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3. PRELIMINARY PROCESSING OF FRESHWATER FISH

3.1 Requirements Related to Freshwater Fish Processing

Freshwater fish processing, like the processing of other food raw materials, should:

- assure best possible market quality

- provide a proper form of semi-processed of final product

- assure health safety of products

- apply the most rational raw processing method

- reduce waste to the extent possible

Due to its chemical composition, fish is a perishable raw material. Fish flavour and texture change rapidly during storage after death. It is thus advisable in freshwater fish processing to keep the fish alive as long as possible. Actions focusing on quality assurance also involve transport and storage/depuration of the fish awaiting processing (described in section 3.2).

In order to reduce the bacterial processes, immediately on death fish should be deheaded, gutted, washed and chilled in order to inhibit unfavourable enzymatic and microbiological processes. If fish is not sold fresh, preservations methods should be applied in order to extend shelf life. These could include freezing, smoking, heat treatment (sterilization, pasteurization, etc.).

Another aspect of fish processing is to give the product a form which is attractive to the consumer, e.g., skinless fillet or deheaded fish with fins removed.

The third main goal of fish processing is high product quality and extended shelf life.

Fresh fish can be stored only for the short time that processing technologies allow for the storage life of fish to be extended without significant loss of quality.

Fish processing must ensure full health safety of fish products and proper sanitary conditions as well as selection of a process (e.g., sterilization, pasteurization) which render impossible the development of harmful micro-organisms and toxins. High quality products which are safe and satisfy the consumer can be reached by compliance with processing parameters, from the start of the operation to the distribution of the final product.

Appropriate processing should enable maximal use of raw material and thus contribute to increased economic profitability. This is a basic approach in modern industry. A filleting operation offers a classic example of such an approach in which, apart from the fillets, minced meat can be produced from the waste material and the remainder sold as animal feed. Thus the process results in practically no unused waste material. However, achievement of this goal essentially requires that mechanization be introduced into processing, albeit on a small scale. At the same time, it is noted that production of value added products is obviously the basis of processing profitability and can be a decisive factor for the survival of many fish processing plants, especially the small ones.

Fishing, processing, transportation and sale of fish products are links in a complete processing chain. Each has its own importance but only together can they form an inseparable process to provide the customer with a top quality product.

3.2 Handling of Freshwater Fish before Processing

The quality of the raw material and its usefulness for further utilization in processing is affected by the fish capture method. Unsuitable fishing methods e.g., catching too many fish in one haul, cause not only mechanical damage to the fish, but also create stress and the conditions which accelerate processes which begin after fish death.

In many countries consumers are used to buying live fish: this assures the highest quality. This habit takes different forms, e.g., the consumer buys live fish, for instance carp or trout and processes it at home. Very often the fish bought live can be partly processed by the shop assistant; for example, it can be filleted. In some restaurants the customer can choose the fish from an aquarium and have it prepared for consumption. Thus the tradition, the quality, and the resultant price, constitute the reason why the preparation of fish for transportation, and the transportation itself, are the preliminary operations of processing of freshwater fish like trout, carp, eel, etc. However, producers should remember that not all fish are suitable for transportation alive. Therefore, just after fishing, fish should be sorted and only those in good condition, healthy and not damaged be destined for sale as live fish. Fish so classified is first conditioned in water of appropriate quality. The conditioning process reduces stress, inhibits metabolism and at the same time food remains are removed from the alimentary ducts and the oxygen demand reduced. During the conditioning process fish is not fed which further inhibits metabolism and also limits the excretion of ammonia and carbon dioxide. In the short conditioning process 1 m of water is sufficient for 50-60 kg of carp, 30-40 kg of pike, 20-25 kg of trout or pike-perch.

Water provided for conditioning must be properly oxidized.

For example, in the case of 1 kg of fish at a temperature of 10 C the oxygen demand is: eel 25 mg, carp 45 mg, pike 50 mg. Young fish need more oxygen than older fish. Oxygen consumption depends also on the liveliness of fish. The amount of oxygen dissolved in water depends on water temperature which should be rather low. But for stenothermal species such as carp water temperature should be not less than 10-12 C in summer and 5-6 C in spring and autumn. Optimal temperature for conditioning and transportation of trout is 5-6 C in summer and 3-5 C in spring. During winter fish tolerates temperatures of 1-2 C.

