CA1107127A

CA1107127A - Process for extracting proteins from milk

Info

Publication number: CA1107127A
Application number: CA303,480A
Authority: CA
Inventors: Bernard Mirabel
Original assignee: Rhone Poulenc Industries SA
Current assignee: Rhone Poulenc Industries SA
Priority date: 1977-05-18
Filing date: 1978-05-16
Publication date: 1981-08-18
Also published as: NL7805357A; IE780993L; AU514604B2; AU3623478A; JPS565499B2; NZ187301A; NL186059C; IE46971B1; IT7849416A0; NL186059B; IT1156754B; DK216578A; GB1563990A; CH631327A5; DK171906B1; JPS54113470A

Abstract

PROCESS FOR EXTRACTING PROTEINS FROM MILK

Abstract of the Disclosure Process for extracting proteins from milk which consists first of extracting the proteins, other than casein, by putting skimmed milk in contact with at least one anion exchanger resin and with silica, fixation of the proteins, then elution and finally separating the casein remaining in solution from the lactose and the mineral salts. This process is used in the dairy industries to prepare proteins usable in the food industry, the dietetic products industry, the phar-maceutical industry, and the animal care products industry.

Description

~1~37~2~

The invention pertains to a new process for extracting proteins from milk.
The processing of skimmed milk generally consists of first extract-ing the casein by acid or enzymatic coagulation~ then extracting the proteins from the milkseru~ by means of thermal coagulation, ultrafiltration or ion exchange and finally separating the lactose, which can be hydrolyzed.
However, this process has its disadvantages. The separated casein is in a precipitated, partially deteriorated state and may contain other entrained proteins; the other proteins extracted by thermal coagulation lose some of their biological properties and the same is true with ultrafiltra- ;
tion, because the length of tl~e operation generally necessitates pasteuri-zation of the milkserum; as for their separation by ion exchange, it is very difficult to achieve this on an industrial scale because the known dextran or cellulose base ion exchangers have weak mechanical properties.
Ion exchangers for separating proteins from milkserum which do not have these disadvantages were described in Canadian Patent No. 1,069,8~3.
However, the casein is still in its precipitated, parlially deteriorated state and may contain proteins carried along with it.
With the process of this invention for ~ractionating milk, these disadvantages are avoided making it possible to obtain industrially pure proteins in their native state, with all their biological properties.
The process consists o extracting all the proteins from the skimmed milk, leaving a solution of mineral salts and lactose. It is characterized in that th~ proteins other kharl , :
.

the casein are first extra.cted by put~ing the skimmed milk in contact first with at least one anion exchanger resin and them with silica, or first with silica and then with at least one anion exchanger resin, fixation of the proteins and elution; the casein remaining in solution is then separated from the mineral salts and the lactose.
As used herein, the term proteins other than casein, or proteins in the milkserum, has reference to lactalbumins, serum albumin, lacto-globulins and immunoglobulins.
The anion exchanger resins are composed of supports: aluminas or silicas, coated with less than 20 mg/m2 of a cross-linked polymer film containing or carrying tertiary amine or quarternary ammonium salts function-al groups with the general formulae -CH2-1-CH2- or C112 N ( )3 ( ) . R
where R, allke or different, represents an alkyl or hydroxyalkyl radical .having 1 to ~ carbon atoms and ~ an orgallic or inorganic anion such as, for example, chloride, sulfate, nitrate, phosphate, citrate. Their exchange capacity is less than 2 meq./g.
The silica and the anion exchanger resin supports llave the follow-: ing charac~eristics: granulometry between 4~n and 5 mm, specific surface :~: 20 area of approximately 5 to lS0 m2/g, a porous volume oE 0~ to 2 ml/g and a pore diameter whlch is less than the size o:E the cascln but more than the s-.ize of the other proteins and is comprised betweell 250 and 2500 ~.
The cross-linked polymers, whi.ch coat the surEace o:E the supports, ; are well known products, obtained from monomers, such as the epoxy compounds, . . .
: which cross-link with polyamines as catalysts; formaldehyde which cross- .
links by polycondensation wi.th urea, melamine, polyamines or phenols; vinyl ~monomers: vinylpyridine, styrene and derlvatives, which cross-link with polyfunctional monomers~such as the diacrylates or dimethacrylates oE mono-, ~
~ ~ or poly- alkylene glycols, divinylbenzene, yinyltr:ialkoxysilane, vinyltri-: : -2-halogenosilane, bismethylene acrylamide, in the presence of an initiator or u]traviolet rays.
The inorganic support :is coated with the cross-linked polymer by means of impregnation of the support with a solution of the monomer or monomers and, i-f need be, with the initiator in a solvent, which is then evaporated and the monomers cross-linked according to known processes. As solvent, any product capable of dissol~ing the monomers and the initiator is used, its boiling point preferably being the lowest possible to promote subsequent evaporation. These are, for example, methylene chloride, ethyl ether, benzene, acetone, ethyl aceta~e.
If the cross-linked polymer coating the surface of the support does not ha~e functional groups in its chain~ it must he modified. This occurs most notably with styrene and derivates base cross-linked ~olymers and polymers of formaldehyde with urea, mekamine, polyamines, phenols.
For polymers of styrene or phenol :Eormaldehyde, tilis modi-Eication consists of flxing chloromethyl groups on the polymer, which are then made to react with a secondary or tertiary amine, a reaction which is carried out accordine to any known technique.
To fix chloromethyl groups on the polymer, it is preferable, in the case of styrene polymers, ~o disperse the polymer coated inorganic support in chloromethyl ether, at a high temperature, :in the presence oE a Lewis acid. On the other hand, :Eor a phenol-Eorlnaldellycle resln, the polymer coatcd inorganlc support can be d:ispersed, for example, in eplchlorllydrill, with the reaction occurring at a high temperature.
For polymers o formald0hyde with polyamines, urea, melamine, this modiEication consists oE changing the pr;mary amines in the chain into tertiary amines or quarternary ammonium salts accordîng to any classical technique, for example, reaction with a sulfate or an alkyl halide.
The skimmed mllk lS put into contact with the anion exchanger resin or reslns and the silica without modifying the milk pH, at temperatures ~ : .
: ~ ,~ :

