Transportation Practices
There are a number of contributing factors as to why fresh produce can degrade in quality while in transit. Being aware of the following factors can aid you in avoiding food loss;
Initial quality
Produce leaving the packing facility must be suited to the handling it will receive as it is transported to market. Locally produced fruits can be fairly mature and ripe because the time to market is short. Produce shipped from great distances is often a little less mature than locally produced fresh produce and must be free of mechanical damage and other conditions predisposing it to a noticeable quality loss in a long postharvest handling period.
Temperature
Produce temperature is the most important factor affecting the quality of horticultural produce. Fresh fruits and vegetables remain alive by respiration, a process where carbohydrate in the produce and oxygen is used and carbon dioxide, water and heat are produced. High respiration rates rapidly deplete stored carbohydrates, shortening produce life. Temperature is the primary factor controlling respiration rate. For fruits and vegetables, respiration increases by a factor of two to five for each 10ºC increase in temperature above its recommended holding temperatures. For example, berries like strawberries, raspberries and blackberries, have a shelf life of 7 days at 0ºC but only 1 day at 20ºC. Longer-lived produce such as green beans, mushrooms, green onions, and pod peas last only 2 to 3 days at 20ºC. Above 30ºC, increases in respiration rate slow down, and at higher temperatures, the produce will die or lose quality very quickly. Many handbooks contain lists of optimal environmental conditions for long-term storage and transport conditions for fruits and vegetables.
At any point in the cold chain, produce should always be held at its lowest recommended storage temperature even if future conditions are not known. A cardinal rule in handling perishable horticulture produce is to keep it as cool as possible for as long as possible, even if it is warmed later in handling.
Temperatures below-recommended levels cause freeze damage or chilling injury to produce with recommended storage temperatures above 0ºC. Chilling injury usually occurs in fruits and vegetables native to tropical or subtropical regions. Signs of chilling injury are:
Tissue darkening
Drying
Surface pitting
Failure to ripen normally
Off flavours
Increased susceptibility to decay
Repeated low-temperature exposures have a cumulative effect on chilling injury.
Maintaining proper temperature can be difficult in some handling systems. For example in air transport, the cargo area is not refrigerated and there can be considerable waiting time outside in ambient conditions varying from hot to freezing, with direct sunlight or precipitation. Once arrived at the destination, internationally shipped produce may be held at ambient for many hours before being cleared by local authorities. Policies and processes must be designed to minimise the time in unprotected environments.
Humidity and water loss
A 90 to 95% relative humidity (RH) environment is needed for the maximal shelf life of most fruits and vegetables. A few fresh produce products such as bulb onions, garlic, winter squashes, and ginger should be kept below 70% RH. Low RH around the produce causes wilting or shrivelling. This is aesthetically displeasing and generally reduces marketability. Loss of produce weight caused by water loss is a direct marketing loss. Water loss also weakens the plant cells, making them more susceptible to decay. In addition, fungal growth causes increased ethylene production, causing chlorophyll loss and yellowing.
Refrigerated highway trailers do not have RH control capability. Produce susceptible to wilting should be waxed or packaged in liners, bags or plastic boxes to slow moisture loss. Special packaging capable of slowing moisture loss is particularly important in air freight. Aeroplanes usually have very low RH, often around 10%.
Some marine containers are equipped with spray humidification systems. These increase RH but air temperatures have to be slightly above 0ºC to prevent ice from clogging the spray nozzle. Packaging material for fresh produce requiring high relative humidity must be selected carefully. Fibreboard and wooden boxes absorb moisture from the fresh produce and can cause more than 1% weight loss from the produce. The absorbed moisture also weakens the box. For example, fibreboard held in a moisture equilibrium of 90% RH retains only 40% of its original stacking strength after a relatively short exposure period. Under high humidity conditions, waxed fiberboard, plastic and wooden boxes would perform better, as they can better withstand high humidity conditions.
Marine containers are much more airtight than highway trailers and they are equipped with an adjustable fresh air exchange vent to control the amount of air entering the container. Excessive air exchange in humid areas results in unnecessary use of refrigeration capacity to condense excess moisture from the air.
Atmospheric composition
As fruits and vegetables respire, CO2 and ethylene levels may increase, and oxygen levels decrease around the produce. CO2 levels should be kept below 0.3 percent in a tightly sealed container. This is much lower than the damage threshold for many commodities. Highway trailers usually have enough air leakage to prevent damage from an accidental build-up of CO2.
Marine containers are sometimes equipped with controlled atmosphere devices. These measure CO2 and O2 concentrations and control them within a specific range depending on the commodity. Controlled atmosphere systems typically increase the storage life of produce by 30%. They add significantly to the cost of using the container and find only limited use in commerce.
Mixed loads
Fresh fruits and vegetables have varying requirements for temperature and humidity and have varying sensitivity to absorbing odours or ethylene-induced damage from other produce. Many trucks have several types of fresh produce in a single trailer. If some fresh produce is mixed with a commodity with differing storage requirements, the quality of the load can be compromised, especially for longer trips. The problem is even more severe in marine containers travelling several weeks to their destination.
