7. Improved fresh fish handling methods
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7.1 Basics of fresh fish handling and use of ice
7.2 Fish handling in artisanal fisheries
7.3 Improved catch handling in industrial fisheries
7.1 Basics of fresh fish handling and use of ice
Throughout history, man has preferred to consume fresh fish rather than other types of fish products. However, fish spoil very quickly and man has had to develop methods to preserve fish very early in history.
Keeping and transporting live fish
The first obvious way of avoiding spoilage and loss of quality is to keep caught fish alive until consumption. Handling of live fish for trade and consumption has been practiced in China with carp probably for more than three thousand years. Today, keeping fish alive for consumption is a common fish-handling practice both in developed and developing countries and at both artisanal and industrial level.
In the case of live fish handling, fish are first conditioned in a container with clean water, while the damaged, sick and dead fish are removed. Fish are put to starve and, if possible, water temperature is reduced in order to reduce metabolic rates and make fish less active. Low metabolic rates decrease the fouling of water with ammonia, nitrite and carbon dioxide that are toxic to fish and impair their ability to extract oxygen from water. Such toxic substances will tend to increase mortality rates. Less active fish allow for an increase in the packing density of fish in the container.
A large number of fish species are usually kept alive in holding basins, floating cages, wells and fish yards. Holding basins, normally associated with fish culture companies, can be equipped with oxygen control, water filtering and circulation and temperature control. However, more simple methods are also used in practice, for instance large palm woven baskets acting as floating cages in rivers (China), or simple fish yards constructed in a backwater of a river or rivulet for large "surubi" (Platystoma spp.), "pacu" (Colossoma spp.) and "pirarucu" (Arapalma gigas) in the Amazonian and Parana basins in South America.
Methods of transporting live fish range from very sophisticated systems installed on trucks that regulate temperature, filter and recycle water and add oxygen (Schoemaker, 1991), to very simple artisanal systems of transporting fish in plastic bags with an oxygen supersaturated atmosphere (Berka, 1986). There are trucks that can transport up to 50 t of live salmon; however, there is also the possibility of transporting a few kilo-grammes of live fish relatively easily in a plastic bag.
By now a large number of species, inter alia, salmon, trout, carp, eel, seabream, flounder, turbot, catfish, Clarias, tilapias, mussels, oysters, cockles, shrimp, crab and lobster are kept alive and transported, very often from one country to another.
There are wide differences in the behaviour and resistance of the various species. Therefore the method of keeping and transporting live fish should be tailored according to the particular species and the length of time it needs to be kept outside its natural habitat before slaughtering. For instance, the lungfish (Protopterus spp.) can be transported and kept alive out of water for long periods, merely by keeping its skin moist.
Some species of fish, noticeably freshwater fish, are more resistant than others to changes in oxygen in solution and the presence of toxic substances. This is probably due to the fact that their biology is adapted to the wide yearly variations in water composition presented by some rivers (cycles of matter in suspension and dissolved oxygen). In these cases, live fish are kept and transported just by changing the water from time to time in the transport containers (See Figures 7.1 (a) and (b)). This method is widely used in the Amazonian, Parana and Orinoco basins in South America; in Asia (particularly in the People's Republic of China, where also more sophisticated methods are used) and in Africa (N'Goma, 1993).
In the case presented in Figure 7.1 (a), aluminium containers with live freshwater fish are stored in the aisles of a public transport vessel. Containers are covered with palm leaves and water hyacinth to prevent the fish from jumping out of the containers and to reduce evaporation. The water in the containers is changed from time to time and an almost continuous visual control is kept on fish. Dead fish are immediately put to smoke-drying (African style) in drum smokers, also transported in the vessels or transporting barges.
In the case presented in Figure 7.1(b), carp is kept in a metal container drawn by a bicycle. This is a rather common practice in China, and other Asian countries; for instance in Bangkok, live catfish is sold daily by street vendors.
The most recent development is the keeping and transporting of fish in a state of hibernation. In this method, the body temperature of live fish is reduced drastically in order to reduce fish metabolism and to eliminate fish movement completely. The method greatly reduces death rates and increases package density, but careful temperature control should be exercised to maintain the hibernation temperature. There is an appropriate hibernation temperature for each species. Although the method is already utilized for instance to transport live "kuruma" shrimp (Penaeus japonicus) and lobster in pre-chilled wet sawdust, it should be considered an experimental technique for most of the species.
