Krill corresponds to a group of small and abundant marine crustaceans in the order Euphasiaceae, living in the pristine Antarctic Ocean that is considered the feed base of the all Antarctic eco-system. The Antarctic krill, in particular those that live at the Antarctic and sub-Antarctic regions, are composed of 11 Euphasia species, being dominant Euphausia superba, Dana and Euphausia. crystallorophias. 
In recent years, krill has acquired great interest as a potential source of protein and other active biological products (Ellingsen, T. and Mohr, V. 1979. Process Bioch. 14:14; Suzuki, T. 1981. Fish and krill protein processing technology. London, Applied Science Publishers). The large expectation ciphered in the South Antarctic krill is based in the large biomass existent at the Antarctic oceans, estimated between 100 to 500 millions tons. It has been suggested that captures of krill could reach up to 50-100 millions tons/year, quantity that is equivalent to the total fish capture in the world (Budzinski, E., Bykowski, P. and Dutkiewicz, D. 1986. Posibilidades de elaboración y comercialización de productos preparados a partir de krill del Antartico, FAO Doc. T,c. Pesca (268):47p).
There are several publications related to the krill lipid content and composition (Grantham, G. J. 1977. The Southern Ocean. The utilization of Krill. Southern Ocean Fisheries Survey Programme, Rome. FAO GLO/SO/77/3:63p; Budzinski et al. 1986. Loc. cit.; Ellingsen and Mohr. 1979. Loc. cit.). The lipid content is about 10-26% of the krill dry weight, depending on the season of the year and its sexual maturity as well as body size. Usually the krill fatty period is between March to June each year. The female krill has near double amount of lipids than the male. The lipid concentration increases with age and decreases rapidly after the spawning. Krill lipids distribution studies showed that lipid rich areas are located along the digestive tract, between the muscle bundles and under the exoskeleton (Saether, O., Ellingsen, T. and Mohr, V. 1985. Comp. Biochem. Physiol. 81B:609).
The main krill lipid fractions include triglycerides, phospholipids, as well as its sterols and esters (Grantham. 1977. Loc. cit.; Budzinski et al. 1986. Loc. cit.). The average content of phospholipids is about 69% and triglycerides about 26%. The krill phospholipids fraction, rich in polyunsaturated fatty acids, particularly 20:5 and 22:6, correspond to approximately 50% of total phospholipids.
There are several publications describing Krill lipid composition. The following being the most relevant among them:    1. Gordeev et al. described that E. superba contains about 5% of its natural weight of extractable lipids, more than half of which are in the form of phospholipids—phosphatidylcholine (33-36% of the lipids total), phosphatidylethanolamine (15-17%), lysophosphatidylcholine (3-4%), others (2-3%)—while triacylglycerols predominate (32-35%) among the phosphorus-free components. In the first two phospholipids the dominating fatty acid residue is the arachidonic acid residue (more than 40% of the acyl residues total) and the amount of eicosapentaenoic acid residues (C20: 5w3) are about 13 and 28%, respectively. (Gordeev, K. Y. et al. 1990. Fatty acid composition of the main phospholipids of the Antarctic krill Euphausia superb. Chemistry of Natural Compounds. 26: 143-147). Fricke et al. described that Euphausia superba Dana lipid composition was phosphatidylcholine (33-36%), phosphatidylethanolamine (5-6%), triacylglycerol (33-40%), free fatty acids (8-16%) and sterols (1.4-1.7%). Wax esters and sterol esters were present only in traces. More than 50 fatty acids could be identified and the major ones being 14:0, 16:0, 16:1(n-7), 18:1(n-9), 18:1(n-7), 20:5(n-3) and 22:6(n-3). Phytanic acid was found in a concentration of 3% of total fatty acids. Short, medium-chain and hydroxy fatty acids (C<10) were not detectable. The sterol fraction consisted of cholesterol, desmosterol and 22-dehydrocholesterol. (Fricke, H. et al. 2006. Lipid, sterol and fatty acid composition of Antarctic krill (Euphausia superba Dana). Lipids, 19:821-827).    