Nowadays, special tanks with aeration system and often with cooling and filtering (activated coal, biological filters) systems are used for transportation of live fish. In simple solutions water is cooled by ice. Cooling is especially important during summer and in transportation over long distances. If all parameters, i.e., temperature, oxygenation, are properly maintained, and when the temperature does not exceed 10 C, the weight loss varies from 1 to 6%, and about 10% of carp and 20% of trout die during a six-day transportation in winter. At present, large valuable fish species are transported via air in which case they are placed in big plastic bags with aeration system.

3.3 Equipment for Preliminary Processing of Freshwater Fish

Preliminary processing of freshwater fish usually consists of the following steps or unit processes: evisceration, deheading, scaling, cutting of fins and belly flaps, slicing of whole fish into steaks, filleting, skinning, grinding of skinned fillets and different combinations of the above (Figure 3.1).
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The products of preliminary processing can be sold or further processed to obtain value added products. In freshwater fish processing, particularly species such as perch, pike-perch and the cyprinids, the processing steps described above are executed manually with a wide variety of knives. Efficient preparation of fish is important when top quality, maximum yield and highest possible profits are to be achieved. This is important when fish is to be exported. Efficient fish preparation is a skill only be acquired with practice. Several perfectly acceptable methods for cutting any fish exist; they may often give the same yield and similar end-products. In the future, the level of mechanization of fish processing in small processing plants will increase due to the constant pressure to reduce production costs and improve economic performance.

The present level of mechanization is low which results from the overall limited production, seasonal availability of the raw product and lack of inexpensive, efficient mechanical equipment adaptable for processing of various fish species.

In practice, most freshwater fish processing is done in small processing plants (with the exception of salmon and trout processing), usually supplying products for local or nearby markets. Manpower capacity in such plants varies, usually not exceeding 10-20 employees. In addition to freshwater fish, frozen marine fish may be processed in the same plant.

3.3.1 Stunning of fish

In many freshwater species the method of stunning is critical for final product quality because prolonged agony of fish causes production of undesired substances in the tissue. Oxygen deficiency in blood and muscle tissue results in accumulation of lactic acid and other reduced products of catabolic processes and consequently in a paralysis of the neural system. Red spots appear on the surface of the skin and in the muscle tissue near the backbone; these reduce quality.

Stunning of freshly caught fish or fish delivered live to a processing plant is best done with an electric current. First, the fish are placed in a tank of water and an electric current is then passed through the water to stun or kill the fish. Live fish are also slaughtered by cutting the aorta and bleeding to death when technological or ritual reasons require the removal of blood from the tissue before further processing.

In some plants, water in the fish tanks is saturated with carbon dioxide which renders the animals unconscious or dead.

3.3.2 Grading

The processing sequence starts from grading the fish by species and size. Sorting by species or on the basis of freshness and physical damage are still manual processes, but grading of fish by size is easily done with mechanical equipment. Mechanical graders yield better sorting precision for fish before or after rigor mortis than for fish in a state of rigor mortis.

Size grading is very important for fish processing (i.e., smoking, freezing, heat treatment, salting, etc.) as well as for marketing. Automated sorters are rarely used in small plants processing freshwater fish because the raw product is usually already sorted on delivery and because of their high costs.

Automated grading is 6-10 times more efficient than manual grading. The sorting speed of different graders varies and depends on the type of device and size of fish sorted. Sorting capacity is 1-15 t/hour, and usually into three size groups.

A combination of conveyor belt and automated sorter shown in Figure 3.2 is used by fish processing plants in the USA. This machine has an interesting design: two smooth rotating rollers are installed above the surface of the conveyor belt and the distance between the rollers and belt can be adjusted according to the maximum thickness of the sorted fish. Thinner animals fall off the belt while the thick ones are retained on it until the end of line. Therefore, one device serves simultaneously as a grading machine and a conveyor.

Figure 3.2 Combination grading machine-conveyor belt:
a - general view, b - cross-section

Most commonly used grading machines consist of a series of compartments connected by slits of varying size (Figure 3.3) with rotating rollers or conveyor belts arranged in a V-shape (Figure 3.4). In such devices fish are sorted according to the maximum thickness which is highly correlated to fish length. The size range to be sorted is easily adjusted.

Figure 3.3 Grading machine with a fan shaped arrangement of rollers:
a - scheme, b - general view.


