GB2100289A - Preparing curd and whey by electrolysis - Google Patents

Abstract

Curd and whey are produced from milk or comparable casein- containing liquids by electrolysis, busing an electrolytic cell in which the anode and cathode are separated by at least one ion-selective, semi- permeable membrane, by generating hydrogen ions in milk or a liquid derived from milk by electrolysis in the anode compartment, using the acid thus formed for acidulating milk or a comparable casein-containing liquid and separating the acidulated milk or liquid into curd and whey by known methods.

Description

SPECIFICATION A process for preparing curd and whey This invention relates to the production of curd and whey from milk by means of electrolysis.

Such a process is described in French patent 0352.531, dating to 1905, and referred to by Scherer in "Das Kasein, dessen Zusammensetzung, Eigenschaften, Herstellung und Verwertung" (Leipzig, 1919), pp. 55, 56. For the electrolysis, a carbon anode and an iron cathode are used, separated by a porous vessel. Sodium hydroxide is present at the cathode, and skim milk at the anode. The milk is heated at 800C. Casein is separated at 11.0 V in 20 minutes, and at 18.0 V in 10 minutes. The amperage is always 1 60 Amp. The resulting product is not pure casein, but a co-precipitate of denatured whey proteins.This explains why the proteins coagulate when the amount of electricity passed is only one-third to one-sixth of the amount required to form sufficient acid to acidulate the milk to the isoelectric point of pure casein. As the porous partition between the electrode compartments has no ion-selective properties, a considerable part of the acid formed at the anode during electrolysis is lost in this process.

In 1935, in Austrian patent specification AT B 154131, the precipitation of casein, which is regarded as a disadvantage in deacidifying milk by electrolysis, is avoided by surrounding the anode with a non-selective diaphragm, and separating it from the milk.

The behaviour of casein solutions in electrolysis is described in the book by Sutermeister and Browne, "Casein and its industrial applications" (New York, 1 939) on pp. 86, 87. In solutions of casein in acid, casein is deposited on the cathode; in a solution in alkali, the casein separates at the anode. The quantity of casein precipitated satisfies Faraday's law, this quantity being directly proportional to the quantity of electricity passed, and inversely proportional to the amount of acid or alkali required to dissolve the casein. In skim milk, casein is present not as a molecular solution, but as a colloidal dispersion of micelles which in addition to protein contain calcium phosphate.

The precipitation of casein by reducing the pH of milk in a kind of electrodialytic cell is described by Kato in J. Soc. Chem. Ind. Japan, 35 (1933) Suppl. binding 158, referred to in Sutermeister and Browne, Casein and its industrial applications (New York, 1939) pp. 21 and 47. This system for concentrating proteins by electrodialysis is also known by the names of electrophoresis-convection and multi-membrane electrodecantation, as described in "Analytical Methods of Protein Chemistry", Vol. 1, pup. 163 ff., by Alexander and Block (London, 1960). In such processes there is no direct contact between the electrodes and the liquid being treated. For a good separation between protein and liquid phase, the liquid must be agitated as little as possible.In this connection, only a low current density can be applied, owing to which the application on a technical scale would require very extensive apparatus.

For these reasons, protein separation according to these principles is exclusively used as a laboratory process.

The recovery of acid casein from milk is generally carried out by reducing the pH to the isoelectric point of casein, which is in the neighbourhood of pH 4.6. Spellacy, in Casein, Dried and Condensed Whey (San Francisco, 1 953) has described that this pH reduction can be achieved by both the conversion of milk sugar in the milk to lactic acid by microbiological fermentation and by the addition of an acid, such as lactic acid, hydrochloric acid, sulphuric acid, acetic acid, and the like. The casein is then precipitated in the form of a curd which with a suitable choice of temperature, acidity and other conditions can be separated by decantation, filtration, sieving or centrifuging from the remaining aqueous phase, called whey.The resulting casein whey contains the residues of the acid added or the lactic acid formed from milk sugar, which considerably reduces the value of such a whey as compared with the value of sweet whey as produced during the production of cheese.

In particular for the use of whey as a food, for example in the production of baby or dietary food, and in the production of cattle food, it is conventional to remove ash constituents by electrodialysis or by cation exchange, because the salts present in whey have an adverse effect on its applicability. Casein whey produced by the most conventional route by adding acid exhibits this drawback to an even greater extent than sweet cheese whey.

