The need for low temperature
Container Design
Loading and Unloading
Recommendations
The storage life of frozen fish depends upon the temperature of storage. For example, white fish frozen within a short time of catching will keep in first class condition for eight months when stored at minus 20°F, but for only one month at plus 15°F.
Any increase in temperature, even for a very short time, has a bad effect on the quality of the product. In a Canadian experiment, fish being stored at 0°F were allowed to rise to plus 15°F for three days and then cooled down again to 0°F. As a result, six weeks of useful storage life were lost.
One of the most difficult problems in the distribution of frozen fish is to move it from one store to another, or from cold store to display cabinet at the retailer's shop, without too great a rise in temperature during the journey.
The warming up of frozen fish during transit should not be confused with thawing out or defrosting. The thawing process takes place very slowly and can easily be seen happening, as the product becomes moist and soft. The warming up of frozen fish from, say, minus 20°F to plus 20°F can take place comparatively quickly with no obvious sign of change. The 'warm' package remains hard, and looks and feels exactly as it did at the lower temperature, but nevertheless the fish is spoiling much more rapidly.
The only way of determining this change is by measuring the temperature of the fish.
In Britain, the longest road journey for frozen fish rarely exceeds thirty hours including waiting time at receiving depots, and more usually is completed within twenty four hours.
Containers should be designed to deliver the last of the load at a temperature not higher than 0°F and preferably at minus 5°F or below.
For such journey times and temperatures, five inches of a suitable insulation is usually sufficient in this country.
The insulation should be of low thermal conductivity, light, strong and resistant to moisture penetration. It should be protected on the outside by a waterproof and heat reflective sheathing. Aluminium is a suitable material. The inner lining must be easily cleaned, waterproof and sufficiently robust to bear the designed loads. Again, aluminium is a practical proposition. Alternatively, if most of the load is being carried on bearers, some of the plastic sheeting materials may answer the purpose. Allowance should be made for the use of fork lift trucks when designing floors for large containers.
A typical vehicle for long distance transport of six tons of frozen fish may have internal dimensions of approximately sixteen feet by six feet by six feet. More than 2,000 British thermal units of heat per hour may pass through the insulation of such a vehicle with an outside temperature of 60°F and an inside temperature of 0°F. This may be more easily understood if one considers the rise in temperature produced by such a rate of heat flow. Assuming for the moment that the temperature of the cargo will rise evenly throughout, such a heat leak would produce a rise in temperature in the fish of ten Fahrenheit degrees, during a twenty four hour trip.
If however the load is packed tightly against the sides of the container, the edges and corners of the load will warm up first, and as the centre of the cargo will be protected by the outer layer, most of the heat that has penetrated through the insulation will affect a layer about one foot thick around the outside of the load.
The temperature of this part of the cargo, which amounts to about 60% of the total load in this case, may rise by as much as 15 Fahrenheit degrees in the same time.
Blower Cooled Container - Sectional diagram
If the fish began its journey at minus 20°F, it would be possible therefore to deliver it at or below 0°F. It is however almost certain that some of the fish will warm up during loading, and in order to keep the temperature rise as small as possible, it is advisable to take other precautions.
Some form of cooling arrangement may be fitted to absorb the heat as it comes in through the insulation. This may be mechanical refrigeration, either with evaporator plates or a cold air blower, or carbon dioxide in liquid or solid form.
Plate Cooled Container - Sectional diagram
In order to take full advantage of whatever cooling medium is provided, an air space at least one and preferably two inches wide, should be left between cargo and floor, walls and roof. This space should allow cold air to circulate around the whole of the cargo and return to the source of cold. The heat coming in through the insulation will then be carried to the cooler and absorbed before it reaches the contents of the container.
If evaporator plates are fitted, they should be placed along the walls of the container, as high up as possible, so that the cold air will fall and pass beneath the floor. Battens and bearers may be so arranged that the air will be free to return to the space above the cargo by way of the ends of the container, thus ensuring continuous natural circulation.
When a blower unit is fitted, the air should be guided by simple ducting, and any battens fitted to support the cargo away from the walls should be so arranged that the air can flow continuously around the cargo and back to the unit.
Dry ice holders should be fitted above the contents, so that the cold air can fall down between walls and cargo to the space beneath the floor. Liquid carbon dioxide may be injected before the start of the journey. The use of either solid or liquid carbon dioxide as a cooling medium is expensive compared with the running costs of mechanical plant to absorb the same heat.
If the load is packed tightly against walls it is of very little value to use refrigeration in the space between the roof and the top of the cargo.
The cold source, however powerful, will have little or no effect on the temperature of the main mass of cargo.
Container cooling using Solid Carbon Dioxide - cross section
In the case of refrigerated containers, it is desirable to run the cooling plant for some time before the container is loaded in order to cool it down. The plant should be switched off before loading begins, and should never at any time run when the container doors are open, since this causes very rapid frosting up of the evaporator.
The load should be assembled and if possible, loaded into the container in a low temperature space. Failing that, every precaution should be taken to ensure that the fish are transferred to the container from the store with as little delay and with as little effect from outside temperature as is possible.
Rapid expansion of the frozen food industry means that many loading bays are now inadequate for proper handling of the goods. In planning future extensions, careful design of loading bay can ensure that the length of carry of the frozen product is as short and well protected as possible.
It has been observed that it may take anything from twenty minutes to two hours to load a container, during which time some of the frozen fish is warming up very rapidly while it stands either on the loading bay or in the container with the door open. It is quite possible for fish to come out of the store at a temperature of minus 20°F and yet be at plus 15°F by the time that loading is completed. No refrigerated container, however well designed, can offset the effect of serious warming up of the load before the journey begins.
Ideally the load should be for one destination only, and unloaded under similar conditions to those prescribed for loading. This is often impractical, but the number of times that the main door has to be opened during the delivery round should be kept to a minimum.
The length of time that the door is open should be kept as short as possible, particularly when the contents of the container are few. The size and capacity of any cooling system fitted should make allowance for the likely number and duration of door openings on a typical journey.
This is a particularly serious problem for vehicles delivering small parcels to a number of retail shops within a few hours. It may be advisable here to reduce the number of door openings that affect a particular parcel, by subdividing the container into a number of smaller compartments, with separate access to each, rather like a mobile locker plant.
The amount of cold air lost each time that the door is opened may be further reduced by fitting suitable screening of the main door, for example by means of a curtain of cold air or by the use of flexible self closing inner doors or screens.
It has been observed that on such retail delivery trips, the main door of the container may be open for as much as 20 % of the total journey time.
It must be remembered that the display units in retail shops have no reserve of refrigeration available for cooling down packages that are loaded into them above the recommended temperature. This means that the cabinet cooling plant becomes overloaded and that it may take an extremely long time to reduce the temperature of the product to the required level, with consequent increase in the rate of spoilage of the stored fish.
Reduce loading and unloading times, and protect the cargo at these times.
Allow cold air to circulate freely around walls, floor and roof of insulated containers.
Do not economise on insulation. Use a good quality material and apply it sufficiently thickly, but not at the expense of the air gap. Four inches of insulation with an air space between cargo and container is much better than five inches of insulation with no air space.
Keep the total opening time of the door to a minimum.
Deliver the goods in the best condition possible - in that way the customer will come back for more.
The work described in this paper was carried out as part of the programme of the Department of Scientific and Industrial Research.
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