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The thermal processing of fish involves packing prepared material, usually in oil or light brine within a metal can, bottle, or pouch; sealing the container completely; and subsequently heating the contents in order to kill most micro-organisms in the products. The oil or light brine is ineffective as a preservative and is added only as a filling medium. The container, which must be able to maintain the “commercial sterility” throughout the product storage life, must be particularly robust and leak-proof. This inevitably results in high processing costs, not only because of the high prices of the container itself, but also because of the high capital investment required for the machinery to process the container.
The following conditions are generally considered as necessary for the establishment of a profitable canning plant (Perovic, 1977):
- A large and regular supply of a suitable species of fresh fish at reasonable cost.- Adequate manpower and infrastructure (i.e. harbours, chill stores, etc.), and
- Large amounts of potable water and electricity energy.
- Large market for canned products given the relatively high retail prices of canned goods (i.e. in relation to prevailing incomes in developing countries).
Due to these requirements, there are very few situations where thermal processing of fish in metal cans may be recommended.
Certain fish are not suitable for canning. For example, the integral cooking operation tends to break up the flesh of white fish before softening the bones, thus making these unsuitable for canning. Fish with a high oil content (mainly pelagic species, e.g. herring, mackerel, tuna and sardine) have much firmer flesh and softer bones after cooking, and may thus maintain their original shapes. A further advantage in canning oily fish is that this method provies protection from oxidation and rancidity development, a feature which is not provided by the simpler curing operations.
Fish quality is of the utmost importance in canning, and good post-harvest handling, including the use of ice, refrigerated sea water or freezing immediately after capture is required. Capture itself also demands speed. For example, methods such as gill netting, which can lead to advanced spoilage in the warm water prior to landing, are not as effective as purse seining where large numbers of fish can be encircled and landed in a short period of time. Oily species of fish tend to deteriorate faster than non-oily species, a particular common feature being the burst abdomen, which is indicative of spoilage. The need for high quality raw materials in large amounts requires the use of large motorised vessels with sufficient space for adequate chill or frozen storage, and fast enough to return to port before spoilage can take place. The proper use of ice is important. Sufficient quantities of flake ice, well mixed in with the fish reduces the temperature of the fish much faster than crushed block ice which may bruise or otherwise damage the fish. The provision of proper landing facilities (e.g. harbour/ice plants, etc.) is necessary if fish are to be delivered in good condition to the processing plant.
In order for the plant to operate efficiently, a regular and large supply of fish is required. Pelagic fisheries are often seasonal and, if other products cannot be canned during the off-season, the plant may have to shut down. For example, on the Mexican Pacific coast, the sardines are only available for a few months and the cannery is closed for the rest of the year.
Conversely, if fish catches are very large, this may present throughput processing problems in the plant and the use of either cold or chilled stores as buffer storage zones may be necessary.
It is also important to take account of the market demand for specific kinds of fish. It has been found in the past that tuna and tuna-like fish, sardines and sardine-like fish, crustaceans, and especially shrimp, and molluscs, are the most profitable types of fishery products used in canning (Da Costa, 1973).
The requirement of adequate quantities of electricity, potable water and other subsidiary services is considerable, and depends on the size of the operation. Energy consumption per kg of product is usually less in larger operations. It is mainly required for the high temperature and washing operations, although there are additional energy requirements for ancillary operations such as fuel for fishing boats and transport, electricity for cold stores, etc.
The running of any canning plant is a complex operation requiring personnel trained in disciplines such as engineering, food technology and microbiology. In many less developed countries such personnel are not available and reliance on foreign expertise will inevitably increase costs.
The fish are often pre-processed after the usual heading, gutting, cleaning, and trimming operations by for example, salting, brining or smoking. These operations denature the protein, and make the flesh firmer and less subject to fragmentation. They also remove some water and thus make the fish less subject to shrinkage in the can, improve the appearance of oil packs and prevent the dilution of sauces. Air, which may produce some rancidity in the product, is removed from the container by sealing under vacuum.
The heat treatment required for sterilisation of the product is determined by the Ph of the content as follows:
- Acid products (i.e. pH less than 4.5) such as fish packed in tomato sauce require very little processing as the heat-resistant, spore-forming pathogenic bacterium, Clostridium botulinum cannot survive under acid conditions. For these products, the centre of the container should be held at 100° C for about 5 minutes.- Medium to low acid foods (i.e. pH 4.5 to 5.3) will support pathogens such as Clostridium botulinum and must be fully heat processed to eliminate all spores. In this case the centre of the container should be held for a period equivalent to 10 minutes at 121° C.