between 0 and 50C, and preferably between 0 and 15C.
The ~uantity of anion exchanger resin or resins is appro~imately 5 to 15 grams per gram oE all the proteins to be extracted, and the quantity of silica is approximately 2 to 7 grams per gram of all the proteins to be extracted.
The proteins retained by the anion exchanger resin or resins are the ~-lactoglobulinsJ the ~-lactalbumins, the serum albumin and a small quantity of the immlmoglobulins. The silica fixes most of the immuno-globulins.
The separation of proteins from the resin or resins and from the silica is obtained by means of elution wi.th either a high ion strength solution or a solution of acid p~l for anion exchanger resins and basic pH
for silica. The solution of acid p~l is an organic or inorgani.c acld solution such as, for example, hydrochloric acid, acetic acid, nitric acid, sulfuric acid and lactic acid. The solution of basic pH i.s a solution o:E alkaline hydroxides such as sodim hydroxide, potassium hydroxide or ammonium hydrox-ide.
For a more:selective separation, it is possible to treat the milk successively with several anion exchanger resins, alike or different, before ,: ~
or aftes treating with silica. Thus, in the case of two anion exchange resins~ the elution solution for the first r~s;.n is very rich in ~-lacto-globulins, while the aluti.on solut:ion for the second resin conta:ins the ~-lactalbum~ns, serum album:in an~l ~ery sma:Ll quantities o:~ ~-lactoglobulins c~nd immunoglobulins. .
The same results can be obtained whether the proteins are extracted .~.
from skimmed milk either discontinuously, semicontinuously in columns or continuously with a series of columns~ Continous operations are particularly suited~for industrial usage because the resins allow for easi.er filling of - -.
: ~ :
the columns,~a high output and they facilitate elution.

3a [he protein solutions obtained contain only traces of lactose and : i :

~ : -4- .::

