Fish oil having decreased fish odor and a method for preparing the same

Abstract: A method for preparing fish oil having decreased fish odor, which comprises slightly hydrogenating fish oil to have decrease rate of iodine value of 15% or less and decrease rate of highly unsaturated fatty acid of 33% or less. A method for preparing fish oil having decreased fish odor, which comprises slightly hydrogenating fish oil under the following non-selective conditions: (1) an amount of catalyst used in the hydrogenation is 0.05% by weight or more to an amount of the fish oil; (2) hydrogen pressure in gaseous phase at the beginning of the hydrogenation is 3kg/cm² or more; (3) reaction temperature of the hydrogenation is in the range from 90 to 150°C; (4) reaction time of the hydrogenation is in the range from 5 to 30 minutes. The fish oil is preferably sardine oil, mackerel oil, skipjack oil, tuna oil, skipjack orbital fat or tuna orbital fat.

Description: BACKGROUND OF THE PRESENT INVENTION The present invention relates to a method for preparing fish oil having decreased fish odor and to the fish oil obtainable by said method. The present invention, in particular, relates to a method for preparing fish oil having decreased fish odor and containing a large amount of highly unsaturated fatty acids such as docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) and to a method for preparing the fish oil. Fat is one of the major three nutrients besides protein and carbohydrate and plays an important role as an energy source. As for Japanese people, the percentage of fat energy in all energy of the diet reaches to about 25% at present. The fat is also an important component constituting organism, and there are many reports that various symptoms and disorders appear when fat digestion from diet is inhibited. In addition, fat has a structure in which three molecules of fatty acids are ester-bonded to a glycerol skeleton and the properties and roles of fats in organisms depend largely on the types and combinations of the fatty acids. Among the fatty acids, there are many highly unsaturated fatty acids which themselves or whose metabolites show useful physiological functions in organisms. Since, for example, the lack of linoleic acid or α-linolenic acid results in symptoms such as dermal disorder, decrement of anagenetic power, increase of sensitivity to infection, and these fatty acids can not be synthesized in organisms and must be ingested from diet, they are called essential fatty acids. DHA and EPA, together with these essential fatty acids, seem to be useful for the prophylaxis and therapy of circulatory system diseases and other geriatric diseases, and thus they are highly unsaturated fatty acids which have been given attention in recent years. In particular, effects on blood circulation system such as platelet aggregation decrease, hemocholesterol decrease, blood sugar decrease, liver neutral fat decrease, prophylaxis and therapy effects on rheumatism, effects to decrease the development rate of various malignant tumors, immunological regulatory actions to atopy, asthma, pollinosis, as well as effects which are given attention recently, effects on nervous system such as development and improvement of learning function and memory, inhibition of dementia, inhibition of increase or decrease of optesthesia, are reported ('Development and Application of Functional Lipid', supervised by K.Sato et al, CMC 'Shokuhin to Kaihatsu' October, 1992, published by Kenko Sangyo Shinbunsha). The DHA and EPA of which various physiological functions have been reported, exist in fats of many fish species, whales, and marine products. The contents thereof are different depending on the fish species or whale species, on their regions, or on place or season of catch. It is known that a large amount of EPA is contained in fat of small-sized fish, such as sardine, mackerel and horse mackerel, and a large amount of DHA is contained in fat of large-sized fish, such as skipjack, tuna, marlin, amber jack and shark (Yushi Kagaku Binran 3rd Ed.). Among these fish species and whale species, sardine, mackerel, skipjack and tuna contain large amounts of highly unsaturated fatty acids, such as DHA and EPA in their body fat. In addition, in orbital fat which is present in the back region of eyeballs of skipjack or tuna, an extremely large amount of highly unsaturated fatty acid such as DHA exists. Generally, these fish oils containing large amounts of DHA and EPA are obtained by squeezing oil from whole fish bodies or part of fish bodies and removing the water-soluble fraction from the oil by an operation, such as decantation and centrifugation. Further, highly unsaturated fatty acids, such as DHA and EPA, may be concentrated by an operation, such as fractionation or wintering, to increase the amounts thereof. Although flavor is an important factor for foodstuffs, fish oil has an unique odor (fish odor) and thus the utilization as foodstuff is limited. At present, as to fish odor, it is attempted to remove it by adsorption to active carbon, active clay and diatomite, molecular distillation or steam distillation. However, even if-these deodorizing treatments are carried out, when the deodorized fish oil or food containing the oil is preserved, fish odor is produced during the preservation. These fish odors are produced by oxidative deterioration of highly unsaturated fatty acids, such as DHA and EPA. It is reported that the odor components are aldehydes, such as nonadienal, decatrienal, hexenal and heptenal, or ketones, such as octadienone (Karahadian and Linsay,J. Am. Oil Chemists' Society, vol.66,No.7,p.953,1989). Therefore, when fish oil is utilized as a foodstuff, there exits a big problem of production of fish odor, and thus removal of these odor components and inhibition of production have been important technical objects. As to sardine oil, mackerel oil, skipjack oil, tuna oil, skipjack orbital fat and tuna orbital fat, the removal or inhibition of production of these odor components have been important objects, and the removal of fish odor by adsorption on active carbon, active clay, and diatomite, molecular distillation or steam distillation have been attempted. However, since the above fish oil treated by these methods also produces fish odor during preservation, it is indispensable at present to control oxidative deterioration using a high amount of vitamin E, ascorbic acid and derivatives thereof, lecithin or many other kinds of antioxidants. On the other hand, hydrogenation of oil is a typical technique concerning the production of processed oil, as well as interesterification and fractionation. A hardened oil obtained by hydrogenation is a useful processed oil together with fractionated oil and interesterified oil, and it plays an important role in the production of edible oils. The hydrogenation is carried out usually at a reaction temperature in the range from 120 to 200°C under a hydrogen atmosphere in the presence of a catalyst with stirring the liquid oil. At the time, the hydrogen pressure is in the range from normal pressure to about 4.9 bar (5 kg/cm 2 ). As a catalyst, nickel catalysts such as reduced nickel, nickel formate, Raney nickel and nickel borate are often used. By the hydrogenation, the C-C double bond in a fatty acid - containing oil is hydrogenated. The hardened oil obtained by hydrogenation has the following characteristics: (1)The melting point the of oil increases and thus it may be used as a plastic oil; (2)Double bonds (unsaturated bonds) decrease, and thus the oxidative stability of the oil is improved; (3)With a selective hydrogenation, the solid fat content (SFC) vs. temperature curve is changed to a sharp vertical curve, and by mixing with another oil or fractionation, it is converted to an oil having good switability for food, such as chocolate, margarine and shortening. Hitherto, it has been attempted to improve the oxidative stability and decrease to the fish odor production by hydrogenating fish oil to obtain hardened fish oil, and this is a conventional method in order to utilize fish oil as foodstuffs. It is described that as a foodstuff a hardened fish oil having an increased melting point from 20 to 45° C, preferably to 35°C or more is easy to use.