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Maintaining Product Quality
During Transportation

Introduction

Effective Packaging and Labeling
Stacking | Packing Methods | Standardization | Unit Loads
Pallets | Slipsheets | Labeling and Branding

Temperature, Humidity, and Other Environmental Controls
Precooling Factors | Precooling Methods | Precautionary Measures

Transportation
Selection Factors | Available Equipment | Refrigeration and Ventilation Systems
Equipment Features | Loading Methods | Mixed Loads | Providing for Air Circulation
Temperature Monitoring and Recording | Cold Treatment and Pest Control
Bracing the Load | Loading Air Cargo Equipment | Loading Trailers and Containers
Trailers and Containers With Top Air Delivery | Trailers and Containers With Bottom Air Delivery
Intransit Procedures | Receiving Procedures | Air Circulation and Sanitation in Storage

In addition to obtaining competitive freight rates and services, a shipper should ensure that the product will arrive in excellent condition. Of particular concern are products of a perishable nature, such as frozen and chilled foods, as well as processed and packaged foods, drinks, and juices. Important considerations include:

• Effective packaging and labeling;
• Temperature, humidity, and other environmental controls;
• Well-maintained transportation equipment; and
• Proper loading, in-transit monitoring, and unloading.

Under the best circumstances, product quality can only be maintained, not improved, during transportation. Initial product quality should be the highest possible. Products in top-quality condition:

• Have a longer shelf life;
• Allow more time for transportation, storage, and marketing;
• Satisfy importers, brokers, and consumers;
• Increase repeat sales and profits; and
• Help expand markets.

Exporters should keep in mind that products must be protected from:

• Rough handling during loading and unloading;
• Compression from the overhead weight of other product containers;
• Impact and vibration during land, ocean, and air transportation;
• Rolling, pitching, yawing, heaving, swaying, and surging during ocean transportation;
• Loss or gain of moisture to or from the surrounding air;
• Higher or lower than recommended temperature; and
• Cross-contamination or odors from other products or residues.

By selecting and packing only top-quality products, shippers can help ensure good arrival condition. Effective packaging, environmental controls, and proper transportation equipment are essential.

Effective Packaging and Labeling

Proper packaging of agricultural products is essential to maintaining quality during transportation and marketing. Packaging serves to enclose the product and facilitate handling. Labeling provides required information and a further opportunity to position the product in the market through attractive graphics. Shipping high-quality, high-value, perishable products in poor-quality packaging can lead to damage, decay, low prices, or outright rejection by the buyer. Improper labeling can also lead to delays and product loss.

Packaging must withstand:

• Rough handling during loading and unloading;
• Compression from the overhead weight of other containers;
• Impact and vibration during transportation; and
• High humidity during precooling, transit, and storage.

Packaging materials are chosen on the basis of the product and environmental considerations. Factors to be considered are method of packing, temperature, humidity, desired atmosphere around the product, packaging strength, cost, availability, buyer specifications, graphics, labeling, freight rates, and government regulations. Packaging manufacturers, foreign buyers, wholesale markets, retail stores, packaging magazines, and consultants are an important source of information on current packaging trends and desires.

All packaging should be recyclable or reusable, and the necessary amount of material should be used to protect the product. The options of incinerating packaging waste or shipping waste to landfills are being reduced throughout the United States and Europe. Mandatory recycling programs, packaging bans, and solid waste reduction programs have been established in many countries.

Packaging also should be standardized to facilitate unit loading on standard-size, reusable pallets in use in the United States, Europe, and other countries. Pallet handling and leasing companies have been established in response to economic as well as environmental concerns.

Boxes should be sized and filled in accordance with the importer's or buyer's desires. Boxes that are very wide and weigh more than 20 kg (44 lb) encourage rougher handling, product damage, and container failure.  Excessive weights and damaged packaging are common complaints of importers of U.S. meat products who receive boxes weighing up to 45 kg (100 lb). In the Netherlands, for example, a single worker does not have to lift over 15 kg (33 lb) unassisted, in accordance with labor regulations and occupational safety concerns.

Overfilling causes product damage and excessive bulging of the box, which leads to reduced compression strength and container failure. Underfilling also may cause product damage. The product may be bruised as it moves around inside the box during transport and handling or during crushing of the available headspace.

Widely used packaging materials include:

• Fiberboard--Pallets, slipsheets, bins, boxes (glued, stapled, interlocking), lugs, trays, flats, dividers, and partitions.
• Wood--Pallets, bins, crates (wirebound, nailed), baskets, trays, and lugs.
• Paper--Bags, sleeves, wraps, liners, pads, excelsior, and labels.
• Plastic--Pallets, bins, boxes, trays, bags (mesh, solid), containers, sleeves, film wraps, liners, coatings, dividers, and slipsheets.
• Polystyrene--Foam boxes, trays, lugs, sleeves, liners, dividers, and pads.

Fiberboard Boxes--Fiberboard boxes are the most widely used packaging, due to their versatility and recyclability. There are many fiberboard box styles and sizes. A minimum 19.3 kg/cm² (275 lb/in²) bursting-test-strength or 7.86 kg/in (44 lb/in) width edge crush test fiberboard is recommended for boxes intended for export. The strength is needed for the handling, transport conditions, and high humidity the boxes must endure. Many boxes are now certified with an edge crush test instead of the bursting strength test. This information is available from the packaging supplier and stamped on each box. Foreign buyers or importers should be consulted about the size, pack, and box style desired.

Fiberboard boxes for products that are precooled in the box, packed wet, or packed with ice are wax-impregnated or coated with water-resistant material. Wax-treated fiberboard is generally not considered recyclable, so polyethylene and other coatings have been developed instead.

Water-resistant coatings may be needed because the compression strength of untreated fiberboard can be reduced by more than one-half in conditions of high relative humidity common in refrigerated environments. In addition to maintaining box strength, coatings help to reduce the loss of moisture from the product to the fiberboard or weakening of the fiberboard from moisture from the product. All glued boxes should be made with a water-resistant adhesive that also is recyclable.