Figure 3.4 Slit grader consisting of two conveyor belts arranged in a V-shape;
1 - rubber belt, 2 - rotating wheel

3.3.3 Removal of slime

Slime accumulating on the skin surface of dying fish is a protection mechanism against harmful conditions. In some freshwater species slime constitutes 2-3% of body weight. Slime excretion stops before rigor mortis. Slime creates a perfect environment for micro-organism growth and should be removed by thorough washing. Eel, trout and carp require special care with regard to slime removal. Even small amounts of slime, which frequently remain after manual cleaning, result in visible yellowish-brown spots (particularly in smoked eel).

Drum-washing with a horizontal rotation axis does not remove slime from some fish, e.g., eel. Eel are best washed in machines which originally serve as scalers (Figure 3.5a, b). The device is loaded with 30 kg of eel and several kilograms of salt, and after about 2-3 minutes the slime is completely removed from the fish skin. This procedure is more efficient than manual washing.

Slime can be removed from eel, trout and other freshwater species by soaking fish in a 2% solution of baking soda and then washing in a cylindrical rotating washer.

3.3.4 Scaling

Many freshwater species are routinely scaled; this is extremely labour-intensive when done manually. Some sources estimate that manual scaling of larger animals requires almost 50% of the total time necessary to produce headed and gutted fish without fins. Fish destined for skinning and filleting or to be smoked or minced in mincing/deboning separator is not scaled. Tools used for manual scaling are shown in Figure 3.6. Tools are moved over the body of fish from tail fin towards the head, pulling out the scales.

Figure 3.6 Tools used for manual scaling

Fish such as perch, bream, pike-perch and carp, are particularly difficult to scale manually. One method includes blanching of fish for 3-6 seconds in boiling water and then scaling by hand with motions perpendicular to the long body axis. Mechanized and power-assisted hand-held scalers are commonly used in small processing plants (Figure 3.7).

Electrical hand-held scalers simplify and speed up the scaling procedure. They are most commonly used for secondary scaling of fish which has left the automated scaling device 80-90% free of scales. Use of electrical hand-held scalers reduces labour intensity and assures complete elimination of scales. The power-assisted tool shown in Figure 3.7 consists of a cylindrical rotating scraper of 30-40 mm diameter powered by an electric motor and connected to it with a flexible rod. The vertical cylindrical scaler with rotating bottom (Figure 3.8 a) and fixed side wall is widely used in small fish processing plants. Fish (usually 30-40 kg) is loaded from the top and unloaded through the door in the side wall. Scales catch on small contoured slits cut in the bottom and side wall of the device, and are thus pulled out of the skin. The same machines can be used for slime removal. Cement mixers are often utilized for scaling after the original cylinder is replaced with a 120-l drum made of stainless steel, with punctured contoured slits of 10 mm diameter (Figure 3.5 b). In addition to devices which have been specifically designed for scaling, a variety of automated tools can be employed, e.g., vegetable peelers. However, their use may result in mechanical damage to the fish even after modifications (Figure 3.8 a).

A semi-automated device, shown in Figure 3.8 b, is used for scaling larger fish; fish is manually passed over the rough surface rotating drums which have contoured slits of 3-4 mm depth. One worker can scale 10-20 fishes/minute (scaling speed varying with species). Special protective gloves must be worn during this procedure.

Various scalers are designed on the same principle. The processing time of a cylindrical rotating scaler with the horizontal rotation axis (Figure 3.9) is from 2 to 7 minutes depending on the species and size as well as on the type of slits on the surface of the drum and the rotational speed. The total weight of fish loaded in one run rarely exceeds 30-60 kg.

Figure 3.9 Cylindrical scaler with horizontal rotation axis

Another kind of cylindrical scaler with a horizontal rotation axis can be periodically tilted during a scaling cycle which causes fish to tumble inside the drum, and consequently scales more efficiently. In some fish species, the scales can be removed from fish with a pressurized stream of water while fish is placed inside the scaler drum. The drums of such devices are made either of stainless mesh with rough edges or of stainless sheets perforated with contoured slits which detach the scales. Water has to be injected into the drum for the machine to operate. Less common are cylindrical scalers with a continuous operating cycle.

3.3.5 Washing

Washing is intended primarily to clean the fish and to remove accumulated bacteria. The effectiveness of the washing procedure depends, inter alia, on the kinetic energy of the water stream, ratio of fish volume to water volume and on the water quality. A proper fish:water volume ratio for achieving the desired level of cleanliness is 1:1, however, in practice more water is usually used (twofold). Washing of gutted and headed fish should be done on termination of the processing operation. To improve the effectiveness of the cleaning procedure, various mechanized scrubbing devices are utilized which can remove up to 90% of the initial bacterial contamination. Potable water is used for washing in freshwater fish processing plants.

