WO1996008547A1

WO1996008547A1 - A method for recovering fish oil having a low cholesterol content from fat fish, and use of the oil in foodstuff

Info

Publication number: WO1996008547A1
Application number: PCT/NO1995/000164
Authority: WO
Inventors: Åge Oterhals
Original assignee: SILDOLJE- OG SILDEMELINDUSTRIENS FORSKNINGSINSTITUTT
Current assignee: SILDOLJE- OG SILDEMELINDUSTRIENS FORSKNINGSINSTITUTT
Priority date: 1994-09-15
Filing date: 1995-09-14
Publication date: 1996-03-21
Anticipated expiration: 1997-03-15
Also published as: DK28897A; NO943424D0; NO180122B; NO943424L; NO180122C; IS4444A

Abstract

A process for extracting fish oil of low cholesterol level from fat fish, especially capelin, where the fish is subjected to coagulation, straining and pressing, whereby strainer/press liquor is formed which contains an oil dispersion. From the strainer/press liquor a fraction of fish oil is separated by primary breakage of the oil dispersion. This fraction has a reduced cholesterol level and is thereby favourable for utilisation in foodstuffs.

Description

A METHODFORRECOVERINGFISHOILHAVINGALOWCHOLESTEROLCONTENTFROM

FATFISH,ANDUSEOFTHEOILINFOODSTUFF

The present invention relates to a process for extracting fish oil having a low cholesterol level from fat fish, as indicated in the introduction to claim 1.

A high intake of cholesterol is regarded as a risk factor with respect to the development of coronary heart diseases. For this reason there is recommended in the U.S.A. a maximum intake of cholesterol of 300 mg. per day. Fish oil contains in the region of 0.4-1.6% cholesterol. The level will be dependent upon the species and the season. On using unhardened or hardened fish oil in foodstuffs, the fish oil will make a significant contribution to the total cholesterol content. In order to increase the utilisation of fish oil in foodstuffs, it is therefore desirable to lower the cholesterol level. A series of patents relate to the reduction of cholesterol in fish oil. These relate to processes which by utilising chemical and/or physical conditions can lower the cholesterol level of fish oil after it is produced from fish and/or fish waste. Fish oil is normally produced by coagulating and straining and/or pressing the fish. The coagulation is effected by heating the fish up to a temperature of above 40°C. The necessary residence time will vary with the temperature level and the size of the fish. The bulk of the oil will follow the liquor phase of the process and be separated away by the use of a high g-field in a three phase decanter centrifuge or plate centrifuge. The process is designed to remove as much as possible of the oil in the liquor phase and to result in one oil fraction. The content of cholesterol in commercial fish oil will vary with the fat % of the fish (A.J. de Konig and T. Mol, "The cholesterol content of South African fish oil and its seasonal variation". Fat. Sci. Technol., Vol 94(2), 1992, p 60-63). Cholesterol and cholesteryl esters are important components of cell membranes and lipoproteins. The bulk of the cholesterol in the fish is bound up in such cell fractions or macromolecules. Depot fat in fish consists of triglycerides and is found primarily under the skin, in the muscles or in the liver. With capelin (Mallotus villosus) significant amounts are also found in the belly cavity wall, and with herring (Clupea harenqus) around the guts (V. Mohr, "Nature of fat depots in fish and principles of fat liberation". Proceedings of the third symposium on the processing of fish meal and oil, IAFMM, London, 1979, p 55- 66) . The distribution of depot fat between the individual fractions will vary with the type of fish.