Specially coated modified atmosphere fiberboard packaging has been developed to slow product respiration rate and eliminate the need for ice and wax coatings in packages of fresh broccoli and other products. Film wraps of individual commodities and film wraps of fiberboard boxes of product have also been developed with the same purpose in mind. Proper product precooling and maintenance of refrigerated product temperatures are required for modified atmosphere packaging to work as intended.

Holes are provided in fiberboard boxes of some products to provide ventilation of product heat (respiration) and allow circulation of cold air to the product when a modified atmosphere is not required. Handholds provide a means of handling boxes during loading and unloading. All holes must be designed and placed in a manner that does not substantially weaken the box.

Wood Crates--Wood crates and wirebound wood crates are popular with some shippers due to their material strength and resistance to high humidity and moisture damage during precooling, transit, and storage. The crates are constructed in a manner that allows a lot of air circulation around the packed product. There is concern over whether wood crates are recyclable or reusable. Machines are available to grind up wood crates for conversion into mulch or other materials, but some countries in Europe have discouraged their use. Fasteners or wire in wood crates to be recycled should be made of steel with a maximum diameter of 10 mm, to allow grinding of the crates and extraction of the fastener particles with magnets.

Stacking

The majority of fiberboard boxes and wood crates are designed to be stacked top-to-bottom. Compression strength and product protection are sacrificed when boxes or crates are stacked on their ends or sides. Misaligned fiberboard boxes can lose up to 30 percent of their strength, while boxes that are not stacked top-to-bottom (stacked either crosswise or off-center) can lose up to 50 percent of their top-to-bottom compression strength.

Various materials are added to boxes to provide additional strength and product protection. Fiberboard trays, dividers, or partitions, and double-or triple-layer sides and ends in boxes provide additional compression strength and reduce product damage.

Pads, wraps, sleeves, and excelsior are used to reduce bruising. Pads also are used to: provide moisture, as with asparagus; absorb moisture as with retail packages of meat, poultry, and seafood; provide chemical treatment to reduce decay, as with sulfur dioxide pads for grapes; and absorb ethylene, as with potassium permanganate pads used in boxes of bananas and flowers, or oxygen in the case of some modified-atmosphere packaging.

Plastic Film Liners or Bags--Plastic film liners or bags are used to retain moisture, provide for a modified atmosphere, or maintain product integrity, such as in a cluster of grapes or tomatoes. Plastic with various size perforations, depending on commodity requirements, is used to let oxygen in and carbon dioxide out. Special films are used to seal the products and provide for a modified atmosphere either by allowing the product to consume oxygen, releasing carbon dioxide and therefore slow product respiration and ripening, or by flushing the package with a modified atmosphere or by vacuum packaging. This is done for bananas, strawberries, cherries, tomatoes, meat, poultry, seafood, and many other products that benefit from a modified atmosphere.

Paper and Polystyrene Foam Liners--Liners help to insulate the product from hot or cold temperatures when they are shipped in unrefrigerated air cargo holds and uninsulated air cargo containers. Wet paper is used to provide moisture to fresh cut herbs and flowers.

Shippers should check with the Foreign Agricultural Service (FAS) or Animal and Plant Health Inspection Service (APHIS) for packaging materials restrictions in foreign countries, especially those made from plant parts such as wood, straw, or leaves. Some of these items are prohibited in other countries or require special documentation or quarantine treatments. Countries with phytosanitary or environmental restrictions on packaging materials include Australia, New Zealand, and the European Community. Soil also is restricted by many countries.

Packing Methods

Types of Packs

Types of packs include:

• Volume Fill--Products are placed by hand or machine into the box until the desired capacity, weight, or count is reached.
• Tray or Cell Pack--Products are placed in molded trays or cells which provide separation and reduce bruising.
• Place Pack--Products are wrapped and carefully placed in the box. This provides reduced bruising and a pleasing appearance.
• Consumer Pack or Prepack--Relatively small amounts of the product are packaged, weighed, and labeled for retail sale.
• Flexible Packaging--Each product is individually wrapped and sealed in film to reduce moisture loss and decay, and in some cases, provide for a modified atmosphere by letting oxygen in and carbon dioxide out through the design of the film layers, at a rate that slows product respiration and ripening. For some commodities the film may be treated with fungicides or other chemicals.
• Modified Atmosphere--Individual commodities, consumer packs, boxes, or pallet loads of commodities are sealed with special plastic film or bags and flushed with nitrogen to slow respiration, ripening, or aging of the product. In each case the packaging is formulated for a specific product.

Packing Fresh Fruits and Vegetables

The packing of fresh fruits and vegetables is of concern due to their highly perishable nature. Growers, processors, packers, shippers, and repackers should:

• Ensure adequate sanitation during harvesting and packing to avoid contamination of produce with pathogenic organisms that can cause food-borne illness;
• Use a chlorinated wash to remove dirt, debris, and organisms present in harvest operations;
• Sort out bruised, cut, decayed, insect-infested, odd-sized, immature, or overripe items;
• Use the minimum amount necessary of fungicides/bactericides to limit decay on certain products, strictly in accordance with label instructions and foreign country restrictions;
• Use the minimum amount necessary of officially approved wax or resin coatings to reduce moisture loss on certain products, strictly in accordance with label instructions and foreign country restrictions;
• Use the minimum amount necessary of officially approved pesticides for certain products to eliminate insect pests, strictly in accordance with label instructions and foreign country restrictions;
• Remove field heat (precool) as soon as possible after harvest, and maintain the cold chain;
• Use grade standards or buyer's specifications in packing;
• Place only uniform sizes or amounts in each box;
• Place only products with a uniform level of maturity in each box; and
• Clearly mark the grade, size, weight, or count on the box, along with any other required label information, such as country of origin, exporter, importer, gross and net weights in kilograms, total number of packages, size of package in centimeters, handling marks (international pictorial symbols), cautionary marks, port of entry, pesticides, and fungicides used, or wax or resin coatings used, in a language accepted by the destination country.

Damaged fresh products can ruin an entire shipment and reduce importers' confidence in the grower and shipper. Products in this condition:

• Spread decay to other products in the load;
• Produce more ethylene gas and heat, which cause further ripening and decay; and
• Lose more water resulting in shriveling and wilting.

