Process for the isomerization of lutein
BACKGROUND OF THE INVENTION 
     An adequate pigmentation is a major concern of the poultry industry. Not only it is synonymous of good healthy birds, but at the same time it is also an important marketing strategy in many cultures. Very frequently, poultry producers make considerable efforts in order to obtain an attractively pigmented bird-carcass that appeals to the eye of the consumer. 
     To satisfy the market preferences, chicken producers have traditionally supplemented the poultry diets with carotenoid (particularly hydroxicarotenoids or xanthophylls) containing extracts or meals of plant origin that are responsible for the desired yellow-orange color in the broiler chickens and in egg yolks. A widely used source of xanthophylls is marigold meal and its extract. The carotenoids from marigold contain approximately from 85 to 90% of lutein (β,ε-caroten-3,3&#39;-diol) and from 2 to 4% of zeaxanthin (β,β- caroten 3,3&#39;-diol) (F. W. Quackenbush, 1972. J.AOAC, 55 (3), 617-21). These hydroxicarotenoids occur in the plant as esters of fatty acids, mainly palmitic, myristic and stearic (W. Gau. 1983, J. Chromatogr. 262, 277-84). 
     However, it has been proved that a more efficient pigmentation is obtained when the xanthophylls are in their free form (Fletcher, D. L. 1986, Poult. Sci., 65(9), 1708-14) after the ester link has been broken by means of a saponification process. As a matter of fact, most of the pigment producers carry on a saponification process in order to improve the bioavailability of the xanthophylls. 
     Furthermore, it has been demonstrated that a synergistic effect occurs when carotenoids like lutein and zeaxanthin are mixed with other natural or synthetic pigments for broiler chicken and egg yolk pigmentation (U.S. Pat. No. 3,539,686 of Nov. 10, 1970 to Ralph Rosenberg). 
     Different methods have been practiced in the past decades for the preparation of xanthophyll concentrates from marigold meal. As an example, in the U.S. Pat. No. 3,523,138 from Aug. 4, 1970. issued to Eastman Kodak Co., a process is described wherein the marigold meal is reacted with an alcoholic alkali solution. After saponification the mixture is diluted with water and the xanthophylls are separated by solvent extraction by means of water insoluble solvent like isopropyl ether. 
     U.S. Pat. No. 3,535,426 dated October 1970, issued also to Eastman Kodak Co., describes a method to stabilize xanthophylls by adding an antioxidant ethoxyquin (6.ethoxy-2,2,4-trimethil-1,2-hydroquinoline) and fat, and heating the mixture afterwards. The product obtained is very stable and does not degrade appreciably. 
     Also Ger. Offen. 2,535,963 dated Mar. 4, 1976, issued to CPC International Inc., describes a process to stabilize xanthophylls by partial saponification using a solution of potassium hydroxide in methanol. 
     U.S. Pat. No. 3,783,099 dated &gt;Jan. 1st 1974 issued to Ralston Purina Co., a process is described where an enzymatic hydrolysis of the cellulosic material of tho marigold meal, improves the extraction of the xanthophylls with a non polar solvent. 
     Recently some fermentative processes have been described wherein the reproduction of improved strains of Flavobacterium Multivorum result in the obtainment of zeaxanthin extracts with a pigment activity 2 to 3 times higher than that of marigold extracts, and are suggested as an alternative to naturally occurring carotenoids; as described on PCT Int. Appln. WO 91 03,571 dated Mar. 21, 1991, issued to Applied Biotechnology, Inc. 
     Traditional sources of zeaxanthin have been the extracts and meals of yellow corn and yellow corn gluten. 
     Years ago, Karrer and Jucker in 1947 (Helv. Chim. Acta. 30,266-7) obtained zeaxanthin by isomerizing lutein in a reaction catalyzed with sodium ethoxide in the presence of ethanol and benzene. A. G. Andrews, also in 1947 (Acta Chem. Scand. B 28 No. 1) obtained zeaxanthin from lutein using potassium methoxide in the presence of methanol and dimethylsulfoxide. In both procedures a poor yield was obtained resulting in a great loss of pigment. The economy of such processes is inadequate for industrial purposes. However, they demonstrated the feasibility to isomerize lutein into zeaxanthin. 
     In other words, isomerization of lutein into zeaxanthin has been carried out in academic research basis in a catalytic organic phase in the presence of solvents with a very strong alcoholic alkaline solutions which could cause a very violent exothermic reaction in the presence of water or humidity. 
     This process is related with the isomerization of lutein into zeaxanthin in a non catalytic aqueous phaso which does not needs the presence of solvents. 
     In the marigolds meal extracts available in the market, zeaxanthin represents only from 4 to 6% of the total xanthophill content. Since the pigment activity of this carotenoid is higher than that of lutein, applicants have developed a process where a higher. concentration of zeaxanthin is obtained, and hence the pigment efficiency of the extract containing this carotenoid is improved. 
     SUMMARY OF THE INVENTION 
     It is therefore a main object of the present invention, to provide a process to isomerize lutein into zeaxanthin in a non catalytic aqueous phase with strongly alkaline aqueous solutions and under controlled conditions which does not need the presence of solvents. 
     It is also a main object of the invention, to provide a process to isomerize lutein into zeaxanthin from marigold (Tagetes erecta L.) meal, its oleoresin or formulations containing lutein. 
     It is still a main object of the invention, to provide a process to isomerize lutein into zeaxanthin to produce a product with a high zeaxanthin content which can be used to pigment broiler chickens and egg yolks, as a pigmenting ingredient in aquaculture and as an ingredient for food consumption. 
     It is a further main object of the present invention, to provide a process to isomerize lutein into zeaxanthin, of the above disclosed nature, wherein the presence of the zeaxanthin in the obtained product represents from 5 to 50% of the total xanthophylls, usually from 8 to 30%, and preferably from 10 to 20%. 
     It is an additional object of the present invention, to provide a process to isomerize lutein into zeaxanthin, of the above disclosed nature, which is safe, and commercially and economically viable because it is carried out in a non catalytic aqueous phase which consequently does not require organic solvents. 
     These and other objects and advantages of the present invention will be apparent to persons skilled in the art, from the following description of the specific embodiments of the invention, represented by the examples. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The standard method of analysis accepted in the industry to quantify total xanthophylls in marigold meals and its extracts is described in the &#34;AOAC Official Method of Analysis&#34; (edited by Sidney Williams, Fourteenth edition, pages 835-6, 1984). 
     The HPLC methods to separate, identify and quantify lutein and zeaxanthin are described in the work of Yitzhak Itta (J. Agric. Food. Chem., 1993, 41, 899-901) and of Lane C. Sander (Anal. Chem., 1994, 66, 1667-1674). 
     The process occurs according to the following mechanism: ##STR1## 
     The extract that contains the lutein can be maintained at any temperature from 25° to 180° C., preferably from 70° to 110° C. The slow addition of strongly alkaline aqueous solution is followed, wherein the alkali can be sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH) 2 ), sodium carbonate (Na 2  CO 3 ), ammonium hydroxide (NH 4  OH), or any other similar compound, or their mixtures. Organic basis like ethylamine, ethanolamine, morpholine or others of a similar nature, or their mixtures can also be used. 
     The alkali concentration in the aqueous solution can be from 5 to 50% by weight, preferably between 25% and 45% by weight. The amount of alkali can be in the range of 0.1 to 1.0 parts for one pan of marigold meal extract, or more preferably from 0.3 to 0.7 parts of alkali to one part of the lutein containing concentrate. 
     The reaction time is strongly dependent on the temperature and amount of alkali used, but can be from 5 to 96 hours, usually from 16 to 36 hours, and preferably from 30 to 48 hours.. 
     The process can be carried out in a reactor or container at atmospheric pressure, but if shorter reaction times are desired the pressure can be increased from 5 to 150 psig, preferably from 20 to 50 psig. By this means the reaction time can be from 5 minutes to 12 hours, usually from 3 to 8 hours. It is desired to employ an inert atmosphere using either steam, nitrogen, carbon dioxide or similar compounds or their mixtures. If desired, the reaction can be carried out at a reduced pressure to avoid xanthophyll degradation. 
    