In Dutch patent specification 138857 and Publication 161,348, processes are described for the production of casein, in which whey is adjusted to a very low pH by means of cation exchange, and the acidified whey is used to acidulate milk to the point at which the casein precipitates. As, in this way, the milk is acidulated indirectly, and the residues of the acid used are washed out during the regeneration of the ion exchanger, the casein whey thus produced contains a considerably smaller quantity of undesirable components than the casein whey produced after the addition of acid or the fermentation of milk sugar. One disadvantage of this method is that the regeneration of the ion exchanger requires considerably more acid than in case the acid is added direct to the milk.The ash composition of the resulting whey corresponds to that of skim milk, in which a portion of the cations have been replaced by hydrogen ions, which are capable of forming acids with the anions present.

In French patent specification 1,479,361 and the appurtenant Additions 90,473 and 94,496, and Dutch patent application 67,03234, which belongs to the same patent family, there is described a process for preparing curdled milk products, in which whey is acidified by electrodialysis, and the acid whey is used for reducing the pH of milk to form a cottage-cheese-like curd. The application of this process requires complex electrodialysis equipment capable of processing simultaneously at least four different liquids with the help of separate supply and discharge lines. The system then requires approximately the same quantity of acid as required for the direct acidifaction of milk and, in addition, electrical energy is used. The resulting whey has, again, virtually the same ash composition as the skim milk raw material in which a portion of the cations has been repiaced by hydrogen ions.

In Dutch patent application 66,09120, a more direct method for the acidification of milk is described by the same inventor. In it, the milk is contained in a compartment of an electrodialysis cell, which compartment is bounded by two membranes, and in which, under the influence of an applied electrical voltage, hydrogen ions are forced to migrate through the membrane located on the side of the positive electrode to the milk compartment.

A disadvantage of this method is that the acidified milk proteins precipitate on the membrane, and even in the pores thereof, thereby obstructing the passage of current and matter. In a particular embodiment of this process, the hydrogen ions are formed in the cell by the electrolysis of water present around the anode in the anode compartment separated by a membrane. As the required cell comprises both such an anode compartment and a cathode compartment, in which water with a relatively low controlled electrolyte content is contained, and owing to a generally rather unfavourable current efficiency resulting from the migration of anions, this process requires a relatively large amount of electricity with a relatively high resistance, having regard to the quantity of required equivalents of acid.

Also, apparatus is used that is unnecessarily complicated and hence expensive.

It has now been found that the above disadvantages of prior methods for casein precipitation can be avoided by using an electrolytic cell in which the anode and the cathode are separated by at least one ion-selective, semi-permeable membrane, generating hydrogen ions in milk or a liquid derived from milk by electrolysis in the anode compartment of such cell, and using the acid thus formed for acidulating milk or a comparable casein-containing liquid, and, by known methods, processing the acidulated milk or liquid and separating it into curd and whey.

In the electrolysis of salt-containing solutions, it can be achieved, with a suitable selection of electrode materials and applied voltage, that the following net reactions take place at the anode and the cathode, respectively: 2H20 < 2 + 4H+ + 4(e)- 4H2O +4(e) e 2H2 + 40H By separating the anode and cathode compartments by means of an ion-selective semi-permeable membrane, it can be achieved that in the liquid surrounding the anode the pH is decreased, while simultaneously in the liquid surrounding the cathode the pH is increased.

Methods of dissolving electrolyte solutions into alkaline and acidic liquids by electrolysis, and equipment that can be used for such processes are described, for example, in French patent specification 1,324,549, based on a US patent application filed in 1961.

In the process according to the present invention, the reduction in pH of the anode liquid is used for acidulating milk and similar casein-containing liquids, such as skim milk, half-skim milk, whole milk, sweet or slightly-soured buttermilk or a concentrate thereof. This can be effected by directly contacting the casein-containing liquid with the anode.

It is also possible to reduce the pH of a casein-free, milk-derived liquid by electrolysis to such an extent that, after mixing with milk or a like casein-containing liquid, the mixture reaches the isoelectric point. By choosing suitable combinations of acidity, temperature and other conditions as conventional in the production of casein, it can be achieved that the casein precipitates at this isoelectric point in the form of a curd that can be separated from the liquid whey phase using current techniques, and can be processed to casein products. At a low temperature and/or high acidity, however, it is also possible using the invented process to produce a softer type of curd, comparable to the curd serving for the preparation of sour cottage cheese.Thereafter, using techniques conventional in cottage cheese production processes, such a curd can be separated from the whey. Generally speaking, such a curd is unsuitable for the preparation of acid casein, unless special af.ter-treatments can be used to obtain better washability. The whey produced as a second product c n be processed into a whey product in the conventional manner. At temperatures above 700 C, whey protein is denatured, so that in that case the process according to the invention does not produce pure casein, but a co-precipitate, while at the same time a deproteinated whey of little value is produced.