Packing methods for fresh produce include:

Field Packing--Products are placed in fiberboard boxes or wood crates during harvesting. Some products are wrapped. The filled containers are then taken to a precooling facility to reduce field heat.

Shed Packing--Products are processed or packed indoors or under cover at a central location. The product is brought from the field to the packing shed in bulk in field crates, bins, or trucks. The products are precooled either before or after they are placed in shipping containers.

Repacking--Products are taken out of one container, regraded, and placed in another. This is often done to make smaller boxes for the retailer or consumer packages.

Fresh-cut Processing--Products are washed, trimmed, shredded, peeled, cut, and otherwise processed into salad mixes or ready-to-eat items under sanitary and temperature-controlled conditions using Hazard Analysis Critical Control Point (HACCP) or similar systems of quality control. These items are then placed in modified-atmosphere consumer and foodservice packages which are then grouped in fiberboard boxes for distribution under constant refrigeration.

Packing Meat, Poultry, and Seafood Products

Through careful sanitation, modified-atmosphere packaging technology, temperature control, and loading of marine containers at meat packing plants, extended shelf life of 70 days or more for chilled beef and lamb has been possible since the late 1980's. This has provided higher revenues for the meat exporter as compared to frozen product shipped by sea or chilled product shipped by air. Recently instituted HACCP procedures may help additional firms get into the export market with extended shelf-life, quality-assured chilled and frozen products.

Standardization

Due to the large number of different box sizes in use, box and pallet standards have been developed by the fresh produce, frozen food, floral, and grocery industries in Europe and the United States to reduce handling damage and packaging waste. Standardized boxes can:

• Reduce box inventory for manufacturers and growers;
• Provide unit loads and more stable mixed pallet loads;
• Reduce transportation and marketing costs; and
• Use 90 to 100 percent of the pallet surface with no overhang and little underhang.

The following are standard pallet sizes:

• Standard Grocery Manufacturers Association (GMA) pallet used in the United States--1219 by 1016 mm (48 by 40 in).
• International Standards Organization (ISO) pallet used in Europe--1200 by 1000 mm (47.24 by 39.37 in).
• Europallet, also widely used in Europe--1200 by 800 mm (47.24 by 31.5 in).

The following five box sizes that fit well on all three of the above pallets are recommended for international trade, especially in Europe.

• 600 by 400 mm
• 400 by 300 mm
• 400 by 200 mm
• 300 by 200 mm
• 200 by 150 mm

Unit Loads

Shippers, carriers, and receivers prefer handling palletized unit loads instead of individual boxes, one at a time. Most distribution centers are set up to store palletized loads in three-tier or higher racks.

Unit loads provide for:

• Reduced handling of boxes;
• Less damage to the boxes and the products inside;
• Faster loading and unloading of transportation equipment;
• More efficient distribution center operations; and
• Reduced pilferage of products.

Unit loads may include some of the following features:

• Standard size reusable or recyclable wood pallets or slipsheets;
• Fiberboard, plastic, or wire vertical interlocking tabs between boxes;
• Boxes with holes for air circulation, which align when the boxes are stacked squarely on top of one another, corner to corner;
• Recyclable glue between boxes to resist horizontal slipping;
• Plastic netting around the pallet load of boxes; and
• Fiberboard, plastic, or metal cornerboards with plastic or metal strapping around the cornerboards and boxes.

Pallets

Wood pallets must be strong enough to allow storage in racks. Pallets also should be capable of being reused a number of times. Provisions for forklift and pallet jack handling are necessary. The design of the bottom of the pallet should not block air circulation.

Block-style 1200 by 1000 mm pallets are standard in European pallet pools and should be used for any exports to Europe. The Europallet, 1200 by 800 mm, also is popular in some European countries. Australia uses an 1100 by 1100 mm pallet.

One-way pallets are increasingly being rejected by importers and receivers due to recycling and disposal costs. Product received in Europe on pallets other than the 1200 by 1000 mm and 1200 by 800 mm pallets used in those countries may be required to be re-palletized at the port of entry, leading to additional handling costs, product damage, and pallet disposal costs.

Pallets must have an adequate number of top deck boards to support fiberboard boxes. Otherwise the boxes may collapse between deck boards, crush the product, or cause the entire load to lean or fall off the pallet. A sheet of fiberboard with holes for air circulation can be used to help distribute weight across the pallet.

Boxes must not overhang the edges of the pallets. Overhang can reduce the strength of fiberboard boxes by one-third. This condition can lead to collapse of the entire load, crushing of the product, and difficulty in loading, unloading, and storage in racks. On the other hand, boxes that use less than 90 percent of the pallet surface and do not align with the pallet edge can shift in transit.

Pallet loads of boxes that are not strapped or netted should have at least the top three layers of containers cross-stacked to provide stability. Some shippers use film wrap, tape, or glue on the top layers in addition to cross-stacking. The boxes must be strong enough to be cross-stacked without collapsing. Film wrap should not be used on boxes of products that need ventilation.

Slipsheets

Slipsheets, made of sheets of fiberboard or plastic, are used by some shippers instead of pallets to reduce transportation costs. Slipsheets eliminate the cost of buying, disposing, or returning pallets. A special forklift with a clamping device is needed to transfer slipsheet loads to and from the warehouse pallets at the shipper's and receiver's distribution center and into the trailer, container, or railcar. If this special forklift is not available, the unit loads must be transferred box by box onto pallets, leading to costly delay and product damage.

Slipsheets made of recyclable fiberboard or plastic must be strong enough to be clamped and pulled onto the forklift tines or plate for lifting. Fiberboard slipsheets should be treated with a recyclable coating for use in wet conditions. Slipsheets used in transportation equipment should have holes for air circulation under the load. The use of slipsheets in refrigerated transportation equipment with shallow floor channels is not recommended due to the need for adequate air circulation under the load. Unit loads of boxes on slipsheets should be netted, stretch-wrapped, or otherwise secured with cornerboards and strapping.