If logistically possible, produce should be loaded as mixed loads only if their temperature and humidity requirements and ethylene sensitivity and odour absorption capability are compatible. Incompatibility between ethylene producing produce and ethylene sensitive produce can be dealt with in several ways. Some fruits and vegetables can be protected from ethylene damage by using 1-MCP products. Damage in refrigerated containers or semi-trailer loads may also be reduced by using a fresh air exchange rate of 0.2 L s-1 or by using ethylene scrubbers. Controlled atmospheres (CA) can allow ethylene producers and ethylene sensitive commodities to be stowed together, but the acceptable fresh produce combinations have not been well researched. Holding produce at its lowest possible temperature reduces ethylene production and ethylene sensitivity.
There are fewer options for dealing with temperature incompatibility. The general rule is to plan the load with produce that has the lowest possible range of recommended temperatures, remembering that long transport times accentuate damage caused by incompatibility. Shipments from local distribution centres to markets or food service institutions are always mixed loads and often have fresh produce with widely varying storage requirements and even have other refrigerated food such as milk, juice and meat. However, transit times are often less than 8 hours.
Transport methods developed for transporting mixed loads:
Multi-compartment vehicles
Mini-containers and insulating covers.
Multi-compartment trucks or semi-trailers have removable partition panels to divide the vehicle into smaller compartments. Mini-containers are usually the size of a standard pallet load. Some are insulated compartments and others are also refrigerated. The mini-containers are highly flexible, easy to handle and can be used in different sizes of vehicles. Insulated-only models can only maintain temperatures for a limited time. Refrigerated models require additional equipment and consume fuel. Their key disadvantages are high cost and non-standardised sizes that do not necessarily match produce container sizes or transport vehicle dimensions therefore resulting in poor space utilisation efficiency. Insulating covers cause a stagnant air layer around the fresh produce or within the cover reducing air infiltration and conductive heat transfer. Their main disadvantages are cost and the fact that they have a life of only 6-12 months.
Some fresh produce has a short postharvest life and is not suited for container shipment. This is particularly true if they are held at non-optimal temperatures. Modified atmosphere (MA) packaging or CA can sometimes increase shelf life and allow fresh produce to be shipped to destinations that require several weeks of transportation time. If a MA environment is used to hold different fresh produce products all together within a single space, it should, as a minimum requirement, not reduce the postharvest life of any of the mixed commodities.
Dried vegetables should not be mixed with other fresh produce when transit times are of a week or more. These vegetables should be held in a 50% to 70% RH environment to prevent decay. Most vegetables in the lowest temperature range (0ºC to 2ºC) are sensitive to moisture loss and should be held at higher than 90% RH or packaged to minimise water loss. The other vegetables and fruits should be held at 85 to 95% RH.
Physical injury
Vibration, compression, and impact cause physical damage to produce. The damage can be minimised by using proper packaging, good package management, and correct placement in the refrigerated vehicle.
Compression damage occurs when the weight of the load is supported by the produce, rather than the produce container. Compression occurs generally when the boxes are overfilled, are not properly palletized, are not strong enough or lose strength through moisture absorption or mechanical damage.
Compression damage can be avoided by not over-filling boxes, properly aligning all boxes and ensuring that the edge of pallet load aligns with the pallet edge. The corners of corrugated fibreboard boxes provide most of their strength and they should not extend over the edge of the pallet. A 25 mm overhang reduces strength by 14% to 34%. Corrugated fibreboard loses strength over time when it is supporting a load. For example, after supporting weight for 10 days, a fibreboard box has only 65% of its original laboratory determined strength. Fibreboard also absorbs moisture and weakens when it is exposed to high RH, generated by the fresh produce within the box.
Reusable and recyclable plastic containers are structurally stronger than corrugated fibreboard boxes and provide more protection against compression damage. If fibreboard boxes are used, they should be designed to be strong enough to withstand the length of the journey under high RH conditions.
Pallet loads should be unitized and secured so that they do not shift during handling or transportation. Stacking tabs or palletizing glue assist in preventing boxes from sliding past each other. The pallet and load can also be unitised, tied together with net wrapping or corner braces and/or banding. Boxes should extend to the edge of the pallet. Free space at the periphery will allow the load to shift in transport. Most reusable plastic containers are designed with an interlocking system and when they are properly stacked they do not shift in transport.
Vibration damage is due to the constant vibrating motion of a vehicle when it is transported over the road. Vibration damage is greatest in locations over steel-spring-suspended axles. Air ride suspension dramatically reduces vibration damage. The frame of a semi-trailer is supported by the tractor and most long haul tractors in North America have air-ride suspension. Most refrigerated semi-trailers manufactured within the last 5 years are equipped with air-ride suspensions. Before loading a trailer check the suspension system of the rear axles. If it has steel springs (leaf springs), do not load vibration-sensitive produce, like berries and Bartlett pears at least in the last two pallet positions.