In tissues which are rich in depot fat the ratio between cholesterol and triglycerides will be significantly lower than in other tissues. Oil extracted from such fish tissues will therefore contain a lower level of cholesterol. Based on conventional process technology, the individual tissue fractions in the fish must be separated mechanically before processing in order to utilise this cholesterol distribution for the production of fish oil having a low cholesterol level. In capelin (Mallotus villosus) 55-70% of the total lipid content exists in fat tissue localised beneath the skin and around the belly cavity wall. The distribution is constant through the whole year (V. Mohr, "Nature of fat depots in fish and principles of fat liberation". Proceedings of the third symposium on the processing of fish meal and oil. IAFMM, London, 1979, p 55-66) . The fat tissue consists mainly of cells where the bulk of the cell volume is filled with one or more fat drops. By utilising the particular fat distribution of fat fish, especially capelin, together with the fact that cholesterol is mainly bound up in cell membranes and lipoproteins, it has been found according to the present invention that it is possible to extract an oil fraction having a low level of cholesterol from strainer/press liquor. The process according to the invention is characterised by the features which are indicated in the characterising portion of claim 1.

The stationary sedimentation rate for a particle which is in a gravitational field having the acceleration g, occurs when the force which causes the movement is equal to the frictional force. In the laminar region the sedimentation rate (V_s) for a sphere is described with reference to Stokes' equation:

V_£

18μ

where D is the diameter of the particle, p_p and p are respec- tively the density of the particle and of the surrounding medium and μ the viscosity of the surrounding medium.

Strainer/press liquor can be characterised as an oil in water (o/w) dispersion and emulsion. Fish oil will have a lower density than the continuous water phase in the strainer/press liquor and dispersed oil drops will therefore rise upwards. This emerges as a negative sedimentation rate in Stokes' equation. Based on the Stokes' equation it can be shown that oil drops having a diameter greater than 0.4 - 0.5 mm will have an elevation rate which results in a relatively rapid separation from the strainer/press liquor, see enclosed Figure. The coalescence of oil drops will furthermore increase the rate of separation. Oil drops having a diameter of less than 0.1 mm will have a very low elevation rate and/or will be emulsified in the strainer/press liquor. Breakage of the oil dispersion in the strainer/press liquor can, based on considerations of this kind, be divided into two periods:

1. Primary breakage with coalescence and rapid separation of large oil drops.

2. Secondary breakage which represents a slow separation of the remaining small oil drops. According to the invention the difference in elevation rate between small and large oil drops in the strainer/press liquor is utilised.

The process according to the invention is characterised in that a fraction of the fish oil is separated from the strainer/press liquor on primary breakage of the oil dispersion.

The invention also relates to the utilisation of the oil produced in the process in foodstuffs.

If the transportation and coagulation of the fish is effected in apparatus which does not result in grinding of the raw material, the cholesterol remains bound in cell fractions and macromolecules. During pressing of the fish oil from fat cells under the skin and around the belly cavity wall will form large oil drops with low cholesterol level. As a result of the close contact between fat and protein in the remainder of the fish, oil from the remaining tissue fractions will form small oil drops which are emulsified in the press liquor.

During separation of the strainer/press liquor large oil drops from fat tissue under the skin and around the belly cavity wall will be separated rapidly (primary breakage of the oil dispersion) and form an oil layer with low cholesterol level. By controlling the residence time and g- field oil drops from the remainder of the fish tissue will remain in the liquid phase because of small size and/or the stabilising effect of emulsifying components (protein, phospholipids etc.). The separation process can be carried out with batch or continuous apparatus described in the literature.

The yield of oil with low cholesterol level will be reduced if the fish is exposed to large shearing forces during transportation and/or coagulation. Such shearing forces can cause oil from fat tissue under the skin and around the belly cavity wall to become completely or partially emulsified in the press liquor. The degree of grinding of the fish will also affect the proportion of the cholesterol bound up in cell fractions and macromolecules, which is released to the oil phase.

Grinding of the fish during transportation and/or coagulation will therefore affect the final cholesterol level in the fish oil. Optimal utilisation of the process is achieved by handling the fish gently. The treatment of the fish in a satisfactory manner can be accomplished based on known technology. Corresponding precautions must be carried out for the strainer/press liquor before and during separation of the oil. After extraction of the first oil fraction from the strainer/press liquor, the remainder of the oil can be separated by the use of a high external g-field, for example by the use of a continuous three phase decanter centrifuge or plate centrifuge. This second oil fraction will have a significantly higher cholesterol level than the first oil fraction.