Labeling and Branding

Labeling of boxes may provide required information for export certification, identify and advertise the products, and assist receivers in storing and retrieving the boxes. Fiberboard boxes can be preprinted with colorful graphics. Wood packaging has glued, stamped, or stenciled labeling. Some high-quality fruits and vegetables are individually branded with small colorful trademark stickers. Some shippers also provide selection, storage, recipes, posters, and other point-of-sale material for the retailer or consumer.

All boxes and consumer packages should be clearly labeled, bar coded, and branded in the language accepted by the destination country. The following information should be included on boxes along with any other data required by the foreign country:

• Common name of the product;
• Net weight, count, and/or volume;
• Brand name as well as name and address of the packer or shipper;
• Country of origin;
• Size and grade, when standards are used;
• Recommended storage temperature;
• Special handling instructions; and
• Name of officially approved fungicides or bactericides used in packaging.

Each country has labeling requirements that must be followed. Labeling of consumer packages is mandatory under most national regulations. For example, the United States requires that in addition to the product name, net weight, and name and address of the manufacturer, packer, or distributor, processed items must have a nutrition label and all ingredients listed in descending order of prominence. The Food and Drug Administration (FDA) classifies fruits and vegetables with wax or resin coatings as processed products which must be properly labeled with names of the coating displayed at the point of retail sale or on the individual items. Many processed products also are labeled with a "sell by" date or "best if used by" date.

Temperature, Humidity, and Other Environmental Controls

Removal of field heat by the process of precooling to a recommended storage temperature and relative humidity is absolutely necessary to maintain the quality of fresh fruits, vegetables, plants, and cut flowers. The quality of most products will rapidly deteriorate if field heat is not removed before loading into transportation equipment. The rate of respiration and ripening increases two to three times for every 10^o C (18^o F) above the recommended storage temperature.

Refrigerated transportation equipment is designed to maintain temperature and should not be used to remove field heat from products packed in shipping containers. The refrigeration units also are not capable of raising or controlling the relative humidity.

A high temperature difference between the refrigeration unit evaporation coil and the product will increase the loss of product moisture. This will cause the evaporator to frost and the products to shrivel or wilt and weigh less. Most fruits and vegetables have a water content between 80 and 95 percent.

Precooling Factors

Precooling extends product life by reducing:

• Field heat;
• Rate of respiration (heat generated by the product);
• Rate of ripening;
• Loss of moisture (shriveling and wilting);
• Production of ethylene (ripening gas generated by the product); and
• Spread of decay.

The success of precooling is dependent on:

• Time between harvest and precooling;
• Type of shipping container, if product is packed beforehand;
• Initial product temperature;
• Velocity or amount of cold air, water, or ice provided;
• Final product temperature;
• Sanitation of the precooling air or water to reduce decay organisms; and
• Maintenance of the recommended temperature after precooling.

Precooling should occur as soon as possible after harvest. Harvesting should be done in early morning hours to minimize field heat and the refrigeration load on precooling equipment. Harvested products should be protected from the sun with a covering until they are placed in the precooling facility.

Many products are field or shed packed and then precooled. Wood wirebound or nailed crates and waxed or coated fiberboard boxes are used for packed products that are precooled with water or ice after packing. This process is being modified in response to the demand for recyclable boxes.

Precooling of products packed in boxes and stacked in unitized pallet loads is especially important as air circulation around and through the packaging may be limited during transportation and storage.

Precooling is particularly important for products that produce a lot of heat. The following are examples of products that have high respiration rates, and short transit and storage lives:

Precooling Methods

The choice of precooling method depends on the nature, value, and quantity of the product, as well as the cost of labor, equipment, and materials. Precooling methods include:

• Room Cooling--Boxes of products are stacked in a refrigerated room. Some products are misted or sprayed with water during room cooling.
• Forced-Air Cooling or Wet Pressure Cooling--Air is drawn through stacks or unit loads of boxes of products in a refrigerated room. For some products, water is added to the air.
• Hydrocooling--Products are flushed with ice water in bulk tanks, bins, or boxes.
• Vacuum Cooling--Heat is removed from products packed in boxes by drawing a vacuum in a chamber.
• Hydrovacuum Cooling--Moisture is added to products packed in boxes before or during the vacuum process to speed the removal of heat.
• Package-icing--Slush or crushed ice is injected into each box of product. Some operations use bulk bins.

Portable ice plants, hydrocoolers, vacuum coolers, forced-air coolers, and package-icing machines are available for use in the fields. This equipment is useful for remote or small-scale operations that cannot justify investment in a fixed precooling facility. Mounted on skids, dollies, or tractor trailers, the equipment can follow the harvest from field to field and be shared by many growers.

Hydrocooling and vacuum cooling are the fastest cooling methods. Cooling times of half an hour are possible. Products and packaging must be able to withstand direct water contact in hydrocooling. In vacuum cooling, the products should have a large surface area, low density, and high moisture content. The boxes and wrapping must allow ventilation of heat.

Forced-air cooling can take 1 or 2 hours, depending on the amount of packaging, while room cooling may take 24 to 72 hours. Packaging must allow ventilation of heat for these methods to be successful. Package-icing provides effective cooling and a high relative humidity for products and packaging that can withstand direct contact with ice.

Many tropical fruits, vegetables, plants, and cut flowers require much less cooling than products that are cooled to 0^o C (32^o F). All products should be precooled as near as possible to the recommended storage temperature and relative humidity. Product temperatures should be taken in sample boxes by inserting an electronic thermometer into the product. The data should be recorded for future reference.

Tables 1-6 at the end of this section provide lists of products and their recommended temperatures, relative humidities, and approximate transit and storage lives.

Precautionary Measures

Products listed in table 7 and  table 8 at the end of this section are sensitive to chilling or freezing injury. Care must be taken not to precool or store the products below the recommended temperature. Often the visible effects of chilling injury are delayed until the product is offered for retail sale. These effects include failure to ripen properly, pitting, decay, watery breakdown, and discoloration in fruits and vegetables. Flowers and plants lose florets or foliage, fail to open, discolor, or wilt.

