A method of processing a raw product in the form of fish fillet and shellfish

The present invention regards a method of processing a raw product in the form offish fillet and shellfish in accordance with the preamble of Claim 1.

Until now, smoked fish fillets have been produced by salting, drying and smoking, and optionally heat treatment.

From NO-A-20026214 there is known a method of producing a smoke extract for use as a flavouring agent and/or colouring agent in foodstuffs. This publication also regards a method of producing a smoked fish product. In this method, the fish meat is cleaned, the smoke extract is applied to the surface, the fish meat is wrapped in plastic foil and the wrapped fish meat is treated with microwaves. The meat is treated with microwaves to achieve the desired texture in the fish meat. The described use of microwaves to soften collagenic fibres in the fish meat will cause boiling of the surrounding meat/fat, with a resulting loss of colour and consistency. The reason for this is that the collagenic fibres are connective tissue proteins that are insoluble in water and also do not contain any water. Thus the microwave energy will not affect the collagenic fibres but will heat the surrounding muscle/fat components, then to melt the collagenic fibres through ordinary heat transmission. The treatment according to this publication does not comprise salting of the fish meat, which is required to bring about the desired flavour and consistency. Furthermore, salt will contribute to liberating the proteins.

From US-A-4353928 there is known a method and apparatus for treating fish material in preparation for consumption. The fish material is salted and/or has food flavouring added. From this publication it further known to use ultrasound in the production of the fish product. It is stated that the use of ultrasound improves the penetration of salt/flavour into the meat. This method is directed especially at the flavouring and maturation of herring, where the fish is placed in a solution of spices and salt, and this solution is subjected to ultrasonic oscillations. This will cause the proteins in the fish

meat also to wash out or leach, making it very difficult to achieve controlled absorption of water into the fish meat. In the case of uncontrolled water absorption, the microwave treatment will cause boiling, especially in the outer parts of the fish, where, inevitably of necessity, the water absorption is at its highest.

It is known, from the abstracts of JP-A-2000245335, JP-A-3117454 and JP-A-5103635, to use ultrasound in the production of various fish products.

The abstract of JP-A-2001178420 describes the application of microwaves to fish products to improve flavour, colour and consistency.

From DE-A-10049839 it is known to combine ultrasound and microwave treatments in the production of sausages in order to achieve a sausage free of skin but with a stable shape. The sausage is also heated.

Traditionally, one has sought to increase the water content of raw products by injecting high pressure brine with no further treatment at the end of the process. This is the case with fish and shellfish to be smoked and marinated. In the case of shellfish, boiling comes in addition. Thus the brine is not bound by the proteins and will for the most part drain away from the fish fillet/shellfish meat. The previously known method of injecting at high pressure with no further processing is not suited for a process which employs microwave energy, as the concentration of water will be too high in the area around the brine injection. Repeated experiments have shown that injection of pressurized brine followed by a treatment consisting of repeated impulses of vacuum and atmospheric pressure, and at the same time subjecting the product to vibrations in the form of ultrasonic or mechanical oscillation, will result in a water distribution that does not lead to localized boiling of the product during the subsequent microwave treatment.

The vacuum chamber should be constructed so as to provide optimal strength under vacuum, and it should have a shape that makes room for the greatest possible number of cases. The cases offish fillet/shellfish are placed in layers on vibrating plates that, either through low frequency, high oscillation amplitude ultrasound or mechanical oscillation,

liberate the proteins that bind the water in the fish/shellfish product during the subsequent microwave treatment.

The spacing of the vibrating plates should be such that each layer of product cases is in direct contact with the plates.

Fish and shellfish normally retain an amount of water equal to three to four times the amount of protein. The amount of retained water depends on many factors, among other things pH- value and salt addition (NaCl). Together, the salt, the oscillations and the vacuum will liberate the proteins in the meat. The vacuum causes the injected brine and any flavouring, air and tissue fluids to be extracted from the fish fillet, in order then to be uniformly absorbed into the fish fillet at atmospheric pressure.

Water soluble protein in fish and shellfish meat is approximately 18-19 grams, connective tissue protein less than 3%. Protein in meat from cloven-hoofed animals is 17-19 grams, and of this approximately 17% is connective tissue protein which is insoluble in water.

