GB2052515A - Coagulation of milk utilizing basic polymers - Google Patents

Abstract

A method for curdling milk consists of contacting the milk with a polymeric substance which has a pH of from 3 to 7 so as to coagulate the milk in order to recover the protein content thereof, the milk to be coagulated being at a pH of 5.0 to 8.0 and at a temperature of -10 DEG C to 80 DEG C. The milk-curdling polymeric substance must be at least fairly water soluble and must display, in an aqueous solution at a pH equal to or higher than 5.0, an excess of positively charged ionizable functional groups over the negatively charged ionizable functional groups. Synthetic polymers which can be used are polyethyleneimine, polyornithine, polyvinylpyridine, polyvinylamine, and natural polymers which can be used are water-soluble proteins and polysaccharides.

Description

SPECIFICATION Coagulation of milk This invention relates to a method of coagulating milk using suitable polymer substances.

Industrially, milk is coagulated in order to recover the proteins contained therein. These are then converted by subsequent technical processes into food products and industrial products such as piastics, paint and adhesives.

In the present state of the art, milk can be coagulated in various ways, all derived from only two basic methods, namely acid coagulation and rennet coagulation. The first method consists of reducing the pH of the milk from its natural value of 6.6-6.7 to 4.5-5.0 (casein isoelectric point). The effect of the acidification is to precipitate the caseins contained in the milk (but not the albumins or globulins).

The product thus obtained, also known as isoelectric casein, does not have the same structure as that which it had in its native state, nor the same composition. In this respect, caseins exist in milk as a colloidal solution in the form of molecular aggregates known as micelles, which have a characteristic structure, and a composition which also includes inorganic elements of which the most important is the Ca++ ion. The change in pH causes the complete destruction of the micellar edifice, and the release of Ca++ ions in solution. Thus, in milk coagulation processes based on the acid method, only the caseins are recovered while the other proteins and inorganic elements are lost. The protein yield of these processes does not exceed 66%. U.S. Patent No. 2,623,038, U.S. Patent No. 2,665,989 and U.S.

Patent No.2,714,068 describes milk coagulation methods which claim protein recoveries exceeding 90%. These are modifications of the acid method, and make use of high temperatures (i.e. greater than 70CC). The conditions are such as to lead to the partial denaturation of the proteins to be recovered, and have the typical drawbacks of acid coagulation.

The second milk coagulation method is the so-called rennet coagulation. This is achieved by the action of certain proteolytic enzymes, which have a specific action on the casein micelles to a lesser or greater extent. The result of this action is an alteration in the state of the electric charge in the micelles, which consequently become destablished and coagulate. This type of coagulation does not lead to large structural alterations of the micellar casein, as it takes place without any pH change. In addition, it leads to products which have a higher content of inorganic elements, and which are generally more valuable.

However, even with this method only the caseins can be recovered, the other proteins of the milk remaining in the whey. Thus in this case, the casein recovery is also low. It never exceeds 95% of the casein (equal to about 84% of the total proteins) because of the partial solubilisation due to the products of the enzymatic reactions.

A milk coagulation method which operates under mild temperature conditions without any pH change, and which allows a greater protein recovery than that obtained by known methods, would therefore be desirable.

According to the present invention, there is provided a method of coagulating milk, which method comprises treating the milk with an aqueous solution of a polymer having a net positive charge at a pH of 5 or more.

The present invention provides a milk coagulation method which may operate under mild temperature conditions, which does not require acidification of the milk to be coagulated, and which usually gives a protein yield of not less than 90% of the total proteins present in the milk. The coagulum obtained by the present method has a very short syneresis time, because of which it is particularly suitable for continuous coagulation processes.