2. Falk-Petersen et al described that lipids of Arctic and Antarctic euphausiids show a seasonally-dependent high lipid content, and neutral lipids, whether wax esters or triacylglycerols, are primarily accumulated for reproduction. The Arctic Thysanoessa inermis and the Antarctic Euphausia crystallorophias contain high levels of wax esters and higher concentrations of 18:4(n-3) and 20:5(n-3) and a lower ratio of 18:1(n-9)/(n-7) fatty acids in their neutral lipids than those in the Arctic Thysanoessa raschii and the Antarctic Thysanoessa macrura and Euphausia superba. Large amounts of phytol in the lipids of T. raschii and E. crystallorophias during winter suggest the ingestion of decaying algae originated from sedimenting material or in sea ice. Thysanoessa raschii, T. macrura, and E. superba have a high ratio of 18:1(n-9)/(n-7) fatty acids, indicating animal carnivory (Falk-Petersen, S. et al. 2000. Lipids, trophic relationships, and biodiversity in Arctic and Antarctic krill. Can. J. Fish. Aquat. Sci. 57: 178-191).    3. Clarke described lipid content and composition of the Antarctic krill Euphausia superba. Female total lipid content increases during the summer as the ovary matures, and there is also some evidence of an increase in the lipid content of males and immatures as winter approaches. The storage lipid is mainly triacylglycerol and there is less than 1% wax ester. Fatty acids are moderately unsaturated, though less so in the ovarian lipid, and the triacylglycerol contains up to 4% phytanic acid (Clarke A. 1984. Lipid content and composition of Antarctic krill, Euphausia superba Dana. J. Crust. Biol. 4:285-294). Phleger et al. described lipid compositions of Antarctic euphausiids, Euphausia superba, E. tricantha, E. frigida and Thysanoessa macrura collected near Elephant Island during 1997 and 1998. Total lipid was highest in E. superba small juveniles (16 mg g-1 wet mass), ranging from 12 to 15 mg in other euphausiids. Polar lipid (56-81% of total lipid) and triacylglycerol (12-38%) were the major lipids with wax esters (6%) only present in E. tricantha. Cholesterol was the major sterol (80-100% of total sterols) with desmosterol second in abundance (1-18%). 1997 T. macrura and E. superba contained a more diverse sterol profile, including 24-nordehydrocholesterol (0.1-1.7%), trans-dehydrocholesterol (1.1-1.5%), brassicasterol (0.5-1.7%), 24-methylenecholesterol (0.1-0.4%) and two stanols (0.1-0.2%). Monounsaturated fatty acids included primarily 18:1(n-9)c (7-21%), 18:1(n-7)c (3-13%) and 16:1(n-7)c (2-7%). The main saturated fatty acids in krill were 16:0 (18-29%), 14:0 (2-15%) and 18:0 (1-13%). Highest eicosapentaenoic acid [EPA, 20:5(n-3)] and docosahexaenoic acid [DHA, 22:6(n-3)] occurred in E. superba (EPA, 15-21%; DHA, 9-14%), and were less abundant in other krill. Lower levels of 18:4(n-3) in E. tricantha, E. frigida and T. macrura (0.4-0.7% of total fatty acids) are more consistent with a carnivorous or omnivorous diet as compared with herbivorous E. superba (3.7-9.4%). The polyunsaturated fatty acid (PUFA) 18:5(n-3) and the very-long chain (VLC-PUFA), C(26) and C(28) PUFA, were not present in 1997 samples, but were detected at low levels in most 1998 euphausiids (Phleger, C. F. 2002. Interannual and between species comparison of the lipids, fatty acids and sterols of Antarctic krill from the US AMLR Elephant Island survey area. Comp Biochem Physiol B Biochem Mol Biol. 131:733-747).    4. Kolakowska described lipid compositions of seven krill (Euphausia superba D.) samples, fresh and after various periods of storage at 251 K (Kelvin Degrees). Fresh krill lipid composition differed from that determined in frozen samples, depending on storage duration, season of harvest, and developmental stage. Phospholipids proved most susceptible to changes, as opposed to triglycerides, which were most resistant; diglycerides and cholesterol esters were also destroyed. The freezing process per-se affected the lipid composition only slightly; however, after 30 days storage the amount of free fatty acids almost doubled. After 6 months storage at 251° K, 70% of phospholipids were decomposed and the amount of free fatty acids increased by a factor of 6 to 20. Monoglycerides, absent from fresh krill, appeared after several months of frozen storage. Juvenile krill were more susceptible to lipolytic changes. Females bearing mature eggs contained stable phospholipids; only triglycerides were hydrolysed (Kolakowska A. 1986. Lipid composition of fresh and frozen-stored krill. Z Lebensm Unters Forsch. 