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2. FRESHWATER FISH AS RAW MATERIAL FOR PROCESSING

2.1 Nutritive and Technological Values of Freshwater Fish

The manufacturing potential of the raw material as food depends on two features - the nutritive and the technological value.

2.1.1 Nutritive value

The nutritive value of dishes prepared from fish and from animal meat is comparable, but in some cases fish-based meals are advisable. In such an evaluation, many parameters, such as energetic value, quality and content of protein components, vitamins and mineral compound content should be examined. The energetic value of eel meat is lower than that of fat beef (1 050 kJ/100 g and 1 250 kJ/100 g respectively); while in the case of trout it amounts to 600 kJ/100 g and it is lower than for lean beef, 735 kJ/100 g. Thus the meat of freshwater fish can be a valuable constituent in low-calorie diets and at the same time has a high energy content.

The composition of amino-acid proteins in fish meat is similar to that of a hen's egg. Consumption of fish together with products of plant origin which are poor in some amino-acids (lysin, threonine), enables not only a complete utilization of plant protein, but also improves the content of a diet.

The biological value of freshwater fish fats is lower than that of marine fish because the former contain fewer unsaturated aliphatic acids. Fish meat is valuable as a source of vitamins and mineral substances. It contains especially the trace metals such as: selenium, molybdenum, cobalt, whose value is emphasized by physiologists.

The definition of food stresses that the basic food ingredients as well as the raw materials used for its production must be wholesome. However, contamination of the environment is fast increasing, especially through the use of chemicals in agriculture or in industry. For that reason, certain countries or groups of countries establish limits and recommendations for permissible levels of chemical contaminants the excess of which leads to exclusion of such raw material from the production of food for human consumption. This problem, for many reasons (diets, habits, analytical methods), is far from being solved as countries have different attitudes in this respect. It may well constitute a non-tariff barrier on a free market in the future.

2.1.2 Technological value

The technological value generally depends on two parameters: the yield of preliminary processing and the quality features of fish meat and by-products. The yield of edible parts of the fish depends, first, on the species and constitution, and also on age and consequently size and maturity.

Yield is affected by the ratio between edible and inedible parts of the fish and this is a decisive factor with regard to the technological value of the fish. This ratio depends on the species. It is most favourable in the Salmonidae family, amounting to approximately 75% of the weight. For most fish species this parameter ranges from 50 to 60%. In the case of perch and most of the Cyprinidae family the yield is less than 50%. More information on the yield of preliminary processing of freshwater fish is given in Table 2.1.

Table 2.1 Average yield of preliminary processing (manual processing) of several species of freshwater fish

SPECIES SIZE OF FISH

[kg]

FORM OF

PREPROCESSED FISH

YIELD

[%]

Trout > 0.35 gutted

74 - 82

Trout > 0.35 h/g (deheaded/gutted)

62 - 74

Trout > 0.35 fillet with skin

50 - 55

Carp > 3.0 gutted

76 - 82

Carp > 1 - 3.0 gutted

73 - 79

Carp > 3.0 deheaded and gutted

55 - 61

Carp > 3.0 chunks

49 - 57

Carp > 3.0 fillet with skin

41 - 49

Pike-perch > 1.0 gutted

79 - 89

Pike-perch > 1.0 deheaded and gutted

66 - 74

Pike-perch 0.35 - 0.5 deheaded and gutted

60 - 68

Pike-perch > 1.0 chunks

56 - 68

Pike-perch > 1.0 fillet with skin

52 - 64

Pike 1 - 3.0 gutted

76 - 84

Bream > 1.0 gutted

68 - 76

Bream 0.5 - 1.0 deheaded and gutted

56 - 64

Bream 0.5 - 1.0 chunks

52 - 64

Evaluation of the technological value of freshwater fish should take account of its possible utilization for different products, considering the sensory properties such as: flavour, texture, appearance, size and bone content. These parameters are decisive as to consumer's interest and thus the market demand.

