BACKGROUND OF THE INVENTION:

Stearidonic is an essential fatty acid of the n-3 series. It is known that essential fatty acids of the n-3 series (nomenclature defined by the position of the first double bond from the methyl group) lead by a chain reaction to series 3 prostaglandins that inhibit the aggregation of blood platelets.

The first element of this chain is a-linolenic acid (ALA, C 18:3 cis 9,12,15) of which the conversion into stearidonic acid (SA, C 18:4 cis 6,9,12,15) is attributable to the activity of the enzyme. DELTA.6 desaturase that is shown to weaken with age and as a result of certain diseases. The synthesis of the series 3 prostaglandins is thus compromised. To overcome this weakness of the organism, it is proposed to introduce stearidonic acid directly into the body.

Among the substances playing an important part in inflammatory processes, for example of the allergic type, such as asthma, of the cutaneous type, such as acne, psoriasis and eczema, or of the rheumatic type, as well as those following traumatisms, states of shock or pathologies, such as mucoviscidose for example, leukotrienes are formed mainly by oxidation by the lipoxygenases of arachidonic acid (AA, C20:4 . DELTA.5,8,11,14) which is released from cell membranes. More particularly, it is known that poly-morphonucleated human leukocytes transform AA into leukotrienes more particularly into stable (5S,12R)-5, 12-dihydroxy-6,14-cis-8,10-trans- eicosatetraenoicacid (LTB4), by way of 5-lipoxygenase. LTB4 plays a predominant part in various inflammatory processes.

SA has the property of inhibiting the biosynthesis of leukotrienes and can be used alone or in combination with one or more polyunsaturated fatty acids, which inhibit the oxygenated metabolism of AA. This unsaturated fatty acid may be a 5- lipoxygenase-inhibiting polyunsaturated fatty acid of the n-3 series, for example EPA, docosahexaenoic acid (DHA, C22:6 Δ4,7,10,13,16,19), Japanese patent application 86 050 752 relates to a process for the separation of stearidonic acid from fish oil which comprises the steps of converting the fatty acids present therein into their ethyl esters, treating the ethyl esters by molecular distillation and collecting the head fraction, reacting it with urea in methanol, collecting the unreacted fraction and, finally, separating a fraction enriched with ethyl ester from the stearidonic acid by two successive operations of reverse-phase partition chromatography. A purity of 85% by weight is thus obtained for a yield of 1.8%, based on the oil used, which represents a recovery of 48% of the stearidonic acid present in the starting material.

In addition, European patent 178 442 relates to the enrichment of blackcurrant seed oil with γ-linolenic acid (GLA, C 18:3 Δ 6, 9, 12) by the double complexing with urea of a mixture of fatty acids eminating from the saponification of blackcurrant oil, followed by high-performance reversephase liquid chromatography. According to this patent, separation appeared particularly difficult, enrichment of the mixture only appearing possible in regard to its major constituent, γ-linolenic acid.

SUMMARY OF THE INVENTION:

Method for obtaining stearidonic acid (SA) ethyl ester that present invention relates to presents clear advantages over the methods above mentioned. The present invention does not use any organic solvent and allows obtaining a product with at least 40% by weight purity of SA.

This method is characterized by a starting point of a mixture of ethyl esters of fatty acids in which it is present as a minor constituent that is subjected to a supercritical chromatography using a supercritical C0 ₂ as mobile phase and irregular porous silicon oxide impregnated with (3-aminopropyl)-triethoxysilane as stationary phase.

Applicants have discovered that taking as a starting point a raw material concentrated in omega-3 fatty acids, where SA is present as a minor constituent, it is possible to reach concentrations of SA ethyl ester of 40% by weight and more. Yield of that DHA concentrate will be between 7 and 20% in weight based on the raw material used. Stationary phase used in the supercritical chromatography object of the present invention is irregular porous silicon oxide impregnated with (3-aminopropyl)- triethoxysilane in a proportion of 10% roughly and a pore size of 6 to 7 nanometers. This is a commercial stationary phase and can be supplied by different suppliers (for instance ProntoPreg. Bischoff Analysentechnick und gerate GMBH, D-7250 Leonburg, Alemania).

Column used at industrial scale has a diameter between 50 and 60 cm and a length between 260 and 280 cm, with opening and closure devices at the top and the bottom. Stationary phase quantity ranges from 180 to 300 kg, preferably from 200 to 240 Kg and is contained between drilled plaques with filtration paper.

Supercritical C0 ₂ circulates through the column pumped using membrane pumps at a pressure ranges from 95 to 150 bars, preferably ranging from 97 to 110 bars, at a temperature ranging from 40^ to 60^ Celsius, preferably from 45^ to 50^ and at a flow rate ranging from 4000 to 7000Kg/h, preferably from 5000 to 6000 Kg/h.

Mixture of ethyl esters where SA is present is pumped at a discontinuous pace to the column in the current of C0 ₂ at a flow rate ranging from 0.1 to 1 Kg/s, using membrane pumps.

For every 1000 Kg of mixture of ethyl esters used, where SA is present, around 150 Kg of ethyl ester of DHA of at least 40% in weight concentration is obtained.

EXAMPLE:

Following description is set only as an example and does not limit the scope of the invention:

A cylindrical column of 540 mm diameter and 2745 mm length with opening and closure devices at the top and with drilled plaques with filtrating paper inside is used. It contains 220 Kg of stationary phase consisting in irregular porous silicon oxide impregnated with (3-aminopropyl)-triethoxysilane with pore size of 6.5 nanometers (ProntoPreg, Bischoff Analysentechnick und gerate GMBH, D-7250 Leonburg, Alemania).

Supercritical C0 ₂ circulates through the column at a pressure ranging 97 to 99 bars, at a temperature ranging from 45^ to 50^ and at a flow rate ranging from 5000 to 6000 Kg/h. With the C0 ₂ current, the mixture of esters is pumped into the column at a flow rate ranging from 8 to 15 Kg in 20-30 seconds through membrane pumps.

Current pressure when exiting the column is reduced to 60 bars, which is equal to pressure condition of C0 ₂ circuit, reheating it afterwards with a steam current at 150^ in a heat exchanger, evaporating the C0 ₂ .

Different fractions obtained from the column during injection time, are separated through a rotating valve with different positions.

182.75 Kg ethyl ester SA 41.3% in weight are obtained from 1019.6 Kg of mixture of esters with an ethyl ester content of SA of 7.54% in weight.

Product obtained is:

Ethyl ester DHA 40 % minimum weight.

Peroxide 10 meq/Kg maximum

Anisidine 20 maximum