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In small freshwater fish processing plants only limited preservation methods are used as compared with marine fish processing establishments. The main methods of freshwater fish processing and technological examples are discussed below.

4.1 Chilling and Storage of Chilled Products

Decreasing the temperature of the fish to about 0° C slows down the microbiological, chemical and biochemical decomposition processes and extends fish stability. Thus when the raw material is cooled quickly, just after capture, and kept at low temperature during transport, processing and distribution, it meets the basic processing requirements. Its usefulness is extended and at the same time fish quality is maintained.

In freshwater fish processing the raw material, and semi-products and final products are almost exclusively ice-cooled. The heat exchange process between fish and ice is complex as it takes place between the fish surface and the ice, between the surface of fish and the melting ice water, and also between the fish and the cool air in spaces between the pieces of ice. Overall, it is a dynamic process, changing minute by minute. Water from the melting ice plays the most important role as it causes a typical convective exchange of heat. But the direct exchange of heat between ice and fish is also important, and thus the ice granulation is very important for the whole process.

In modern fish processing plants, especially the small ones, flake ice generators dominate as flake ice ensures major contact surface with fish and its production cost is low. Flake ice production consists in freezing a thin layer of water on the cooled surface of a cylindric evaporator and then scraping off the ice with a knife.

Modern ice generators generally comprise a vertical cylindric evaporator. Ice is formed on the outer, inner, or on both the surfaces of evaporators (Figure 4.1).

Ice production is a continuous process and ice is collected in an insulated container. When the container is full the mechanism stops functioning. Capacities of flake ice generators vary from 100 kg/24 h to 60 t/24 hours. However, due to the high cost of equipment, fish producer should rather consider purchasing flake ice from the nearest cold store plant.

When the producer decides for organizational reasons (e.g., production unevenly distributed in time) to buy an ice generator it is advisable to buy two small capacity generators instead of one of a greater capacity.

The effectiveness of temperature exchange depends on the thickness of the layers of fish and the distribution of ice. For example, an 80 mm layer of fish requires two hours to decrease the temperature from 10° C to 17° C when exposed to double-sided cooling, and about 24 hours when exposed to one-sided cooling.

To evaluate optimal conditions for fast cooling of fish, many parameters (degree of ice granulation, temperature of the fish and the environment), which influence the activity of the process, should be known.

Greater amounts of ice do not shorten the process. It was ascertained that use of 25% ice in relation to the amount of fish causes temperature to drop to 5° C after 3.3 hours, for 50% ice - cooling down to 1° C takes 6 hours, but for 75% ice - 2.25 hours.

Standards for use of ice should be set individually for different types of fish and fish products, different conditions, seasons, etc. The ambient temperature does not affect the cooling rate of the fish, but considerably affects the amount of ice necessary to maintain a low temperature. It is difficult to determine the exact amount of ice needed to keep the fish temperature at about 0° C. In short-distance transportation (up to 24 hours) during the cold season (up to 10° C) 1 kg of flake ice is sufficient to cool 8 kg of fish. When ambient temperature exceeds 10° C, 1 kg of flake ice suffices for 4 kg of fish.

Proper handling of freshwater fish as raw material and its products ensures continuous cooling with ice and maintenance of temperature. All processing phases should be as short as possible and if for any reason a surplus of raw material occurs this should be sent to the cold stores.

Raw materials and products should be transported so as to ensure the maintenance of temperature close to 0° C; this involves both the most simple isothermal vehicles and mechanically-cooled containers. Fish and fish products should reach the buyer without delay. In practice, in freshwater fish processing the wholesale storage phase is omitted due to the small scale of this kind of production. Products are delivered direct to shops where they should be placed in cold stores and if necessary ice should be added. Good trade practice indicates that retailers should only keep a one-day stock of cooled fish or fish products such as fillets, deheaded and gutted fish.

The following diagrams show the flow of technological processes for chilled products (Figures 4.2, 4.3, 4.4).