Although keeping and transporting live fish is becoming more and more important, it is not a viable solution for most of the bulk fish captures in the world.
Chilling fish with ice
Historical evidence proves that the Ancient Chinese utilized natural ice to preserve fish more than three thousand years ago. Natural ice mixed with seaweed was also used by the Ancient Romans to keep fish fresh. However, it was the development of mechanical refrigeration which made ice readily available for use in fish preservation.
In developed countries, particularly in USA and some European countries, the tradition of chilling fish with ice dates back more than a century. The practical advantages of utilizing ice in fresh fish handling are therefore well established. However, it is worthwhile for young generations of fish technologists and newcomers to the field, to review them, paying attention to the main points of this technique.
Ice is utilized in fish preservation for one or more of the following reasons:
(i) Temperature reduction. By reducing temperature to about 0°C, the growth of spoilage and pathogenic micro-organisms (see section 6) is reduced, thus reducing the spoilage rate and reducing or eliminating some safety risks.
Temperature reduction also reduces the rate of enzymatic reactions, in particular those linked to early post mortem changes extending, if properly applied, the rigor mortis period.
Fish temperature reduction is by far the most important effect of ice utilization. Therefore, the quicker the ice chills the better. Although cold-shock reactions have been reported in a few tropical species when iced, leading to a loss of yield of fillets (Curran et al., 1986), the advantage of quick chilling usually outweighs other considerations. The development of ad hoc fish handling methods is of course not ruled out in the case of species that could present cold- shock behaviour.
(ii) Melting ice keeps fish moist. This action mainly prevents surface dehydration and reduces weight losses. Melting water also increases the heat transport between fish and ice surfaces (water conducts heat better than air): the quickest practical chilling rate is obtained in a slurry of water and ice (e.g., the CSW system).
If, for some reason, ice is not utilized immediately after catching the fish, it is worthwhile keeping the fish moist. Evaporative cooling usually reduces the surface temperature of fish below the optimum growth temperature of common spoilage and pathogenic bacteria; although it does not prevent spoiling.
Ice should also be utilized in relation with chilling rooms to keep fish moist. It is advisable to keep chilling room temperature slightly above 0°C (e.g., 3-4°C).
However, water has a leaching effect and may drain away colour pigments from fish skin and gills. Ice melting water can also leach micronutrients in the case of fillets and extract relatively large amounts of soluble substances in some species (e.g., squid).
Depending on the species, severity of leaching and market requirements, an ad hoc handling procedure may be justified. In general, it has been found that drainage of ice meltwater is advisable in boxes and containers and that permanence of fish in chilled sea water (CSW) and refrigerated seawater (RSW) should be carefully assessed if leaching and other effects (e.g., uptake of salt from the seawater, whitening of fish eyes and gills) are to be avoided.
During the past there was much discussion about allowing drainage from one fish box to another, and consequent reduction or increase of bacterial load by washing with drainage water. Today, apart from the fact that in many cases box design allows for external drainage of each box in a stack, it is recognized that these aspects have less importance when compared with the need for quick reduction in temperature.
(iii) Advantageous physical properties. Ice has some advantages when compared with other cooling methods, including refrigeration by air. The properties can be listed as follows:
(a) Ice has a large cooling capacity. The latent heat of fusion of ice is about 80 kcal/kg. This means that a comparatively small amount of ice will be needed to cool 1 kg of fish.
For example, for 1 kg of lean fish at 25°C, about 0.25 kg of melted ice will be needed to reduce its temperature to 0°C (see Equation 7.c). The reason why more ice is needed in practice is mainly because ice melting should compensate for thermal losses.
The correct understanding of this ice characteristic is the main reason for the introduction of insulated fish containers in fish handling, particularly in tropical climates. The rationale is: ice keeps fish and the insulated container keeps ice. The possibility to handle fish with reduced amounts of ice improves the efficiency and economics of fresh fish handling (more volume available for fish in containers, trucks and cold storage rooms, less weight to transport and handle, reduction in ice consumption, less water consumed and less water drained).
(b) Ice melting is a self-contained temperature control system. Ice melting is a change in the physical state of ice (from solid to liquid), and in current conditions it occurs at a constant temperature (0°C).
This is a very fortunate property without which it would be impossible to put fresh fish of uniform quality on the market. Ice that melts around a fish has this property on all contact points. In the case of mechanical refrigeration systems (e.g., air and RSW) a mechanical or electronic control system (properly tuned) is needed; nevertheless, controlled temperature will be always an average temperature.