The most important advantage of the process according to the invention is that no acid, or in any case considerably less acid, is required for the production of curd than in prior techniques. As a result, the whey produced as a by-product is of better quality than with most of the conventional techniques, or smaller amounts of salt-containing or acid-containing waste liquids are produced.

For carrying out the process according to the invention, many conventional electrolytic cells provided with a semi-permeable membrane can be used. Ion-selective membranes on the basis of organic polymers having ion-exchange properties have been found to be particularly satisfactory for the purpose. A preferred cell type is of the design used in particular for carrying out electrodialysis processes. In it, the two electrodes consist of flat metallic plates with membranes between them also formed as flat plates, on the basis of an organic polymer having ion-exchange properties. Interposed between electrodes and membranes are flat spacer elements of a somewhat flexible synthetic plastics material which simultaneously provide for as uniform spacing between electrodes and membranes as possible throughout, enclose the liquid compartments between them, and seal these leakage-tight.

These spacer elements and the other parts have cut-outs in register with each other to define passageways for the supply and discharge of the liquid being treated. By pressing the elements of such a cell together by lateral pressure with a suitable construction, there is produced a cell of simple construction, which can be combined in a relatively simple manner with other similar cells to form an electrolytic apparatus of small volume and large active surface area, i.e. with a large processing capacity and low energy consumption.

It has been found that, in an electrolytic cell in which anode and cathode compartments are separated by an anion-selective membrane, milk can be acidulated in the anode compartment by electrolysis until the casein precipitates as a properly dewatering and washable curd.

If the cathode compartment contains a salt solution, whey is produced having the same composition as casein-whey produced by adding the acid corresponding to the salt solution to skim milk. It is only in special cases that this embodiment of the process according to the invention has an advantage over and above conventional techniques, for example, when the price of the acid is considerably higher than that of the corresponding salt.

When a cation-selective membrane is used between the two electrode compartments, during the electrolysis of milk in the anode compartment and of salt in the cathode compartment, cations from the milk will migrate through the membrane to the salt solution. Accordingly, the whey produced after curd separation contains, as compared with skim milk, as many equivalents of cations less as equivalents of acid were required for the reduction in pH. The decrease in ash content thus achieved makes the whey more valuable than acid casein whey, produced after the addition of acid to skim milk. It is also possible, however, for the acid whey or a liquid derived therefrom to be neutralized by means of the hydroxyl ions formed in the cathode compartments.If that is effected in an electrolytic cell equipped with a cationselective membrane, it is possible to produce a neutral whey with substantially the same ash composition as that of skim milk. The same is effected in an electrolytic cell equipped with an anionselective membrane. In that case, the acidulated mixture contains more salt than in case cationselective membranes are used. As a consequence, on the one hand the separated curd will contain more salt and require more intense washing, but on the other hand, the electrical resistance of the cell will be less. Depending on the importance of, and the cost of washing the curd, and depending on the cost of electrical energy, it is therefore possible to choose one of these two possible embodiments of the process according to the invention.

If as many equivalents of hydroxyl ions are added to the separated whey as there are supplied equivalents of hydrogen ions to the milk, it is possible for the neutralized whey to acquire a higher pH than is desirable for most uses. In fact, when the curd is separated, a portion of the acid produced is withdrawn from the mixture, so that the neutralized whey may get a pH higher than that of milk.

This drawback can be avoided by acidulating a portion of the milk by means of electrolysis, whereby a salt solution flows through the cathode compartments. This alkalized salt solution is in this case produc,,d as a by-product that can be used for the production of alkali or as a cleaning agent.

Instead of a salt solution, acid whey of other origin can be cathodically neutralized. By varying the ratio between the number of cells in which the whey produced from skim milk is cathodically neutralized and the number of cells in which a different liquid flows through the cathode compartments, the pH of the neutralized whey can be adjusted to suit requirements.