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Figure 4.4 Production of chilled fillets of trout and carp (technology used in Poland)

4.2 Freezing and Refrigerated Storage

Even when the most effective chilling methods and further chilled storage of raw fish and fish products are applied, shelf life is limited. Freezing is needed to extend shelf life for long periods. This can be achieved by changing two parameters: first, a considerable decrease in temperature, and second, by freezing the water in the fish tissue. The second is of particular importance because water in the fish tissue acts as a solvent for many organic and mineral compounds which are a suitable environment for the growth of micro-organisms and also because they influence the biochemical processes. At the same time, the frozen water in the tissue causes changes in muscle tissue as a result of damage of cell structure during the formation of ice crystals. Further, the denaturation of proteins takes place during this process. An increased drain of tissue fluids, fat oxidation and dehydration are the effects of denaturation which are visible after the defrosting process. During the freezing process the majority of micro-organisms is inactivated and only psychrotrophic bacteria can develop in such conditions and to a limited degree. A temperature of about -10° C is a limit for growth of such micro-organisms. Some moulds and yeasts multiply very slowly at -15 to -18° C.

Fish should be frozen rapidly in order to produce the highest quality frozen products. Quick freezing implies a fast change from cryoscopic temperature to -5° C. During this period (about 2 hours) the main changes take place in fish tissue. A faster freezing process is linked to the formation of smaller ice crystals which damage the cellular membranes to a lesser degree, especially if freezing takes place before rigor mortis sets in.

The size of the ice crystals depends on the duration and temperature at which the fish was chilled/stored prior to freezing. The longer the time and the higher the temperature the bigger the crystals. Changes of proteins and oxidization of lipids in muscle tissue are the results of slow freezing process and unsuitable conditions (time, temperature) of fish storage before freezing. These affect the quality of the final product.

In small fish processing plants there are usually two kinds of freezing equipment: chamber freezers and contact-plate freezers. The simplest is the chamber freezer-batch air blast freezer which consists of a battery of evaporators, a ventilator for air circulation and a rack for trays with fish products or for unpacked raw material. Versatility is the main advantage of such freezers as they make it possible to freeze different kinds of products, for example, regular shape blocks of fish/fillets and individual fish/fillets on the wire nets.

For that reason such freezers can be used in small plants; but high energy consumption and their large size are the main disadvantage. Contact freezers are far less common in fish processing plants with low daily production. Their operation consists in placing the fish for freezing between two plates which are cooled mechanically. This device is installed exclusively for freezing fish which is in regular blocks. In these freezers, good contact between the plates and the fish is essential to ensure rapid removal of heat from the product. Many kinds of such freezers are available including those with limited capacity, e.g., 1 500 kg/24 h, and requiring little space, about 1.2 mē.

Even properly frozen fish has limited storage life. Low temperatures inhibit processes of microbiological decomposition but do not protect against fat oxidation and loss of water. The stability of frozen fish depends on the initial quality of the raw material, the rancidity, the drying process and the storage temperature.

Glazing is the simplest and cheapest method which effectively prevents water loss of from fish tissue and prevents rancidity. Glazing consists of forming a very thin adherent layer of ice on the fish's surface. This method is used especially for freezing of whole fish or in fish/fillet blocks. Individual portions of fish or individual fillets are packed in plastic material characterized by low permeability of water vapour and oxygen. This prevents rancidity and loss of water.

The storage temperature of frozen products is the next factor which influences the quality and stability of frozen products. Table 4.1 shows the practical storage life of fish products in relation to temperature. Unfortunately, industrial practice shows that the basic principles of freezing process are often not complied with, especially in small and poorly equipped establishments. Fish is frequently frozen in store chambers, home freezers, etc. The capacity of such chambers is limited, temperature is not stable and generally lower than required. Further, no temperature recording is made. Low quality of products results from such practice, particularly texture and flavour; fish becomes dry and very often discoloured.

Table 4.1 Practical storage life (PSL) of fish products in relation to storage temperature

Fish product Storage life in months
-18 ° C -24 ° C -30 ° C
Fat fish glazed 5 9 > 12
Lean fish fillets 9 12 24

4.3. Smoking of Freshwater Fish

Smoked freshwater fish such as eel or trout, and less often carp, are the most popular fish products. Saturation of raw material with wood smoking is the main principle of the smoking process. During this process, some water is removed from the tissue and changes of proteins occur. The smoked fish is then ready for consumption without further culinary treatment.