All products are sensitive to decay. Precooling equipment and water should be sanitized continuously with a hypochlorite solution to eliminate decay-producing organisms. Care also must be taken not to allow products to warm up after precooling. Condensation on cool product surfaces at higher air temperatures also spreads decay.

Harvesting and packaging of most products should be closely coordinated with transportation to minimize time in transit and storage, and maximize product freshness in the hands of consumers. After precooling, the products must be properly loaded and transported at or near the recommended storage temperature and relative humidity to maintain quality.

Transportation

The design and condition of the transport equipment, and the loading method used, are critical to maintaining product quality. The mode of transportation and the carrier should be chosen carefully.

Selection Factors

The mode of transportation and type of equipment used should be based on:

• Destination;
• Value of the product;
• Degree of product perishability;
• Amount of product to be transported;
• Recommended storage temperature and relative humidity;
• Outside temperature conditions at origin and destination points;
• Time in transit to reach destination by air, land, or ocean transport;
• Freight rates negotiated with the carriers; and
• Quality of transportation service.

The reliability and quality of transportation services provided by different carriers must be carefully considered along with the rates charged. Services and schedules are subject to change. Shippers should contact air and ocean port authorities at their origin and destination locations to receive the most current information on available services. Local trade publications also are excellent sources of information, as many carriers and their agents advertise their schedules and destinations.

Refrigerated trailers and containers are recommended for products shipped in large volumes with transit and storage lives of 1 week or more. After transit, there must be enough remaining product life for marketing. Carriers using trailers and containers may offer door-to-door service, which reduces handling, exposure, damage, and theft of the products.

Air cargo containers also can be used to provide door-to-door service. Products transported by air are generally high in value, highly perishable, but shipped in lower volumes. Freight costs are higher by air, but transit time is reduced considerably.

Many products are shipped in unrefrigerated air containers or on air cargo pallets. This requires close coordination at the origin and destination airports to protect the products when flights are delayed. Cold storage facilities are needed at airports to ensure product quality. Refrigerated air containers, insulated blankets, or gel pack refrigerants should be used when possible.

Products that can be shipped in refrigerated trailers and van containers are sometimes shipped by air to take advantage of brief market opportunities, such as the beginning of a season when prices are high and supply is limited. Often an importer who is first to receive a certain product is able to build goodwill and increase sales throughout the season.

Available Equipment

The following transportation equipment is available:

• Air Cargo Containers--For air and highway transport.
• Air Cargo Pallets With Netting--For air and highway transport.
• Highway Trailers--For highway transport only.
• Piggyback Trailers--For rail, highway, and roll-on/roll-off ocean transport.
• Containers--For rail, highway, and lift-on/lift-off ocean transport.
• Breakbulk Reefer Vessels--Handling palletized loads in refrigerated holds.
• Bulk Vessels--Handling dry and liquid products in holds.
• Railroad Boxcars--Handling palletized or individual shipping containers.

Refrigeration and Ventilation Systems

The following systems are available:

• Mechanical--Diesel-generated electric power is used over the road and aboard ocean vessels. Van containers are plugged into electrical power at depots and aboard ships.
  
• Cryogenic--Liquid or gaseous nitrogen or carbon dioxide, which is released into cargoes of frozen food and occasionally chilled food under controlled conditions. Some products, such as leafy green vegetables, are not compatible with carbon dioxide refrigeration.
  
• Dry Ice--Solid blocks of carbon dioxide in special trays or compartments are used in air cargo containers, pallet containers, and within individual shipping containers of frozen product. Shippers must check with airlines prior to using dry ice, as it is considered a hazardous material. If permitted, the containers and accompanying documents must be properly marked to show the amount of dry ice used. Some products, such as leafy green vegetables, are not compatible with dry ice. Direct contact with dry ice will injure fresh products.
  

• Wet Ice--Ice is used within individual shipping containers or on top of a load of containers, either as a supplement or instead of mechanical refrigeration. Many airlines refuse to handle shipping containers with wet ice due to the risk of expensive damage from leaking containers. Airlines that do permit wet ice require that it be placed in sealed polyethylene bags inside a leakproof container with a moisture absorbent pad.
  
  Top-ice is used for certain fresh products to supplement mechanical refrigeration and help maintain a high humidity. Most refrigeration units on trailers and containers cannot control relative humidity and actually remove moisture from products as a natural part of the cooling process.

Top-ice on loads should be applied in rows instead of a solid mass, especially in bottom air delivery equipment. Air circulation should not be blocked. The thermostat on top-iced loads should be set at 2^o C (35^o F) to prevent freezing of the ice into a solid mass which would block air circulation. Table 9 at the end of this section provides a list of products that benefit from top-icing. Products that can be top-iced also can be package-iced, provided the correct packaging materials are used.

• Gel Refrigerant--Frozen containers of chemical eutectic gel are used to maintain temperature within shipping containers. This is the refrigeration system preferred by most airlines.
  
• Ventilation, Ethylene Scrubbing, Humidity Control--Fresh air exchange in the refrigeration system or vents in dry or insulated containers can be used to protect products from a buildup of heat, moisture, carbon dioxide, or ethylene. Leafy green vegetables are sensitive to carbon dioxide, while many products are sensitive to ethylene.

In lieu of ventilation, potassium permanganate pads can be installed in the trailer or container at the refrigeration unit to absorb ethylene. These ethylene scrubbers also can be placed inside individual shipping containers.

Moisture absorbents and kraft paper liners are used to control condensation on products during transportation through different climates. Some refrigeration units are capable of maintaining humidity at optimum levels.

• Multitemperature--A mechanical or cryogenic system provides two or three temperature conditions in separate compartments of a trailer or container which can be used to carry loads of frozen, chilled, and dry products in one vehicle. Advance planning is required when loading multitemperature trailers. The conditions provided by three compartment trailers may include -18^o C (0^o F), 0^o C (32^o F), 10^o C (50^o F), or ambient temperature for products not requiring refrigeration.

The frozen compartment is usually located at the front of the trailer closest to the refrigeration unit. Movable bulkheads are placed between the compartments. Separate evaporators or ventilation between compartments provide temperature control for the nonfrozen products. Side doors are needed to access the forward compartments when the trailers are inspected at ports of entry or used to make multiple deliveries on a single run.