A different manner of preventing the neutralized whey from acquiring an unduly high pH is to adjust the milk by anodic acidulation to a pH above the isoelectric point of casein. By virtue of this, no casein will be precipitated in the anode compartments, so that the disadvantages of an increase in electrical resistance in the anode compartment owing to the presence of a casein deposit, and the risk of clogging are avoided. After the electrolytic acidulation, so much further acid is added as required to reach the isoelectric point. After the separation of the curd, the whey produced can be neutralized in the cathode compartments of the electrolytic cells used for the acidulation, to a pH value that depends on the pH reached in the anode compartments.The neutral whey thus produced has a higher ash content than the milk serving as a raw material, it is true, but such ash content is still considerably lower than that of whey produced by the most conventional manner by adding acid to skim milk, followed by neutralization.

According to French patent application 2,418,627, milk is also acidulated in apparatus equipped with ion-selective membranes to a pH at which no protein precipitation occurs yet. In this whey, the disadvantage occurring in the comparable process of Dutch patent application 66,09120, i.e. the formation of protein deposits on and in membranes through which hydrogen ions have to migrate, is avoided. In the process according to the present invention, such a migration does not occur, and, indeed, the drawback to be avoided is in this case of a different nature, namely, obstructions of the liquid current and affecting the conductivity of the milk.For that matter, the process of the above French application 2,418,627 concerns electrodialytic metathesis, which requires as much added acid as direct acidulation of milk by the addition of acid, and furthermore also results in as much waste solution of salts as acid is added to the milk by electrodialysis. In the process according to the present invention, such waste solutions are only formed if, at the same time, a corresponding quantity of salt is removed from the milk, that is to say, if a de-salted whey is produced.

The ash content of the whey can be reduced still further in a simple manner by using electrolytic cells each consisting of three compartments. In such a cell, in which the anode compartment is bounded by a cation-selective membrane and the cathode compartment by an anion-selective membrane, there is a concentration compartment between these membranes. Simultaneously with the acidulation in the anode compartment and the neutralization in the cathode compartment, the liquid in this concentration compartment will be enriched with and equivalent quantity of salt formed from cations from the anode liquid and anions from the cathode liquid.

Whereas a neutral casein whey produced in a conventional casein production process by the addition of acid to skim milk contains more salt than the milk raw material, the neutral casein whey produced according to the last-mentioned embodiment of the process according to the present invention will contain approximately as much less salt than the milk.

The afore-mentioned disadvantages of the precipitation of casein in the anode compartment, namely, the increase in electrical resistance and the risk of clogging, can also be avoided by acidulating in the anode compartment a liquid derived from milk, which contains no casein, to a pH below the isoelectric point of casein, and mixing milk with this liquid to form an isoelectric mixture, whereby the casein coaguiates as a curd. This liquid may, for example, be the whey produced after the separation of the curd or whey of different origin. Mother liquor produced after the recovery of lactose from whey or a permeate produced by ultrafiltration of milk or whey, or a concentrate of whey or of one of the other liquids mentioned can also be used for this purpose, provided the whey produced is processed further in a manner in which this admixture has no adverse effect.

The acidulation of milk with a liquid reduced in pH by electrolytic means can be combined with the various embodiments of the processes of the present invention as described hereinbefore. For this indirect acidulation, electrolytic cells with two compartments and a cation-selective membrane or an anion-selective membrane can be used, and it is possible to neutralize the whey produced with the cathodically formed hydroxyl ions, with or without a simultaneous de-salting by using a threecompartment electrolytic cell.

It has been found that, if milk has been directly acidulated by the anodic formation of hydrogen ions to the isoelectric point of casein, the presence of gaseous oxygen in the liquid can be used for a simple separation between curd and whey. If, immediately after leaving the anode compartment, the mixture is passed to a flotation tank with as little mechanical disturbance as possible, the gas remains attached to the curd, which as a consequence comes to float on the whey as a foamy layer. If the flotation tank is equipped with the conventional means for the removal of the liquid and the floating curd, a preseparation between these two phases can be achieved in this way. Using conventional means, such as pressing or centrifuging, the curd can subsequently be further liberated from appendant whey and gas.

The reduction in volume achieved by this flotation separation makes it possible to use smaller centrifuges for the dewatering process, which consume less energy, and to do without the stationary or vibratory sieves otherwise used for this purpose. Comparable processes in which casein is separated by flotation are known, for example, from French patent 1 ,367,739. Unlike the process of the present invention, it is there required to have provisions for dispersing air into the mixture of whey and curd. The process of the present invention makes such provisions superfluous. The many possible embodiments of the process according to the present invention offer the possibility of adapting the process to suit the various requirements that may be imposed on the products, thereby optimizing the cost of equipment and processing.In all embodiments, less acid is used than in comparable prior processes. As a result, the whey products obtained contain less ash than is the case in conventional processes, or in any case the same ash content can be achieved without thereby producing an acid-containing and/or saltcontaining waste liquid.