There are two methods of fish smoking: hot and cold, which give very different products. The difference lies in stability and sensory properties which in turn depend on a degree of fish dryness and saturation with smoke components.

Smoke is produced by a not complete burning of some type of wood and is a mixture of more than a hundred chemical components. The chemical composition of smoke depends on the type of wood and traditionally deciduous tree wood is used.

During the smoking process sensory features such as colour and flavour undergo changes. The colour of properly smoked fish depends on the quantity and composition of the smoke components absorbed through the fish surface; the higher the smoke density the darker the colour of the fish. Smoke density and humidity inside the smokehouse influence smoked fish characteristics. Flavour is the most typical feature of smoked products. It is generally considered that phenol compounds and other components soluble in water are the most important criteria in creating flavour in smoked products.

The presence of antioxidants in smoke renders smoked products resistant to rancidity. Hot-smoking reduces microbiological growth thanks to high temperature (close to 80° C in tissue) and the antiseptic components of smoke. Generally, after hot-smoking fish products contain only meso- and thermophilous micro-organisms, resulting from heating the product and not the antiseptic action of smoke components and salt content. Cold-smoking enables preservation of the product by smoke components. Their concentration in the product is higher than in hot-smoked fish and the product is drier. The vegetative forms of micro-organisms are the most sensitive to smoke treatment but spores of moulds are relatively resistant. For that reason, smoked products often grow with mould - the main disadvantage.

The hot-smoking process includes the preliminary processing of raw material, brining, drying to a certain loss of water content, the actual smoking process and thermal treatment at temperatures above 30° C, usually 70-80° C (Figure 4.5, 4.6, 4.7).

The cold-smoking process involves no thermal treatment and the entire process is carried out at temperatures below 30° C (Figure 4.8).

During hot-smoking, brining is carried out to ensure penetration of about 2% of salt into the fish tissue; the salt gives the desired taste to the product. During cold-smoking, salt is required for the conditioning process which favours the action of the enzymes. However, the brining process can be a source of microbiological reinfection. It was shown that multiple use of brine, 20% salt content, may produce a source of many micro-organisms including spores of Clostridium botulinum. The brine thus needs to be changed frequently.

Drying is carried out in order to reduce the water content in fish tissue to a level which ensures product stability and texture. Usually 25-30% weight loss takes place during hot-smoking and 40-45% during cold-smoking.


* See section 3.3, preliminary processing covers: deheading, cutting, gutting, removing of kidney, cutting off fins; big fish can be cut into pieces 50-70 mm thick


* See section 3.3, preliminary processing covers: scaling, cutting, gutting, removing of kidney, blood and slime from the surface of fish


* See section 3.3, preliminary processing covers: scaling, removing slime from skin, filleting, removing blood, clotted blood and peritoneal traces

During hot-smoking thermal treatment should be continued until the temperature inside the thickest part of the fish reaches about 70° C. This ensures the denaturation of proteins and destruction of micro-organisms to a high degree. In some countries, e.g., the USA, fish originating from the Great Lakes could be infested with C. botulinum. Thus fish with minimum 3.5% salt content should be heated up to 82.2° C and thermal treatment continued for about half an hour. That process should be followed by very rapid cooling and storage at temperature below 4° C or preferably freezing. Thermal treatment should be conducted at humidity lower than 70% because of bacteriological effect. Thermal treatment in the modern smoking house (Figure 4.9), very often equipped with an automatic control stem and adjustment of processing parameters, like air and smoke, can be programmed to maintain optimum temperature. Traditional methods of smoking do not ensure the same results but the traditional process, carried out in smoking chambers, is much cheaper. Wood is a source of smoke and energy necessary for this process. The effectiveness of traditional method depends on the experience of the operator.

Packaging materials and packaging methods of smoked products are described in section 5.

4.4 Production of Fish Silage from Offal

During fish processing, a large quantity of offal is produced and its proper utilization poses a problem, particularly for smaller processing plants. Fishmeal production is not profitable because of a low supply of the raw material, and thus production of a liquid form of this fish product is the only simple solution.