('Yushi,Yuryo handbook', supervised by A.Yoshiro, published by Saiwai shobo). However, highly unsaturated fatty acids, such as DHA and EPA contained in fish oil will disappear by hydrogenation of the fish oil. Thus, hardened fish oils which are available at present, contain no highly unsaturated fatty acids, such as DHA and EPA, or contain little of these fatty acids. There has been no report about an attempt to prepare fish oil in which a large amount of highly unsaturated fatty acids, such as DHA and EPA remain, and the fish odor production is inhibited by hydrogenation and which has no organoleptic problems. GB-A-382 060 discloses a method of improving the taste and smell of fish oils at a temperature not exceeding 100°C in which a non-noble metal catalyst is used and the quantity of hydrogen taken up by the oil does not substantially exceed 15 liters of hydrogen/500 g of the oil. Further, the results from the treatment of cod oil using a nickel catalyst (1%) under a pressure of hydrogen of 9,8 bar (10 atmospheres) at 40°C to 50°C, is disclosed in the Example. Although it is disclosed that the iodine value of the oil is slightly reduced, the rate of reduction of the iodine value is not described concretely. Further, it is disclosed in that the desirable quantity of hydrogen taken up by the oil is 5 to 10 liters/500g of fish oil. Since the initial iodine value of cod oil used in the Example is not described, the invention of GB-A-382 060 cannot be compared with the present invention. JP-A-05 117 686 discloses a cosmetic material containing oil which is colorless and clear without fish oil odor and highly stable after prolonged storage, obtained by hydrolysis with lipase and deoxidization, after purification of crude oil or hydrogenation of crude fish oil; and a method of preparing the cosmetic material. In addition the hydrogenation is carried out at a temperature of 130 to 200°C, a hydrogen pressure of 2,94-4,9 bar (3-5 kg/cm 2 ) with Ni, Pt and Pd catalyst(s). Since the obtained material does not contain triglyceride, it is assumed that the material is a mixture of free fatty acid and glycerol. GB-A-658 189 describes the partial hydrogenation of unsaturated glyceride oils to obtain oil material for plastic shortenings. Polyvalent unsaturated fatty acid-containing oils, such as cottonseed oil and soybean oil, are used and linoleic or linolenic acid is converted to oleic acid. The hydrogenation is controlled at a temperature of 60 to 180°C, using a nickel catalyst to obtain an oil having a iodine value of 60 to 90 which is suitable for shortening. SUMMARY OF THE INVENTION The present invention was made in view of the above mentioned problems and the purpose of the present invention is to provide a method for preparing fish oil which has decreased fish odor and contains a high amount of highly unsaturated fatty acids such as DHA and EPA, by under non-selective conditions hydrogenating fish oil, which may be a useful foodstuff owing to its many physiological functions but which cannot be utilized easily owing to its specific odor or whose utilization is limited, and the fish oil which may be prepared by the method. Another object of the present invention is to provide sardine oil having decreased fish odor and containing a high amount of highly unsaturated fatty acids, such as DHA and EPA, and a method for preparing the sardine oil. Still another object of the present invention is to provide mackerel oil having decreased fish odor and containing a high amount of highly unsaturated fatty acids, such as DHA and EPA, and a method for preparing the mackerel oil. Still another object of the present invention is to provide skipjack oil having decreased fish odor and containing a high amount of highly unsaturated fatty acids, such as DHA and EPA, and a method for preparing the skipjack oil. Still another object of the present invention is to provide tuna oil having decreased fish odor and containing a high amount of highly unsaturated fatty acids, such as DHA and EPA, and a method for preparing the tuna oil. Still another object of the present invention is to provide skipjack orbital fat having decreased fish odor and containing a high amount of highly unsaturated fatty acids, such as DHA and EPA, and a method for preparing the skipjack orbital fat. Still another object of the present invention is to provide a tuna orbital fat having decreased fish odor and containing a high amount of highly unsaturated fatty acids, such as DHA and EPA, and a method for preparing the tuna orbital fat. DETAILED DESCRIPTION OF THE INVENTION The present invention in order to attain the above objects comprises a method for preparing fish oil having decreased fish odor, which comprises slightly hydrogenating fish oil to reduce the iodine value by 15% or less and to reduce the highly unsaturated fatty acids by 33% or less, under the following non-selective conditions: (1) the amount of catalyst used in the hydrogenation is 0.05% by weight or more, based on the weight of the fish oil; (2) the hydrogen pressure in the gaseous phase at the beginning of the hydrogenation is 2.94 bar (3kg/cm 2 ) or more; (3) the reaction temperature of the hydrogenation is in the range from 90 to 150°C; (4) the reaction time of the hydrogenation is in the range from 5 to 30 minutes. The catalyst is preferably a nickel catalyst, and the highly unsaturated fatty acid is preferably docosahexaenoic acid (DHA) or eicosapentaenoic acid (EPA). The fish oil is preferably sardine oil, mackerel oil, tuna oil, skipjack oil, tuna orbital fat or skipjack orbital fat. According to a particularly preferred embodiment of the present invention, the fish oil having decreased fish odor is sardine oil or mackerel oil having the following characteristics: (1) the concentration of DHA contained in the fatty acid residue of the oil is in the range from 1 to 13% by weight; (2) the concentration of EPA contained in the fatty acid residue of the oil is in the range from 3 to 18% by weight; (3) the content of trans-isomer is 4% by weight or more. Furthermore, the fish oil having decreased fish odor may be skipjack oil or tuna oil having the following characteristics: (1) the concentration of DHA contained in the fatty acid residue of the oil is in the range from 15 to 25% by weight; (2) the concentration of EPA contained in the fatty acid residue of the oil is in the range from 1 to 10% by weight; (3) the content of trans-isomer is 4% by weight or more. Furthermore, the fish oil having decreased fish odor may be skipjack orbital fat or tuna orbital fat having the following characteristics: (1) the concentration of DHA contained in the fatty acid residue of the oil is in the range from 25 to 38% by weight; (2) the concentration of EPA contained in the fatty acid residue of the oil is in the range from 2 to 8% by weight; (3) the content of trans-isomer is 4% by weight or more. The invention also comprises fish oil having decreased fish odor which is obtainable by the method as defined above, including sardine oil, mackerel oil, tuna oil, skipjack oil, tuna orbital fat, and skipjack orbital fat. Since the slightly hydrogenated fish oil obtained by the method of the present invention has little fish odor having a undesirable organoleptic effect, and disappearance of highly unsaturated