1.1 Composition of Fish
1.2 Spoilage of Fish
One of the important issues affecting fish preservation is the large biological variations existing from one region of the world to another and from one species of fish to another. This, combined with the fact that catching methods and consumption habits vary, has a considerable influence on the handling and preservation of the product.
In order to choose and operate refrigeration systems in the best possible way, some knowledge of the fish biology and factors influencing the quality are essential.
The fish has a skeletal or cartilaginous structure which provides support for the body. The muscles which form the edible part account for most of the weight of the fish. The skin forms a cover, often with an outer layer of scales, and secretes a slimy mucus, which lubricates the fish and seals the surface. The gills are the main part of the breathing mechanism and take up oxygen from the water. The organs in the body cavity, including the stomach, intestine and liver are known as the guts. Removal of the guts is normally the first step in handling and preservation. Shell fish has no backbone, but a hard outer cover or shell exoskeleton, which gives the necessary support and protection.
The principal components of the fish muscle - water, fat and protein - must be preserved with little or no changes. The protein content is usually in the region of 15-20 percent, whereas the fat content varies widely from species to species and from season to season. It can be as low at 0.5 percent in lean starved fatty fish and can reach over 20 percent in some species. In lean fish the bulk of the fat is stored in the liver and not in the muscle. Water is the main constituent, with considerable variations, typically 80 percent in lean fish and 70 percent in fatty fish. Carbohydrates, minerals, vitamins and some water extractable components are examples of other minor substances present.
As soon as a fish dies, spoilage begins. Spoilage of fresh fish is a rather complex process and is caused by a number of inter-related systems, some of which are suppressed by others. The factors which principally contribute to the spoilage are the degradation of protein with a subsequent formation of various products like hypoxanthine, trimethylamine, development of oxidative rancidity and the action of micro-organisms.
In live fish, food in the gut is reduced to simple substances, such as sugar and aminoacids, which are absorbed into the blood stream. The blood conveys these essential substances to sites where they are required, notably in the muscles. Production of these substances is induced by enzymes, which act as catalysts to chemical reactions, both in the gut and in the flesh. The enzymes remain active after death and thus bring out self-digestion, affecting the flavour, texture and appearance of the fish. After a fish dies, stiffening of the muscle called rigor mortis sets in and commences, due to the action of enzymes. Subsequently softening of the flesh occurs as self-digestion proceeds.
Self-digestion can take place rapidly in fish, especially in small fatty fish full of feed, where the gut enzymes are particularly active. The well-known phenomena "Burst Belly", which can occur in only a few hours after catch in sardines, herring and some other fish, is caused simply by a weakening of the belly wall due to self-digestion. The rate of selfdigestion is much dependent on temperature. Chilling of the fish to just above the freezing point does not stop, but retards self-digestion. Enzyme action can be stopped by heating; it is controlled to some extent by other methods, such as salting, frying, drying and marinating.
Micro-organisms are present in the surface slime, on the gills and in the intestines of the fish, but the muscle is sterile. Although it is not known with certainty how long it takes for the bacteria to penetrate the skin of the muscle three to four days is a reasonable estimate, but each species may be somewhat different. Fresh fish is seldom the cause of food poisoning, since the bacterial growth tends to make the muscles or flesh unpalatable before any toxins develop.
The environmental microflora introduced by cooling medium and by handling are also responsible for spoilage after the initial phase of self-digestion. Soon after the fish dies, bacteria will enter at a number of points, through the gills and into the blood vessels, through the lining of the belly cavity and eventually through the skin. Once in the flesh they can grow and multiply rapidly, producing disagreeable odours and flavours.
There are many different types of micro-organisms, each type having particular conditions for optimum growth. Thus it has been found that certain types of microorganisms dominate, depending on the initial infection, the properties of the food material, the temperature and other conditions. By cooling the fish to around 0°C, some of the bacteria groups responsible for the spoilage will cease to grow and the rate of spoilage will thereby be reduced.
Ambient conditions, such as the amount of moisture and oxygen available, have a marked effect on the microbiological activity. In melting ice the rate of fish spoilage is to some extent dependent on the rate of melting. Providing there is sufficient ice to maintain the desirable fish temperature of 0°C a higher melting rate can give slightly better results than a lower melting rate presumably due to the washing effect. Where the fish is in contact with surfaces such as wood, metal or other fish, foul odours can arise due to the action of certain anaerobic bacteria, which thrive in the absence of oxygen.
As microbiological action is the main and fastest cause of spoilage, great care must be taken to avoid conditions which accelerate the growth of micro-organisms. The growth rate is highly temperature-dependent and the principal preservative measure, besides good hygienic conditions, is to cool the fish as soon as possible after catching. Other supplementary measures have been tried, for example the use of antibiotics and different gases. So far only marginal improvements have been achieved and these supplementary methods have not found any wide applications.
Some of the chemical changes are caused by enzymatic reactions, the first taking place even before any serious changes are caused by microbiological activity. These enzymatic reactions are associated with rigor mortis. The result of those changes is that some constituents are chemically altered and some even disappear, altering the sensory properties -odour and flavour. Some of these substances, commonly known as extractives, are the first to be changed by the microbiological activity and the protein of the muscles will change considerably later.
The extractives are present in varying amounts from species to species. Herring and mackerel contain large amounts of amino-acid histamine, whereas cod and haddock only contain traces. Skate, dogfish and shark contain large quantities of urea which is absent in cod.
Trimethylamine oxide, which is available in all the salt water fish is usually absent from fresh water species. The breakdown of trimethylamine oxide into trimethylamine (TMA) is an important reaction, as the chemical determination of TMA may be used in quality assessment of salt water fish. Equally important is the determination of ammonia in some species, eg., sharks. Ammonia being formed during the breakdown of urea.
Chemical denaturation of proteins to a noticeable degree appears normally late in the deterioration process, as does oxidation of fat.
Development of oxidative rancidity is extremely variable in fresh fish. The ease with which some fish undergo oxidative rancidity is in part, explained by the large proportion of the highly unsaturated fats that many fishes contain. There is, however, a great difference between fatty species, such as mackerel and herring and fish such as cod and haddock. The former group have a high lipid content, free fat content and proportion of triglycerides, while the latter have a low lipid content, chiefly in the form of phospho lipids and lipoprotein immediately associated with muscle proteins. Even within a single fish itself there is a difference in the ease with which different portions undergo rancidity. Seasonal variations in susceptibility to rancidity have also been found.