COLD CHAIN PERFORMANCE ISSUES

The cold chain remains one of the most important ways to preserve perishables and deliver them to market in good condition. The various requirements for success in the cold chain are identified, and examples of what can go wrong are given. Environmental issues are considered. Solutions for the future relating to produce, equipment, stowage and handling, and logistics are identified. The main requirements are for education and training and for provision of good operating systems.

1. INTRODUCTION

The cold chain is generally considered as the transport and storage chain between the initial production and the final consumer of temperature-controlled perishable goods. This is not a complete definition, as there are other items carried under temperature control, for example works of art. For the purposes of this paper, only the transport and storage of perishable foodstuffs and pharmaceuticals will be considered. There are many types of cold chain. They may be categorised by carriage mode, by commodity, or by location and duration of transport.

Air freight is generally limited to goods at chilled or ambient temperatures. Sea containers are universal in their application. Refrigerated ships carry bulk commodities in their holds, though increasingly these ships also carry containers. International land transport by road and/or rail is able to carry anything. Local distribution generally excludes the movement of bulk frozen goods.

At the present time there are approximately 1300 specialised refrigerated cargo ships, 80 000 refrigerated railcars, 650 000 refrigerated containers and 1.2 million refrigerated trucks in use worldwide. They carry vast amounts of foodstuffs and other perishables and mostly do so very successfully.

Storage is an important part of the overall cold chain. It has been reported by one company that, in the transport of pharmaceuticals during the last five years, critical or major problems arising in storage are more than four times as frequent as problems arising in transport. This emphasizes the point that cold chains should always be considered as a whole, if reliable results are to be obtained.

2. COLD CHAIN REQUIREMENTS FOR SUCCESS

There are many requirements which have to be met for successful handling of perishables. Some of these are as follows:

2.1 Produce temperatures
All refrigerated produce deteriorates with time at a rate dependent on the temperature of storage. For most frozen goods at normal storage temperatures of -18°C or below, maximum times for high-quality storage are appreciably longer than transport times, so there are no special difficulties. Frozen fish requires lower temperatures to achieve this situation.
Refrigerated chilled and fresh produce, stored and transported at temperatures between -1.5°C and 14°C dependent on produce type, frequently has a high quality storage life measured in weeks rather than months, and the relatively slow pace of marine transport frequently requires much closer control of temperature than is achieved in road, rail or air transport if goods are to be carried successfully. Data on storage lives and temperatures show that produce such as oranges and apples are readily transportable by sea, whereas a very short life produce such as watercress could not survive more than a very short sea journey. Many subtropical fruits have lives comparable to the length of time needed to transport them by sea from growing areas to major markets, so special attention to quality of both produce and handling is essential for successful marketing.
Many types of chilled and fresh produce have lower temperature limits below which they are damaged. Clearly, for items carried near their freezing points, lower temperatures will give freezing damage, and this is as true for chilled meats as for other more obvious products. For many fruits, there are temperatures well above their freezing points at which low temperature physiological injuries occur.
There is a temptation if cargoes are loaded at a high temperature to reduce cargo space temperatures to below carriage temperature to achieve more rapid cooling. This can only be done at the risk of damaging part of the cargo.

It must be remembered that, for many cargoes, safe carriage temperatures depend on the variety as well as the type of produce, and may also depend on growing area and on whether early, main, or late harvested produce is being carried. Also, it may be necessary to mix produce with slightly different temperature requirements, in which case the highest temperature must be chosen, with consequent effects on the storage life of the products requiring lower temperatures.

It is essential that the storage characteristics and optimal storage temperature are correctly specified and maintained. Specification is the responsibility of the shipper; temperature maintenance is the responsibility of the carrier. If the shipper specifies the wrong temperature in an obvious way, the carrier will question it, but in less obvious cases the shipper has no reason to over-ride the instructions given. The carriage instructions must be clear and unambiguous.

2.2 Transport equipment
It is essential to use equipment which can do the job. The correct equipment will have good insulation, good temperature control, adequate airflow and refrigeration capacity, and will be well maintained. It will be able to achieve all that is requested in the carriage instructions. In the case of refrigerated containers, it should have been subject to a pre-trip inspection not longer than a month before loading.
The operation of transport refrigeration in ships’ holds, in containers, and in trucks differs, and the shipper would be well advised to be familiar with the equipment to be used. As a rule of thumb, ships should be most reliable because of the size of cargoes they carry, and road vehicles may be most liable to failure as they carry out many journeys often on bad road surfaces.

2.3 Produce quality
At best, refrigerated transport can maintain quality, it cannot improve it. Poor quality produce cannot justify the costs of transportation, and can give the likelihood of an unsuccessful outturn, leading to loss of confidence in the producer. Long-distance transportation should be seen as a means of obtaining premium prices for top quality produce, and never as a means of disposing of second-rate goods for which no local market exists.

The quality of produce may be subject to standards and inspections by producer boards or export control boards in the country of origin. It may also be subject to import requirements in the receiving country, details of which must be determined by potential exporters.