182:475-478. Bottino described lipid compositions of two Antarctic euphausiids. In Euphausia superba complex lipids were the major lipid class followed by triglycerides. In E. crystallorophias the complex lipids were also the major lipid class, but the second major constituent was wax. The complex lipids of both Euphausiids consisted mostly of phosphatidylcholine with smaller amounts of phosphatidylethanolamine and lysophosphatidylcholine. The phospholipids of E. crystallorophias were less unsaturated than those of E. superba. The waxes of E. crystallorophias were mostly esters of oleic (84%) and palmitoleic (10%) acids with n-tetradecanol (69%) and n-hexadecanol (28%) (Bottino, N. R. 1975. Lipid composition of two species of Antarctic krill: Euphausia superba and E. crystallorophias. Comp Biochem Physiol B. 50:479-484).    5. EP1997498 and WO02/102394 owned by Neptune Technologies & Bioressources Inc. Relate to Neptune krill oil that corresponds to acetone extracted krill lipids. Proteins and krill material are removed from the lipid extract through filtration. The acetone and residual water are removed by evaporation. The phospholipids content is 38-50%, and EPA and DHA is 22-35%, wherein the major amount of these omega-3's are attached to phospholipids.    6. US2008/0274203 and WO 2008/117062 owned by Aker Biomarine ASA. These applications disclose a new krill oil composition characterized by high amounts of phospholipids, astaxanthin esters and/or omega-3. This krill oil is characterized by comprising about 30-60% w/w phospholipids and about 20-35% omega-3 fatty acid wherein the major amount of these omega-3 lipids are attached to phospholipids.
The high content of polyunsaturated fatty acids in the phospholipidic fraction could be necessary to keep the plasmatic membrane fluidity at low temperatures in the Antarctic oceans. A high unsaturation level might be required to give the krill phospholipids deposits the necessary plasticity for the animal flectation and motion at low temperatures.
An increase of the total lipids present in the krill is accompanied with both a decrease in the phospholipids and an increase in the triglycerides. The polyunsaturated fatty acid content decreases as the content of total lipids increase (Saether et al. 1985. Loc. cit.). Post-mortem changes that occur in krill lipids showed that during the krill storage at 0° C. the polyunsaturated fatty acids (20:5, 22:6) compared to the content of fatty acids (16:0) do not decrease. These data suggest that during the krill storage at 0° C. a large oxidation of polyunsaturated fatty acids after the crustacean death does not occur.
There are several documents describing industrial processes to obtain krill oil. These documents include:    1. Budzinski et al. (1986. Loc. cit.) and Saether et al. (1985. Loc. cit.), described procedures for krill lipid extraction with different organic solvents.    2. CA2346979, ES2306527 or UA75029, documents presented by Universite de Sherbrooke. This document described a method to extract lipid fraction from marine and aquatic animals, including krill, using acetone extraction.    3. WO2006/106325 presented by Pro-Bio Group AS. This document described a process to obtain phospholipids from krill. This process comprises contacting the krill meal with an organic solvent to produce a lipid containing liquid. Optionally this liquid is extracted with other organic solvents to extract neutral lipids. The remaining fraction is a phospholipids enriched fraction.    4. EP1997498 and WO02/102394 presented by Neptune Technologies & Bioressources Inc. These documents describe lipid extraction from krill or krill derived material utilizing ketone solvents, preferably acetone.    5. GB407729 presented by Johan Olsen Nygaard. This patent describes a method to extract oil from marine animals, in particular whale and other sea mammals, by heating in bath of oil. This document is different from our invention because is not applied to krill and utilizes heated oil for lipid extraction. Our invention does not use heated oil for phospholipids extraction.    6. WO2009/027692 discloses a multiple step process for obtaining krill meal and related products. The process includes a first heating step in which krill is brought to a temperature of about 75° C. and thereafter sieved to obtain phospholipid free krill. This phospholipid free krill is then heated a second time to 85° C., sieved again and pressed. The krill liquid (milk) left behind from the first heating step is then coagulated to remove the proteins and phospholipids.