The method according to the present invention preferably comprises bringing the milk to be coagulated, at a pH of 5.0 to 8.0, preferably 6.0 to 7.Q, and at a temperature of 00C to 80a C, -20C to --500C, into contact with an aqueous solution of a suitable concentration of polymers which are either pure or in mixture with each other or in mixture with other substances, and having a pH of 3 to 7, preferably 4.5 to 7.The properties necessary for the pure or mixed polymers to be suitable for use in the present invention are (1) they must be soluble in water under the aforesaid temperature and pH conditions, the solubility preferably being greater than 0.1% weight/volume, more preferably greater than 0.5% weight/volume: and (2) when in aqueous solution at a pH of 5.0 or more, they must possess either an excess of positively charged ionisable functional groups over those negatively charged, or only positively charged ionisable functional groups, this condition preferably persisting a pH of 6.0 or more.

In the present invention, therefore, there can be used any natural polymer in its natural state or chemically modified, or any artificial or synthetic polymer, which is soluble in water and which possesses a net positive charge at a pH of 5.0 or more.

Examples of synthetic polymers which can be used in the present invention are polyethyleneimine, polylysine, polyornithine, polyvinylpyridine, polyvinylamine or any other polymer which possesses the aforesaid properties. Examples of natural polymers which can be used in the present invention are all substances belonging to the water-soluble protein and polysaccharide classes. It is however not necessary for these to possess the aforesaid properties in their natural state. They can be used in the present invention after chemical change. For example, a soluble starch which does not possess positive charges can be used after conversion into an aminoethoxy derivative. Likewise, all proteins having an isoelectric point of 6.0 or more can be used after total or partial esterification of the carboxyl groups with an alcohol.In particular, the chemical change makes it possible to use proteins which are widely available at low cost, such as isoelectric casein, powdered milk, proteins recovered from milk whey, egg white, proteins recovered from animal blood and vegetable proteins.

Partial or total esterification of the carboxyl groups of a protein subtracts negative charges from it, so increasing its isoelectric point. If it did not previously possess it, a protein modified in this manner acquires the capacity for coagulating the casein micelles of milk. This chemical change is very useful for the purposes of the present invention, because it can be carried out under very mild and economically advantageous conditions on isoelectric casein and powdered milk. This fact is of fundamental importance, because it makes it possible to coagulate milk us;ng casein or powdered milk itself, as will be more apparent from the Examples below.

The total or partial esterification of the carboxyl groups of a given protein is preferably carried out with low cost alcohols such as ethanol and methanol. However, if desired, any other alcohol can be used such as glycerine, athanolamine, choline, and glucose. It is therefore possible to choose the alcohol to be used according to the purpose for which a given coagulation process is carried out.

Milk coagulation according to the present invention can be carried out on fresh milk, whole milk, partly skimmed or skimmed milk, or on milk regenerated from powdered milk. it can be carried out on natural milk or more advantageously on concentrated milk, whether the concentration has been carried out by evaporation or by ultrafiltration.

Milk coagulation according to the present invention is preferably carried out at varying temperature by adding the cold coagulating substance to cold milk (O5 C), and then heating the mixture to a temperature greater than 200 C, by which means it is possible to obtain a continuous gel.

Alternatively, it is preferably carried out by adding the coagulating substance to the milk at a temperature of between 200C and 500 C. This gives a granular coagulum, the granules of which vary in size depending upon the type of stirring used. They are larger the gentler the stirring, and their size can vary from a fraction of a millimetre to 2-3 centimeters in diameter. The pH of the coagulation can be varied, and it is not necessary for the pH of the solution of coagulating substance to be equal to the pH of the milk to be coagulated, provided that the pH of the resultant mixture is 5.5 to 7.0.

The yield of this method of coagulation is greater than that of the acid or rennet method. At a temperature of 30 to 400C and at a pH of 5.5 to 7.0, it is possible to recover in the coagulum up to 99% of the proteins initially present in the milk. A high temperature is not necessary for increasing the yield, as in the case of certain acid coagulation processes. Furthermore, in contrast to the rennet method which requires a long period of hardening for the gel and a laborious rennet breaking operation, the method according to the invention, if carried out at a temperature of 300C to 500 C, provides a granular coagulum which can be separated from the whey by filtration just a few minutes after coagulation has taken place.This fact is very important because it makes it possible to build a coagulation plant for continuous operation, which obvious economical advantages.