2.4 Pre-shipment handling
For fruits and vegetables, this can include application of appropriate post-harvest treatments (e.g. fungicides), short pre-shipment storage times, and avoidance of mechanical damage such as bruising from either sorting or grading machinery or from rough handling of cartons.
For frozen goods, the principal requirement is maintenance of temperature throughout all links in the transport chain, without excessive periods lacking refrigeration.

2.5 Packaging
Packaging has to protect produce, so must have adequate strength, if necessary when damp, for the anticipated use. It may also need to incorporate wraps or liners to minimise loss of moisture. However, it must also in the case of respiring produce provide sufficient ventilation to allow heat and respiration products to be removed. Packaging materials must not contain substances likely to produce taints or odours. Cartons must be of a suitable size and shape to protect the product from movement and also from external pressure. For palletised transport, cartons must be adequately secured to pallets without jeopardizing any air movement designed into the cartons.

Successful packaging systems have been developed over the years, and these are frequently more sophisticated in their requirements than is immediately obvious. Changes to such systems should be considered with care. 2.6 Pre-cooling
For some fruits, such as citrus and banana in marine transport, it is normal practice to load them warm, and cool them during transportation. These are exceptions; generally it is essential to cool fruits to their transport temperature as soon as possible after harvest. For this purpose air-cooling tunnels, hydro-coolers, or vacuum coolers may be used, all of which can extract heat far more rapidly than is possible during transportation.

In all transport equipment, cooling rates will be relatively slow, and very dependent on position within the stow. Whilst cooling tunnels may achieve 7/8 cooling times of only a few hours, in transport it is more usual to achieve times of 2 to 6 days.

If the cooling demands on the transport equipment are increased excessively due to inadequate pre-cooling, there will be uneven cooling through the cargo, and some areas will be delivered with reduced residual storage life. This loss is due to the loading condition and no amount of care in transport can overcome it. For produce with a high metabolic rate, if it is not yet adequately pre-cooled, the respiratory heat can be so high that correct storage temperatures in transport are unachievable.

2.7 Air circulation
For frozen goods, air circulation is necessary to remove heat entering from the outside of the storage space through insulated walls. For respiring fresh produce, it is also necessary to remove respiratory heat. For some produce, cooling is also necessary.

Transport equipment is designed to provide adequate air circulation in properly packaged and properly stowed cargoes. Wrong packaging or careless stowage can negate this, and can give overall or localised inadequacies of air movement.

Air circulation is also important in retail display – open multi-deck cabinets depend on proper air circulation to provide reasonably uniform product temperatures. 2.8 Temperature control
Temperature control for frozen goods is less critical than that for chilled goods. The former require a maximum temperature (e.g. -18°C) not to be exceeded, whereas chilled goods on long journeys often require temperatures maintained within a band of 2°C or less. In retail display, chilled goods are normally expected to be kept within a 5°C band or less.
The controllability of transport equipment designed for use with chilled cargoes is of necessity good. However, if equipment primarily designed for frozen goods is used for chilled transport, it may be possible to obtain wider temperature swings or lower temperatures than are desirable. Wide swings mean an appreciable time at higher-than-planned temperatures, with consequent loss of storage life. Low temperatures may mean freezing or damage from chilling injury.

2.9 Air freshening
Air freshening during transport is necessary for cargoes of respiring produce to take away carbon dioxide and ethylene before they reach excessive levels. Air freshening, or fresh air ventilation, requirements are usually clearly specified by shippers, and these requirements should be adhered to.

2.10 Cross-contamination from other cargo
Cross-contamination can be of two types. The most obvious is the transfer of taint or odours from one cargo to another. The other is the transfer of ethylene from produce with high ethylene production rates to ethylene sensitive goods, leading to premature ripening. Carriers usually have clear instructions to prevent inappropriate mixing of cargoes, but care may be necessary to ensure that these instructions are also available to transhipment terminals inland or at ports.

2.11 Insect infestation
Where problems of insect infestation are likely, procedures will have been established by importing countries which may require fumigation or cold treatment of particular produce from some growing areas. It is essential that would-be exporters are aware of requirements of importing countries in this respect.

2.12 Journey time
Marine transport is generally very reliable, but by the nature of things, breakdowns and storms can sometimes create delays. So also can industrial action, often beyond the control of the carrier. Where such things lead to appreciable delays, damage to more perishable produce can occur.

2.13 Time without refrigeration
Goods can be without refrigeration for short periods during land journeys to and from ships, and can occasionally be without refrigeration due to mechanical breakdown of equipment. Breaks in refrigeration of a few hours may be acceptable for many, but not all, produce types. Breaks of a few days are likely to pose problems for all types of produce.

2.14 Retail sale
Following transportation, the cold chain continues through the wholesale handler and the retail store to the customer. In the store, proper equipment sensibly sited and properly used is necessary. Losses at this stage waste all the effort and energy which has been expended earlier in the chain.