6. ICING OF FISH

6.1 At Sea

6.1.1 Bulk stowage
6.1.2 Boxed stowage
6.1.3 Chilled sea water

6.2 Ashore

6.3 Handling Ice

6.3.1 Manual operation
6.3.2 Conveyors
6.3.3 Pneumatics

In order to maintain optimum fish quality, fish, once caught should be chilled to 0�C as quickly as possible. Ice chills fish by surface heat transfer either by direct contact between fish and ice or by cold melt water running over the fish surface. Therefore the more ice in contact with the fish the quicker the cooling rate. Even quicker cooling can be achieved by immersing the fish in a mixture of iced water which allows maximum surface heat transfer.

6. 1 At Sea

6.1.1 Bulk stowage.

Bulk stowage of fish is the stowage of iced fish in pounds usually formed by fitting portable pound boards into fixed vertical stanchions in the fish room. First a bed of Ice is laid in the pound then a layer of fish, followed by further layers of ice and fish to a maximum depth of half a metre. A second platform is then formed of pound boards and the process repeated with generous applications of ice to the top, bottom and sides of each pound.

Pound boards are commonly made of aluminium, plastic or wood; the first two being preferable for reasons of hygiene and handling. Corrugated extrusions give strength to the boards and provide for better draining of contaminated melt water to the edges of the pounds.

As a guide to icing, an ice to fish ratio of 1:2 is commonly used in temperate climates for voyages up to 18 days and 1:1 for tropical climates. Ice requirements can be theoretically calculated from a knowledge of the thermal properties of the fishroom, the length of voyage and the ambient temperatures but in practice the requirements can only be realistically based on experience. At the end of the voyage at least a little ice should be remaining in each pound. It is better to use a little too much ice than not enough.

The stowage density of bulked fish is about 0.50 t/m� of fishroom.

The major disadvantage of bulked stowage is the problem it presents on unloading. Digging out of the fish by hook and shovel is a laborious, slow and difficult process that results in a greater degree of damage than that caused by unloading of boxed fish. It is also a common cause of industrial injury. The rate of unloading depends upon the method used, the vessel, quality and number of the work force and the size of the catch. As an example, 60 men (22 fishroom, 5 hatchway, 5 landing boards, 5 winch, 23 dragging baskets) can unload 125 t from a distant water vessel in 6 h using baskets on gin wires from a landing leg above the hatches. This does not include for setting-up or weighing, sorting or laying out on the market.

6.1.2 Boxed stowage.

Boxed stowage is similar in principle to bulk stowage except that instead of building up pounds in the fishroom the fish is mixed with ice in boxes. Similar quantities of ice are used for boxing as used for bulking except when the box is to be used for onward road transportation on landing without additional icing. In such cases extra ice is required to suit the onward journey. Types and costs of boxes are discussed in Section 8.

The stowage density of boxed and iced fish is in the order of 0.37 t/m� depending on the size and design of the box. The advantages of boxed stowage are: easier and quicker unloading, reduced handling of the fish and reduction in the damage to fish. As an example of the unloading rate, 8 men (3 shore, 3 fishroom, 1 hatch, 1 winch) can unload 10 t of boxed fish from an inshore vessel, using a gin wire and hooks from the vessels derrick, in less than an hour.

6.1.3 Chilled sea water.

Stowage of fish in sea water chilled by ice and held in insulated fixed tanks offers several advantages over other methods of stowage, particularly for pelagic species not normally gutted at sea but which are caught in bulk and require rapid chilling.

Stowage in an ice-water mix minimizes crushing which causes damage and weight loss and rapidly cools the fish by efficient heat transfer. By design of the deck pounds and scuttles or pumping system the fish can be rapidly transferred from the deck or net to the tanks with the minimum delay and disruption to fishing effort.

The amount of Ice required to cool a tank of fish and water and maintain it in a chilled condition will depend upon the size of the tank, the effectiveness of the insulation, the ambient temperatures and the length of the trip. Allowance should also be made for ice loss during the outward passage to the fishing grounds. As a guide to icing a water: Ice: fish ratio of 1: 1: 4 is commonly used for 3-4 days stowage in insulated fixed tanks in temperate climates and 1: 2: 6 in tropical climates. For a given tank installation and prevailing conditions the quantity of Ice can be estimated In a similar manner to that of an insulated container for which an example calculation is given in Section 11. The calculation of the ice required to combat the heat gain to a fixed tank system is more complex and depends upon the method of construction of the tanks and particularly the way in which the tank lining is supported and insulated. The total heat gain to the tank Is the sum of the heat gains through each tank surface and for each tank surface the gain must be calculated depending upon the conditions that exist on that surface. For example, if we consider an outer wing tank side, we must calculate for the transfer of heat through the tank surface and frames above the water line on the outer surface of the vessel, and the transfer through the surface and frames below the water line, as each surface area will be subject to different outer temperatures and will have different surface heat transfer coefficients. The overall heat transfer coefficient for a tank surface, or part of, is a function of the outside heat transfer coefficient, the inside heat transfer coefficient and the thickness and conductivity of the insulation, the tank lining and the outer steel surface, as given by:

where,

= overall heat transfer coefficient (kcal/m� h deg C)
= outside heat transfer coefficient (Kcal/m� h deg C)
= thickness of plate, ship's side (m)
= conductivity. of plate, ship's side (kcal m/m� h deg C)
= thickness of insulation (m)
= conductivity of insulation (kcal m/m� h deg C)
= thickness of tank lining (m)
= conductivity of tank lining (kcal m/m� h deg C)
= Inside heat transfer coefficient (kcal/m� deg C)

For each surface then or part of surface the heat transfer can be calculated through the surface, and frames, from the above given the temperatures that exist on each surface. From the total heat transfer to the tank the quantity of ice needed to offset the gain can be calculated from the latent heat of the ice. Examples of surface heat transfer coefficients for a typical steel tank installation are given in Table 15.