The process according to the invention is illustrated in and by the following examples.

EXAMPLE I The electrolytic cell used was built up from seven elements having the form of rectangular flat plates. These elements were, in succession: 1. A support plate of synthetic plastics material.

2. An anode plate of platinum-coated titanium.

3. A synthetic plastics plate with a cut-out, provided with supply and discharge ducts, serving as an anode compartment.

4. An anion-selective semi-permeable membrane on the basis of ion exchange resin.

5. A plate of synthetic plastics material similar to (1), serving as a cathode compartment.

6. A cathode plate of stainless steel.

7. A support plate of synthetic plastics material.

The cut-out in the two spacer elements of synthetic plastics material had such a form as to leave a free effective transport surface area of 1 67 cm2 at the electrodes and the membrane. The spacing between anode and membrane was 1.8 cm, that between cathode and membrane 1.2 cm. The seven elements were pressed together to form a leakage-tight unit by lateral pressure on the support plates.

Both electrodes were connected to a source of controllable DC voltage. A buffer vessel of 5 litres was connected to the supply and discharge ducts of the anode compartment. A similar vessel was connected to the cathode compartment. Both vessels were provided with an overflow permitting upon the supply of fresh liquid the excess to flow away in volume. The anode liquid was circulated through the anode compartment at a rate of 800 I/h, the rate of the cathode liquid being 500 I/h. As soon as the liquid in the buffer vessels reached the desired pH by electrolysis, the supply of fresh liquid to the vessel was started at such a rate as to keep the pH constant.

At the commencement of the test, the buffer vessels were respectively filled with fresh neutral skim milk and whey produced by acidulating skim milk with hydrochloric acid to pH 4.6 and separating the precipitated curd. Both liquids were kept at 400 C. After sufficient whey had been produced by the electrolysis of skim milk, this whey was used for maintaining the pH of the cathode liquid. The anode liquid was controlled at pH 4.6, and the cathode liquid at pH 7.0.

The acidulated skim milk from the overflow of the anode buffer vessel was collected in a different vessel. In it, the casein curd floated from the buoyancy of the appendant gas bubbles, so that the separation of whey was possible in a simple manner. The remaining curd lent itself well to being processed to a purified casein using conventional methods of washing, sieving, centrifuging, pressing, and drying.

At an amperage of 1.5 Amp, corresponding to a current density of 9.0 mA/cm2, it was possible to acidulate 1 kg skim milk per hour and neutralize 1.44 kg acid whey to pH 7.0. The electricity supplied per hour corresponds to the formation of 56 meq of hydrogen ions per litre of skim milk. In conventional processes for preparing casein, generally 50 to 60 meq/kg mineral acid or 77 to 111 meq/kg lactic acid is added to the skim milk. In this test the efficiency of the formation of hydrogen ions was more than 90%, calculated on the amount of electricity supplied.

The analyses specified in Table I show that the transmission of electricity between the compartments can be explained predominantly from the migration of chloride ions.

TABLE I Skim milk Acid whey Neutral whey Dry matter wt% 9.16 6.61 6.32 Protein wt% 3.58 0.68 0.65 Ash wt% 0.78 0.78 0.73 Chloride wt% 0.10 0.28 0.16 The electrical resistance of the cell was dependent on the voltage applied as specified in Table II.

TABLE II Voltage Volts 5.7 8.5 11.3 14.2 1 6.9 1 9.6 22.3 24.8 Amperage. Amperes 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 Current density mA/cm2 6.0 12.0 18.0 24.0 29.9 35.9 41.9 47.9 EXAMPLE II Using the same apparatus and the same procedure as in Example I, skim milk was acidulated to pH 5.3 by electrolysis, which did not result in the precipitation of casein. By adding hydrochloric acid to the skim milk acidulated to pH 5.3, casein was caused to precipitate at pH 4.6. After separation of the curd, whey of pH 4.6 was produced, which was neutralized to pH 7.0 by electrolysis in the cathode compartment.

At an amperage of 1 Amp, it was possible to acidulate 1 kg skim milk to pH 5.3 and to neutralize 1 kg whey of pH 4.6 to pH 7.0 per hour. During the electrolysis, the chloride content of the skim milk increased from 0.10 percent by weight to 0.21 percent by weight. This shows that the transmission of electricity through the membrane can be explained predominantly from the migration of chloride ions.

