Technical field of the invention

The present invention relates to a method for ex- tracting oil from fish offal, comprising the step of heating the offal to a temperature above +30°C while keeping the offal in a liquid-permeable container from which oil extracted from the offal by heating is sepa¬ rated by dripping off, the rest of the offal remaining in the container.

Description of the prior art

When fish, such as different species of the salmon family, e.g. of the type cultured in special fish breed¬ ing stations, are slaughtered, one obtains, apart from pure fish meat and bones, an offal chiefly consisting of the viscera of the fish. At least for salmon, the weight of this offal amounts to about 20% of the total weight of the fish.

Efforts have previously been made to make use of this offal in various ways, for example by treating the offal with agents (such as formic acid) preventing putrefaction and mincing it, such that it can be used as animal feed or as filler in such feed. Another alternative is to use the offal as a fertiliser to be spread in finely divided form in fields and the like. In both cases, the handling costs are, however, so considerable that the economic yield be¬ comes insignificant. In actual practice, the material is quite simply thrown away.

A considerably more interesting way to make use of the fish offal is to extract oil from it, and various methods for extraction have indeed been suggested. US 2,614,110, for example, discloses a method in which a slurry of fish offal and water is subjected to the action of a centrifugal force sufficient to separate the liquid from the solids in the slurry, whereupon the separated liquid is divided into an oil phase and an aqueous phase, also by being subjected to the action of a centrifugal

force. In this connection, the slurry of water and fish offal may be heated to temperatures above +30°C. This method is, however, rather complicated and costly, and since the fish, offal is mixed with substantial amounts of water, there is a risk that the oil prepared will be of poor quality, unless the separation of the two liquid phases is carried very far. Summary of the invention

The invention aims at providing an easily practicable method for extraction, which allows cost-effective use of fish offal while yielding high-quality oil. This object is achieved by a method which is characterised in that the fish offal in an initial step, before being heated to the temperature stated by way of introduction, is deep-frozen, i.e. frozen to a temperature below -18°C, and thereafter thawed from the deep-frozen state. Most suitably, the deep¬ freezing temperature is kept between -20°C and -30°C.

The fish offal is stored in its deep-frozen state, preferably for at least one day, and suitably for a longer period, e.g. 2-4 days. Then, the material is placed in an environment having a temperature of above +5°C so as to thaw, whereupon it is heated to a temperature exceeding +30°C, preferably a temperature of between +40°C and +50°C, at which it is kept for at least one hour. Advan- tageously, this heating can be carried out at such a rate that the temperature increases by about 5-30°C/hour. During this heating, the fish offal is kept in a first, liquid-permeable container, for example a string bag con¬ sisting of a fine-meshed net or a vessel with a straining cloth, which itself is disposed inside or above a second, liquid-impermeable container. As found in practical tests, a fat-containing liquid in the form of oil will gradually be extracted from the heated material and allowed to drip off from the first, liquid-permeable container into the second container for collection of the liquid. In the tests, it has been observed that the amount of oil or oily liquid obtained by the method may be up to 70-80% of the

weight of the fish offal in solid form. At room tempe¬ rature, the resulting oil has low viscosity (about the same viscosity as sewing-machine oil) and high storage _l stability, so that its properties will not change. If 5 blood-containing matter, such as heart and liver, are removed from the starting material, i.e. the fish offal, the resulting oil will be clear and of a pale yellow, but with an increasing amount of blood-containing matter in the offal, the red-colouring of the oil is intensified.

10 The resulting oil contains many useful substances, such as unsaturated fatty acids, for which reason it can be used in a number of technical fields, e.g. as raw mate¬ rial for the production of foodstuffs, medicines, cosme¬ tics, lubricants and fuels.

15 The following results were obtained from an analysis of two samples of oil obtained by using the method accord¬ ing to the invention:

Sample No. 1 Sample No. 2

Total content of fatty

20 acids as glyceryl esters,% 87.1 90.0 saturated, % 18.3 polyunsaturated, % 24.4 linoleic acid (C18.-2) 8.7

EPA (C20:5) 2.7

DHA (C22:6) 6.4

25 Free fatty acids (FFA), % 6.8

Vitamin A, mg/kg 10.0

30

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