Depending on the volume, design and control scheme of mechanical refrigeration systems, different temperature gradients may appear in chill storage rooms and RSW holds, with fish slow freezing in one corner and maybe above 4°C in another corner. Although the need for proper records and control of temperature of chill storage rooms has been emphasized recently in connection with the application of HACCP (Hazard Analysis Critical Control Point) to fresh fish handling, it is clear that the only system that can assure accurate temperature control at the local level (e.g., in any box within a chill storage room) is ice melting.
Ice made of sea water melts at a lower temperature than fresh water ice, depending on the salt content. Theoretically with 3.5% of salt content (the average salt content of seawater) seawater ice will melt at about - 2.1 °C. However, as ice made out of seawater is physically unstable (ice will tend to separate from salt), brine will leach out during storage lowering the overall temperature (and this is the reason why sea water ice always seems wet). In these conditions, fish may become partially frozen in storage conditions and there may be some intake of salt by the fish muscle. Therefore, it cannot be said that ice made out of seawater has a proper self-controlled temperature system.
There is a narrow range of temperature below 0°C before fish muscle starts to freeze. The freezing point of fish muscle depends on the concentration of different solutes in the tissue fluids: for cod and haddock, it is in the range of -0.8 to -1°C, for halibut -1 to -1.2°C, and for herring about -1.4°C (Sikorski, 1990).
The process of keeping fish below 0°C and above the freezing point is called super-chilling, and it allows achievement of dramatic increases in overall keeping times. In principle it could be obtained using seawater ice or mixtures of seawater and freshwater ice, or ice made out of a 2% brine and/or mechanical refrigeration. However, in large volumes it is very difficult to control temperature so precisely and temperature gradients, partial freezing of fish in some pockets and hence lack of uniformity in quality are unavoidable (see section 6. 1).
(iv) Convenience. Ice has a number of practical properties that makes its use advantageous. They are:
(a) It is a portable cooling method. It can be easily stored, transported and used. Depending on the type of ice, it can be distributed uniformly around fish.
(b) Raw material to produce ice is widely available. Although clean, pure water is becoming increasingly difficult to find, it is still possible to consider it a widely available raw material. When there is no assurance that freshwater to produce ice will be up to the standard of drinking water, it should be properly treated, e.g., chlorination.
Clean seawater can also be utilized to produce ice. Ice from seawater is usually produced where freshwater is expensive or in short supply. However, it should be remembered that harbour waters are hardly suitable for this purpose.
(c) Ice can be a relatively cheap method of preserving fish. This is particularly true if ice is properly produced (avoiding wastage of energy at ice plant level), stored (to avoid losses) and utilized properly (not wasted).
(d) Ice is a safe food-grade substance. If produced properly and utilizing drinking water, ice is a safe food substance and does not entail any harm either to consumers or those handling it. Ice should be handled as food.
(v) Extended shelf life. The overall reason for icing fish is to extend fresh fish shelf life in a relative simple way as compared to storage of un-iced fish at ambient temperatures above 0°C (see Chapter 6). However, extension of shelf life is not an end in itself, it is a means for producing safe fresh fish of acceptable quality.
Most landed fish can be considered a commodity, that is, an article of trade. Unlike other food commodities, it is usually highly perishable and it is thus in the interest of the seller and the buyer to ensure fish safety at least until it is consumed or further processed into a less perishable product. Ice and refrigeration in general, by making possible extension of fish shelf life, convert fresh fish into a true trade commodity, both at local and international level.
Ice is used to make fish safe and of better quality to consumers. It is also used because otherwise the current fish trade at local and international level would be impossible. Shelf life is extended because there is a strong economic reason to do so. Fishermen and fish processors who fail to handle fresh fish appropriately ignore the essence of their business. The inability to recognize fresh fish also as a trade commodity is at the root of misunderstandings and difficulties linked to the improvement of fish handling methods and prevention of post-harvest losses.
Types of ice
Ice can be produced in different shapes; the most commonly utilized in fish utilization are flake, plate, tube and block. Block ice is ground before being utilized to chill fish.
Ice from freshwater, of whatever source, is always ice and small differences in salt content or water hardness do not have any practical influence, even if compared with ice made out of distilled water. The physical characteristics of the different types of ice are given in Table 7.1.
