|Small-Scale Processing of Fish (ILO - WEP, 1982, 140 p.)|
|CHAPTER I. INTRODUCTION|
|III. PREVENTION OF LOSSES AND SPOILAGE OF FISH|
Since fish are the most important animal protein in the diet of many people in the tropics, it is important to reduce wastage and losses to the lowest possible level. Fish spoil very quickly and small-scale fish processing enterprises can easily loose profits through wastage. In general, it has been estimated that approximately 25% of a catch of fish may be lost through one cause or another before consumption.
Immediately after a catch, a complicated series of chemical and baterial changes begin to take place within the fish. If these changes are not controlled the fish quickly become spoiled (e.g. within 12 hours at tropical temperatures). Thus, the need to process fish according to some of the curing methods described under section II soon after the catch.
Spoilage of fish may take place before, during or after processing. The reasons for such spoilage and measures to prevent or minimise it are briefly described below.
A great deal of spoilage may occur before the fish is processed. The bacterial and chemical changes which cause spoilage proceed rapidly at the temperature at which tropical fish normally live (in the range of 25-30°C). In general, the lower the temperature of the fish, the slower the change which causes spoilage. Furthermore, spoilage may be reduced if fish are handled properly, and good hygienic measures are adopted. A few measures for avoiding or minimising spoilage are briefly described below.
(i) Improvement of landing facilities and distribution. Very often, whenever unexpectedly large catches are taken, landing facilities and the distribution system cannot handle the surplus of fish. Thus, a long period of time may elapse before the fish can be processed. Consequently, a high percentage of the fish may become unsuitable for processing. It is therefore important to expand cold storage facilities in proximity of the catch areas whenever sufficient and/or adequate transport facilities (e.g. trucks equipped with a refrigeration system) are not available. Alternatively, processing plants may be located near the catch areas in order to avoid the need for extensive transport facilities.
(ii) Maintaining the fish at low temperatures. To minimise spoilage, fish should be kept as cool as possible immediately after catching until processing starts. If tropical fish are chilled with ice, they may be kept in an edible condition for an increased period. The actual length of time depends very much on the type of fish, but may be as long as three weeks. However, in many areas far away from major towns, ice may not be available in sufficient quantities. Fish may then be kept relatively cool by other means, including the following:
- keeping the fish in the shade out of direct sun,
- placing damp sacking over the fish. This helps reduce the temperature as the water evaporates. The sacking must be kept wet and the fish must be well ventilated.
- mixing the fish with wet grass or water weeds in an open-sided box so that the water can evaporate and cool the fish. In this method, the fish should be kept continuously wet.
(iii) Maintaining a hygienic environment. Fish which have been handled cleanly and carefully will be in a better condition than fish which have been handled carelessly; they can, therefore, be worth more money.
Before processing starts, attention to the following points is important.
- To keep the fish as clean as possible. Washing with clean water will remove any of the bacteria present on the fish skin, especially in the presence of mud.
- To keep the fish cool, chilled in ice or chilled water, if possible, at all stages before processing starts. Fish spoilage is a continuing process: once a particular stage of spoilage has been reached no amount of good practice or processing can reverse it.
- To avoid damaging fish by careless handling. If the skin is broken this will allow bacteria to enter the flesh more quickly and spoilage will be more rapid. This sort of damage can be caused by walking on fish and by the use of a shovel. If the guts can be removed and the gut cavity washed carefully, this will reduce the number of spoilage bacteria present; however, in some areas, the purchaser requires whole fish so that this practice may lower the value of the catch.
A number of measures may be adopted in order to minimise the spoilage of fish during processing. They may include the following:
- To keep all tools, fish boxes, boat holds, cutting tables etc. clean by washing with clean water. Where drinking water is available, to use it to wash the fish before and during processing, for example, after gutting or splitting the fish.
- To prevent fish offal (guts, heads, gills, etc.) from coming into contact with cleaned fish. Also, the fish working area should be cleaned regularly, at least once a day by removing all offal and dirt which might contain bacteria or attract insect pests such as flies. All offal should be removed from the working site. It may be used as fertiliser, or buried. It should not be thrown into the water near the work site as this practice fouls the water and may attract insects.
- To ensure that high standards of personal hygiene are maintained. Fish processors are handling food, and hands should always be washed before starting work and particularly after visiting the toilet. People who have infected wounds, stomach complaints or any contagious disease, should not be allowed to handle the fish.
- To ensure speed during processing. The longer the time required by processing, the greater the amount of spoilage which will occur before processing is completed.
- If possible, to keep fish in boxes and off the ground. Work, such as cutting fish prior to salting or drying, must be carried out on tables, not on the ground where the fish will become dirty and pick up bacteria.
- To protect the fish from rain and to use salt during drying in order to avoid the spoilage of fish through bacterial, mould or insect attack.
- To use well-designed smoking kilns or ovens in order to avoid the over-cooking of fish which may catch fire or become excessively brittle.
- To protect the fish against insect infestation during processing. Blow-flies lay their eggs in the fish while they are still moist and the larvae eat the flesh. Beetles, such as the hide beetle, lay eggs in the fish as they are drying and the larvae eat the flesh even when it is quite dry. Damage can be reduced by ensuring that processing waste is properly disposed of so that there are no places for insect to breed. Using better salting techniques may help since insect larvae are not attracted by heavily salted fish. Techniques which speed the drying process are useful in countering blowflies. Temperatures in excess of 45°C decrease infestation by fly larvae although 20 hours at this temperature is required for complete de-infestation. Fumigation or heat treatment such as resmoking can be used to kill beetle pests. 40 minutes at 70° C is generally sufficient to kill insects in a dried product.
Although the spoilage process of fresh fish can be inhibited through various curing methods, cured products may still become inedible due to other causes such as mould or insect attack. The subject of losses in cured fish has been reviewed in detail by FAO (1981). It is hoped that the research and development programme recommended in the FAO study will yield improved techniques and reduced losses.
The storage life of cured fish will depend on the adopted curing methods and packaging. Cooking inhibits spoilage by destroying bacteria and preventing certain chemical changes. Tropical fish cook at temperatures over 50° C, although higher temperatures are usually used to reduce the time required to complete the process. Boiling fish in water for a few minutes is, for example, a popular process in South-East Asia. It must be stressed, however, that the preservation of fish due to cooking alone is short term unless recontamination by bacteria is prevented by canning or a similar process. Canned fish can be kept for a long time but the process is expensive and may not be suitable if retail prices are to be kept low. Cooked fish, such as boiled or hot-smoked products, must also be salted and/or dried if a storage life of more than two days at tropical temperatures is required.
The storage life of cured fish may be increased if the following measures were adopted:
- To ensure sufficient drying of the fish in order to avoid attacks by certain bacteria or moulds during storage.
- To use appropriate packaging, and to store packaged goods in cool storage areas protected from dust, insects, etc.
- To avoid excessive smoking and drying of fish if losses are to be avoided through the breaking of fish into small pieces.
- In general, it would be useful to process the right amounts of fish at a time, so that the whole output can be sold within the estimated storage life of the processed fish. It would not make sense to process much more than can be marketed within a given period of time, and have the surplus thrown away because it is not anymore fit for consumption.
More information on the various measures needed to prevent the early spoilage of fish will be further elaborated in the following chapters.