    
     Typical examples are illustrated on Table 1, indicating some of the results that have been obtained. These are examples only, and should not be interpreted as limiting the scope of this invention. 
     
                       TABLE 1                                                     
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Lot No.       1       2        3      4                                   
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Alkali        KOH     NaOH     Na.sub.2 CO.sub.3                          
                                      KOH                                 
Alkali/Extract, w/w                                                       
              0.3     1.0      0.5    0.5                                 
Temperature °C.                                                    
              80      90       105    95                                  
Pressure, atm.                                                            
              0.5     1.0      2.0    1.0                                 
Time, Hr,     48      30       3      36                                  
.sup.1 Initial Lutein %                                                   
              85.5    85.5     85.5   85.5                                
Final lutein %                                                            
              68.9    66.2     60.8   68.3                                
.sup.2 Initial Zeaxanthin %                                               
              4.5     4.5      4.5    4.5                                 
Final zeaxanthin %                                                        
              16.1    18.7     24.0   15.8                                
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 .sup.1,2 : the initial extract contents 97 grams of total xanthophylls pe
 kilo of extract, and on this value are calculated the percentages.   1, 2
 the initial extract contents 97 grams of total xanthophylls per kilo of
 extract, and on this value are calculated the percentages.
 
    
     The product obtained can be formulated as a water dispersion by means of suitable emulsifiers or can be dispersed on a carrier to make a premix or can be further concentrated by solvent extraction.