mineral salts. They can be used as they are, or the proteins can be sepa-ra-ted by ~ny known technique and more particularly by atomization.
After ex*raction by the anion exchange resin or resins and the silica, the remaining solution which is composed of casein, lactose and mineral salts no longer contains other proteins. The casein can be extraeted by exclusion chromatography or more particularly by ultraEiltration, accord-ing to any known technique suited to this specific case. ~xtraction may be aehieved either discontinuously or continuously.
In this way, a solution of native casein in water is obtained, the concentration of which varies according to the extraction process used. A
solution of lactose and mineral salts is also obtained.
The solution of noncoagulated, native casein no longer contains anything but traces of lactose and it can be used as is, or else the casein may be separated from the solution, notably by atomizatlon.
In a varieant of the process of the invention, the lactose and the mineral salts are separated from the skimmed milk; the proteins other than the casein are then extracted by putting the milk ;nto contact first with at least one anion exchanger resin and then with silica, or first with silica and then at least one anion exchanger resin, fixation of the proteins -~
followed by elution; the casein remaining in solution.
The lactose and mineral salts are separated from the skimmed milk by extraction by means of exclusion chromatography or ultraEiltration.
~ 'he extraction oE proteins by the anion exchnnger resin or resins and the silica is carried out as stated above.
After extraction of proteins other than casein, a solution of native, noncoagulated casein is obtained.
In another variant, the process in the invention can be applied to milkserum; i.e., to skimmed milk from which the casein has been eliminated.
In this case, the milkserum is put into contact f.irst with at least one anion exchanger resin and then~with silica, or first wi*h silica and then at _5_ least one anion exchanger resln, fixatlon of the proteins and elution.
In a copendlng appllcation, descriptlon is made of the separation of proteins from milkserum by using anion exchanger resins and cation exchanger resins. However, silica used instead of a cation exchanger resin makes elution much easier, yieldlng protein solutions whlch are more con-centrated. Consequently, when the proteins are dried, there is less water to be eliminated, hence a shorter processing time, less harmful to the proteins.
Furthermore, silica is a simpler product than cation exchanger resins and it has been authorized for use in the food industry.
The process for extracting proteins from milkserum by anion exchanger resin or resins and silica is the same as the process described eor miik, with the diference that contact of the lactoserum with the anion ; exchanger resin or resins and the silica is carried out at a p~l h;gher than and preferably between 5.5 and 7.5, at temperatures between 0 and 50C
and preferably between 0 and 30C. :
Once the proteins have been extracted by anlon exchanger resln or resins and s`illca, the remaining solution contains lactose and mineral salts, but no more proteins.
Whatever its origln, ~he lactose in solutlon can be hydrolyzed chemically or en~ymatically according to any known process, to obtaln a soll~tion of glucose and galactose.
The process of the lnvention is used in the dair~ industries to prepare prote.ins, including casein, which are particularly suitable to be - used in the food industry, the dietetic products industry, the pharmaceutical industry and the animal care products industry.
, ~ These examples for carrying out the invention are given by way of illustratlon and not by way of limitation.
EXA~PLE 1:
20 g of an anion exchanger resin, composed of a silica with a granulometry of 100 to 200 ~m, a specific surface area of 24 m2/g, an average pore diameter of 1400 A and a porous volume of l ml/g, coated with 3.3 mg/m2 of a styrene vinyltriethoxysilane copolymer carrying the functional groups F~l3 CH N~+) CH Cl(~) are placed in column l which is 2.5 cm in diameter.
This resin has the following characteristics:
- proportion of carbon........................... 4.8%
- proportion of chlorine.......................... 2 %
- proportion of nitrogen....................... ~ 0.9%
-exchange capacity......................... 0.6 me~./g lO g oE silica balls with a granulometry of lO0 to 200 ~m, a specific surface are of 25 m2lg) a pore diameter of 1400 A and a porous ; ~ volume of l.l ml/g arD placed in column 2 which is 2.5 cm in diameter.
After the two columns are arranged in series, the resin and the -~
silica are washed by runnincg through 500 ml oE water.
250 ml of skImmed milk, containing 7 g of casein, 1.6 g of other proteins and 12 g oE lactose, are percolated f:irsk in column 1, then in column 2, at the rate oE 100 ml/h.
~ ~le resin and the silica in the two columns are washed by running through 200 ml of watDr.
When an N/lO0 solution of hydrochloric acid is run through column 1, the fixed proteins are eluted 33 ml of solution, containing 1.3 g of .
; proteins are obtained. These proteins are the ~-lactalbumins5 the ~-lacto-:. .
globulins, serum albumln and a small amount of immunoglobulins.
; ~ When~a N solution of ammonium carbonate is run through column 2, the fixed proteins are eluted: 12 ml of solution containing 0.3 g of immuno-`: ~ :