The following washers are commonly used: vertical drum (Figure 3.10 a), horizontal drum (Figure 3.10 b) and a combination washer-conveyor belt (Figure 3.10 c).

The operation cycle for these machines is 1-2 minutes. The vertical drum washer is frequently used because of its conveniently small size. The most common is the horizontal tumbler washer. A rotating perforated drum constitutes the main component of this device; the drum is usually 2-4 m long, with round holes 10 mm in diameter. Inside the drum there are metal or rubber bars which facilitate tumbling and mixing of fish. Rotation of the drum, its tilted axis and the arrangement of internal bars result in a movement of fish towards the outlet of the device. Washing is continuous and is accomplished by spraying pressurized water through the perforated pipe installed inside the drum. Dirty water collects in the waste basins.

The mechanized washers described can be used to process whole fish, deheaded and gutted fish as well as boneless fillets because the washing action generates no physical damage to the product. Due to their continuous operating cycle, horizontal-axis drum washers are particularly suitable for production lines requiring constant product flow. A combination washer-conveyor is less popular but can serve to separate fish from ice: ice, having lower density than water, floats to the water surface from where it is removed, while fish falls onto the meshed conveyor and leaves the washing basin. Although there is an additional water jet at the exit from the water basin, the effectiveness of washing in this washer is lower than in the drum washers; fish on the conveyor belt is not exposed to scrubbing which is so important in the tumbler washers. The meshed conveyor (stainless steel or plastic mesh) with a water spraying system shown in the Figure 3.10 d, can also serve as a washer but its use is limited.

3.3.6 Deheading

The head constitutes 10-20% of the total fish weight and it is cut off as an inedible part. Although many mechanized deheading machines had been developed for processing marine fish, freshwater fish are usually deheaded manually. The main reason is the lack of inexpensive equipment offering minimal tissue loss during this procedure. Different cutting techniques used for deheading are shown in Figure 3.11.

A cut around the operculum, a so-called round cut, results in lowest meat loss. This technique is 4-5% more efficient than the straight cut commonly used in mechanized systems. The contoured cut, which runs perpendicular to the fish's backbone and then at an angle of 45o (Figure 3.11 II), is also advantageous. This particular deheading technique is used when fillet, mainly boneless and skinned, is the final product. The head is removed with the pectoral bones and fins.

In small freshwater fish processing plants, small fish are frequently deheaded manually. Deheading of larger fish requires much more effort and automated heading devices are essential. Unfortunately, a single deheading machine which would cover a broad spectrum of fish sizes, i.e., 20-110 cm, does not exist. An average deheading device can usually be used to process fish for which a difference between minimum and maximum length does not exceed 30-40 cm. The cutting elements used in the deheading machines are either disc, contoured, cylindrical knives, band saws or guillotine cutters. A machine operator adjusts the position of the cutting element according to the fish size. Thus the amount of meat lost during the deheading procedure depends not only on the type of head cut but on the experience and skill of the operator. The speed of a deheading device depends on the size of fish processed and is usually 20-40 fish/minute.

In some plants, simple - and sometimes rigged by an amateur - deheading devices are used which can potentially cause severe physical damage to the operator's hands. It is very important to examine safety problems associated with handling of the device before making a final decision about its purchase.

The deheading machine with a guillotine cutter is used for deheading larger freshwater fish (Figures 3.12 a, 3.12 b); cutters are changed according to species and size range. Economical cuts such as contoured cut or cut around operculum can be performed by changing the cutters.

In one type of deheading device with cylindrical rotational saw (Figure 3.12 b) the round cut is used. The most commonly utilized saw sizes are 12, 15 and 18 cm in diameter; saw size is adjusted to the fish species and size. The simplest designs are represented by the deheading machines with a circular saw (manually operated by pushing the fish under the saw - Figure 3.13 a) and with a disc saw which also acts as a guillotine (Figure 3.13 b).

3.3.7 Gutting

The purpose of gutting is to remove those fish body parts most likely to reduce product quality, as well as to remove gonads and sometimes the swim bladder. Evisceration of freshwater fish is labour-intensive and usually performed by hand. Gutting consists of cutting down the belly (fish may be deheaded or not), removal of internal organs, and, optionally, cleaning the body cavity of the peritoneum, kidney tissue and blood. Fish is cut longitudinally up to the anal opening, and special care is taken to avoid cutting the gall bladder. This procedure is performed on a table made of special material which is hard, easy to wash and does not absorb fluids. The table surface should be frequently rinsed and periodically disinfected.