By virtue of a favourable distribution of the fat tissue, capelin appears to be particularly suited to use of the process, but other fatty fish species, such as herring (Clupea harenqus) . mackerel (Scomber scombrus) . horse mackerel (Trachurus trachurus) and sand eel (Ammodvtes tobianos and Ammodvtes marinus) can be considered for use. The yield of fish oil with a low cholesterol level will be dependent upon the distribution of the fat tissue in the respective fish raw material.

The invention will be further elucidated in the following Examples.

Example 1

100 kg of winter capelin was coagulated in a continuous band cooker at about 85°C. The residence time in the water bath was 20 minutes. The fish was strained and pressed in a twin screw press. The press liquor was collected in a container and allowed to stand at rest for about 20 minutes. Separated oil on top of the liquid surface was skimmed off (skimmed press liquor oil) . From the skimmed press liquor a second oil fraction was extracted (separated press liquor oil) by pumping the press liquor through a strainer for removing suspended solids and separating out the oil in a continuous plate centrifuge.

Results:

Oil Fraction Cholesterol Yield %) (kq/100 kg raw material)

Skimmed press liquor oil 0.21 4.8 Separated press liquor oil 0.79 2.0

The cholesterol analyses show a significantly lower level in skimmed press liquor oil compared to oil separated from skimmed press liquor.

Example 2

100 kg of winter capelin from the same load of raw material as in Example 1, was coagulated in a continuous band cooker at about 85°C. The residence time in the water bath was 20 minutes. The fish was strained and pressed in a twin screw press. The press liquor was collected in a container. 1 litre of press liquor was transferred to a measuring cylinder in a water bath at 70°C. The volumes of separated oil and an intermediate phase of oil and water below the pure oil layer, were recorded as a function of time.

Results:

Time Oil Intermediate Phase Oil + Intermediate Phase

(min) (ml) (ml) (ml

6.8 60 140 200

9.9 120 80 200

12.8 150 50 200

18.8 170 30 200

22.3 180 20 200

29.2 180 30 210

41.3 190 20 210

50.0 200 10 210

The results show that the amount of separated oil reached a relatively stable level after about 20 minutes. This is a result of large oil drops in the press liquor rising rapidly to the surface of the liquor (primary breakage of the oil dispersion) and forming a pure layer of oil. The formation of the pure layer of oil proceeds via an intermediate phase which constitutes a layer of large, separated oil drops where the film of continuous phase between the oil drops is broken down with the formation of a pure layer of oil.

Example 3

100 kg of winter capelin from the same load of raw material as in Example 1 was coagulated in a continuous band cooker at about 85°C. The residence time in the water bath was 20 minutes. The fish was strained and pressed in a twin screw press. The press liquor was pumped through a screen cloth in order to remove suspended solids, and oil thereafter separated out in a continuous plate centrifuge. Results:

Oil Fraction Cholesterol Yield L%J (kg/100 kg raw material) Separated press liguor oil 0.55 7^0

The cholesterol level lies clearly above the level which is measured in skimmed press liquor oil in Example 1. Separated press liquor oil is here representative of all oil which can be extracted from the press liquor by the use of the indicated process conditions. The oil is extracted as one fraction without prior skimming.

Example 4

100 kg of capelin from the same load of raw material as in Examples 1, 2 and 3 was transferred to a batch cooker and 27 kg of water added. The fish was coagulated by heating the water to about 90°C over the course of 20 minutes. During the heating the fish was kept moving by the use of strong mechanical stirring. The stirring resulted in the fish going partially into solution. The fish was strained and pressed in a twin screw press. The strained/press liquor was collected and divided into two equal portions. From the first half of the strainer/press liquor 1 litre was taken out and transferred to a measuring cylinder. The measuring cylinder was kept at rest in a water bath at 70°C for about 20 minutes. The separated oil on the top of the water column was skimmed off (skimmed strainer/press liquor oil) .