- Low acid foods (i.e. pH higher than 5.3) will support both Clostridium botulinum and the highly heat-resistant spoilage bacteria Bacillus stearothermophilus. Spores of the latter only germinate and grow at temperatures greater than 37° C, and thus the product should be stored at temperatures lower than this. It is not possible to ensure that all spores of these bacteria are destroyed by heat as the severe processing required would result in a total breakdown of the product.
In practical terms, fish canning normally involves holding the product in a retort (steam pressure vessel used for thermal processing of food) for up to an hour at a temperature of 115-116° C under pressure. The process includes a pre-heating stage since the centre of the product is the slowest to attain the temperature of the retort. Tanikawa (1971) gives an excellent review of canning and processing times for fishery products, and a recommended international code of practice for canned fish is available from FAO (See Appendix I).
Cooling of the sterilised product should be carried out in the retort, using chlorinated water and a gradual reduction of pressure. The chlorine ensures that accidental introduction of pathogens, especially in faulty containers, does not occur.
Quality inspection at regular intervals of raw materials, including fish, the cans and the water supplies is essential. Monitoring of the process is also important in quality control in order to ensure that all inputs have been correctly processed. In normal practice, it is recommended to hold back samples of product from each retort batch for incubation tests. These tests should be made routinely in order to confirm that processing has been satisfactory.
The high cost of metal cans is one of the major drawbacks of canning as a means of preserving fish. Two general types of alternative containers may be considered in order to reduce costs: the reusable container and the flexible pouch.
The adoption of reusable containers has a number of advantages both to the manufacturer and consumer. These are:
- The reduced cost of the product once an efficient return system has been established;- The reduced onus on the manufacturer to keep buying new stocks of cans, thus allowing more self-sufficiency; and
- The reduced pollution from waste packaging;
However, a number of disadvantages are evident:
- A reusable container is useful to the consumer (e.g. for the storage of kerosene, etc.). Thus a return system could be difficult to operate;- Rusable containers are invariably manufactured from glass. This requires a greater energy input to ensure adequate sterilisation because heat will not pass through thick glass as quickly as through thin tin plated steel;
- Developments of an efficient return system in a large rural developing area would present collection and transport problems;
- The washing of used containers is very energy-intensive,
- Glass containers, which can be resealed easily, may create a potential micro-biological hazard to uneducated consumers who may reseal the bottle after an initial opening, thus allowing spoilage to occur; and
- Glass bottles are generally much more fragile than metal tins.
A fairly recent development in the area of thermal processing is the foil/plastic laminated pouch. This type of container can provide a shelf life comparable to that provided by the metal cans for most foodstuffs. It is impermeable to oxygen, prevents the ingress of harmful spoilage bacteria and can withstand temperatures in the order of 125° C, thereby allowing heat sterilisation (Turtle, 1970). The materials used in their production include 3 thin layers laminated together. These are made of:
- nylon or polyester for the outside layer in order to resist abrasion.- aluminium foil for the middle layer in order to prevent light and oxygen transmission, and
- heat-sealable plastic, for the inside layer, such as polythene. The combined thickness of the three layers is about 0.5 mm. The pourches can be filled and sealed with a hot bar vacuum sealer, and then sterilised in a retort. A brief economic assessment of this type of packaging is given in Chapter V.
Pouches have a number of advantages over cans. These are:
- A lower weight and volume, especially when empty;- A wide size range;
- Easy opening;
-Boil-in-bag facility,
- Reduced processing time due to a larger surface area/volume ratio than in the case of cans,
- Attractive appearance; and
- Capital manufacturing costs should be low if a high volume can be achieved.
However, they also present a number of disadvantages namely:
- They are more easily ruptured than cans: this disadvantage requires a code of practice for handling;- As yet, they are not much chapter than cans: the requirements for a cardboard box for further protection of the pouches increases the price;
- They still require energy-intensive thermal processing;
- They are not conducive to self-sufficiency: they rely on advanced technological developments;
- They are more difficult to fill; and
- Obtaining a satisfactory seal can be difficult.
Details for processing such pouches are available commercially and will not be dealt with in this memorandum.
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