fatty acids such as DHA and EPA in fish oil is inhibited at the minimum, the fish oil is suitable as foodstuff and may be applied to medical products. Further it is possible to reinforce the effect of decreasing fish odor by using the slightly hydrogenated fish oil obtained by the method of the present invention together with an antioxidant, and by combining with another purified fat, and such fish oil may be used as a good-tasting and stable oil. The fish oil used as raw material in the present invention is collected from bodies of small-sized or middle-sized blueback fish such as sardine, horse mackerel, mackerel, or of big-sized blueback fish such as tuna, skipjack and marlin. However the fish oil as a raw material is not limited thereto and may be collected from shark, whale and cuttlefish. Further the fish oil used as raw material may be collected from parts of fish bodies such as internal organs e.g. liver, head and eye and not from whole fish bodies. In the present invention, the fish oil collected from sardine, mackerel, tuna, skipjack and further tuna orbital fat or tuna orbital fat is most preferable. Sardine generally means spotlined sardine of the species Clupeidae and round herring of the species Dussumieriinae, Japanese anchovy of the species Engraulidae and related species thereof but scientifically spotlined sardine is classified in the Sardinops genus, round herring is classified in the Etrumeus genus, and Japanese anchovy is classified in the Engraulis genus. Sardine is distributed in all oceans over the world, and is called Sardine, Pilchard, Anchovy, Clupeoid, and herring-like fish, depending on the species. As for the sardine, the fish body contains approximately 10% by weight of fat and the sardine contains a large amount of highly unsaturated fatty acids, i.e., it has 4 to 14% of DHA and 10 to 23% of EPA in body fat. The sardine has been considered to be a useful fish from old times as a highly available fish. Sardine is marketed and eaten by processing into Namasu (a dish of fish and vegetables seasoned with vinegar); by baking, grilling, broiling; treating for preservation such as into a salted food, a food preserved in sake lees, a food preserved in malted rice, a salted and dried food; or by processing into canned or bottled food in oil. In addition, sardine is used as feed or fertilizer. The production quantity of sardine is large and the catch quantity of sardine in Japan is about 2,720,000 metric tons (1980) and, in addition, about 30,000 metric tons of sardine is imported at present ('Shokuhin, Seisan, Yunyu, Shohi, 1993' edited by Shokuhin Ryutsu Kenkyuukai (1993). Mackerel generally is a generic name of Lateolabrax japonicus Scombridae 15,48 and it primarily means chub mackerel and spotted mackerel. Mackerel is distributed in all tropical and subtropical oceans; its Latin name is Scomber, and it is called mackerel (English), maquereau (French), makrele (German) and makreel (Dutch) and is an object of fishery. As for the mackerel, the fish body contains approximately 10 to 15% by weight of fat and the mackerel contains a large amount of highly unsaturated fatty acids, i.e., it has 4 to 18% of DHA and 7 to 20% of EPA in body fat. The mackerel is used to migrate in large groups, and thus it has been an important edible fish since old times in Europe, the Mediterranean area and Japan. The domestic production of mackerel in Japan exceeded 1,000,000 metric tons (1980) and after that it has been decreasing, but it keeps about 300,000 metric tons (1992). The import quantity of mackerel is also large, and about 140,000 metric tons of mackerel is imported from Norway and other countries at present ('Shokuhin, Seisan, Yunyu, Shohi, 1993', edited by Shokuhin Ryutsu Kenkyuukai (1993). Skipjack generally has the scientific name of Lateolabrax japonicus Scombridae Katsuwonus pelamis 1 and has the Latin name of Katsuwonus. The skipjack contains a large amount of highly unsaturated fatty acids, i.e., it has 20 to 25% of DHA and 5 to 10% of EPA in body fat. The skipjack is distributed in all tropical and temperate oceans and is called Skipjack, Bonito (English), Bonite,Listao (French) and Bonito (German) and is an object of fishery. The catch quantity of skipjack in Japan is about 320,000 metric tons (1992) and further about 30,000 metric tons of skipjack is imported at present ('Shokuhin, Seisan, Yunyu, Shohi, 1993', edited by Shokuhin Ryutsu Kenkyuukai (1993). Tuna generally has the scientific name of Lateolabrax japonicus Scombridae Thunnus 7 and has the Latin name of Thunnus. Tuna contains a large amount of highly unsaturated fatty acids, i.e., it has 20 to 30% of DHA and 3 to 10% of EPA in body fat. The tuna is distributed in all tropical and temperate oceans and is called Tuna (English), Thon (French) and Thun (German), and is an object of fishery. The catch quantity of tuna in Japan is about 340,000 metric tons (1992) and further about 250,000 metric tons of tuna is imported at present ('Shokuhin, Seisan, Yunyu, Shohi, 1993', edited by Shokuhin Ryutsu Kenkyuukai (1993). In particular, the catch quantities and import quantities of bigeye tuna and yellowfin tuna are both large. Highly unsaturated fatty acids, especially DHA exist in a very high amount in orbital fat which is the fat existing in the back position of skipjack and tuna eyeballs. The highly unsaturated fatty acid content in these orbital fat varies depending upon fish species, fishery sea area and fishery season, but DHA exists in an amount in the range from 30 to 40% and EPA exists in an amount in the range from 4 to 10% in skipjack orbital fat and tuna orbital fat. The orbital fat collected from skipjack or tuna may be obtained by removing the water-soluble fraction from the oil by centrifugation after acid treatment and treatments, such as degumming and deacidification. In the present invention, these fish oils as raw materials may be directly slightly hydrogenated. However, it is desirable to purify these fish oils used for slight-hydrogenation as much as possible since complex lipids typically exemplified by phospholipids or proteins existing in fish oils poison the catalyst used in the slight-hydrogenation and deteriorate the catalytic activity to inhibit the slight-hydrogenation progress. In the present invention, a fish oil as raw material and a catalyst for hydrogenation may be added to a reaction vessel to carry out a slight (mild) hydrogenation reaction. As the catalyst for hydrogenation, a reduced catalyst may be used, and it may include a nickel catalyst having nickel as main constituent element such as reduced nickel, nickel formate, Raney nickel, nickel borate; a metal catalyst formed from platinum, palladium, iron or copper; and a hydrogen storage (occlusion) alloy such as a lanthanum series alloy and a calcium series alloy. The catalysts may be selected for use depending on the catalytic activity and the reaction condition desired. In the present invention, it is preferable, in particular, that one, two or more nickel catalysts may be used. These catalysts are used in an amount of 0.05% by weight or more based on the weight of the fish oil in order to achieve non-selective mild hydrogenation, although these catalysts are used in an amount of 0.02 to 0.20% by weight based on the weight of the oils in conventional hydrogenation. A reaction vessel which is resistant to pressure and is equipped with a stirring device is preferably used, and the shape or size of the vessel is not limited. In addition, batch type reaction vessels and continuous type reaction vessels may be used. In the present invention, the fish oil and catalyst, when added to the reaction vessel, are deaerated and dehydrated sufficiently by reducing the pressure, preferably to 6.7 mbar (5 torr) or less with stirring, and