Fish with high bone content are not so popular as a product for consumption. Therefore, the technological value of roach (Rutilus rutilus) is lower than that of pike-perch (Stizostedion lucioperca). The taste of freshwater fish depends mainly on the quality of their water habitat and on their food. It is known that fish (for example, carp) living in dirty and muddy ponds, have an unpleasant flavour. The flavour of wild trout from streams is better than that of fish from aquaculture. The opposite is true for eel resulting from the fact that the aquaculture eel has a more tender tissue and thinner skin.

Freshwater fish are classified according to size, larger individuals usually being preferred to small fish. This is also connected with bone content: e.g., trouts weighing about 300 g are very popular as single portions, prices increasing with popularity. The most expensive are fish weighing over 500 g which are destined for smoking. The best market value are carp of 1-2 kg, but those exceeding 3 kg have less customer appeal.

The sanitary and hygienic condition of fish and fish meat also influences the technological value. This relates to the presence of parasites and pathogenic micro-organisms.

However, the main role in evaluating technological value and usefulness is played by a set of features termed freshness. These features change during storage after the death of the fish and the intensity of the changes depends on the species, fishing conditions and storage conditions immediately after capture.

2.2 Post mortem Changes and Fish Quality Assurance Methods

On the death of the fish, processes of physical and chemical change caused by enzymes and micro-organisms begin to occur. The complete decay of the fish is the final result of those changes.

Post-mortem changes which take place in fish tissue occur in the following phases:

- slime secretion on the surface of fish

- rigor mortis

- autolysis as enzymatic decomposition of tissues

- microbiological spoilage

The duration of each phase can change or phases can overlap. This depends on storage conditions, especially the temperature which greatly influences these processes.

2.2.1 Slime secretion

Slime is formed in certain cells of fish skin and the process becomes very active just after fish death. Some of the fish, for example eel, secrete more slime than, for comparison, Salmonidae and perch. Fish which secrete great quantities of slime have poorly developed scales; very often the quantity of slime reaches 2-3% of the fish mass and that in turn creates problems during processing. The secretion process stops with the onset of rigor mortis.

Slime contains large amounts of nitrogenous compounds and these provide good nourishment for micro-organisms originating from the environment. Therefore, the slime spoils quickly: first giving an unpleasant smell to the fish, and second opening the way for further and deeper bacterial penetration into the fish.

2.2.2 Rigor mortis

Rigor mortis is a result of complicated biochemical reactions which cause muscle fibres to shorten and tighten, and finally the fish becomes stiff. Rigor mortis has many technological consequences. If, for example, the bones were removed prior to rigor mortis the length of the fillet shortens by 30%. At the same time, the fillet becomes wider and thicker because its volume does not change.

This tightness very often causes the connective tissue of individual myomeres to break; this process is termed "gaping" and results in muscle separation which is considered a quality defect. "Gaping" depends on temperature; the higher the temperature of fish at the beginning of the rigor mortis process the greater the gaping of the muscle. Therefore, during rigor mortis fish temperature should be as low as possible. For example, for roach and perch kept at 0 C rigor mortis begins 24 hours after death and lasts for 72-80 hours. When the same species is kept at 35 C it begins 20-30 minutes after death and stops after about 3 hours. The time rigor mortis begins and its duration depend on the fish species (e.g., for carp at 0 C it starts after 48 hours, for roach and perch at 0 C after 24 hours), on the fish catching technique, and on fish temperature. It was also found that fast swimmers, for example trout, undergo rigor mortis faster but for a shorter duration than slow swimmers like carp.

In those fish which are in good condition (well-nourished) rigor mortis is more intensive. Fish put to death just after removal from the water reach a state of rigor mortis later than those fish which died after a long agony. In the case of carp put to death just after capture rigor mortis begins after 48 hours, but if the carp died after a long agony it sets in after 24 hours (at 0 C).

Unnecessary and rough handling of the fish can shorten the time of occurrence and duration of rigor mortis. Such treatment causes stress in live fish.