, ~
~ -7-: :

globulins are obtained.
The two protein solutions contain less than 1% by weight of fatty substances and lactose. The electrophoretic migration of proteins is identical to the one they had in the milk. Therefore, they are not denatured.
I`he solutions leaving column 2 (milk ~ wash waters), containing only casein and no other proteins, undergo ultrafiltration.
The retained product contains approximately 7 g of native casein at a concentration of 20% by weight.
The ultrafiltrate contains almost all the lactose at a concentration of about 42 g/l and the mineral salts.
AMPLE 2:
Example 1 is repeated, but the processing of skimmed milk by the anion exchanger resin and silica is carried out at 50C
The results are identical to those in Example 1.
EXAMPLE 3:
Example 1 is repeated, by placing column A, which is 1 cm in dia-meter~ containing 3 g of the sarne anion exchanger resin as that in colurnn 1, before column 1. By elution in column A with an N/100 solution of HCl, 15 ml of a solution containing 0.3 g o:E proteins made up almost exclusively of ~-Iactoglobulins is obtained.
Elution in column I with an N/100 solution oE ~ICl yiclds 33 ml o~
a solution containing 1 g of proteins consisting of mostly ~-lactalbumins, serum albumin and a small quantity of ~-lactoglobulins and immunoglobulins.
Elution in colwnn 2 yields the same results as in example 1.
EXAMPLE ~
10 g of silica balls with a granulometry of 100 to 200 ~m, a specific surface area of 25 m2~g, a pore diameter of 1400 A and a porous volume of l.I ml/g, are placed in column 1 which is 2.5 cm in diameter.
15 g of an anion exchanger resin composed of a silica with a ~ 30 granulometry of 100 to 200 ~m, a specific surface area of 24 m2/g, an -~ ~ -8-.. ~
~ .

:' ~ ~ . , - . .

7~

average pore diameter of 1400 A and a porous volume of l ml/gg coated with 6 mg/m2 of a styrene-vinyltriethoxysilicane copolymer carrying the func-tional groups /
- ~ - CH2 - N \

~2 5 and having the following characteristics:
- proportion of carbon........... ......... 7.5%
- proportion of nitrogen................... 1.5%
- exchange capacity........................ 1.07 meq./g are placed in column 2~ which is 2.5 cm in diameter.
After the two columns are placed in series, the silica and the resin are washed by running through 500 ml of water.
Z50 ml of skimmed milk, containing 7 g of casein, 1.6 g of other proteins and 12 g oE lactose are percolated at the rate of 100 ml/h first in column 1, then in column 2.
The silica and the resi.n in the two columns are then washed by running througll 200 ml oE water.
When a N/L00 solut;on o:E ammonium hyclroxicle is run througll column 1, the fixed proteins are eluted. 12 ml of solution containing 0.4 g, i.e.
the majority of immunoglobulinsg are obtained.
When a N/lO solution of hydrocloric acid is run through column 2, the fixed proteins are eluted: ~-lactalbumins, ~-lactog:Lobulins, serum albumin and traces of immunoglobulins; 23 ml o solution contain 1.2 g of these proteins.
The two protein~solutions contain less than 1% by weight of lactose.
The electrophoretic migration of proteins is the same as the one they had in the milk. Therefore, they are not denatured.
The solution leaving column 2, approximately 260 ml~ no longer ~9~

~7~

containing proteins except for casein, is percolated in column 3 in order to separate the casein by exclusion chromatography. This column, which is 3 cm iTI diameter, contains ~50 g oE silica with a granulometry of 100-200 ~m, a specific sur-face area o:f ~00 m2/g, an average pore diameter o~ 80 A
and a porous volume of 1 ~l/g and is eluted with water.
330 ml of a solution containing almost all the native casein and a solution containing the lactose and the mineral salts are obtained.
~ XAMPLE 5:
15 g of the same anion exchanger resin as that in column 2, Example 4, are placed in column 1, which is 2.5 cm in diameter.
10 g of silica balls with a granulometry of 100 to 200 ~m, a - specific surface area of 50 m2/g, a pore diameter of 800 A and a porous volume of 1.1 ml/g, are placed in column 2, which is 2.5 cm in diameter.
~ fter the two columns are placed ln series, the resin and the silica are washed by running through 500 ml of water.
300 ml of milkserum with a pH of 6.5, containing 1.5 g of proteins and 11 g of lactose, are percolated at ambient temperature, first in column 1, then in column 2, at the rate of ~00 ml/h.
The resin and the silica in the two columns are washed by rLmning :~ 20 through 200 ml of ~ater.
When a N/100 solution o:~ hydrochloric acid is run through column I, the -Eixecl proteins are eluted. 33 ml of solut;on contalning 1.25 g of proteins are obtained. These proteins are the ~-lactalbumins, serum albumin, the ~-lactoglobullns and a small quant:ity of :immunoglobulins.
~ When a N/lOO solution of ammonium hydroxide is run through column i : :

2, the fixed proteins are eluted: 10 n~l of solution containing 0.25 g of proteins composed almost exclusively of immunoglobulins are obtained.
Ihe two protein solutions contain less than 1% by weight in fatty , ; substances and lactose. The electrophoretic migration of proteins is the si~ne as the one they had in the milkserl~n. Thereore, they are not denatured. :