then they are preferably heated to a predetermined reaction temperature while keeping them at the reduced pressure. However, if the fish oil used is already sufficiently dehydrated, the reduction of pressure is not necessary. The fish oil and catalyst are not necessarily filled into a reaction vessel at the same time and the catalyst may be filled into a reaction vessel after the fish oil is filled into it and has reached the predetermined conditions. The operation may also be reversed. After the fish oil and catalyst have reached the predetermined temperature, hydrogen gas is supplied to the reaction vessel to start mild hydrogenation. At the time, the hydrogen pressure of gaseous phase in the reaction vessel is set at 2.94 bar (3kg/cm 2 ) or more. The hydrogen pressure is preferably kept while the mild hydrogenation is carried out. As a reaction temperature, it is preferable to keep a temperature at which the catalyst exhibits its activity and a temperature as low as possible. These optimum reaction temperature is determined depending on the catalyst species but is preferably in the range from 90 to 150°C when a nickel catalyst is used. In the present invention, after a predetermined time has passed from the beginning of the hydrogenation, the stirring is stopped and hydrogen gas is removed from the reaction vessel to stop the hydrogenation reaction. If a reaction vessel is used in which a rapid temperature change may be carried out the hydrogenation reaction may be stopped by cooling the temperature of the fish oil rapidly to 50°C or less, preferably to 10°C or less. In the present invention, the reaction time is in the range from 5 to 30 minutes in order to keep the extent of the hydrogenation in the range of slight-hydrogenation. When the slight-hydrogenated fish oil is taken out from the reaction vessel, the fish oil is most preferably cooled to 20°C or less in order to inhibit oxidative deterioration of the mildly hydrogenated fish oil. An adsorbent such as active clay may be added to the mildly hydrogenated fish oil which is thus taken out from the reaction vessel, and the adsorbent and the fish oil are stirred. The adsorbent may be used in an amount of 1 to 5% by weight, based on the weight of the fish oil, but it is not limited thereto. As the adsorbent, diatomite may be used besides active clay, and silica gel and Florisil (TM) may be mixed with active clay or diatomite. Then the catalyst and the adsorbent are removed by filtration, using e.g. a filter press to collect the mildly hydrogenated fish oil. On the other hand, vacuum drying is conveniently used to remove water, but freeze drying and dehydrating agents may be used. Further, depending on the necessities, deodorizing treatments such as steam distillation, may be used on the mildly hydrogenated fish oil. The mildly hydrogenated fish oil having reduced fish odor may be stored in a refrigerator after adding an antioxidant to it and blowing an inactive gas into it. The fish oil having decreased fish odor of the present invention may, if necessary be used by mixing with another food oil. In addition, after a fish oil as a raw material is mixed with another food oil, the method of the present invention may be carried out to obtain the fish oil having decreased fish odor. By the above mentioned mild hydrogenation operation, fish odor components or precursors thereof are reduced, isomerized or decomposed, and are converted into chemical components producing no fish odor. Therefore, since the mildly hydrogenated fish oil of the present invention has decreased fish odor and production of fish odor during storage is inhibited, the fish oil of the present invention has no organoleptic problems. Further, in the mildly hydrogenated fish oil of the present invention, the highly unsaturated fatty acids, such as DHA and EPA, hardly disappear, and these acids remain in the fish oil in high amounts. Further, the decrease of the iodine value and the increase of the melting point are small. Namely, in the mildly hydrogenated fish oil obtained in the present invention, the decrease rate of iodine value from fish oil as raw material is preferably 15% or less but most preferably in the range from 5 to 10% in order to exhibit the effect of decreasing fish odor effectively and to inhibit the disappearance of highly unsaturated fatty acids, such as DHA and EPA. The sardine oil or mackerel oil having decreased fish odor which may be obtained by the method of the present invention each contains 1 to 13% of DHA in the fatty acid residue and 3 to 18% of EPA in the fatty acid residue. The trans-isomer content of each oil is 4% or more and the each oil was changed to sufficiently stabilized fish oil by the present method. On the other hand, usual sardine oil or mackerel oil contains little positional isomer and the trans-isomer content is 1 to 2% or less. The skipjack oil or tuna oil having decreased fish odor obtained by the method of the present invention contains 15 to 25% of DHA in the fatty acid residue and 1 to 10% of EPA in the fatty acid residue. The trans-isomer content of each oil is 4% or more and the each oil was changed to sufficiently stabilized fish oil. On the other hand, usual skipjack oil or tuna oil contains little positional isomer and the trans-isomer content is 1 to 2% or less. The skipjack orbital fat or tuna orbital fat having decreased fish odor obtained by the method of the present invention contains 25 to 38% of DHA in the fatty acid residue and 2 to 8% of EPA in the fatty acid residue. Further, the trans-isomer content of the each orbital fat is 4% or more and the each fat was changed to sufficiently stabilized fish oil. On the other hand, usual skipjack orbital fat or tuna orbital fat contains little positional isomer and the trans-isomer content is 1 to 2% or less. The measurements of trans-isomer content were carried out in accordance with Standard Oil Analysis Test method 2.3.24 established by Nihon Yukagaku Kyokai, or with Official and Tentative Methods of the American Oil Chemists' Society, Official Method Cd 14-61. The fish oils of the present invention may be used alone or may be used by mixing with one or more other fish oils of the present invention. Further, by the preservation tests and organoleptic evaluation made with each fish oil having decreased fish odor of the present invention, it was confirmed that the fish oils of the present invention produce little fish odor and are excellent also in flavor. Therefore, the fish oils having decreased fish odor are suitable for use as foodstuffs and are useful as raw materials for any type of foods, for example beverages such as milk shake, coffee beverages and lactic acid beverages; desserts such as ice cream, jelly, mousse, yogurt; Miso, meat products, fish meat products; milk products, such as powder milk, cheese food, fat spread; or baby food. In addition, the fish oils of the present invention may be used as materials for medical products. By using the fish oil of the present invention, the amount of antioxidant which has been used to maintain the flavor-stability, may be decreased. The present invention will be described in more detail by referring to the following examples. EXAMPLE 1 600g of purified sardine oil (iodine value:162, DHA: 8.4%, EPA:15.2%) was ,filled into a 1L reaction vessel, and 0.6g (0.1% by weight) of Raney nickel catalyst was added to it. Then, after deaerating and dehydrating at a pressure of 6,7 mbar (5 torr) or less with stirring, a hydrogenation reaction was carried out under a hydrogen atmosphere of 3,92 bar (4kg/cm 2 ) at 100°C for 30 minutes. Then the hydrogenation reaction was stopped by removing hydrogen gas from the reaction vessel, and after cooling the oil to 20°C or less, the oil was treated with active clay to obtain 515g of slightly hydrogenated sardine oil. The iodine value of the slightly hydrogenated sardine oil was 149, the contents of DHA and EPA were 6.8% and 12.2%, respectively. A preservation test was carried out with the purified sardine oil used as a raw material, and the slightly hydrogenated sardine oil obtained in the example. 