From the other half of the strainer/press liquor oil was extracted by pumping the liquor over a strainer in order to remove suspended solids before separation in a continuous plate centrifuge (separated strainer/press liquor oil) . Results: Oil fraction Cholesterol Yield Li) (kg/100 kg raw material)

Skimmed strainer/press liquor oil 0.31 1.7 Separated strainer/press liquor oil 0.53 7.0

The cholesterol analyses show a significantly lower level in skimmed strainer/press liquor oil compared with separated strainer/press liquor oil. Strong mechanical stirring during coagulation of the fish involved the oil being emulsified in the liquor phase to a significantly greater degree. This resulted in a lower yield and a somewhat higher cholesterol level in skimmed strainer/press liquor oil compared with Example 1. Separated strainer/press liquor oil is here representative of all oil which can be extracted by the use of the indicated process conditions. The cholesterol level in separated strainer/press liquor oil is comparable with the level which is observed in Example 3.

Example 5

Capelin from the same load of raw material as in Example 1 was processed in a commercial processing plant. The fish was coagulated in a continuous screw cooker, strained and pressed in a twin screw press. Suspended solids in the strainer/press liquor was removed in a two phase decantation centrifuge and oil separated out in a continuous plate centrifuge. Results:

Oil fraction Cholesterol Yield¹ il (kg/100 kg raw material) Separated strainer/press liquor oil 0.77 7.2

1) Calculated on the basis of material balance in the processing plant.

The cholesterol level is significantly higher than that which was achieved in the respective skimmed press liquor and strainer/press liquor oil in Example 1 and 4.

Example 6

100 kg of summer capelin was coagulated in a continuous band cooker at about 85°C. The residence time in the water bath was 20 minutes. The fish was strained and pressed in a twin screw press. The press liquor was collected in a container and divided into two equal portions. One portion was kept at rest in a container, and oil on the surface of the press liquor skimmed off (skimmed press liquor oil) after about 20 minutes. The other half of the press liquor was pumped through a strainer in order to remove suspended solids before separation of oil in a continuous plate centrifuge (separated press liquor oil) .

Results:

Oil fraction Cholesterol Yield L% (kg/100 kg raw material)

Skimmed press liquor oil 0.19 5

Separated press liquor oil 0.34 11 This Example shows that a significantly lower cholesterol level in skimmed press liquor oil also can be achieved by utilising the process on summer capelin. The observed level in separated press liquor oil is representative for all oil which can be extracted by using the described process conditions. A lower cholesterol level in the separated press liquor oil compared with oil extracted from winter capelin, is due to a higher percentage of fat in the fish.

Because of the low cholesterol level, oil extracted according to the invention is well suited for use in foodstuffs. The oil can be considered used in both a refined unhardened and hardened form.

Claims

AMENDED CLAIMS [received by the Internatiorral Bureau on 28 February 1996 (28.02.96) Original claims 1 and 2; replaced by amended claim 1; claims 1-5 renumbered 1-4 (1 page)]

1. A process for extracting fish oil with low cholesterol level from fat fish, where the fish is subjected to coagulation, straining and pressing, whereby strainer/press liquor is formed which contains an oil dispersion, characterised in that a fraction of the fish oil is separated from the strainer/press liquor by primary breakage of the oil dispersion, and that the fraction is collected on the surface of the strainer/press liquor from which it is skimmed off.

2. A process in accordance with claim 1, characterised in that as fat fish capelin is utilised.

3. A process in accordance with claim 1 or 2, characterised in that the coagulation is carried out without the fish being exposed to large shearing forces.

. The use of the oil produced by the process according to claim 1, 2 or 3 in foodstuffs.