Cooling capacity is expressed by weight of ice (80 kcal/kg); therefore it is clear from Table 7.1 that the same volume of two different types of ice will not have the same cooling capacity. Ice volume per unit of weight can be more than twice that of water, and this is important when ice stowage and volume occupied by ice in a box or container are considered. Ice necessary to cool fish to 0°C or to compensate for thermal losses is always expressed in kilogrammes.
Under tropical conditions ice starts to melt very quickly. Part of the melted water drains away but part is retained on the ice surface. The larger the ice surface per unit of weight the larger the amount of water retained on the ice surface. Direct calorimetric determinations show that at 27°C the water on the surface of flake ice at steady conditions is around 12-16% of the total weight and in crushed ice, 10-14% (Boer) et al., 1985). To avoid this problem, ice may be subcooled; however, under tropical conditions this effect is quickly lost. Therefore a given weight of wet ice will not have the same cooling capacity as the same weight of dry (or subcooled) ice, and this should be taken into account when making estimations of ice consumption.
Table 7.1 Physical characteristics of ice utilized in chilling fish. Adapted from Myers (1981)
|Approximate||Specific volume||Specific weight|
- Crushed block
10/20 - 2/3 mm
30/50 - 8/15 mm
50(D)- 10/12 mm
2.2 - 2.3
1.7 - 1.8
1.6 - 2.0
1.4 - 1.5
0.45 - 0.43
0.59 - 0.55
0.62 - 0.5
0.71 - 0.66
Notes: (1) They depend on the type and adjustment of the ice
(2) Indicative values, it is advisable to determine them in practice for each type of ice plant.
(3) Usually in blocks of 25 or 50 kg each.
There is always the question of which is the "best" ice to chill fish. There is no single answer. In general, flake ice will allow for an easier, more uniform and gentle distribution of ice around fish and in the box or container and will produce very little or no mechanical damage to fish and will chill fish rather more quickly than the other types of ice (see Figure 7.2). On the other hand, flake ice will tend to occupy more volume of the box or container for the same cooling capacity and if wet, its cooling capacity will be reduced more than the other types of ice (since it has a higher area per unit of weight).
With crushed ice there is always the risk of large and sharp
pieces of ice that can damage fish physically. However, crushed
ice usually contains fines that melt quickly on the fish surface
and large pieces of ice that tend to last longer and compensate
for thermal losses. Block ice requires less stowage volume for
transport, melts slowly, and contains less water at the time it
is crushed than flake or plate ice. For these reasons, many
artisanal fishermen utilize block ice
(e.g., in Colombia, Senegal and the Philippines).
Probably tube ice and crushed ice are more suitable for use in CSW systems if ice is wet (as it normally is under tropical conditions), since they will contain less water on their surfaces.
There are also economic and maintenance aspects that may play a role in deciding for one type of ice or another. The fish technologist should be prepared to analyze the different aspects involved.
Cooling rates depend mainly on the surface per unit of weight of fish exposed to ice or chilled ice/water slurry. The larger the area per unit of weight the quicker the cooling rate and the shorter the time required to reach a temperature around 0°C at the thermal centre of the fish. This concept is also expressed as "the thicker the fish the lower the cooling rate".
Small species such as shrimp, sardines, anchovies and jack mackerels cool very quickly if properly handled (e.g., in CSW or CW). Large fish (e.g., tuna, bonito, large sharks) could take considerable time to cool. Fish with fat layers and thick skin will take longer to cool than lean fish and fish with thin skin of the same size.
In the case of large fish, it is advisable to gut them and to put ice into the empty belly as well as around it. In large sharks, gutting alone may not be enough to prevent spoilage during chilling, and therefore it is advisable to gut the shark, to skin it and to cut the flesh into sizeable portions (e.g., 2-3 cm thick) and to chill them as soon as possible. Chilled sea water (CSW) has in this case the advantage of extracting some of the urea present in shark muscle (see section 4.4). However, this is an extreme case, since in current situations fillets kept in ice will last less time than gutted fish or whole fish (because of the unavoidable microbial invasion of the flesh) and will lose soluble substances.
Typical curves for cooling fish in ice, using different types of ice and chilled water (CW) are shown in Figure 7.2.
From Figure 7.2 it is clear that the quickest method to chill fish is with chilled water (CW) or chilled sea water (CSW), although the practical difference with flake ice is not great. There are, however, noticeable differences after the quick initial drop in temperature with crushed block ice and tube ice, due to differences in contact areas between fish and ice and flow of melt- water.