50g of each oil was filled into a 100ml glass vessel having a cap, 30mg of tocopherol was added to each oil as an antioxidant, and the vessels were stored in a temperature-controlled oven which was kept at 30±1°C and was protected from light to carry out the preservation test. An organoleptic evaluation was made by a panel of ten well-trained professional persons using the following evaluation standards on the fish odor strength and preferability listed in Table 1. Evaluation Fish Odor Strength Preferability 0 no odor at all extremely bad 1 very little odor bad 2 slight odor a little bad 3 distinct odor not bad and not good 4 somewhat strong odor good 5 strong odor extremely good The results are shown in Table 2. An average of the evaluation points made by all panel members is shown as the organoleptic evaluation point. Oil Fish Odor Strength Preferability 0 day 7th day 0 day 7th day Purified Sardine Oil 1.5 3.2 3.9 2.6 Sardine Oil of the Present Invention 1.0 2.5 4.3 3.1 As apparent from the above results, the sardine oil of the present invention had low fish odor at day zero of the preservation test, as compared with the purified sardine oil. Further, even at the 7th day of the preservation test, the sardine oil of the present invention had low fish odor and the production of fish odor during the test was inhibited. In addition, the sardine oil of the present invention had always high evaluation points of preferability reflecting the behavior of the fish odor strength. EXAMPLE 2 2kg of purified sardine oil (DHA:6.5%, EPA:19.3%, trans-isomer content:0.8%) was filled into a 4L reaction vessel, and 1.5g (0.075% by weight) of reduced nickel catalyst was added to it. Then, after deaerating and dehydrating of a pressure of 6.7 mbar (5 torr) or less with stirring, a hydrogenation reaction was carried out under a hydrogen atmosphere of 2.94 bar (3kg/cm 2 ) at 130°C for 15 minutes. Then the hydrogenation reaction was stopped by removing hydrogen gas from the reaction vessel and after cooling to 20°C or less, the oil was treated with active clay to obtain 1.75kg of sardine oil having decreased fish odor. The DHA content of the sardine oil obtained was 3.2%, the EPA content was 14.9% and the trans-isomer content was 4.2%. A preservation test was carried out with the purified sardine oil used as a raw material, and the sardine oil obtained in the example. 50g of each sardine oil was filled into a 100ml glass vessel having a cap, 15mg of tocopherol was added to each oil as an antioxidant, and the vessels were stored in an oven kept at 50±1°C to make a preservation test. An organoleptic evaluation was carried out in the same manner as described in Example 1, according to the standards shown in Table 1. The results are shown in Table 3. Oil Fish Odor Strength Preferability 0 day 7th day 0 day 7th day Purified Sardine Oil 1.5 3.8 3.9 1.8 Sardine Oil of the Present Invention 1.0 1.9 4.2 3.6 As apparent from the above results, the sardine oil of the present invention had weak fish odor from day zero of the preservation test, compared with the purified sardine oil. In addition, even at the 7th day of the test, the production of fish odor in the sardine oil of the present invention was inhibited and the evaluation point of preferability was high. EXAMPLE 3 500g of purified sardine oil (DHA:6.5%, EPA:19.3%, trans-isomer content:0.8%) was filled into a 1L reaction vessel, and 0.25g (0.050% by weight) of Raney nickel catalyst was added to the oil. Then, after deaerating and dehydrating at a pressure of 6.7 mbar (5 torr) or less with stirring, a hydrogenation reaction was carried out under a hydrogen atmosphere of 3.97 bar (4kg/cm 2 ) at 110°C for 10 minutes. Then the hydrogenation reaction was stopped by removing hydrogen gas from the reaction vessel, and after cooling the oil to 20°C or less, it was treated with active clay to obtain 432g of sardine oil having decreased fish odor. The DHA content of the sardine oil thus obtained was 3.9%, the EPA content was 11.4% and the trans-isomer content was 8.8%. Then a preservation test was carried out on the purified sardine oil used as a raw material and the sardine oil obtained in the example. The preservation test was made as a forced-deterioration test in the same manner as described in Example 2 except that the temperature of the oven was 30±1°C, and an organoleptic evaluation was made. The results are shown in Table 4. Oil Fish Odor Strength Preferability 0 day 7th day 0 day 7th day Purified Sardine Oil 1.4 4.1 4.0 2.0 Sardine Oil of the Present Invention 0.9 2.5 4.3 3.4 As apparent from the above results, the sardine oil of the present invention had weak fish odor from day zero of the preservation test, compared with the purified sardine oil. In addition, even at the 7th day of the test, the production of fish odor in the sardine oil of the present invention was inhibited and the evaluation point of preferability was high. EXAMPLE 4 500g of purified mackerel oil (DHA content :12.8%, EPA content:16.4%, trans-isomer content:1.8%) was filled into a 1L reaction vessel, and 0.375g (0.075% by weight) of reduced nickel catalyst was added to the oil. After deaerating and dehydrating the oil at a pressure of 6.7 mbar (5 torr) or less with stirring, a hydrogenation reaction was carried out under a hydrogen atmosphere of 2.94 bar (3kg/cm 2 ) at 130°C for twenty minutes. Then hydrogen gas was removed from the reaction vessel to stop the hydrogenation reaction, and after cooling to 20°C or less, the oil was treated with active clay to obtain 373g of mackerel oil having decreased fish odor. The DHA content of the mackerel oil thus obtained was 8.1%, the EPA content was 13.9% and the trans-isomer content was 6.2%. Then a preservation test was made with the purified mackerel oil used as a raw material and the mackerel oil obtained in the example. The preservation test and the organoleptic evaluation were made in the same manner as described in Example 2. The results are shown in Table 5. Oil Fish Odor Strength Preferability 0 day 3rd day 0 day 3rd day Purified Mackerel Oil 1.4 3.7 3.6 1.8 Mackerel Oil of the Present Invention 1.1 2.0 4.0 3.4 As apparent from the above results, the mackerel oil of the present invention had weak fish odor from the day zero of the preservation test compared with the purified mackerel oil and had a high evaluation point of preferability. In addition, even at the 3rd day of the test, the production of fish odor in the mackerel oil of the present invention was inhibited and the evaluation point of preferability was high. EXAMPLE 5 500g of mixed oil (DHA content :8.8%, EPA content:18.7%, trans-isomer content:1.0%), in which purified sardine oil (DHA content:6.5%, EPA content:19.3%, trans-isomer content:0.8%) and purified mackerel oil (DHA content:12.8%, EPA content:16.4%,trans-isomer:1.8%) were mixed in the ratio of 80:20 by weight, was filled into a 1L reaction vessel, and 0.5g (0.10% by weight) of reduced nickel catalyst was added to the mixed oil. After deaerating and dehydrating the mixed oil at a pressure of 6,7 mbar (5 torr) or less with stirring, a hydrogenation reaction was carried out under a hydrogen atmosphere of 2.94 bar (3kg/cm 2 ) at 130°C for fifteen minutes. Then hydrogen gas was removed from the reaction vessel to stop the hydrogenation reaction, and after cooling to 20°C or less, the oil was treated with active clay to obtain 380g of sardine and mackerel mixed oil having decreased fish odor. The