Finally, the quantity of polymer necessary for coagulating a certain quantity of milk depends on various factors such as the type of milk, its Ca++ ion content, the temperature and pH at which the coagulation is conducted, and the type of polymer used.

The invention wilt now be illustrated by the following Examples.

EXAMPLE 1 (Milk coagulation with synethtic polymers) In this Example, there was used milk regenerated from powdered milk, prepared in the following manner, 120 g of powdered skimmed milk (commercial product MAG-IST of SITIA--YOMOO-Milan) were suspended in 1000 ml of an 0.01 M aqueous solution of CaCI2, and the suspension was stirred for five minutes. The milk thus obtained was left standing for one hour at ambient temperature, and then used within three hours from preparation. Its pH was 6.5. Its total nitrogen content (TN) and non-protein nitrogen content, i.e. nitrogen soluble in 2% trichloroacetic acid solution in water (NPN), as determined by the Kjeldahl method, were 0.576 g/1 00 cc in the case of TN and 0.040 9/100 cc in the case of NPN.

The difference (0.576-0.040) of 0.536 9/100 cc is due to the proteins in the milk, i.e. 6.38 x 0.536 = 3.42 g of proteins per 100 cc of regenerated milk, 6.38 being the factor for converting nitrogen content into casein content, as 6.38 g of milk casein contain 1 g of nitrogen.

(I) Coagulation of regenerated milk with polyethyleneimine A solution of polyethyleneimine was prepared in the following manner. 2 g of a 50% (weight/weight) aqueous solution of polyethyleneimine (commercial product of Fluka AG, Buchs, Switzerland having a declared molecular weight of 30,000-40,000 Dalton) were diluted to about 30 ml with water, and phosphoric acid was added until the pH was 6.4. The solution obtained was made up to a volume of 50 ml with water, so that it contained 20 mg/ml of polymer.

25 ml of the polyethyleneimine solution were added over a period of about one minute to 100 ml of the regenerated milk prepared as described above, and temperature controlled at 300C under mild stirring. This addition caused the immediate coagulation of the milk into granular form, with separation of the whey, which was perfectly clear and had a characteristic yellow-green colour. Thus 0.5 g od polymer were required to coagulate 100 ml of milk. The granules obtained were separated from the whey by filtration through a porous G-3 filter, and washed twice with 30 ml of distilled water. The whey obtained was added to the wash water for the solid, the total was diluted quantitatively to a volume of 300 ml with water, and then analysed.The washed solid was also quantitatively dissolved to a final volume of 250 ml in 0.2 M soda, and analysed. A Kjeldahl nitrogen analysis was carried out on the whey solution and on the coagulum dissolved in soda. For the serum, the analysis results were 0.049 g of TN and 0.015 of NPN, giving 0.049-0.015 g = 0.034 g of protein nitrogen, corresponding to 0.034 x 6.38 = 0.22 g of non-coagulated protein, equal to 0.22/3.43 x 100 =6.3% of proteins initially present in the milk.For the coagulum, the analysis result was 0.678 g of total nitrogen, of which 0.16 g are due to the added polyethyleneimine (0.5 g with a declared nitrogen content of 32%); thus 0.518 g of protein nitrogen were recovered in the coagulum, corresponding to 0.518 =6.30 x 3.31 g of proteins initally present in 100 ml of milk, representing 96.6% of the total proteins.

(II) Coagulation of regenerated milk using poly-L-lysine A solution of poXy-L-lysine hydrobromide was prepared as follows. 2.5 g of the polymer (produced by Khock-Light Laboratories Ltd, Colnbrook, England, having a declared molecular weight of about 54,000 Dalton) were dissolved in 30 ml of water, adjusted to a pH of 6.4 with 1 M soda, and finally diluted to a volume of 50 ml with water. The solution obtained thus contained 50 mg/ml of polymer.