EXAMPLE Ill Using similar apparatus as described in Example I, except that, instead of an anion-selective membrane, a cation-selective membrane was interposed between the electrode compartments, skim milk was acidulated by electrolysis.

At the commencement of the test, the anode buffer vessel was filled with neutral skim milk, and the cathode buffer vessel with a 2 wt percent potassium chloride containing salt solution. As soon as the pH in the anode buffer vessel had decreased to 4.6, the supply of fresh neutral skim milk to the vessel was started to control the pH at that value. The pH in the cathode buffer vessel was controlled between pH 3 and 4 by adding dilute hydrochloric acid. By adding water, the conductivity in this vessel was controlled at 25 mS cm-l. Both liquids were maintained at a temperature of 440 C. The further test conditions were the same as in Example I.

The precipitation and separation proceeded as described in Example I. After washing and drying, the product was casein containing 94% by weight of dry matter, 92% by weight of protein and 1.8% by weight of ash.

At an amperage of 1.5 Amp, 1 kg skim milk could be acidulated to pH 4.6 per hour.

The electrical resistance of the cell depended on the voltage applied as specified in Table Ill.

TABLE Ill Voltage Volts 6.5 9.5 13.4 16.5 19.0 21.8 24.5 Amperage Amperes 1 2 3 4 5 6 7 Current density mA/cm2 6 12 18 24 30 36 42 The analytical results specified in Table IV show that the transmission of electricity between the compartments can be explained predominantly from the migration of sodium, potassium, and calcium ions.

TABLE IV Skim milk Acid whey Dry matter wt% 9.17 6.00 Protein wt% 3.68 0.68 Ash v,,t% 0.72 0.40 pH 6.7 4.6 Sodium wt% 0.04 0.02 Potassium wt% 0.15 0.06 Calcium wt% 0.14 0.08 Chloride wt% 0.11 0.10 An analysis of the salt solution in the cathode buffer vessel as to protein and lactose content showed that the losses thereof from the milk were negligible.

EXAMPLE IV The electrolytic cell used was built up from nine elements having the same shape as described in Example I. Between the two electrode compartments, however, there was additionally provided a concentration compartment of equal form, bounded at the anode side by a cation-selective membrane and at the cathode side by an anion-selective membrane. The thickness of the anode compartment was 1.2 cm, that of the concentration compartment 1.8 cm, and that of the cathode compartment 0.6 cm.

The concentration compartment was connected to a buffer vessel having a capacity of 1.5 litres. The rates of circulation of the liquids were: anode liquid 520 I/h, concentration liquid 250 I/h, cathode liquid 220 I/h. For filling the anode buffer vessel, and later keeping it at pH 4.6, neutral skim milk was used; for filling the cathode buffer vessel, and later keeping it at pH 6.9, electrolytically acidulated whey of pH 4.6 was used. The concentration buffer vessel was initially filled with a 2 wt% potassium chloride containing salt solution. This vessel was not controlled as to pH or conductivity. All liquids were kept at a temperature of 420C.

The precipitation and separation of the curd proceeded as described in Example I.

At an amperage of 1.5 Amp, 1 kg skim milk could be acidulated to pH 4.6 per hour. The analyses specified in Table V show that the transmission of electricity between cathode compartment and concentration compartment can be explained predominantly from the migration of chloride ions and the transmission between the anode compartment and the concentration compartment from the migration of sodium, potassium, calcium, and magnesium ions.

TABLE V Skim milk Acid whey Neutral whey Dry matter wt% 9.08 6.40 6.24 Protein wt% 3.52 0.72 0.70 Ash wt% 0.75 0.45 0.36 pH 6.7 4.6 6.9 Sodium wt% 0.05 0.03 0.03 Potassium wt% 0.17 0.08 0.08 Calcium wt% 0.13 0.08 0.08 Magnesium wt% 0.02 0.01 0.01 Chloride wt% 0.10 0.10 0.03 During the test, the salt content in the concentration compartment increased from 2% by weight to 18% by weight. The pH varied between 5.5 and 6.1.

Per kg of skim milk processed, an average of 3.8 g salt was concentrated in this compartment, 1.6 g of which was in the form of cations from the skim milk, and the balance in the form of anions from the whey.

At a salt content of 2% by weight in the concentration compartment, the resistance of the cell was dependent on the voltage applied, as specified in Table VI.