DHA content of the sardine and mackerel mixed oil thus obtained was 4.2%, the EPA content was 16.6% and the trans-isomer content was 5.9%. Then a preservation test was made with the purified sardine and mackerel mixed oil used as a raw material and the sardine and mackerel oil obtained in the example. A preservation test and organoleptic evaluation were made in the same manner as described in Example 2. The results are shown in Table 6. Oil Fish Odor Strength Preferability 0 day 5th day 0 day 5th day Purified Sardine/Mackerel Mixed Oil 1.4 3.6 3.5 1.8 Sardine/Mackerel Mixed Oil of the Present Invention 1.1 2.2 3.9 3.2 As apparent from the above results, the mixed oil of the present invention had weak fish odor from day zero of the preservation test compared with the purified mixed oil and had high evaluation point of preferability. In addition, even at the 5th day of the test, the production of fish odor in the mixed oil of the present invention was inhibited and the evaluation point of preferability was high. EXAMPLE 6 2kg of purified skipjack oil (iodine value:182, DHA content:23.5%, EPA content:6.2%, trans-isomer content:1.4%) was filled into a 4L reaction vessel, and 1.5g (0.075% by weight) of reduced nickel catalyst was added to the oil. After deaerating and dehydrating at a pressure of (5 torr) or less with stirring, a hydrogenation reaction was carried out under a hydrogen atmosphere of 2.94 bar (3kg/cm 2 ) at 130°C for fifteen minutes. Then hydrogen gas was removed from the reaction vessel to stop the hydrogenation reaction, and after cooling to 20°C or less, the oil was treated with active clay to obtain 1.7kg of slightly hydrogenated skipjack oil. The iodine value of the slightly hydrogenated skipjack oil thus obtained was 171, the DHA and EPA content were 17.2% and 5.2%, respectively. Then a preservation test was made on the purified skipjack oil used as a raw material and the slightly hydrogenated skipjack oil obtained in the example. The preservation test by and the organoleptic test were made in the same manner as described in Example 2 except that the amount of tocopherol used was 30ml. The results are shown in Table 7. Oil Fish Odor Strength Preferability 0 day 3rd day 0 day 3rd day Purified Skipjack Oil 1.0 3.0 4.5 3.0 Skipjack Oil of the Present Invention 0.6 1.8 4.8 4.0 As apparent from the above results, the slightly hydrogenated skipjack oil of the present invention had weak fish odor at day zero of the preservation test, compared with the purified skipjack oil and had high evaluation point of preferability. In addition, even at the 3rd day of the test, the production of fish odor in the slightly hydrogenated skipjack oil of the present invention was inhibited and the evaluation point of preferability was high. EXAMPLE 7 500g of purified skipjack oil (DHA content:23.5%, EPA content:6.2%, trans-isomer content:1.4%) was filled into a 1L reaction vessel, and 0.25g (0.050% by weight) of Raney nickel catalyst was added to the oil. After deaerating and dehydrating at a pressure of 6,7 mbar (5 torr) or less with stirring, a hydrogenation reaction was carried out under a hydrogen atmosphere of 3.92 bar (4kg/cm 2 ) at 110°C for ten minutes. Then hydrogen gas was removed from the reaction vessel to stop the hydrogenation reaction, and after cooling to 20°C or less, the oil was treated with active clay to obtain 440g of skipjack oil having decreased fish odor. The DHA content of the skipjack oil was 16.8%, the EPA content was 5.8%, and trans-isomer content was 6.8%. Then a preservation test was made with the purified skipjack oil used as a raw material and the skipjack oil obtained in the example. The preservation test and the organoleptic evaluation were made in the same manner as described in Example 2, except that the oven temperature was 30±1°C. The results are shown in Table 8. Fish Odor Strength Preferability Oil 0 day 7th day 0 day 7th day Purified Skipjack Oil 1.0 4.1 4.5 2.0 Skipjack Oil of the Present invention 0.6 2.0 4.8 3.9 As apparent from the above results, the skipjack oil of the present invention had weak fish odor from day zero of the preservation test, compared with the purified skipjack oil, and had high evaluation point of preferability. In addition, even at the 7th day of the test, the production of fish odor in the skipjack oil of the present invention was inhibited and the evaluation point of preferability was high. EXAMPLE 8 2kg of purified tuna oil (DHA content:26.5%, EPA content:7.2%, trans-isomer content:1.1%) was filled into a 4L reaction vessel, and 1.5g (0.075% by weight) of reduced nickel catalyst was added to the oil. After deaerating and dehydrating at a pressure of 6,7 mbar (5 torr) or less with stirring, a hydrogenation reaction was carried out under a hydrogen atmosphere of 2.94 bar (3kg/cm 2 ) at 130° C for fifteen minutes. Then hydrogen gas was removed from the reaction vessel to stop the hydrogenation reaction, and after cooling to 20°C or less, the oil was treated with active clay to obtain 1.7kg of tuna oil having decreased fish odor. The DHA content of the tuna oil was 21.2%, the EPA content was 6.2%, and trans-isomer content was 4.8%. Then a preservation test was made with the purified tuna oil used as a raw material and the tuna oil obtained in the example. The preservation test and the organoleptic test were made in the same manner as described in Example 2. The results are shown in Table 9. Fish Odor Strength Preferability Oil 0 day 4th day 0 day 4th day Purified Tuna Oil 1.0 3.3 4.4 2.9 Tuna Oil of the present Invention 0.8 1.7 4.6 3.9 As apparent from the above results, the tuna oil of the present invention had weak fish odor from day zero of the preservation test compared with the purified tuna oil and had high evaluation point of preferability. In addition, even at the 4th day of the test, the production of fish odor in the tuna oil of the present invention was inhibited and the evaluation point of preferability was high. EXAMPLE 9 500g of purified tuna oil (DHA content:26.5%, EPA content:7.2%, trans-isomer content:1.1%) was filled into a 1L reaction vessel, and 0.25g (0.050% by weight) of Raney nickel catalyst was added to the oil. After deaerating and dehydrating at a pressure of 6.7 mbar (5 torr) or less with stirring, a hydrogenation reaction was carried out under a hydrogen atmosphere of 3.92 bar (4kg/cm 2 ) at 110°C for ten minutes. Then hydrogen gas was removed from the reaction vessel to stop the hydrogenation reaction. After cooling it to 20°C or less, the oil was treated with active clay to obtain 435g of tuna oil having decreased fish odor. The DHA content of the tuna oil was 20.1%, the EPA content was 4.3%, and trans-isomer content was 6.5%. Then a preservation test was made with the purified tuna oil used as a raw material and the tuna oil obtained in the example. The preservation test and organoleptic evaluation were carried out in the same manner as described in Example 2, except that the oven temperature was 30±1°C. The results are shown in Table 10. Fish Odor Strength Preferability Oil 0 day 10th day 0 day 10th day Purified Tuna Oil 1.0 4.0 4.5 2.5 Tuna Oil of the Present Invention 0.7 2.5 4.7 3.6 As apparent from the above results, the tuna oil of the present invention had weak fish odor from the day zero of the preservation test compared with the purified tuna oil and had high evaluation point of preferability. In addition, even at the 10th day of the test, the production of fish odor in the tuna oil of the present invention was inhibited and the evaluation point of preferability was high. EXAMPLE 10 2kg of mixed oil (DHA content :24.0%, EPA content:7.6%, trans-isomer content:1.4%), in which purified skipjack oil (DHA content:22.5%, EPA content:7.0%, trans-isomer content:2.0%) and purified tuna oil (DHA content:26.5%, EPA content:8.6%,trans-isomer:1.4%) were mixed in the ratio of 60:40 by weight, was filled into a 4L reaction vessel, and 1.5g (0.075% by weight) of reduced nickel catalyst was added to the oil. After deaerating and dehydrating at a pressure of 6.7 mbar (5 torr) or less with stirring, a hydrogenation reaction was carried out under a hydrogen atmosphere of 2,94 bar (3kg/cm 2 ) at 130° C for fifteen minutes. Then hydrogen gas was removed from the reaction vessel to stop the hydrogenation reaction, and after cooling it to 20°C or less, the mixed oil was treated with active clay to obtain 1.7kg of skipjack and tuna mixed oil having decreased fish odor. The DHA content of the skipjack and tuna mixed oil thus obtained was 18.6%, the EPA content was 6.3% and the trans-isomer content was 5.6%. Then a preservation test was carried out on the purified mixed oil used as a raw material and the mixed oil obtained in the example. Corn oil was added to each mixed oil to have DHA content in each mixed oil of 15%. With each mixed oil, a preservation test and an organoleptic evaluation were made in the same manner as described in Example 2. The results are shown in Table 11. Fish Odor Strength Preferability Oil 0 day 5th day 0 day 5th day Mixed Oil containing the Purified oil 1.0 3.2 4.4 2.9 Mixed Oil containing the Oil of the Present Invention 0.7 1.6 4.7 3.9 As apparent from the above results, the mixed oil of the present invention had weak fish odor from day zero of the preservation test, compared with the mixed oil containing the purified oil, and it had high evaluation point of preferability. In addition, even at the 5th day of the test, the production of fish odor in the mixed oil of the present invention was inhibited and the evaluation point of preferability was high. EXAMPLE 11 500g of purified tuna orbital fat (DHA content:35.5%, EPA content:7.2%, trans-isomer content:1.4%) was filled into a 2L reaction vessel, and 0.50g (0.10% by weight) of reduced nickel catalyst was added to the fat. After deaerating and dehydrating at a pressure of 6.7 mbar (5 torr) or less with stirring, a hydrogenation reaction was carried out under hydrogen atmosphere of 2.94 bar (3kg/cm 2 ) at 130° C for ten minutes. Then hydrogen gas was removed from the reaction vessel to stop the hydrogenation reaction. After cooling to 20°C or less, the fat was treated with active clay to obtain 378g of tuna orbital fat having decreased fish odor. The DHA content of the tuna orbital fat was 29.3%, the EPA content was 4.9%, and trans-isomer content was 6.3%. Then a preservation test was made on the purified tuna orbital fat used as a raw material and the tuna orbital fat obtained in the example. The preservation test and the organoleptic evaluation were carried out in the same manner as described in Example 2, except that the amount of tocopherol was 20mg. The results are shown in Table 12. Fish Odor Strength Preferability 0 day 3rd day 0 day 3rd day Purified Tuna Orbital Fat 0.9 2.9 4.1 3.3 Tuna Orbital Fat of the Present Invention 0.6 1.5 4.5 3.9 As apparent from the above results, the tuna orbital fat of the present invention had weak fish odor from day zero of the preservation test compared with the purified tuna orbital fat, and it had a high evaluation point of preferability. In addition, even at 3rd day of the test, the production of fish odor in the tuna orbital fat of the present invention was inhibited and the evaluation point of preferability was high. EXAMPLE 12 300g of purified skipjack orbital fat (DHA content:36.5%, EPA content:9.8%, trans-isomer content:1.7%) was filled into a 1L reaction vessel, and 0.15g (0.050% by weight) of Raney nickel catalyst was added to the fat. After deaerating and dehydrating at a pressure of 6.7 mbar (5 torr) or less with stirring, a hydrogenation reaction was carried out under hydrogen atmosphere of 3.92 bar (4kg/cm 2 ) at 110° C for 30 minutes. Then hydrogen gas was removed from the reaction vessel to stop the hydrogenation reaction, and after cooling to 20° C or less, the fat was treated with active clay to obtain 225g of skipjack orbital fat having decreased fish odor. The DHA content of the skipjack orbital fat was 30.3%, the EPA content was 7.6%, and trans-isomer content was 7.5%. Then a preservation test was made with the purified skipjack orbital fat and the skipjack orbital fat obtained in the example. The preservation test and the organoleptic evaluation were carried out in the same manner as described in Example 2, except that the amount of tocopherol was 20mg and the oven temperature was 30±1°C. The results are shown in Table 13. Fish Odor Strength Preferability Oil 0 day 10th day 0 day 10th day Purified Skipjack Orbital Fat 1.0 3.6 4.0 2.2 Skipjack Orbital Fat of the Present Invention 0.7 2.5 4.4 3.4 As apparent from the above results, the skipjack orbital fat of the present invention had weak fish odor from day zero of the preservation test, compared with the purified skipjack orbital fat, and it had a high evaluation point of preferability. In addition, even at the 10th day of the test, the production of fish odor in the skipjack orbital fat of the present invention was inhibited and the evaluation point of preferability was high. EXAMPLE 13 500g of mixed fat (DHA content :35.3%, EPA content:7.8%, trans-isomer content:1.8%), in which purified tuna orbital fat (DHA content:36.0%, EPA content:7.0%, trans-isomer content:1.4%) and purified skipjack orbital fat (DHA content:34.5%, EPA content:8.9%, trans-isomer content:1.8%) were mixed in the ratio of 50:50 by weight, was filled into a 1L reaction vessel, and 0.5g (0.10% by weight) of reduced nickel catalyst was added to the fat. After deaerating and dehydrating at a pressure of 6.7 mbar (5 torr) or less with stirring, a hydrogenation reaction was carried out under a hydrogen atmosphere of 2.94 bar (3kg/cm 2 ) at 130° C for fifteen minutes. Then hydrogen gas was removed from the reaction vessel to stop the hydrogenation reaction, and after cooling to 20° C or less, the mixed oil was treated with active clay to obtain 380g of mixed tuna and skipjack orbital fat having decreased fish odor. The DHA content of the mixed tuna and skipjack orbital fat thus obtained was 27.9%, the EPA content was 4.3% and the trans-isomer content was 6.8%. Then a preservation test was made with the mixed purified tuna and skipjack orbital fat and the mixed tuna and skipjack orbital fat obtained in the example. The preservation test and the organoleptic evaluation were made in the same manner as described in Example 2, except that to 50g of each oil corn oil was added to have a DHA content of tuna and skipjack orbital fat of 25% and that the amount of tocopherol was 20mg. The results are shown in Table 14. Fish Odor Strength Preferability Oil 0 day 5th day 0 day 5th day Mixed Purified Orbital Fat 1.0 3.4 4.0 2.8 Mixed Orbital Fat of the Present Invention 0.6 1.8 4.3 3.7 As apparent from the above results, the mixed tuna and skipjack orbital fat of the present invention had weak fish odor from day zero of the preservation test compared with the mixed purified tuna and skipjack orbital fat, and it had high evaluation point of preferability. In addition, even at the 5th day of the test, the production of fish odor in the mixed tuna and skipjack orbital fat of the present invention was inhibited and the evaluation point of preferability was high. The fish oil having decreased fish odor of the present invention produces little fish odor which has bad organoleptic influences. Further, since the fish oil having decreased fish odor contains a high amount of highly unsaturated fatty acids, such as DHA and EPA, the fish oil of the present invention is suitable for use as foodstuff and it may be used also as material for medical supplies.