Proceeding exactly as described in test (1), 12 ml of a solution of poly-L-lysine hydrobromide containing 0.6 g of polymer were added to coagulate 100 ml of milk. They whey and coagulated proteins were recovered exactly as described in test (1), and were analysed for nitrogen. For the serum, the analysis results were 0.060 g of TN and 0.023 g of NPN. This is equivalent to 0.060--Q.023 = 0.037 g of protein nitrogen, equal to 0.037 x 6.38 = 0.24 g of non-coagulated proteins, equal to 6.9% of the proteins initially present in the milk. For the coagulum, the analysis result was 0.568 g of total nitrogen, of which 0.08 g were due to the polylysine (0.6 g having a nitrogen content of 13.4%). Thus 0.488 g of protein nitrogen were recovered in the coagulum, corresponding to 0.488 x 6.38 = 3.11 of proteins.

When compared with the 3.42 g of proteins initially present in 100 ml of milk, this value represents 910/0 of the total proteins.

(III) Acid coagulation for comparison purposes 2.3 ml of 1 M HCI were added over about three minutes to 100 ml of regenerated milk the same as used in tests (I) and (II) above, temperature controlled at 300C and with stirring, the resultant pH being 4.7. The mixture was heated to 500C for about 30 minutes in order to increase the size of the precipitate, which was otherwise unfilterable, and was then processed as described in tests (I) and (if).

The whey obtained in this case was not perfectly clear as in the previous cases. For the serum, the analysis results were 0.144 g of TN and 0.038 g of NPN, equivalent to 0.114 - 0.038 = 0.076 9 of protein nitrogen, equal to 0.076 x 6.38 =0.49 g of non-coagulated proteins, corresponding to 14.2% of the proteins initially present in the milk. For the coagulum, the analysis result was 0.459 g of TN, corresponding to 0.459 x 6.38 = 2.39 g of proteins, equal to 85.6% of the proteins initially present in the 100 ml of milk.

EXAMPLE 2 (Milk coagulation usung natural polymers) A solution of protamine was prepared in the following manner. 2.5 g of protamine (free base protamine from salmon sperm, produced by Sigma Chemical Company, St. Louis, USA) were dissolved in about 30 ml of water and adjusted to a pH of 6.4 with N HCI. The solution obtained was diluted to a total volume of 50 ml with water, so that it contained 50 mg/ml of protein. Proceeding exactly as described in tests (I) and (II) of Example 1, 17.5 ml of protamine solution containing 0.875 of free base were added in order to coagulate 100 ml of regenerated milk. The whey and coagulated proteins were recovered by exactly the same method as the tests of Example 1, and a nitrogen analysis was carried out.For the serum, the analysis results were 0.g64 g of TN and 0.37 g of NPN, the protein nitrogen oozing therefore 0.064-0.037 =0.027 g corresponding to 0.027 x 6.38 = 0.17 g of non-coagulated proteins, representing 5% of the proteins initially present in the milk. For the coagulum, the analysis result was 0.776 g of total nitrogen, of which 0.275 g were due to the protamine (0.875 g with a nitrogen content of 31.4%), corresponding to a recovery in the coagulum of 0.776 - 0.275 = 0.501 of nitrogen from milk proteins, this corresponding to 0.501 x 6.38=3.20 g of proteins. This value, with respect to the 3.42 g of proteins initially present in 100 ml of milk, represents 93.5% of the total proteins.

EXAMPLE 3 (Regenerated milk coagulation using chemically modified natural polymers) The regenerated milk used in the coagulation tests of this Example was prepared by the method described in Example 1.

(I) Milk coagulation using casein esterified by methyl alcohol 5 g of isoelectric casein (nitrogen content 14.8%), prepared by precipitation at a pH of 4.7 from regenerated powdered milk, were suspended in 40 ml of absolute methyl alcohol, and 10 ml of a 1.05 M solution of HCI in methanol were added. The suspension was kept stirred at ambient temperature for 48 hours. The suspension was then filtered through a porous filter. The esterified casein obtained was washed three times with 30 ml of methyl alcohol, and dried at 400C under vacuum. The yield of dry product was 4.95 g.