Cooling curves may also be affected by the type of container and external temperature. Since ice will melt to cool fish and simultaneously to compensate for thermal losses, temperature gradients may appear in actual boxes and containers. This type of temperature gradient could affect the cooling rate, particularly in boxes at the top or side of the stacks, and more likely with tube and block crushed ice.
Curves such as those shown in Figure 7.2 are useful to determine the critical limit of chilling rates when applying HACCP to fresh fish handling. For instance, in specifying a critical limit for chilling fish "to be at 4.5°C in the thermal centre in no more than 4 hours", in the case of Figure 7.2 it could be achieved only by using flake ice or CW (or CSW).
In most cases the delay in reaching 0°C in the thermal centre of the fish may not have much practical influence because the surface temperature of the fish will be at 0°C. On the other hand, warming-up of the fish is much riskier because the fish surface temperature (which is actually the riskiest point) will almost immediately be at the external temperature, and therefore ready for spoilage. As large fish will take longer than small fish to warm up and also have less surface area (where spoilage starts) per unit of volume than small fish, they usually take a little longer to spoil than small fish. This circumstance has been widely used (and abused) in practice in the handling of large species (e.g., tuna and Nile perch).
Figure 7.2 Chilling of large yellow croaker (Pseudosciaena crocea) with three different types of ice and chilled water (CW). Ice-to-fish ratio 1:1; the same type of insulated containers (with drainage) was used in a parallel experiment (data obtained at the FAO/DANIDA National Workshop on Advances in Chilling and Processing Technology of Fish, Shanghai, China, June 1986)
Small species will warm up very quickly and definitely more quickly than large species (warming-up the same reason for which they cool faster). Although warming-up studies of fresh fish have received little attention in the past, they are necessary within an HACCP scheme, to determine critical limits (e.g., maximum time fish can be handled without ice in a fish processing line).
With application of HACCP and HACCP-based systems, thermometers including electronic thermometers, should be a standard tool in fish processing plants. Therefore, it is advisable to perform fish cooling and warming-up trials on actual conditions.
Ice consumption can be assessed as the sum of two components: the ice necessary to cool fish to 0°C and the ice to compensate for thermal losses through the sides of the box or container.
Ice necessary to cool fish to 0°C
The amount of ice theoretically necessary to cool down fish from a temperature Tf to 0°C using ice can easily be calculated from the following energy balance:
L · mi = mf · cpf · (Tf - 0) 7a where:
L = latent heat of fusion of ice (80 kcal/kg)
mi = mass of ice to be melted (kg)
mf = mass of fish to be cooled (kg)
cpf= specific heat capacity of fish (kcal/kg °C)
From (7.a) it emerges that:
mi = mf · cpf · Tf/L 7.b
The specific heat capacity of lean fish is approximately 0.8 (kcal/kg. °C). This means that as a first approximation:
mi = mf · Tf/100 7.c
This is a very convenient formula, easily remembered, to quickly estimate the quantity of ice needed to cool fish to 0°C.
Fatty fish have lower cpf values than lean fish and, in theory, require less ice per kilogramme than lean fish; however, for safety purposes it is advisable to make calculations as if fish were always lean. Refinements in the determination of cpf are possible; however, they do not drastically alter the results.
The theoretical quantity necessary to cool fish to 0°C is relatively small and in practice much more ice is used to keep chilled fish. If we relate the proper fish handling principle of surrounding middle and large sized fish with ice, to the approximate dimensions of ice pieces (see Table 7.1), it is clear that with some types of ice (tube, crushed block and plate) greater quantities are required for physical considerations alone.
However, the main reason for using more ice is losses. There are losses due to wet ice and ice spilt during fish handling, but by far the most important losses are thermal losses.
Ice necessary to compensate for thermal losses
In principle, the energy balance between the energy taken by the melted ice to compensate heat from outside the box or container could be expressed as follows:
L · (dMi/dt) = - U · A · (Te - Ti) 7.d where:
Mi = mass of ice melted to compensate for thermal losses (kg)
U = overall heat transfer coefficient (kcal/hour m² °C)
A = surface area of the container (m²)
Te = external temperature
Ti = ice temperature (usually taken as 0 °C)
t = time (hours)
Equation (7.d) can be easily integrated (assuming Te = constant) and the result can be expressed as:
Mi = Mio - (U·A·Te / L)·t 7.e
It is possible to estimate thermal losses, calculating U and measuring A. However, this type of calculation will seldom give an accurate indication of ice requirements, for a number of practical factors (lack of reliable data on materials and conditions, irregularities in the construction of containers, irregular geometric shape of boxes and containers, influence of lid and drainage, radiation effect, type of stack).