Claims: A method for preparing fish oil having decreased fish odor, which comprises slightly hydrogenating fish oil to reduce the iodine value by 15 % or less and to reduce the highly unsaturated fatty acids by 33 % or less, under the following non-selective conditions: (1) the amount of catalyst used in the hydrogenation is 0.05% by weight or more, based on the weight of the fish oil; (2) the hydrogen pressure in the gaseous phase at the beginning of the hydrogenation is 3kg/cm 2 or more; (3) the reaction temperature of the hydrogenation is in the range from 90 to 150 °C; (4) the reaction time of the hydrogenation is in the range from 5 to 30 minutes. The method of claim 1 wherein the catalyst is a nickel catalyst. The method of claims 1 or 2 wherein the highly unsaturated fatty acid is docosahexaenoic acid (DHA) or eicosapentaenoic acid (EPA). The method of any one of claims 1 to 3 wherein the fish oil is sardine oil, mackerel oil, tuna oil, skipjack oil, tuna orbital fat or skipjack orbital fat. The method of any one of claims 1 to 4 wherein the fish oil having decreased fish odor is sardine oil or mackerel oil having the following characteristics: (1) the concentration of DHA contained in the fatty acid residue of the oil is in the range from 1 to 13 % by weight; (2) the concentration of EPA contained in the fatty acid residue of the oil is in the range from 3 to 18 % by weight; (3) the content of trans-isomer is 4 % by weight or more. The method of any one of claims 1 to 4 wherein the fish oil having decreased fish odor is skipjack oil or tuna oil having the following characteristics; (1) the concentration of DHA contained in the fatty acid residue of the oil is in the range from 15 to 25 % by weight; (2) the concentration of EPA contained in the fatty acid residue of the oil is in the range from 1 to 10 % by weight. (3) the content of trans-isomer is 4 % by weight or more. The method of any one of claims 1 to 4 wherein the fish oil having decreased fish odor is skipjack orbital fat or tuna orbital fat having the following characteristics: (1) the concentration of DHA contained in the fatty acid residue of the oil is in the range from 25 to 38 % by weight; (2) the concentration of EPA contained in the fatty acid residue of the oil is in the range from 2 to 8 % by weight; (3) the content of trans-isomer is 4 % by weight or more. Fish oil having decreased fish odor which is obtainable by the method as claimed in any one of claims 1 to 7. Sardine oil having decreased fish odor which is obtainable by the method as claimed in any one of claims 1 to 5, and which preferably has the following characteristics: (1) the concentration of DHA contained in the fatty acid residue of the oil in the range from 1 to 13'% by weight; (2) the concentration of EPA contained in the fatty acid residue of the oil is in the range from 3 to 18 % by weight; (3) the content of trans-isomer is 4 % by weight or more. Mackerel oil having decreased fish odor which is obtainable by the method as claimed in any one of claims 1 to 5, and which preferably has the following characteristics: (1) the concentration of DHA contained in the fatty acid residue of the oil is in the range from 1 to 13 % by weight; (2) the concentration of EPA contained in the fatty acid residue of the oil is in the range from 3 to 18 % by weight; (3) the content of trans-isomer is 4 % by weight or more: Tuna oil having decreased fish odor which is obtainable by the method as claimed in any one of claims 1 to 4 and 6, and which preferably has the following characteristics: (1) the concentration of DHA contained in the fatty acid residue of the oil is in the range from 15 to 25 % by weight; (2) the concentration of EPA contained in the fatty acid residue of the oil is in the range from 1 to 10 % by weight; (3) the content of trans-isomer is 4 % by weight or more: Skipjack oil having decreased fish odor which is obtainable by the method as claimed in any one of claims 1 to 4 and 6, and which preferably has the following characteristics: (1) the concentration of DHA contained in the fatty acid residue of the oil is in the range from 15 to 25 % by weight; (2) the concentration of EPA contained in the fatty acid residue of the oil is in the range from 1 to 10 % by weight; (3) the content of trans-isomer is 4 % by weight or more. Tuna orbital fat having decreased fish odor which is obtainable by the method as claimed in any one of claims 1 to 4 and 7, and which preferably has the following characteristics: (1) the concentration of DHA contained in the fatty acid residue of the oil is in the range from 25 to 38 % by weight; (2) the concentration of EPA contained in the fatty acid residue of the oil is in the range from 2 to 8 % by weight; (3) the content of trans-isomer is 4 % by weight or more. Skipjack orbital fat having decreased fish odor which is obtainable by the method as claimed in any one of claims 1 to 4 and 7 and which preferably has the following characteristics: (1) the concentration of DHA contained in the fatty acid residue of the oil is in the range from 25 to 38 % by weight; (2) the concentration of EPA contained in the fatty acid residue of the oil is in the range from 2 to 8 % by weight; (3) the content of trans-isomer is 4 % by weight or more.

Inventors: KONISHI HIROAKI, SATO NORIFUMI, TATSUMI KIYOSHI, KONISHI, HIROAKI, SATO, NORIFUMI, TATSUMI, KIYOSHI, KONISHI, HIROAKI, RIVERSIDE ICHIBANGAI
Applicants: SNOW BRAND MILK PROD CO LTD, SNOW BRAND MILK PRODUCTS CO., LTD.
Application year: 1995
Main classification: C11C3
IPC codes: C11C3