Modified or Controlled Atmosphere--Nitrogen and some carbon dioxide gas is added to pallet bags, or to the cargo compartment of refrigerated van containers, displacing oxygen. This reduces product decay, respiration, and ripening of certain products. Controlled-atmosphere systems monitor and replenish the nitrogen, scrub carbon dioxide, and adjust the level of oxygen according to specific product requirements. This allows products to be shipped at a higher level of maturity. Controlled atmosphere also allows for longer transit times enabling shippers to use less costly land and sea transport instead of air transport for highly perishable products and to allow the transport of riper fruit. Products shown to benefit include apples, asparagus, avocados, bananas, cherries, kiwifruit, mangoes, pears, and strawberries.

Modified or controlled atmospheres of reduced oxygen, and elevated nitrogen and carbon dioxide, are provided to specially equipped containers of certain fruits, vegetables, meat, and seafood after loading is completed. The atmospheres are tailored for each commodity. Three controlled atmosphere systems are available: gas injection, membrane air separation, and pressure swing absorption. Ethylene scrubbers can be added to these systems.

For modified and controlled-atmosphere gas injection systems, containers are equipped with channels at the doorway. A plastic curtain is sealed in the channels to reduce air leakage. Gas ports in the side of the container are used for the injection of the desired atmosphere and discharge of the existing air. In the case of controlled atmosphere, an electronic controller, a scrubber for absorbing excess carbon dioxide, and an air exchange port to allow in more oxygen are provided.

Trailers are generally considered too leaky, and transit times too short to benefit from modified or controlled atmospheres. Instead, shippers may use a gas injection system of modified atmosphere, which is applied either to pallet loads enclosed by a plastic bag or applied to individual modified-atmosphere shipping containers. The pallet application is primarily used for strawberries. The packaging application has been used for tomatoes and broccoli.

The above systems are proprietary and, applied under a service agreement, available at select ports and shipping points around the world. In the case of container loads, the service applicators must check the container for excessive air leakage prior to the application and correct major problems.

The membrane air separation system is either built into the refrigeration unit or clipped on to it. The pressure swing absorption system is installed separately in the container in addition to the refrigeration unit. These systems generate nitrogen, reduce oxygen, and add or remove carbon dioxide as needed. These systems can be programmed by the shipper and are able to overcome some air leakage. Membrane air separation is also available to service individual holds in refrigerated vessels or a number of porthole containers in the hold of a vessel.

Research continues to be conducted on using controlled atmosphere systems to reduce or eliminate insect infestations. Warning labels must be applied to containers and vessel holds with controlled atmospheres to caution employees that the atmosphere will not support human life. The cargo area must be ventilated properly before personnel can enter to unload the cargo.

Equipment Features

Long-distance transportation through tropical and frigid climates requires rugged, well-designed equipment to withstand the transit environment and protect the products. Desirable features in refrigerated trailers and containers include:

• Adequate refrigeration capacity to hold frozen food at extreme ambient temperatures;
• Adequate air circulation for uniform product temperature and high relative humidity throughout the load;
• A solid return air bulkhead at the front of the trailer to ensure air circulation throughout the load;
• Vertical ribs on side walls and the rear door to assist in air circulation;
• Adequate insulation and provisions for heating in areas with extreme cold weather;
• Deep floor grooves or channels to provide an adequate cross-sectional area for air circulation under loads placed directly on the floor;
• Supply-air temperature sensing of the operation of the refrigeration unit to reduce product chilling and freezing injury;
• Provisions for ventilation to prevent ethylene or carbon dioxide buildup;
• Provisions for application of controlled or modified atmospheres; and
• Adequate suspension to reduce the amount of shock and vibration transferred to the shipping containers and the products inside.

The capacities and dimensions of air cargo containers, air cargo pallets, refrigerated trailers, and refrigerated van containers vary among carriers due to differences in equipment design and manufacture. Sample specifications are provided at the end of this section.

Carriers should be consulted for specifications, availability, and rates well in advance of shipping. Many carriers provide valuable assistance and information on loading and operating their equipment.

Room for air circulation must be provided in transport equipment loaded with agricultural products. The nature of the product, packaging type, and loading method affect air circulation, as well as the total weight and volume occupied by the load.

Maximum cargo weights are limited by carriers to comply with restrictions on particular transport and handling equipment, or limits enforced by Government agencies to protect roads and bridges. Due to light product density or load limits, many loads do not use the maximum rated-weight capacity of the transport equipment.

Most carriers check their transport equipment before delivery to the shipper for loading. Good equipment condition is critical to maintaining product quality. The shipper also should check the equipment to ensure it is in good working order and meets the needs of the product. Carriers provide guidance on checking and operating the refrigeration systems.

All transportation equipment should be checked for:

• Cleanliness--The load compartment should be regularly steam cleaned.
• Damage--Walls, floors, doors, and ceilings should be in good condition.
• Temperature Control--Refrigerated units should have been calibrated recently and capable of supplying continuous air circulation for uniform product temperatures.

Shippers should insist on clean equipment. A load of products can be ruined by:

• Odors from previous shipments;
• Toxic chemical residues;
• Insects nesting in the equipment;
• Decaying remains of agricultural products; and
• Debris blocking drain openings or air circulation along the floor.

Shippers should insist on well-maintained equipment and check for the following:

• Damage to walls, ceilings, or floors, which can let in the outside heat, cold, moisture, dirt, and insects;
• Operation and condition of doors, ventilation openings, and seals; and
• Provisions for load locking and bracing.

For refrigerated trailers and containers, the following additional checks are important:

• With the doors closed, the cargo area should be checked from inside for light--door gaskets must seal. A smoke generator also can be used to detect leaks.
• The refrigeration unit should cycle from high to low speed when the desired temperature is reached and then back to high speed.
• The location of the sensing element that controls the discharge air temperature must be located. If it measures return air temperature, the thermostat will have to be set higher to avoid a chilling injury or freezing injury to the products.
• A solid return air bulkhead should be installed at the front of the trailer.
• A heating device should be available for transportation in areas with extreme cold weather.
• Equipment with a top air delivery system must have a fabric air chute or metal ceiling plenum in good condition.