If the fish is filleted prior to rigor mortis, the contraction of the dark muscles in particular may be very strong, up to 50 percent shortening of the dark muscles, as opposed to 15 percent for the white muscles. At high temperatures (10-15 degrees) the contraction in the fish meat may therefore be violent enough to cause rupturing of the tissue (gaping).

Moreover, as it is difficult to fillet fish in the actual rigor state, the filleting is normally carried out post-rigor. The generally low glycogen content offish meat causes a smaller drop in pH- value than that which occurs in meat from cloven-hoofed animals. The pH- value drops to approximately 7, which is neutral. Consequently the meat is less able to retain water, and this can be connected with the above rupturing (gaping). This is caused by ruptures in the myocomata.

Fish meat reaches a minimum for water retention at a pH of approximately 5, which coincides with the isoelectric point of the protein. Here, the number of positive and

negative charges is at a minimum. Thus there is only room for a few water molecules. Above pH = 5 there will be an excess of negative charges, below 5 an excess of positive charges, and in both cases the filaments will repel each other and make more room for water. For that reason it is important to denature the proteins and turn them into jelly in order to prevent drainage. The fish fillets can be finished in the vacuum chamber by from time to time extending the use of microwaves in this. In a preferred method of treatment however, the microwave treatment takes place in a separate oven. Prior to this one has the option of adding spices and food dyes etc. It is also possible to smoke the fillet in the traditional way. This allows better product control than final treatment in the vacuum chamber.

Shellfish and prawns in their shells may be treated without penetrating the shell with needles, in order to achieve the same increase and penetration of flavourings. Prawns and shellfish are treated in a drum alternating between vacuum and atmospheric pressure. The treatment takes place under mechanical action, for as long as a person skilled in art deems necessary to achieve a level of salt and flavourings in the product which will denature the proteins. Following this process, prawns and shellfish are treated in a microwave oven. Advantageously, and as a preferred treatment, prawns and shellfish will be subjected to microwaves in a microwave oven in which there is provided a device which agitates the prawns and shellfish to ensure that everything receives the same microwave effect.

Prawn and shellfish meat has a low water retention capacity. However, denaturation of the proteins will make it possible to reduce/stop the drainage with the effect of microwaves that turn the proteins into gelatine.

Prawns and shellfish may be processed in several stages, with microwaves of various intensity and duration, optionally with intervening breaks. Prawns and shellfish may also undergo final boiling during this microwave process.

In the development of this salting and aromatization technique, it has become clear that there is a process here which may be used as a quick steeping method for salted fish. Trends in the main markets for salted fish and clipfish, where the products are moving

from traditional shops into supermarkets and hypermarkets, have resulted in a greater demand for quick and easy, ready to cook products with a long shelf life. This development has led to a greater interest in industrial steeping, both in consumer and producer countries.

The present invention and technological solutions may also be applied to the steeping of salted fish and clipfish. It will improve the weight yield and achieve a more uniform salt content throughout the fish meat. The invention will also reduce the steeping time while preserving the sensory quality.

The raw materials for salted fish mainly consist of cod with a NaCl-content of 20-21%. In order to accomplish a quick steeping process it is important to know the limitations of the rate at which water and salt are transported during the steeping. Steeping of salted fish and clipfish can be viewed purely as a transport process between solid phase and liquid phase.

The difference between the concentration of salt in the fish and that in the liquid phase drives salt out of the meat through diffusion. Correspondingly, water is driven into the meat because the concentration of water is lower in the muscle than in the aqueous phase, and because salt draws water.

The problem with the steeping is that agitation of the aqueous phase does not increase the rate of steeping. It would therefore seem obvious to assume that the limitation of the steeping rate lies either in the stagnant liquid film or inside the fish. The stagnant liquid film is not visible but differs from the rest of the aqueous phase in that the water molecules are barely moving - also termed "stationary water". As a result, agitation of the water will affect the film to only a small degree, and the salt can move through the film by diffusion only.