Fish body temperature is a decisive factor in the onset and duration of the rigor mortis process. The higher the temperature the sooner it begins and the faster it ceases. This is evidenced by enzymatic reactions whose speed increases with increased temperature. At high temperatures it results in greater changes in proteins, the latter causing higher loss of tissue juices, e.g., during processing. Usually, the later rigor mortis begins and the longer it lasts, the longer are the storage life of the fish and its use for consumption.

2.2.3 Autolysis

On the death of the fish, a complicated biochemical process starts, leading to a decomposition of basic compounds of tissues which takes place under the influence of enzymes. This decomposition involves proteins, lipids and carbohydrates. Its intensity is not the same for all compounds and the decomposition of one can influence the decomposition of the others.

The quality of fish as a raw material for consumption or for processing depends largely on proteolysis, that is, the decomposition of proteins. This process follows rigor mortis. The final products of protein hydrolysis, under the influence of enzymes, are: amino-acids and other low-molecular substances which have an impact on the sensory features of fish. A similar situation concerns the products of lipid autolysis: thus autolysis cannot be qualified as a phase in the spoilage process.

During autolysis, great changes occur in the structure of muscle tissue which becomes softer, and very often falls into layers along the myosepts. In small fish, perforation of the belly occurs. From the technological view, it is negative because the proteolysis process leads to a decrease in the capacity of tissue to retain tissue juice, resulting in toughness of texture of the final product. The degradation of proteins creates ideal conditions for the growth of spoilage bacteria.

2.2.4 Microbiological decomposition

The muscle tissue of live fish is generally sterile but bacteria thrive in the alimentary tract and on the skin, and from there they penetrate into the muscles; for example, through the blood vessels. This process is further favoured by structural changes in the tissue as a result of rigor mortis and autolysis. Bacteria are able to decompose proteins, but products of proteolysis such as amino-acids and other low-molecular nitrogenous compounds provide better nourishment. Thus it was found that, due to lower content of these substances, freshwater fish tissue undergoes microbiological decomposition more slowly than marine fish tissue. Micro-organisms cause decomposition of not only proteins but other compounds containing nitrogen, lipids to peroxides, aldehydes, ketones and lower aliphatic acids. However, the decomposition of nitrogenous compounds occurs much faster than in the case of lipids.

Compounds such ammonia, hydrogen sulphide and mercaptans, indole, skatole, etc., are the final products of microbiological spoilage of fish, which produces an unpleasant and then disgusting flavour.

Penetration of bacteria into fish tissue and microbiological decomposition begins with autolysis and these processes are practically parallel. However, their rate and intensity strictly depend on the storage temperature. Low temperature strongly inhibits the activity of micro-organisms in which case the autolysis process dominates.

2.3 Indicators of Fish Freshness

Freshwater fish, as other fish species, are raw material which fast deteriorates. This implies that both the producer and the consumer are very often exposed to the risk of buying fish which is not fresh or has even deteriorated. Knowledge of the average shelf life for individual fish species - depending on storage conditions - is a basic principle applied in the food - and the fish - industry. Effective, objective and repeatable methods for evaluation of raw material freshness should be specified, but attempts so far are only now showing positive results. Thus, sensory analysis is the main method of evaluating fish freshness. It enables differences in texture, flavour, and taste to be determined, and subsequently the usefulness of the raw material. Sensory properties change during storage from the desired very high standard, through neutral or average, and finally to undesirable or disgusting. It is generally assumed that prior to disappearance of desirable features the fish is considered to be fresh, while the appearance of undesirable or disgusting features disqualifies the raw material. The most difficult step is to determine an intermediate state in which the fish is not entirely fresh. Sensory analysis is thus carried out on raw fish and cooked fish. Flavour, appearance and state of abdominal cavity (for not eviscerated fish) are the main indicators of quality in the case of raw fish. For cooked fish, smell is the most important indicator. These problems are covered in section 6.5.2 Quality control.

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