Products requiring refrigeration should be thoroughly precooled prior to loading into transportation equipment. Product temperatures should be taken with an electronic probe thermometer and recorded on the bill of lading for future reference.

The load compartment in the equipment also should be precooled to the recommended transport or storage temperature for the product. Ideally, the loading area should be enclosed and refrigerated, with dock seals at the trailer or container doors.

Proper loading practices are critical to maintaining temperature and relative humidity, protecting the products from impact and vibration forces in transit, and preventing insects from entering the load. Special care must be taken when shipping mixed loads--the products must be compatible.

Loading Methods

Basic loading methods include:

• Bulkloading, by machine or hand, of unpackaged commodities;
• Hand loading individual boxes with or without pallets; and
• Unit loading of palletized or slipsheet loads of boxes with pallet jacks or forklifts.

Mixed Loads

Many products are often transported in mixed loads or stored with other products. They must be compatible in terms of:

• Recommended temperature and relative humidity;
• Production and sensitivity to ethylene; and
• Production and absorption of odors.

Groups of fresh products suitable for transportation and storage together have been identified and are listed in table 13 at the end of this section. Products sensitive to chilling, freezing, moisture loss, ethylene, and odors are listed in tables 7-12.

Many products are subject to chilling injury when transported or stored at lower than recommended temperatures (table 7, table 8). This damage often becomes apparent after the products warm up. Products injured may show pitting, discoloration, water-soaked areas, decay, and failure to ripen.

Many products are recommended to be transported or stored at temperatures only 1^o C to 3^o C (2^o F to 6^o F) above their freezing points. Thermostats on some trailers and van containers are set 1^o C to 3^o C (2^o F to 6^o F) higher than the recommended temperature of 0^o C (32^o F) for chilled products. Most tropical products that freeze are first damaged by chilling injury.

Most products need to be transported and stored at a high relative humidity. Some products are more susceptible to moisture loss than others (table 9, table 10). Moisture loss results in wilting and shriveling. To reduce moisture loss, products must be adequately precooled before transit. Some products also are waxed, film-wrapped, package-iced, or top-iced. Relative humidity during transit and storage must be maintained as much as possible.

Never transport or store fruits and vegetables that produce a lot of ethylene with products that are sensitive to it (table 11). Ethylene can cause premature ripening of some products and will ruin others, such as plants and cut flowers. Cucumbers and celery turn yellow, while lettuce will turn brown, in the presence of ethylene. Potassium permanganate pads can be used to absorb ethylene during transit and storage.

Never transport or store odorous products with products that will absorb the odors (table 12). Never load fruit, vegetables, or other food products with nonfood products that provide any risk of contamination through transfer of toxic chemical residues.

Similar-sized shipping containers should be loaded together in mixed loads for increased stability. Heavier shipping containers of products should be loaded first and distributed evenly across the floor of the trailer or container. Lighter shipping containers can then be placed against or on top of the heavier products.

Load lock bars, load gates, and pallets placed in a vertical position can be used to separate and secure stacks of different-sized shipping containers. To facilitate inspection of mixed loads at ports of entry, a representative sample of each commodity should be available near the door. This can minimize the unloading of cargo for examination.

The longer the transit time, the higher the risks in transporting mixed loads of agricultural products. Therefore, it is essential that guidelines be followed closely to maintain quality in distant markets.

Providing for Air Circulation

Inadequate provisions for air circulation will ruin a load, even in well-designed transportation equipment. When possible, boxes should be kept off shallow floors and away from flat sidewalls by using pallets, racks, and dunnage. Room for air circulation must be provided under, around, and through the load to protect the products from:

• Heat gain from the outside air during hot weather;
• Heat generated by the produce through respiration;
• Ethylene produced by certain products;
• Heat loss to the outside air during extreme cold weather; and
• Chilling injury or freezing injury during operation of the refrigeration unit.

Temperature Monitoring and Recording

Shippers should follow the carrier's recommendations on loading and setting the temperature of the equipment's load compartment to avoid chilling or freezing injury to fresh products. Discharge air may be colder than the set-point temperature if the refrigeration system operates on return-air temperature sensing. The temperature should be clearly marked on the bill of lading. Drivers and shipper should check product temperatures with a pulp thermometer and record the temperatures during the loading process.

Many carriers advise setting the thermostat temperature 1^o C to 3^o C (2^o F to 6^o F) higher than the recommended temperature of 0^o C (32^o F) for chilled products. This depends on the design of the transportation equipment. Newer equipment with supply-air temperature sensing and good air circulation can be operated closer to the recommended temperature.

For most tropical fruits and vegetables and plants that have recommended temperatures in the 10^o C to 21^o C (50^o F to 70^o F) range, the thermostat is set at or near the recommended temperature.

It is now possible to monitor refrigeration unit operating conditions from a central control room on a ship or by satellite transmission.

Refrigeration units for trailers and containers may have an electronic recorder which can monitor up to three different points in the load. This data can be downloaded and analyzed on a computer. Mechanical temperature recorders which place data on a circular chart also are used.

In addition to trip insurance, all loads should have a small portable air-temperature recorder (supplied by the shipper) placed between packages in the area where the warmest temperatures occur. Recorder companies recommend placement on top of the load, near a sidewall, one-third of the way in from the rear doors, and away from any direct discharge of refrigerated air.

Railcars should have two or three portable recorders. In loads with top-ice or humidity above 95 percent, the recorders should be waterproof or enclosed in a plastic bag. Models are available for frozen food applications.

Shippers and receivers must follow the recorder company's instructions on documenting the load, starting the recorder, reading the results, and returning it for calibration and certification. The temperature recorder chart and/or instrument number should be clearly marked on the bill of lading. These steps are essential for settling claims over temperature management during transportation.

In the European Union temperature recording is mandatory. It also is necessary in the case of cold treatment quarantine shipments in which fruit is held at specific temperatures to kill fruit flies.