The transport time in the stagnant liquid film is a rather abstract concept but says something about the rate at which the salt is transported into the water. If the film resistance is high, the transport will be slow. Stab injection of water into the fish meat

will reduce the effect of the liquid film, as the salt will be rinsed out with the injected water.

The design of the vacuum chamber is the same for both salting desalting. Cases used for salting should be smooth and flat inside. Fish meat for salting should fit the size of the plastic case to allow the fish meat to absorb the tissue fluid and brine sucked out by the vacuum.

Desalting cases should have a profiled bottom. The bottom consists of profiles two centimetres high and two centimetres wide, with a spacing of three centimetres. The ends of the profiles are connected to two manifolds. One manifold is connected to an external water supply, the other drains to a space at the bottom of the vacuum chamber. Following the oscillation and vacuum treatment, the salt will be extracted from the fish meat, and due to the density of the salt this will deposit between the profiles after a short while. After the salt has deposited, the profiles are flushed with clean, unsalted water via the one manifold. At the same time, vacuum is used to compensate during the inflow of clean water, to ensure that the fish does not absorb salt water during the process. At the end of the process, and before returning to atmospheric pressure, the fish meat will absorb unsalted water. This process is repeated until the product achieves the desired salt content. Traditional steeping causes a protein loss of between 8 and 10%. Tests carried out with the above invention show a protein loss of 1.2% on average. This is connected with the use of microwaves, short-term use of which will cause muscle proteins to swell/absorb water. Steeping time in this process is approximately two hours, compared with 36 to 48 hours for conventional steeping. This process will also result in a significantly more uniform salt content throughout the fish meat. Use of microwave energy in the vacuum device will also affect the ions in the salt, which are susceptible to electrical influences. The properties of the salt are changed by the effect of the microwaves, increasing the penetration and discharge of salt in the cells.

With fresh fish fillet and when cutting thin slices, the problem is that salt has liberated proteins and enzymes that stick to the treatment device in an undesirable fashion. The knife used to cut fresh fish fillet into thin slices in principle consists of at least two blades extending in parallel and in separate directions. When the knives move against

each other, frictional heat is generated, and this affects the albumen, enzymes and proteins in the fish meat. These form a coating on the knife, which results in the knife motion tearing the loose and layered fish meat to pieces.

Viewed in the light of the above, an object of this invention is to provide a method of achieving a stable consistency, quality and flavour in raw fish fillet.

A solution which entails the knives having an adhesion minimizing microstructure has been tried, but with no apparent good results. A solution where the knife is kept clean with water during production will not yield a good result either, as this will allow recontamination and damage colour effects.

Thus this invention provides a method by which the dissolute proteins are bound, the enzymes form gelatine and denaturation is achieved.

These and other objects and advantages are achieved by a method of the type mentioned by way of introduction, characterized in that the method comprises the steps of:

(i) bringing the raw product into contact with one or more ultrasonic sonotrodes;

(ii) applying ultrasonic energy in the lower frequency range, with a high oscillation amplitude; and (iii) applying microwaves at a frequency of 2.45 GHz.

The raw product is fish fillet, and injection of salt or dry salting and any addition of flavouring is preferably carried out prior to step (i). Any traditional smoking is preferably carried out between steps (ii) and (iii).

The raw product is shellfish, and the raw product is subjected to mechanical action such as from a rotating drum and alternating pressures, plus addition of salt and any flavouring, preferably prior to step (i).

The raw product is salted fish fillet, and, preferably, addition of water, through e.g. stab injection, is carried out prior to step (i), steps (i) - (iii) are alternated and repeated several times until the raw product has achieved a predetermined salt content.

Preferably the sonotrodes have a shape which is complementary to that of the raw product.

At the same time, the raw product is preferably subjected to a subsequent treatment of liquid smoke or other pressurized flavour additives or food dyes etc.

Preferably the raw product is briefly subjected to a high surface temperature by use of radiant heating or a gas-flame.

Preferably, the surfaces of the sonotrodes, which contact the raw product, have an adhesion minimizing finish or an equivalent textural coating.