These five documents describe the utilization of organic solvents, particularly acetone, to extract krill lipid fractions. These methods being different from the procedure disclosed herein, as the method declared in the present invention does not utilize organic solvents for extracting or purifying lipid fractions from Antarctic krill.    1. JP58008037, Nippon Suisan Kaisha Ltd. This document describes a method to obtain eicosapentaenoic acid or derivatives from Antarctic krill oil. Krill oil is converted into free fatty acid or an ester thereof by the conventional method, e.g. saponification or alcoholysis, being the resultant product continuously distilled to collect a main fraction of distillate containing 40 wt % or more titled substance. The main fraction of distillate is then treated with urea to remove low unsaturated fatty acids. This document differs from the present invention through the use of a distillation process for lipid purification.    2. JP2004024060 (Nippon Suisan Kaisha Ltd). This document describes a procedure for obtaining an astaxanthin enriched lipid fraction from Antarctic krill. This document declares a different process from the one declared in the present invention in that the instant process aim is not the astaxanthin production.    3. US2008/0274203 (Aker Biomarine ASA). This document describes a procedure for obtaining krill oil from krill meal using supercritical fluid extraction in a two-stage process. Stage 1 removes the neutral lipid by extracting with neat supercritical CO2 or CO2 plus approximately 5% of a co-solvent. Stage 2 extracts the actual krill oils by using supercritical CO2 in combination with approximately 20% ethanol. This document differs from the present invention as this procedure does not use supercritical fluid extraction.            WO2007/080514 (Krill A/S and Alfa Laval Copenhagen A/S), describes a method for extracting lipid fractions from krill, wherein freshly captured krill is grinded to obtain a slurry, which is gently heated to a temperature below 60° C., preferably bellow 30° C., for less than 45 minutes, thereafter the liquid splits into an aqueous phase and a krill oil phase from which a krill oil extract is derived without the use of organic solvents. This document reveals a process different from the one declared in the instant invention as the present procedure does not grind the captured krill before heating. This document is also different since the heating temperature declared in the instant process is >90° C. Another main difference regarding the process declared in this document is that grinded krill used to produce slurry before heating produces an emulsification that impedes the phospholipids separation by centrifugation. In the present invention a slurry is not directly or indirectly produced. Further in WO2007/080514 is used ultrasound for krill oil separation while the present invention does not use any type of ultrasound technology. It also proposes a simple extraction while the present invention is based in a double-extraction principle.            4. WO2007080515 (Aker Biomarine ASA) describes a process for obtaining krill lipids by processing the krill at a temperature below 60° C. with mechanical and physical disruption of the lipid cell membrane to facilitate low temperature extraction. This process takes place under inert gas atmosphere to prevent oxidation or denaturation of fat and proteins. Intermediate processing tanks are kept at a minimum level to reduce residence time; and the oil after recovery is immediately frozen to stabilize it. This document differs from the present invention since the instant procedure does not grind the captured krill to facilitate lipid extraction, instead a heating temperature of >90° C. is used; further the process of the present invention does not use any type of gases or freezing technologies.    5. WO2008/060163 (Pronova Biopharma Norge AS), describes a procedure for obtaining krill oil-using CO2 at supercritical pressure containing ethanol, methanol, propanol or isopropanol. This document is different from the present invention because the present procedure does not use CO2 supercritical lipid extraction with or without solvent.    6. WO2009/027692 (Aker Biomarine ASA), describes a method for krill meal production. This procedure uses a two-step cooking process. In the first step the proteins and phospholipids are removed from the krill and precipitated as a coagulum. In the second stage the krill without phospholipids are cooked. Following this, residual fat and astaxanthin are removed from the krill using mechanical separation methods. In this method, krill is heated to 60-75° C. in the presence of water to dissolve/disperse lipids and proteins to the water phase, called krill milk. This krill milk was heated to 95-100° C. to remove as a precipitate the krill protein and lipids from the water phase. The processes of the present invention differ from those disclosed in this document since the krill oil is not separated from the crustacean by precipitation and the multiple heating steps are avoided.