More accurate calculations of ice requirements can be made if meltage tests are used to determine the overall heat transfer coefficient of the box or container, under actual working conditions (Boer) et al., 1985; Lupin, 1986 a).
Ice meltage tests are very easy to conduct and no fish are needed. Containers or boxes should be filled with ice and weighed before commencing the test. At given periods, the melted water is drained (if it has not already drained) and the container is weighed again. The reduction of weight is an indication of the ice lost due to thermal losses. In Figure 7.3 the results of two ice meltage tests obtained under field conditions are presented.
Initially, some ice will be melted to cool down the walls of the box or container; depending on the relative size and weight of the container, wall materials and thickness and entity of the thermal losses this amount may be negligible. If it is not, the container can be cooled down before starting the test, or the ice necessary to cool down the container can be calculated by the difference disregarding the first part of the meltage test. A constant air surrounding temperature would be preferable and it can be achieved during short periods (e.g., the testing of a plastic box in tropical conditions). However, reasonably constant temperatures may be achieved during the intervals between weight loss measurements and an average used in the calculations.
Results as shown by Figure 7.3 can be interpolated empirically by a straight line equation of the form:
Mi = Mio- K · t 7.f
Comparing Equations 7.e and 7.f, it is clear that:
K = (Uef · Aef · Te / L ) 7.g where:
Uef = overall effective heat transfer coefficient Aef = effective surface area
Figure 7.3 Results of ice meltage tests under field conditions. (·) standard plastic box (not insulated) 40 kg total capacity, (x) insulated plastic fish container (Metabox 70, DK). Both kept in the shade, un-stacked, flake ice, average external temperature (Te) 28°C. (Data obtained during the FAO/DANIDA National Workshop on Fish Technology and Quality Control, Bissau, Guinea-Bissau, March 1986)
From Expression 7.g it follows that:
K = K' · Te 7.h and eventually K' could be determined, if experiments can be conducted at different controlled temperatures.
The advantage of meltage tests is that K can be obtained experimentally from the slope of straight lines, as appears in Figure 7.3, either graphically or by numerical regression (now found as sub-routine in common pocket scientific calculators). In the ease of the straight lines appearing in Figure 7.3 the correlations found are as follows:
Mi = 10.29 - 1.13 · t, r = -0.995 7.i
K = 1.13 kg of ice/hour
Mi = 9.86 - 0.17 · t , r = - 0.998 7.j
K = 0.17 kg of ice/hour where r = correlation coefficient.
From 7.i and 7.j it follows that the ice consumption due to thermal losses in these conditions will be 6.6 times greater in the plastic box than in the insulated container. It is clear that under tropical conditions it will be practically impossible to handle fish in ice properly utilizing only non-insulated boxes, and that insulated containers will be needed, even if additional mechanical refrigeration is used.
The total amount of ice needed will be the result of adding mi (see Equations 7.b and 7.c) to Mi (according to expression 7.f) once t (the time fish should be kept chilled in the box or container in the particular case) has been estimated.
Under tropical conditions it may happen that, depending on the estimated t, total available volume in the box or container might not be enough even for ice to compensate for thermal losses, or the remaining volume for fish could be insufficient to make the chilling operation attractive.
In such cases it might be feasible to introduce one or more re-icing steps, or to resort to additional mechanical refrigeration (see Figure 7.5 to observe the effect of storage in a chill room on ice consumption). In practice, an indication of when re-icing is needed would be given to foremen or people in charge.
An analytical approach to this problem in connection with the estimation of the right ice-to- fish ratio in insulated containers can be found in Lupin (1986 b).
Ice consumption in the shade and in the sun
An important consideration, particularly in tropical countries, is the increased ice consumption in boxes and insulated containers when exposed to the sun. Figure 7.4 gives the results of a experimental meltage test conducted with a box in the shade and the same box (same colour) in the sun.