EXA~PL.E 6:
Example 5 is repeated, but processlng the milk serum with the anion exchanger resin and the silica is carried out at 50C.
'['he results are the same as those in Example 5.
EXAMPLE 7:
Example 5 is repeated, by placing column A, which is 1 cm in diameter, contalning 3 g of the same anion exchanger resin as that in column l, before column 1. By elution in column A with a N/100 solution of HCl, 15 ml of a solution containing 0.3 g of proteins composed almost exclusively of ~-lactoglobulins are obtained.
Elu~ion ;.n column 1 with a N/100 solution of ~ICl yields 33 ml of a solut:ion containing 0.95 g of proteins composed mostly of ~-lactalbumins and serum albumin and a very small quantity of ~-lactoglobulins and immuno-globuliTIs.
Elution in column 2 yields the sam~ results as in Example 5.
EXAMPLE 8:
10 g of silica balls with a granulometry of 100 to 200 ~m, a specific surface area of 25 m2/g, a pore diameter of 1400 A and a porous volwne of l.l ml/g~ are placed in column 1, which is 2.5 cm in diameter.
20~ ~20 g of the same anion exchanger resin as that ~ound in column 1,example 1~ are placed in column 2 which is 2.5 cm :i.n d:iametcr.
After the kwo columns are placcd in sar:ies, the sllica and the resin are washed by runnlng through 500 ml oE wate~r.
300 ml of milkseruIll o:E plI 6.5, contain.ing 1.5 g of prokeins and 11 g of lactose, are percolated at ambient temperature, at the rate of 600 ml/h first in column l, then in column 2.
I'he silica and the resi.n in the two columns are then washed by running through 200 ml of water.
When a N/lOO~solution~ of ammonium hydroxide is run through column 1, the fixed proteins~ are eluted. 12 ml of solution containing 0.4 gJ i.e., .: :

the majority of immunoglobulins~ are obtained.
When a N/10 solution of sulfuric acid is run through column 2 the :Eixed proteins are elutecl~ lactalbumins, ~-lactoglobulins, serum albumin and traces of immunoglobullns; 20 ml o:F solution contain 1.1 g of these proteins.
The two protein solutions contain at least 1% by weight of lactose.
The electrophoretic migration o proteins is the same as the one they had in the milkserum. Therefore, they are not denatured.
XAMPLE 9:
20 g of the same anion exchanger resin as the one in column 2, example 1, are placed in column 1, which is 2.5 cm in diameter.
10 g of silica balls with a granulometry of 100 to 200 ~ImJ a specific surface of 25 m2/g, a pore diameter of 140~ A and a porous volume of 1.1 ml/g are placed in column 2, which ;.s 2.5 cm in d:ialneter.
After the two columns are placed in series, the resln and the silica are washed by running through 500 ml oE water.
500 ml of milkserum, which is adjusted to a p~l 7.5 by adding 0.1 N sodium hydroxide, are filtered to eliminate the insoluble substances and contain 2.5 g oE pr~teins and 18 g of lactose. This is percolated at ambient temperature first in column 1, then in column 2, at a rate of 300 ml/ll.
The res:in and the silica in the two columns are washed by running through 100 ml oE water.
When a N/100 solution oE hydrochloric ac:icl is run through column 1J the eixed proteins are eluted. 35 ml o:E solution conta:ining 2.05 g of proteins are obtained. These proteins are the c~-lactalbumins, the ~-lacto-globulins, serum albumin and a small quantity of immunoglobulins.
When a molar ammonium carbonate solution is run through column 2, ~-~
the fixed proteins are eluted: ].2 ml of solution containing 0.45 g of proteins composed almost exclusively of immunoglobulins are obtained.
For comparative purposesj this example is repeated, but instead .' ~ :

~ .
.. ~ . .

of 10 g of silica in column 2, 10 g of a cation exchanger resin are used, the resin is composed of a silica with a granulometry of 100 to 200 ~m, a speciEic surface area of 25 m2/g, an average pore diameter oE 1400 A and a porous volume oE 1.1 ml/g, coated with 7.2 mg/m2 of a copolymer of acrylic acid and dimethacrylate of diethylene glycol carrying the functional groups -COOH.
I`his resin has the following characteristics:
- proportion oE carbon..................... 10.55%
- exchange capacity........................ 1.05 meq /g By elution in the first column, the same results are obtained as in Example 5.
By elution in column 2, 17 ml of solution containing 0.45 g of proteins composed almost exclusively of immunoglobulins are obtained.
It is noted that when using silica, a much simpler resin than the cation exchanger resin, it is possible to obtain a more concentrated solution of immunoglobulins, - 2.65 g/100 ml with the exchanger resin

- 3.75 g/100 ml, which is 41.5% more proteins, with the silica.