The quantity of esterified carboxyl groups was determined in the followidng manner. 200 mg of casein esterified by methanol were dissolved in 20 mi of water, the spontaneous pH of this solution being 2.6. The pH was raised to 3.0 with 0.1 N NaOH, and the solution was then titrated to pH 8.0 with 0.01 N NaOH. This solution required 16.3 ml of 0.01 N NaOH to increase the pH from 3.0 to 8.0.

220 mg of isoelectric casein dissolved in 20 ml of water and adjusted to pH 3.0 by 0.1 N HC1 required 30.0 ml of 0.01 N NaOH to increase the pH from 3.0 to 8.0. These data give (30.0--16.3) x Q.01/0.2 = 0.68 meq of ester groups/g of product. The esterified casein thus obtained had a nitrogen content of f 4.196.

4 g of this product was dissolved in 200 ml of water and adjusted to a pH of 4.7 with 1 N NaOH.

The solution therefore contained 20 mg/ml of chemically modified protein. Proceeding in the manner described in Examples 1 and 2, 90 ml of this solution were used to coagulate 100 ml of regenerated milk. The whey obtained was perfectly clear, and had the characteristic yellow-green colour. The whey and coagulum were recovered quantitatively using the procedure of Example 1. The following results were obtained. In the whey, both the total nitrogen and the non-protein nitrogen had the same value of 0.029 g, indicating that there was complete precipitation of the milk proteins in this case. The total nitrogen contained in the coagulum was 0.779 g, of which 0.254 g was due to the added esterified casein (1.8 g containing 14.1 % of nitrogen).This therefore represents 0.779-0.254=0.525 9 of protein nitrogen, equal to 0.525 x 6.38 = 3.35 g of coagulated proteins, representing 98% of the initial proteins in 100 ml of milk.

(II) Milk coagulation using casein esterified by ethyl alcohol 5 g of isoelectric casein prepared as described in the first part of this Example were suspended in 45 ml of absolute ethyl alcohol, and 5 ml of 3.2 N HC 1 in absolute ethyl alcohol were added. The suspension was kept stirred at ambient temperature for 5 days. The product was recovered exactly as described in the first part of this Example, and was characterised in the same manner. It contained 0.65 meg/g of esterified carboxyl groups, and had a nitrogen content of 14.09/0.

80 ml of an aqueous solution containing 20 mg/ml of casein esterified by ethanol and having a pH of 4.7 were added to coagulate 100 ml of regenerated milk, under the conditions of Example 1. An analysis of the whey and coagulum carried out as in the preceding Examples gave the following values.

In the whey, the total nitrogen content was 0.058 g and the non-protein content was 0.042 g, so that the quantity of non-coagulated proteins was ((0.058-0.042) x 6.38 = 0.10 g, equal to 3% of the total proteins. In the coagulum, 0.735 g of nitrogen were recovered, of which 0.224 g related to the added casein (1.6 g containing 14.0% of nitrogen), the protein quantity recovered in the coagulum therefore being (0.735-0.224) x 6.38 = 3.26 g, equal to 95.3%.

(III) Milk coagulation using powdered milk esterified by methyl alcohol 32 g of powdered milk dried in an oven at 600C under vacuum until of constant weight, were suspended in 88 ml of absolute methyl alcohol, and 12 ml of 3.2 N HC1 in absolute methanol were added. The suspension was then centrifuged, and the produce obtained was suspended in 100 ml of methyl alcohol and recentrifuged. This operation was carried out three times. After drying the product in an oven at 500 under vacuum until of constant weight,27.5 g of chemically modified powdered milk were recovered. 20 g of this product were dissolved in 200 ml of water, and adjusted to a pH of 4.5 with a few drops of 1 N NaOH. The solution therefore contained 100 mg/ml of powdered milk modified by reaction with CH30H and HC1.Operating in accordance with the conditions described in Example 1, 65 ml of the solution of esterified powdered milk were added to coagulate 100 ml of regenerated milk.