Production of fish hydrolysate (silage) to be used as feed is the cheapest way of utilizing offal. Considering the capital needed and the operating costs for fishmeal and hydrolysate production (cost ratio 4:1), production of the liquid form of this by-product is very profitable and it can be done by small plants. It is a simple technological process, but several rules must be observed to obtain a satisfactory final product.

The raw material, the, offal, must be fresh; decomposing offal should not be processed. The main phases of offal processing are: grinding of offal or whole fish, acidifying of the pulp and liquefying it which results from a self-digestion (autolysis) process. Adequate grinding is a basic operation of the process.

The following preservatives are used to produce pyrosilage:

- sodium pyrosulphite (Na2S2O5), 1% for fatty and medium fatty offal, and 1.3% for lean product,

- sulphuric or hydrochloric acid, both at 1% concentration in the mix

The measured pH should always be the final indicator of a proper level of acidification and should range from 3.5 to 4.5. The pH should never exceed 4.5.

The basic requirement of the process is to obtain homogeneity of the mix consisting of the fish, inorganic acid and sodium pyrosulphite. Homogeneity can be achieved by using slowly revolving mixers or other methods (turbulent mixing causes aeration of the mix and consequently oxidation of fatty acids). When mixing is too gentle, pockets of mix occur which do not contain preservatives, and decomposition of the product by the bacteria may begin. Each day the end-product is pumped into the retaining tank(s). These tanks should be equipped with mixers or recirculating systems powered by pumps. The tanks should be located under a roof to avoid solar radiation. The silage can be stored for up to 6 months if it is stirred periodically and kept at about 15-20 oC. In small freshwater fish processing plants where the volume of offal and fish not used for consumption is low (i.e., 1-2 t/shift), the production of fish hydrolysate is simplified (Figure 4.10). The processing equipment consists of a grinder (sieve openings 6-10 mm in diameter, processing capacity circa 400 kg/hour), dispenser with a worm-wheel unloading conveyor, rotating mixer made of suitable materials with a 150-l volume drum, and 120-l plastic barrels.

This equipment (Figure 4.10) is manned by an operator who can produce 2 t of liquid feed per shift.

A production cycle consists of the following stages:

- grinding of the raw product in a grinder

- loading of circa 100 l of ground product from the dispenser into the mixer drum, and adding 1.6-2.0 l of sulphuric acid at density 1.28-1.3

- mixing for about 10 minutes and adding a solution of sodium pyrosulphite (1 kg of pyrosulphite dissolved in 3-4 l of water)

- additional mixing for 5-7 minutes and pouring of the product from the mixer drum straight into the 120 l barrels

An approximate chemical composition of fish silage is:

- protein - about 15%

- fat - 6-14% (depending on raw material)

- ash - 2.4%

- micro-elements and vitamins

Different forms of fish hydrolysate are used for feeding pigs, poultry, fur animals and fish. Hydrolysates contain very valuable, easily assimilated proteins and fatty acids, unaltered vitamins, micro-elements and digestive enzymes. For pig and poultry feed, fish hydrolysates can be substituted for fish meal, meat and bone meal, and powdered blood. Experiments showed a 10-20% increase in weight and a lower feed use per weight gained by an animal. It was determined that 1 kg of hydrolysate equals 0.3 kg of fish meal, and its use reduces the need for feed by 0.66 kg per 1 kg of weight gained. Polish scientists reported 0.7 kg/day of weight gained when bacon-type pigs were fed fish hydrolysates.

According to Danish researchers, no more than 15% of the total feed given to pigs should consist of fish hydrolysates, and these should be detracted from the diet several weeks before slaughter. The Polish and Danish experiments confirmed the positive results of feeding poultry with fish hydrolysates instead of fish meal (chickens were fed hydrolysates in amounts equal to 50% of the daily protein requirement). Substitution of dry animal and fish meal with hydrolysates gave very good production results:

- use of feed per 1 kg of weight gained equal to 2.54 kg

- mean body weight of an 8-week old chicken was 1.20 kg

- slaughter efficiency higher by 23%

- costs of the components used in the feed lowered by 20%

- content of additional animal feed lowered by two-thirds, that is, by 110 kg/1 t of combined feed, fish and meat meal, and powdered milk

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