Cold Treatment and Pest Control

Shippers should avoid loading at night. Insects attracted by light can enter the load and cause problems upon inspection at destination. The loading area should be enclosed to prevent insects from reinfesting treated and packaged products. The Animal and Plant Health Inspection Service (APHIS) operates a preclearance program in which product is inspected and certified to be free of injurious insect pests and plant diseases. Such shipments are marked and sealed to prevent tampering and infestation.

Fumigations for pest control inside loaded transportation equipment are usually done under APHIS supervision in accordance with the necessary treatment schedule for a particular product and insect. Cold treatment of certain products during transportation also is used to kill insects. This involves strict temperature control throughout the load for up to 2 weeks. APHIS maintains a list of refrigerated vessels and refrigerated containers certified as capable of maintaining intransit cold treatment temperatures.

Bracing the Load

Loads should be secured with some of the following materials to prevent vibration and impact damage in transit:

• Aluminum or wood load locks;
• Fiberboard honeycomb fillers;
• Wood blocking and nailing strips;
• Inflatable kraft paper air bags;
• Cargo nets and straps; and
• Wood load gates constructed of 25 by 102 mm (1 by 4 in) material.

Loading Air Cargo Equipment

Air cargo containers are loaded by hand or with forklifts when using fiberboard LD-3 container inserts. Polystyrene foam triangular inserts, wood blocking, and fiberboard dunnage are recommended to brace boxes and provide a level platform on the sloped surface of LD-3 containers. Refrigerated air cargo containers should be used when available.

Air cargo pallets are loaded by hand or with forklifts. The loads should be secured with straps, tape, or cross-stacking of the boxes. A weatherproof or insulated cover can be placed over the load along with the required cargo netting, provided the pallet load is protected from sunlight.

Loading Trailers and Containers

For refrigerated trailers and containers the following loading practices are recommended:

• Precool the trailer or container to the recommended transport or storage temperature. Turn off the refrigeration unit during loading if the loading area is not refrigerated; otherwise, the evaporator will frost due to the warm air drawn in by the unit.
• Thoroughly precool unit loads, as air circulation to some of the shipping containers may be limited. The containers should have openings for cooling and ventilation of product heat.
• Avoid loading tightly against flat sidewalls. Use centerline loading for unit loads.
• Secure unitized loads with dunnage between the walls and load.
• Do not block air circulation at the rear door.
• Secure the rear of both hand-stacked and unitized loads with straps, load gates, or load lock bars to prevent the load from shifting against the rear doors. Figures 1-5 at the end of this section illustrate unit loading and hand loading patterns.

Trailers and Containers With Top Air Delivery

For refrigerated trailers and containers with air delivered to the top of the load by chutes or ceiling ducts, these additional practices are necessary to maintain product quality:

• Hand-stacked loads should be evenly spaced, with lengthwise air flow channels created on every other layer, to ventilate product heat (respiration).
• Header stacks must be provided at the front bulkhead of the trailer or container in hand-stacked loads to connect all the lengthwise channels and allow the air to return to the evaporator.
• Loads of frozen food or nonrespiring products can be loaded solidly without air channels in the middle of the load.
• Fiberboard boxes must be strong enough to counter the reduced stacking strength due to the boxes being offset to provide airflow channels.
• Pallets should be used with hand-stacked loads to provide adequate air circulation in equipment with flat or shallow grooved floors.
• The load should not block the ceiling air chute or plenum.

Trailers and Containers With Bottom Air Delivery

For refrigerated trailers and containers with air delivered to the bottom of the load through the floor channels, these additional practices are necessary to maintain product quality:

• Hand-stacked and unit loads of boxes should have bottom-to-top ventilation slots that align in the stacks. Otherwise, small vertical air flow channels are needed between boxes as a result of the slight bulge in the box sides.
• At least 13 mm (5 in) of space should be provided at the ceiling for return-air circulation.
• The load should cover most of the floor surface to force more air through the load. Pieces of fiberboard can be used to cover any remaining floor space next to unit loads or stacks of boxes. Only the floor area next to the doorway should be open to permit return air flow.

Intransit Procedures

During transportation of refrigerated loads in trailers and containers, the carrier should check the operation of the refrigeration unit and temperature of the load compartment regularly.

Receiving Procedures

Before completely unloading a shipment for storage, receivers usually check the load to determine if it meets specifications for quality, grade, and packaging. The receiver also will note whether the load was adequately braced and the correct temperature was maintained.

Product temperatures in sample shipping containers throughout the load should be taken and recorded, using an electronic probe thermometer. The air-temperature recorder should be read, if one was placed in the load. Shippers and carriers should be notified of any problems with the product, packaging, loading method, or transportation equipment, so corrective action can be taken.

If there is a problem with the load, the receiver, carrier, or shipper can request an inspection by a licensed inspector. Unresolved disputes over product quality or payment can be referred to arbitration or other legal avenues.

Unloaded products need to be protected from direct sun, condensation, ethylene, and contamination. Products needing refrigeration or protection from hot or cold temperatures should be placed in the recommended storage conditions as soon as possible. Otherwise, the efforts of growers, shippers, and carriers to maintain product quality will have been in vain.

Air Circulation and Sanitation in Storage

Uniform air circulation in the storage room at the proper temperature and relative humidity is important to remove product heat that occurs from respiration, and from outside heat that enters through door openings and building surfaces. Doors to refrigerated storage areas should be protected with plastic strip curtains to reduce heat gain during operations. Warm air will quickly reduce relative humidity in the cold storage area.

To maintain temperature and relative humidity, the storage room refrigeration system should have a large evaporator surface area, an adequate number of fans, and a humidifier. Temperature control should be by an electronic thermostat. The system must be carefully balanced to avoid free moisture or excessive air flow.

Electric forklifts and pallet jacks should be used with horticultural products to reduce ethylene. Periodic sanitation of the storage room walls, ceilings, floor, and refrigeration units is necessary to reduce decay organisms and odors. Carbon filters can be used to absorb odors and volatile gases, while potassium permanganate pads and air exchange can reduce ethylene.

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