Table 15 Surface heat transfer coefficients

Tank stowed fish for human consumption is usually unloaded by brail or by pump. Unloading rates of up to twenty t/h can be achieved using a brail of one t capacity. Rates of 30 t/h or more are possible with pump discharge but with a higher degree of damage to the fish, particularly if the fish has been previously pumped from the net.

6.2 Ashore

Upon landing the fish should be maintained at or as near as possible to its chilled temperature of 0�C through the market, processing and distribution chain.

If the fish is to be laid out on the market for display prior to sale additional ice or re-icing may be required to prevent the fish becoming warm and spoiling. This practice however is often ignored because of the problem it causes in viewing the fish on sale and checking the fish weight. This can be overcome by weighing the fish prior to icing if a market kit is used, or check weighing a small number of boxes if the vessels boxes are used for display. In any instance, the fish should be moved as quickly as possible and kept out of direct sunlight.

If the fish is to be re-iced at the market after sale for transport to the processing factory or secondary market the quantity of ice required will depend on the temperature of the fish, the length of the journey. The ambient temperature and the thermal protection given to the fish by the fish box and the transportation vehicle. The quantity of ice theoretically to chill the fish can be calculated from Table 16 which gives the weight of ice required to cool 1 kg of fish from selected temperatures. In practice more may be required.

Table 16 Quantity of ice theoretically required to cool 1 kg of fish to 0�C

The quantity of ice required to maintain the fish in chill will depend on the ambient temperature, the insulative properties of the box and vehicle, and the position of the individual box within the load. A box in contact with the sides or base of the lorry will require more ice than a box in the centre of a stacked load which is protected from heat ingress by the boxes surrounding it, but it is not always practicable to ice boxes according to their later loading position within a lorry. In practice the amount of ice used on any given journey is based on experience but it can be estimated given the box and vehicle dimensions, thickness of insulation and thermal conductivities.

Typical conductivities of fish box materials are as follows:

Table 17 shows the quantity of ice required in practice to chill and maintain in chill boxed fish under selected surrounding temperatures, for both a single box and for a stack of 35 boxes.

Table 17 Ice requirements to chill and maintain in chill fish held in individual boxes and within a stack of boxes

Not only is the quantity of ice of importance in a box, also the distribution of ice within a box.

If the fish is already chilled then the ice is required to combat the heat gain through the box and can be packed round the fish against the box surfaces, but if the ice has to chill the fish also then the ice should be well mixed with the fish.

If a box of fish was iced on the top only for example, even with the calculated and correct amount of ice for cooling and maintaining the fish in chilled condition. the fish would not be efficiently chilled even if the box was well insulated. This is because the cooling is largely achieved by surface heat transfer and if the bulk of the fish are not in contact with the ice, the transfer of heat through the bulk of the fish will require a considerably longer cooling period. (Some cooling may be achieved by the passage of melt water.) In effect the fish at the bottom of the box would be 'insulated' from the ice by the fish above them. For the same reason it takes a substantially longer time to cool a large fish of a given weight than to cool the same weight of smaller fish to a uniform chilled temperature.

6.3 Handling Ice

6.3.1 Manual operation.

The simplest form of delivery and icing system is a manual operation. Ice can be removed from bins using hand rakes and shovels and carried to the point of use in tubs, carts or box pallets. Such handling is satisfactory for small operations. Icing at sea is still most commonly carried out with a shovel using an axe to break the surface crust. It is a laborious and physically exhausting job.

6.3.2 Conveyors.

Straight line fixed conveyor systems are the most common means of handling ice within the ice factory and for delivery of the ice to the fishing vessel or road vehicle. Where the ice factory is sited a long distance from the quayside specially designed tipping trucks or tow units can be used to deliver ice to petrol-driven inclined conveyors that deliver the ice through the fishroom hatch to the hold. Conveyors are normally supplied with galvanized finish and can be Insulated. Simple screw conveyors can act as elevators up to a maximum angle of approximately 300 to the horizontal depending on the type of ice although other types of screw conveyor will deliver vertically. Dished belt conveyors will handle ice horizontally and slated belt conveyors at inclinations up to approximately 400 or more depending on the design.

Screw conveyors should not be left with ice in them when stopped but should be run for a short period (depending on the length of the conveyor) after the supply of ice to them has been stopped to ensure that all the ice is discharged. Failure to do so can result in jamming the conveyor and burning out the drive motor. It is possible to fit an auto-delay to the conveyor motor to ensure this.

6.3.3 Pneumatics.

Pneumatic conveying of ice is often used to deliver ice from the ice factory to the fishing vessel or road carrier but can also be used for Icing of fish at sea. Pneumatic conveying on shore is more viable at greater distances (over 30 m), and when multiple stations are to be served. Delivery rates of 10-60 t/h are obtainable.

The system must be carefully designed for the handling of ice, particularly flake ice which can be reduced to a slush by impact at high velocity on the duct walls. To reduce the heating effect of the blown air it is usual to precool the air through a heat exchanger. Bends should be a minimum radius of 1.5 m.

At sea a pneumatic delivery system with flexible discharge hose greatly simplifies the task of icing fish in the hold. The hose has only to be directed at the fish and velocity of discharge throws the ice to the required box or pounds eliminating the laborious task of shovelling ice. In a large fishroom hold numerous coupling points can be provided for connexion of the discharge hose. Figure 18 shows a pneumatic distribution system for a distant water wet fish trawler fitted with its own ice plant.

Figure 18. Pneumatic distribution system