The plastic box in the shade is the same plastic box of Figure 7.3 (see Equation 7.i). The correlation for the plastic box in the sun is:
Mi = 9.62 - 3.126 · t 7.k
This means that for this condition and this type of box, the ice consumption in the sun will be 2.75 times that in the shade (3.126/1.13). This considerable difference is due to the radiation effect. Depending on the surface material, type of material, colour of the surface and solar irradiation, it will be a surface radiation temperature, that is higher than dry bulb temperature. Direct measurements on plastic surfaces of boxes and containers on field conditions, in tropical countries, have given values of surface radiation temperature up to 70°C.
Figure 7.4 Results of ice meltage tests under field conditions. (·) plastic box in the shade, (x) plastic box in the sun. Plastic boxes, 40 kg capacity, red colour, unstacked, flake ice, external average temperature (dry bulb) 28°C. (Data obtained during the FAO/DANIDA National Workshop on Fish Technology and Quality Control, Bissau, Guinea-Bissau, March 1986)
It is clear that there is little practical possibility in tropical countries to handle chilled fish in plastic boxes exposed to the sun. An increase in ice consumption, even if less dramatic than in plastic boxes, can be measured in insulated containers exposed to the sun.
The obvious advice in this case is to keep and handle fish boxes and containers in the shade. This measure can be complemented by covering the boxes or containers with a wet tarpaulin. The wet tarpaulin will reduce the temperature of the air in contact with boxes and containers to the wet bulb temperature (some degrees below the dry bulb temperature, depending on the Equilibrium Relative Humidity - ERH - of the air), and will practically stop noticeable radiation effect (since there are always radiation effects between a body and its background).
Ice consumption in stacks of boxes and containers
In a stack of boxes or containers not all of them will lose fee in the same way. Figure 7.5 gives the results of an fee meltage test conducted on a stack of boxes.
Boxes or containers at the top will consume more ice than boxes and containers at the bottom, and those in the middle will consume less than either.
Figure 7.5 Results of ice meltage tests during storage in a stack of plastic boxes. Plastic boxes 35 kg in a chill storage room at 5°C, flake ice (from Boeri et al. (1985))
Jensen and Hansen (1973) and Hansen (1981) presented a system ("Icibox"), mainly for artisanal fisheries. In this system, a stack of plastic boxes were insulated by placing wooden frames, filled with polystyrene, at the top and at the bottom of the stack, and covering the whole with a case made out of canvas or oil skin. A similar system, composed of stacks of styropor boxes, accommodated in a pallet, and covered by an insulated mat of high reflective (Al) surface, is used in practice for shipment of fresh fish by air (e.g., it is utilized to ship fresh fillets of Nile perch from Lake Victoria to Europe).
Results of Figure 7.5 are also of interest to demonstrate the effect of a chill room on fresh fish handling. The use of chill rooms drastically reduces the ice consumption in plastic boxes, avoiding the need of re-icing. In a fish handling system chilling fish with ice, mechanical refrigeration is used to reduce the ice consumption and not to chill fish.
Although analytical models of ice consumption (e.g., Equations 7.a to 7.h) can be applied directly to estimate the ice consumption in simple and repetitive fish handling operations, their main importance is that they can help in arriving at solutions for the proper handling of chilled fish in rational way (as seen from Figures 7.3, 7.4 and 7.5).
Ice consumption in the sides of boxes and containers
It is necessary to bear in mind that ice will not melt uniformly in the interior of a box or container, but meltage will follow the pattern of temperature gradients between the interior of the box/container and the ambient. In Figure 7.6, a commercial plastic box with chilled hake shows the lack of ice in the sides due to the temperature gradients at the walls.
Following Figure 7.5, and supposing that a simple box could be devided into five sub-boxes, it is clear that the bottom and top of boxes and containers should receive more ice to compensate for thermal losses, the top receiving more ice than the bottom. However, in practice more ice should also be put in the sides of boxes and containers.
The box of Figure 7.6 was initially prepared with enough ice, and it can be seen that ice is still abundant on top of the box. However, after a period of storage in a chill room, ice has melted, mainly on the sides, leaving some fish and parts of fish exposed to the air with a consequent rise in temperature and dehydration. In addition, ice and fish have formed a compact mass that can produce physical damage to exposed fish when the box is moved.
In chilled fish onboard fishing vessels or transported by truck, this problem may not exist if there is a continuous gentle movement which allows for ice melt water from the top to move to the sides. However, in chill rooms or storage rooms (insulated containers) it would be advisable to re-ice if this problem is observed. Under tropical conditions this effect is observed, even with insulated containers, in less than 24 hours of storage.
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