From current traditional krill meal processing on board, in some factory vessels, only a small amount of krill oil is produced. This krill oil is usually enriched in neutral lipids with very low or undetectable amount of phospholipids (<0.5%). Normally, during the traditional on board krill process, fresh krill is heated using an indirect heating cooker with rotating screw conveyor, followed by a twin-screw press and drier. The press liquid obtained by the twin-screw press is passed through a decanter to remove the insoluble solids. The clarified decanter liquid is then used to feed separators centrifuges to separate the krill oil normally enriched with neutral lipids and astaxanthin. In this traditional process the phospholipids are bound to the proteins in the press cake. Therefore, phospholipids are usually found associated to the hill meal. In the krill fatty period, the fat content in the krill meal is usually 16-18%. In the same krill fatty period, the yield of the neutral lipid-enriched krill oil obtained using the traditional krill meal plant is low, ranging usually between 0.3-1.0% from raw krill. This neutral lipid-enriched krill oil contains astaxanthin ranging between 700-1.500 mg/kg depending on the season and the fishing ground catching.
Moreover, when a non-traditional krill meal processing layout is used, a similar situation explained above was obtained. Normally, the non-traditional krill meal plant considered a contherm cooker system, a two-phase decanter or three-phase decanter and a drier. These decanters are used for de-watering and de-fatting the cooked krill. The decanter liquid is used to feed the centrifuge separators to obtain usually a neutral lipid-enriched krill oil with low or undetectable levels of phospholipids (<0.5%). In this case, the phospholipids are also bound to the proteins in the decanter solids. As described above, phospholipids are found in the krill meal.
In this case, the yield of neutral lipid-enriched krill oil from the non-traditional krill meal plant is much lower, in the fatty period ranging from 0.1 to 0.4% of raw krill. In this process a conventional contherm cooker system is used which has inherent agitation (scraped knife). Therefore, the processed krill is agitated and also minced. This agitation/mincing process produce lipid emulsification along with krill proteins and water. Besides, krill phospholipids catalyze the emulsification because these lipids act as an emulsifier agent. For this reason, using a non-traditional krill meal process, higher lipid content is bound to the decanter solids. In the krill fatty period, the fat content of the decanter liquid has a lipid content ranging from 1.0 to 2.5%, the resultant stickwater has a yellow color and it is emulsified, with a fat content ranging from 0.7 to 1.6%, the krill meal has a fat content ranging from 20 to 26%, and the krill oil recovery by this non-traditional process is very low.
The lipid composition and fatty acid profile of neutral lipid-enriched krill oil obtained using the traditional and non-traditional krill meal processing are very similar: triglycerides about 86-89%, phospholipids not detected (<0.5%), and DHA and EPA about 4-6%.
Several efforts have been made to produce a krill meal with a lower fat content and phospholipids enriched krill oil containing DHA and EPA, through an industrial scale method associated to a krill meal plant. Several different cooking temperatures, different decanting torque, strong pressing, using two decanting steps, washing the first decanter solids with stick water before the second decanter, electroplasmolysis and so on have been tested. However, the results have not been successful.
Focusing on the problem to separate phospholipids-enriched krill oil with DHA and EPA, some extraction methods have been developed and patent protected. The patent application CL 1021 1995 (Compañía Tepual S.A) refers to a method for obtaining krill oil using thermal fractionation and centrifugation. This oil obtaining process claims that the fatty acids types and lipids composition can be regulated controlling the krill cooking temperature during the process of oil production. At high cooking temperature (95° C.) lower yield of poly-unsaturated and phospholipids fractions were obtained as compared when the cooking temperature was reduced (75° C.). Also this oil process claims that main components of its fractional composition correspond to triglycerides being 35 to 96%; and phospholipids from 4 to 28%. The poly-unsaturated fatty acids ranged from 4 to 46%. Basically this process used for obtaining krill oil, considers krill cooking, pressing the cooked krill and the passing the press liquid through a two-phase decanter to separate the insoluble solids, being the oil separated from the decanter liquid using centrifuge separator. During this process only one type of krill oil was obtained.
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