TABLE Vl Voltage Volts 6.0 8.8 14.6 20.6 26.5 31.6 36.3 Amperage Amperes 0.6 1 2 3 4 5 6 Current density mA/cm2 3.5 6 12 18 24 30 36 The reduction in ash content of the whey that could be achieved in this test and in the tests described in the preceding examples, relative to the ash content of a conventional type of casein whey is apparent from the following Table VII.

TABLE VII Dry Protein Manner of content % of dry acidulation Membranes used wt % content Ash pH H2SO4 addition none 6.4 11.4 12.5 4.6 H2SO4 addition none 6.4 11.4 14.0 7.0 Electrolysis anion-selective 6.6 10.6 11.9 4.6 Electrolysis anion-selective 6.3 10.3 11.5 7.0 Electrolysis cation-selective 6.0 11.3 6.7 4.6 Electrolysis cation + anion-selective 6.4 11.3 7.0 4.6 Electrolysis cation + anion-selective 6.2 11.2 5.8 7.0 EXAMPLE V The electrolytic cell used was as described in Example I. At the commencement of the test, both buffer vessels were filled with whey of pH 4.6, produced by using electrolysis as described in Example Ill. The liquid in both vessels was kept at a temperature of 420C. As soon as the pH in the anode buffer vessel had decreased to 1.2, fresh whey of pH 4.6 was supplied at such a rate as to maintain the pH at a constant value of 1.2.The pH of the cathode liquid was controlled in the same manner to be maintained at 6.8.

Whey of pH 1.2 thus produced was mixed with skim milk at a temperature of 420C, to produce a mixture of pH 4.6. The curd thereby formed was separated by sieving and washed and dried in the conventional manner. The whey remaining after curd separation was used for controlling the pH of the liquids in the electrode buffer vessels.

At an amperage of 1 Amp, it was possible to produce 0.34 kg whey of pH 1.2 per hour, with which 0.68 kg skim milk could be acidulated to pH 4.6. At the same time, 1.1 kg whey of pH 4.6 could be neutralized to pH 6.8.

The analyses listed in Table VIII show that the transport of electricity between the electrode compartments could be explained predominantly from the migration of chloride ions.

TABLE VIII Whey isoel. Acid whey Neutral whey Dry matter wt% 6.43 6.33 6.12 Protein wt% 0.60 0.63 0.60 Ash wt% 0.75 0.73 0.70 pH 4.6 1.2 6.8 Chloride wt% 0.29 0.65 0.17 The electrical resistance of the cell was dependent on the voltage applied as specified in Table IX.

TABLE IX Voltage Volts 5.3 8.2 10.0 12.0 14.0 15.9 17.5 Amperage Amperes 1.0 2.0 3.0 4.0 5.0 6.0 7.0 Current density mA/cm2 6.0 12.0 18.0 24.0 29.9 35.9 41.9 EXAMPLE VI Whey of pH 1.2, produced as described in Example V, was added to fresh skim milk until the pH thereof was 5.3. No curd was precipitated. Subsequently, dilute hydrochloric acid was added to the mixture to adjust the pH to 4.6. The curd thereby formed was separated from the mixture by sieving, and subsequently washed and dried to form casein. A portion of the separated whey was passed to the anode buffer vessel of an electrolytic cell as described in Example I, and the balance to the cathode buffer vessel, whereafter the admixture of skim milk with whey, after-acidulation with hydrochloric acid, separation of curd, and electrolysis were continued in a continuous manner.At an amperage of 1.0 Amp, 0.34 kg whey of pH 1.2 could be produced, with which 1.11 kg skim milk could be acidulated to pH 5.3. Per hour, 1.35 kg whey of pH 4.6 was produced, 1.01 kg of which was neutralized to pH 7.0 by electrolysis. In this way,100 kg skim milk was processed to produce, in addition to the casein, 91.0 kg neutralized whey of pH 7.0 + minor quantities of neutral and acid whey, which together were equal to the quantity of whey with which the test was started (20 litres).

The analysis specified in Table X show that the transport of electricity between the electrode compartments can be explained predominantly from the migration of chloride ions.

TABLEX Whey isoel. Acid whey Neutral whey Dry matter wt% 6.40 6.35 6.20 Protein wt% 0.64 0.65 0.62 Ash wt% 0.76 0.74 0.73 pH 4.6 1.2 7.0 Chloride wt% 0.28 0.66 0.15 The electrical resistance of the cell was dependent on the voltage applied as specified in Table Xl.