The whey, which was perfectly clear and had the characteristic yellow-green colour, was recovered quantitatively by washing the precipitate twice with 30 ml of water, and diluting the initial whey together with the wash water to a total volume of 300 ml. Determinations of total nitrogen and nonprotein nitrogen carried out by the Kjeldahl method on this solution gave values of 0.205 g and 0.163 g respectively. The residual non-coagulated proteins in the whey were therefore (0.205-0.163) x 6.38 = 0.270 g, equal to 7.8% of the proteins initially present in 100 ml of milk.

(IV) Rennet coagulation for comparison purposes 0.1 ml of liquid veal rennet (strength 1:10,000) were added to 100 ml of milk temperature controlled at 300 C. The mixture was kept stirred for two minutes and then left standing. After about fifteen minutes, the milk had coagulated into a continuous gel. This gel was left to harden for about twenty minutes, and was then crushed with a spatula. The mixture was then raised to 500C and left standing for thirty minutes. The solid was then separated by filtration from the whey, which in this case was turbid (not perfectly clear as in the preceding Examples).The solid was washed twice with 30 ml of water, and dissolved quantitatively in 250 ml of 0.2 N NaOH, the serum obtained being recovered quantitatively and added to the wash liquids for the solid, the mixture being then diluted to a total of 300 ml with water. A nitrogen determination by the Kjeldahl method gave the following values. For the whey, the total nitrogen was 0.092 g and the non-protein nitrogen was 0.036 g. Thus (0.092-0.036) x 6.38 of protein remained in the whey, equal to 10.4% of the proteins initially present in the milk. For the coagulum, the total nitrogen was 0.469 g, corresponding to 2.99 g of coagulated proteins, equal to 87.5% of the initial proteins in the milk.

EXAMPLE 4 (Coagulation of fresh pasteurised whole milk by esterified proteins) Fresh pasteurised whole milk from the Rome Milk Centre was used for the coagulation tests of this Example.

(I) Coagulation of whole milk at varying temperature usung alburnin esterified with methanol 10 g of crystalline albumin from oxen serum (produced by Merck, Darmstadt, W. Germany) were suspended in 90 ml of absolute methanol, and 10 ml of a 3.27 M solution of HCI in methanol were added. The mixture was kept stirred at ambient temperature for 48 hours. 200 ml of H20 were then added to the mixture, and the pH of the resultant solution was adjusted to 5.5 with 1 M NaOH. The resultant solution was concentrated by evaporation under vacuum at a temperature of 30.40 C to about 100 ml in volume, then dialysed for 24 hours against water, and finally lyophilysed. In this manner, 10.05 g of albumin esterified with methanol were obtained.This protein when titrated as described for the casein in Example 3, was shown to contain 0.55 meg/g of esterified carboxyl groups.

Its nitrogen content was 15.19/0.

2.5 g of this protein were dissolved in 50 ml of water and adjusted to a pH of 6 to give a solution which contained 50 mg/ml of protein. 100 ml of whole milk (TN = 0.541; NPN = Q.024; total proteins = (0.541-0.024) x 6.38 = 3.30 g) having a pH of 6.6, were temperature controlled at 600C, and 22 ml of the solution of esterified albumin, also at 6 C, were added. This addition of the albumin solution gave no formation of coagulum in the cold milk. On heating the mixture to 300C for twenty minutes, a continuous gel was obtained, similar to that obtained in the rennet coagulation. Proceeding exactly as described in test (IV) of Example 3, the following results were obtained.For the whey, the total nitrogen was 0.048 g, and the non-protein nitrogen was 0.031 g, so that (0.048-0.031) x 6.38 = 0.110 g of non-coagulated proteins were present, equal to 3.3% of the initial proteins. For the coagulum, 0.665 g of nitrogen were found, of which 0.168 g were due to the esterified albumin (1.1 g with 15.1% of nitrogen). The total recovered proteins where thus 0.497 = 6.38 = 3.17 g, equal to 96% of the proteins initially present in the milk.