:::

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~ -13-.. . .

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Process for extracting proteins from milk, characterized in that the proteins other than the casein are first extracted by putting skimmed milk into contact first with at least one anion exchanger resin and then with silica, or first with silica then with at least one anion exchanger resin, fixing the proteins and eluting and separating the casein remaining in solution from the mineral salts and lactose, the anion exchanger resins having an exchange capacity less than 2 meq/g and being formed of a support selected from the group consisting of an alumina and silica and coated with less than 20 mg/m2 of a cross-linked polymer film containing or carrying tertiary amine or quaternary ammonium salts functionnal groups; and the anion exchanger resin supports and the silica having a granulometry between 4 µm and 5 mm, a specific surface area of approximately 5 to 150 m2/g, a porous volume of 0.4 to 2 ml/g and a pore diameter between 250 and 2500 .ANG..

2. Process for extracting proteins from milk comprising separating the lactose and mineral salts from skimmed milk by ultrafiltration or exclusion chromatography; extracting the proteins other than casein by put-ting the milk into contact first with at least one anion exchanger resin and then with silica, or first with silica and then at least one anion exchanger resin, fixing of proteins and eluting, and the natural casein stays in solution; the anion exchanger resins having an exchange capacity less than 2 meq/g and being formed of a support selected from the group consisting of an alumina and silica and coated with less than 20 mg/m2 of a cross-linked polymer film containing or carrying tertiary amine or quaternary ammonium salts functional groups, and the anion exchanger resin supports and the silica having a granulometry between 4 µm and 5 mm, a specific surface area of approximately 5 to 150 m2/g, a porous volume of 0.4 to 2 ml/g and a pore diameter between 250 and 2500.ANG..

3. The process as claimed in claim 1, in which in the extraction of proteins from lactoserum, the lactoserum is put into contact with first at least one anion exchanger resin and then with silica, or first with silica and then at least one anion exchanger, resin, at a pH higher than 4, fixing of proteins and eluting; the anion exchanger resins having an exchange capa-city less than 2 meq/g and being formed of a support selected from the group consisting of an alumina and silica coated with less than 20 mg/m2 of a cross-linked polymer film containing or carrying tertiary amine or quaternary ammo-nium salts functional groups, and the anion exchanger resin supports and the silica having a granulometry between 4 µm and 5 mm, a specific surface area of approximately 5 to 150 m2/g, a porous volume of 0.4 to 2 ml/g and a pore diameter between 250 and 2500 .ANG..

4. Process as claimed in claim 1, 2, 3, in which tertiary amine and quaternary ammonium salts functional groups of the anion exchanger resins have the general formulae:

in which R, alike or different, represents an alkyl or hydroxyalkyl radical having 1 to 4 carbon atoms and X an organic or inorganic anion.

5. Process as claimed in claim 1, in which the cross-linked polymer is selected from the group consisting of epoxy compounds cross-linked with polyamines as catalyst; formaldehyde cross-linked by polycondensation with urea, melamine, polyamines and phenols; vinyl monomers: vinylpyridine, styrene and derivatives, cross-linked with polyfunctional monomers: diacrylates or dimethacrylates of mono- or polyalkylene glycols, divinylbenzene, vinyl-trialkoxysilane, vinyltrihalogenosilane, bis-methylene acrylamide.

6. Process as claimed in claim 1, in which contact of the skimmed milk with the anion exchanger resins and silica is carried out at a temperature between 0 and 50°C

7. Process as claimed in claim 1, in which the quantity of anion exchanger resin or resins is 5 to 15 grams per gram of proteins to be extracted and the quantity of silica is 2 to 7 grams per gram of proteins to be extracted.

8. Process as claimed in claim 1, in which the proteins retained by the resins and silica are eluted with either a high ion strength solution or a pH solution which is acid for the anion exchanger resins and basic for the silica.

9. Process as claimed in claim 1, in which the protein extraction is carried out discontinuously, semicontinuously or continuously.

10. Process as claimed in claim 1, in which after the proteins have been extracted, the natural casein is extracted by exclusion chromatography or ultrafiltration.