A specialized gutting work station shown in Figure 3.14, allows to safely cut fish down the belly (used mainly during processing of trout), remove the guts by vacuum suction and quickly wash and rinse the body cavity with a rotational brush and a water spray, including kidney tissue removal.

Simple systems consisting of rotating brushes and water sprays are widely used (Figure 3.14 a). They facilitate the work and increase the product quality. Protective gloves, periodically disinfected and replaced, should be worn during gutting, especially when mechanized devices are used.

It is likely that the vacuum suction tools (kidney and blood removal) used to clean the body cavity in processing salmonids, will find an application for other freshwater fish species (Figure 3.15 b).

Gutting machines for processing trout, eel and a couple of other species, have been constructed in several countries, but high price renders them unsuitable for smaller plants. The cutting of the body cavity, removal of guts and kidney tissue with brushes and vacuum suction can be performed in these multi-application machines.

Some freshwater fish species, in particular bream, perch, roach, carp of length 20-40 cm, can be deheaded and gutted in a machine which employs a so-called American cut (Figure 3.16). Although the technological efficiency of this cut is not high, the processing speed reaches up to 40 fishes/minute.

3.3.8 Cutting away the fins

Manually cutting away the fins with either a knife, special mechanized scissors or rotating disc knives, is a labour-intensive and strenuous operation when handling larger fish. This operation is most frequently done after gutting during the production of deheaded whole fish and fish steaks. An automated device consisting of the rotating disc knives with a slit cutting edge, powered by electric motor (Figure 3.17), facilitates and speeds up the fin removal procedure. The knife slot has a horizontal opening through which the dorsal and ventral fins are passed manually and cut out.

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3.3.9 Slicing of whole fish into steaks

Slicing of deheaded whole fish into steaks with a cut perpendicular to the animal's backbone is a very common fish processing method. The high technological efficiency of this processing technique compared to filleting and automated cutting into pieces, makes it popular with retail markets and the canning industry. The fish pieces obtained average 2.5 to 4.5 cm thick.

Smaller and medium size fish are cut manually in concave basins which have slots evenly spaced to facilitate slicing into steaks of equal thickness. A knife or a band saw is used to slice the fish. Sometimes a band saw is used to remove the head and cut the body into two parts, one retaining the backbone.

Larger fish, particularly cyprinids, which have a massive and more solid backbone, need slicing mechanically. Numerous designs of such machines exist (Figure 3.18 a,b,c), and generally utilize multiple rotating circular saws attached to the drive. The distance between the saws as well as the elements moving the fish along the line can be adjusted. The deheaded whole fishes are placed into an automated cutter oriented so that the last piece cut has a prescribed length. A mechanized cutter can process 20-40 fishes/minute, depending on the fish size.

Figure 3.18 a. Cutter used for slicing whole fish into steaks,
b. Cutter with a drum-type loading system,
c. Cutter with a loading conveyor belt.

3.3.10 Filleting

A fillet which is a piece of meat consisting of the dorsal and abdominal muscles has been a most sought-after fish product in the retail market. Filleting efficiency depends upon fish species, its sex, size and nutritional condition.

Manual filleting is very labour-intensive and largely depends on the skills of the workers. However, filleting of freshwater fish is not as widely applied as for marine fish.

Filleting machines for processing marine fish are quite costly and are not suitable for freshwater species; in the case of trout, for example, expensive multi-function devices have been designed which are not used in small processing plants.

Some fish markets sell fillets of carp, perch, pike-perch and smoked single or block fillet of trout. Besides fillets, other forms are processed, e.g., block fillet retaining some bones (boned fillet) and the simplest type of processed carp which is the deheaded whole fish cut into two halves, one retaining the backbone. Restaurants and fish stores use simple tools to streamline the manual longitudinal cutting of fish. The same result is obtained by using a filleting device with a single rotating disc knife and two conveyor belts (Figure 3.19 b).

Manual filleting and deboning are time- and labour-consuming procedures, and are usually carried out using simple and inexpensive machines. In small plants processing freshwater fish, a type of machine which separates fillets and bones, sometimes with part of the backbone left near the head region, is increasingly more common.

The demand for freshwater fish fillets increases interest in simple and inexpensive single-purpose machines for filleting of deheaded and/or gutted fish. Different species (trout, perch, pike-perch, pike, cyprinids, etc.) can be processed in these devices as long as they are in the same size range. The remaining ribs and pin bones are manually removed from the fillets, and sometimes, as in case of cyprinids, perch and roach, the bones are cut by machine as shown in Figure 3.20.

