_Transport.shtml 2. http://www.partlow.com/content.aspx?id=99