TABLE Xl Voltage Volts 5.5 8.2 10.4 12.4 14.3 16.3 18.0 Amperage Amperes 1.0 2.0 3.0 4.0 5.0 6.0 7.0 Current density mA/cm2 6.0 12.0 18.0 24.0 29.9 35.9 41.9 EXAMPLE VII For the acidulation of whole milk, an electrolytic cell was used, formed by a cathode compartment and an anode compartment with a cation-selective membrane between them.

The cathode compartment was a closed synthetic plastics chamber comprising supply and discharge ducts for the circulation of a salt solution from a buffer vessel having a capacity of 5 litres.

The anode compartment was an open-topped synthetic plastics chamber also provided with supply and discharge ducts for the circulation of milk, and also with an overflow for carrying off acidulated product at a rate equal to that at which fresh milk was added to the circulating liquid. Placed in these compartments were a stainless steel cathode and a platinum-coated titanium anode, both having the form of a flat plate.

The liquids circulated through the anode compartment at a rate of 120 I/h and through the cathode compartment at 250 I/h. The temperature was maintained at 200 C, At the commencement of the test, the compartments were respectively filled with whole milk and a 2 wt% potassium chloride containing salt solution. After the application of a voltage of 1 6 V, with a measured amperage of 10 Amp, the supply of fresh milk to the anode liquid was started as soon as the pH decreased to 4.4.

The pH in the cathode liquid was controlled between 3.0 and 4.0 by supplying hydrochloric acid.

The acidulation of 1 kg whole milk required the passage of 59.5 milliequivalents of electricity.

The curd resulting from the acidulation floated owing to the presence of gas bubbles, and flowed out of the anode compartment via the overflow. Whey was separated from the curd by pressing in the manner conventional in the production of cottage cheese.

The pressed curd contained 25.8% dry matter, 9.6% protein and 10.4% milk fat. The product had a structure comparable to that of cottage cheese made by a conventional process. The taste was mildly sour, without further characteristic off-flavours, by virtue of which the curd lent itself well to be processed to a tasteful food by being mixed, for example, with fruit, herbs and the like.

Claims (12)

1. A process for producing curd and whey from milk by means of electrolysis, characterized by using an electrolytic cell in which the anode and the cathode are separated by at least one ion-selective semi-permeable membrane, generating hydrogen ions in milk or a liquid derived from milk by electrolysis in the anode compartment of such cell, and using the acid thus formed for acidulating milk or a comparable, casein-containing liquid, and, by methods known per se, processing the acidulated milk or liquid and separating it into curd and whey.

2. A process according to claim 1, characterized by using an electrolytic cell with an anionselective membrane as the separation between cathode and anode compartment.

3. A process according to claim 1, characterized by using an electrolytic cell with a cationselective membrane as the separation between cathode and anode compartment.

4. A process according to claim 1, characterized by using an electrolytic cell comprising a concentration compartment separated by a cation-selective membrane from the anode compartment, and by an anion-selective membrane from the cathode compartment.

5. A process according to claim 1, 2, 3 or 4, characterized by generating in milk or a comparable casein-containing liquid so many hydrogen ions by the anode reaction as to reach the isoelectric point of casein.

6. A process according to claim 1 , 2, 3 or 4, characterized by generating hydrogen ions in milk or a comparable casein-containing liquid by the anode reaction to a pH value at which no casein precipitation yet occurs, and after electrolysis adding an acid or a liquid having a pH below the isoelectric point of casein so that the mixture acquires the isoelectric pH of casein.

7. A process according to claim 1, 2, 3 or 4, characterized in that, in a casein-free liquid derived from milk, hydrogen ions are generated by the anode reaction to a pH value below the isoelectric point of casein, and this liquid is admixed with milk or a comparable casein-containing liquid so that the mixture acquires the isoelectric pH of casein.

8. A process according to claim 7, characterized in that the casein-free liquid derived from milk is whey produced by the process according to the preceding claims, or a product produced from such a whey.

9. A process according to any of the preceding claims, characterized by generating in the cathode compartment hydroxyl ions in whey produced by a process according to any of the preceding claims, or in a liquid derived from such a whey.

1 0. A process according to claim 5, characterized in that the curd from the acidulated milk is separated from the whey by flotation by means of the gas formed during electrolysis.

11. A process according to claim 1, substantially as hereinbefore described with reference to any of the Examples.

12. Curd or whey produced by a process according to any of claims 1 to 11.