The simplest filleting machine (Figure 3.21) for gutted and deheaded fish has two disc knives set from each other at a distance equal to the thickness of the fish's backbone. Filleting speed of these devices is 30-40 fishes/minute: they are efficient and the quality of the final product is good. However, manual processing yields better results. The size range of the processed fish is 20-45 cm. Machines of different design and with bigger knives are used for processing larger fish (Figure 3.19 c). Filleting devices are produced in several countries (Germany, Poland, Russia) and are increasingly used in small processing plants.

Meat left on the fish's backbone after filleting can be recovered to a high degree using a meat-bone separator (Figure 3.23). Up to 50% of the total mass of processed backbones can be recovered as meat.

Boned fillets with ribs are subsequently processed by cutting the ribs in an automated system consisting of several disc knives 100-200 cm in diameter, set on a drive every 4-5 mm. After cooking, particularly after frying, the tiny cut rib pieces are barely noticeable and cause no discomfort during consumption. In the machine used for cutting ribs (Figure 3.20), the boned fish fillets lie skin-down on a conveyor belt which drives them under the disc knives; the ribs are cut and incisions of determined depth are made in the meat.

3.3.11 Skinning

Only recently has skinning of freshwater fish fillets been introduced into processing plants. Manual fillet skinning is labour-intensive and difficult; a sharp knife and flat board made of metal or plastic are needed. The fillet is placed on the board skin-down, the meat is grasped in the left hand and the knife is drawn between the skin and meat.

The simplest and most inexpensive automated tool for skinning of fillet with or without scales has been in use since 1992, and it can be attached to the processing table. This tool consists of an oscillating knife powered with a small electric motor and a system of compression springs operated with a foot pedal. Water is not needed to operate this device. One end of the fillet is placed in a slit between the knife and compression element and the tip grasped manually in a wrench which allows the skin to be pulled off the meat from under the oscillating knife. Various freshwater and marine fish species can be processed in this machine, including larger fish. Its use is recommended for small processing plants, fish markets, fishmongers, supermarkets, restaurants and catering sectors. Compared with manual operations, this machine facilitates and speeds up skinning. Some devices are small and can be placed directly on the processing table; running water and electricity are necessary for their operation. Efficiency varies depending upon the fish species. The price of these devices varies; some are quite expensive and their use is profitable only when a certain level of production is maintained. Depending on fillet size and type of machine, 20 to over 40 fillets/minute can be skinned; faster machines require a conveyor to move the fillets. Skinning machines (see Figure 3.22) are produced in many countries.

3.3.12 Meat-bone separation

In recent years a new trend has emerged to effectively process raw fish products which resulted in production of minced meat separated from inedible parts, such as bones, skin and scales. During filleting a considerable amount of meat is usually left along the ribs and backbone (30-50%). The carcasses are a source of minced meat. Minced meat is also produced from less valuable fish species after deheading, their body cavities carefully cleaned and kidney tissue removed. Meat is separated from the bones, skin and scales, in automated devices called separators. In the separator shown in Figure 3.23, meat is squeezed through holes into the cylinder under pressure applied by a conveyor belt partially encircling the cylinder (about 25% of the cylinder's perimeter). The cylinder rotates slightly faster than the conveyor. The openings in the cylinder are usually 3-7 mm in diameter. For processing of freshwater fish, the holes are 4 and 5 mm in diameter. The smaller the holes, the stronger the grinding action. Pressure applied by the conveyor to the cylinder can be regulated depending on the type and size of the raw product and on the hole diameter.

The use of separators for processing such freshwater species as perch, bream and tench, offers a new perspective on production of novelty products which could gain customer approval and be successfully marketed. Minced meat can be either frozen in cardboard or foil containers, or used immediately to produce fishburgers, fish sticks, canned fish, vegetable mixes and fish dumplings. The technological efficiency attained during the production of ground meat from bream not larger than 1 kg, was 40% of total body weight. For example, in Poland in a small fish processing plant which employs 8 workers, 1 t of frozen ground bream meat can be produced during one shift. According to routine practice, ground meat can be stored at -25oC to -28oC for up to 6 months.

In Hungary, minced fish meat is made from freshwater species, mostly cyprinids 1-3kg in weight. Halves (fillet with backbones) obtained mechanically, are the raw material. The minced meat is dried and later added to fish soups.

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