(II) Coagulation of whole milk using casein esterified with methanol An aqueous solution having a pH of 4.7 was prepared, containing 20 mg/1 00 ml of Ca++ ions (added as chloride) and 2 g/1 00 ml of casein esterified with methanol (the same product as used in test (I) of Example 3). 100 ml of this solution were added over about three minutes to 100 ml of whole milk temperature controlled at 300 C. The milk thus coagulated into granular form, with the separation of a perfectly clear whey. Proceeding exactly as described in the preceding Examples, the following results were obtained. For the whey, the total nitrogen was 0.037 g and the non-protein nitrogen was 0.025 g.

The non-coagulated proteins were thus 0.08 g, equal to 2.4% of the initial proteins. For the coagulum, the nitrogen found was 0.775 g, of which 0.282 g were due to the added casein, giving (0.775-0.282) x 6.38 = 3.15 g of cogulated proteins, equal to 93.3%.

The following Table summarises the most significant results of the Examples.

TABLE

K of milk proteins recovered by coagulation 1 calculated by calculated by Coagulating substance Example coagulum whey average REGENERATED MILK

Polyethyleneimine 1-1 96.6 93.7 95.1 Poly-l-lysine hydrobromide 1~ill 91 93. i 92 Protamine 2 93.5 95.0 94.3 Casein esterified with methanol 3-1 98 100 99 Casein esterified with ethanol 3-11 95.3 97.0 96.2 Powdered milk esterified with methanol 3-Ill - 92.2 92.2 Acid coagulation with HCI 1-Ill 85.6 85.8 85.7 Rennet coagulation 3-IV 87.5 89.6 88.6 FRESH WHOLE MILK

Albumin esterified with methanol 4-I 98 96.7 96.3 Casein esterified with methanol 4-lI 95.3 97.6 96.5

Claims (15)

1. A method of coagulating milk, which method comprises treating the milk with an aqueous solution of a polymer having a net positive charge at a pH of 5 or more.

2. A method according to claim 1, wherein the milk is fresh milk (either whole, partly skimmed or skimmed), milk regenerated from powdered milk, or milk concentrated by evaporation or ultrafiltration.

3. A method according to claim 1 or 2, wherein the coagulation is carried out at a pH of 5.0 to 8.0.

4. A method according to claim 3, wherein the coagulation is carried out at a pH of 5.5 to 7.0.

5. A method according to any of the preceding claims, wherein the coagulation is carried out at a temperature of 00C to 800C.

6. A method according to claim 5, wherein the coagulation is carried out at a temperature of -2a to 500C.

7. A method according to any of the preceding claims, wherein the coagulation is carried out at constant temperature.

8. A method according to any of the preceding claims, wherein the polymer has a water solubility exceeding 0.196 weight/volume under the temperature and pH conditions of the coagulation.

9. A method according to claim 8, wherein the polymer has a water solubility exceeding 0.5% weight/volume under the temperature and pH conditions of the coagulation.

10. A method according to any of the preceding claims, wherein the polymer is a natural polymer, either in its natural state or chemically modified.

d A method according to claim 10, wherein the polymer is a chemically modified protein.

12. A method according to claim 11, wherein the chemical modification of the protein is a total or partial esterification of its carboxyl groups with an alcohol.

13. A method according to claim 12, wherein the esterified protein is pure, or is in admixture with another esterified protein, or is in admixture with an other substance (which need not be a polymer).

14. A method according to claim 12 or 13, wherein the esterified protein is the ethyl or methyl ester of a substance selected from isoelectric casein, powdered milk, egg albumin, proteins recovered from milk whey, proteins recovered from animais' blood, vegetable proteins, and hydrolysed protein products of any one of these substances.

15. A method according to any of claims 1 to 9, wherein the polymer is a synthetic polymer of the polyamine class.

1 6. A method according to any of claims 1 to 14, wherein the polymer is a natural protein of the histone and protamine classes.

1 7. A method according to claim 1 , substantially as described in any of the foregoing Examples.

1 8. A protein recovered from milk which has been coagulated by a method according to any of claims 1 to 17.