The raw products are brought into direct contact with the surface of one or more ultrasonic sonotrodes having an anti-adhesion coating. Oscillation will ensure an even distribution of sprayed spices/flavouring and a certain penetration into the product, the penetration depth being controlled through selection of time and frequency.

The ultrasonic energy applied to the raw products has a low frequency but high oscillation amplitude. The ultrasound treatment of the raw products is based on the thickness, age and consistency of the raw products. Greater efficiency may be achieved by placing product-shaped ultrasonic sonotrodes in contact with the top side of the raw product. These sonotrodes must also have an anti-adhesion coating.

The molecules in the raw product exposed to ultrasonic energy will set up natural oscillations upon receiving this treatment. This, in combination with the salt that has penetrated into the raw product, will liberate the proteins.

Spices/flavouring may be applied to the surface of the raw products under pressure in order to affect the flavour or colour. This is done prior to the entry into the microwave

oven and the subsequent continuous process. This can eliminate the requirement for any after-treatment to add aroma/flavour (e.g. with smoke) and colour, or will help optimize this.

However, it is also possible to chill and pack the stream of ultrasound-pretreated fish fillets at the outlet from the sonotrodes, or subject it to further treatment or processing, e.g. hot smoke, liquid smoke or similar flavouring or food dyes etc.

The above described liberation of proteins and surface treatment may be followed by a described further treatment.

The raw product is conveyed into a microwave oven in which it is subjected to microwaves at a frequency of approximately 2.45 GHz. The effect of the microwaves will be to initiate denaturation of the proteins, bringing about firmness and stability when the proteins coagulate to form gelatine, by the water molecules in the electrical field in the microwave making the molecules unidirectional and generating heat through oscillation.

When the electrical field is subjected to oscillation, the water molecules, which in liquid water are randomly oriented, become unidirectional electric dipoles. The electrical forces from the field acting on the charges of the water molecule change direction at the rate of the microwave frequency. The water molecules interact with other types of molecules in the raw product. The other molecules are also activated, and the heating starts as a result of the oscillation/friction that arises from this. The selected frequency is 2.45 GHz, which is a frequency to which no regulations on use pertain. Moreover, it is well suited for oscillation of water molecules. At the above frequency the electrical field will just manage to turn the water molecules around before the direction of the field changes again. This provides rapid heating around the water molecules in the fish.

The penetration depth of the microwaves into the material to be treated, which is a mixture of protein, fat, water, salt and spices, is small, so as to prevent undesirable through-boiling of the material. The highest concentrations of water molecules exposed to the microwaves will, after the ultrasonic treatment/salting, be located between the fat

and the meat lamella in the raw product, and through the effect of the salt, the highest concentrations of proteins will also be located in the layers of fat between the muscular meat.

When exposed to microwaves, the areas in which binding of the structure through coagulation is desirable, and which contain the most water molecules and protein, will be affected most quickly and with the greatest effect. There will also be very little heat transmission to the surrounding muscular meat, and consequently no loss of colour. Use of the described method can reduce the otherwise unavoidable loss of protein, fat, flavour etc. and increase the quality and uniformity of the product.

At the same time, the throughput time in a continuous process can be reduced, thus from a technical point of view facilitating a reduction in costs. The risk of recontamination is also considerably less with the present invention than in the course of a traditional process.

An exemplary embodiment describes the invention in greater detail.

The salmon fillet is dry salted and kept in a refrigerated storage room at 4°C for 24 hours. The fillet is then rinsed to remove excess salt and treated as described above, with ultrasound and microwaves, in addition to spices and flavouring. In addition, some fish fillets were briefly subjected to intense heat from an electric grill, while others were subjected to an open gas flame to cause the proteins in the surface to form a film (a so- called smoke film) through coagulation. Ultraviolet radiation at a wavelength in the range 200-260 nm may also be applied to achieve sterilization of the surface.

Sensory- wise, the products that were treated in the described manner were judged to be better than traditional cold smoked salmon. The panel of tasters particularly emphasized the fact that there was no sense or taste of raw fish meat from the products, neither from the products with a smoke film nor from the ones without.

Unexpectedly, it was observed that even large tears in the salmon fillet had been closed, and that the edges of the tear were bound together by coagulated proteins.