IE914026A1 - Continuous process for washing proteins and an installation¹for carrying out this process - Google Patents

Abstract

The invention relates to a process for the continuous washing of proteins, particularly milk proteins and more particularly casein, consisting in circulating a suspension of the said proteins in one or more tangential filtration modules. It also relates to an installation for the continuous washing of proteins, a protein suspension concentration installation and a casein manufacturing installation.

Description

The invention has as its object a continuous process for washing proteins, in particular milk proteins and more particularly casein, and it also has as its object the installation for carrying out this process.

Proteins are frequently isolated from their natural medium and brought by washing to a state of purity acceptable for use in, for example, human and animal foods, pharmaceuticals or cosmetics or for industrial purposes. This applies particularly to milk proteins and of these particularly casein and whey proteins which are isolated from, respectively, milk and whey by chemical, enzymatic or thermal coagulation, and to proteins of soya beans, wheat, blood and fish, which are also isolated or separated from their natural medium by coagulation, agglomeration or precipitation. As a general rule, the stage of isolation, separation or concentration of the soluble or insoluble proteins obtained must be followed by washing to bring the said proteins to the degree of richness or purity required for their use.

Referring, for example, to casein, its manufacture requires three stages - a stage of precipitation for separating the insoluble curds from the soluble elements of the milk and whey, - a washing stage -a drying stage.

The washing stage is obviously a decisive stage for obtaining as pure a casein as possible; it provides for lft the separation of impurities substantially consisting of lactose, mineral salts, amino acids and soluble proteins.

This washing stage has traditionally been carried out as a discontinuous operation in which the curds obtained from the stage of coagulation are washed several times with water in a vat. It is obvious that these operations of washing by successive emptying and filling use up large quantities of water and that the productivity of such an installation is low.

This process was therefore subsequently Improved in that washing was carried out in a series of vats arranged as a cascade with intermediate transfer and drying of the casein between one vat and the next. The system was also mechanised to operate continuously. Thus the casein progressed down the line, passing successively through the various stages and becoming more and more pure in the process while the wash water , which was Introduced into the last stage, was moved by a set of separators and pumps up the chain from one stage to the next to become progressively charged with impurities. The larger the number of stages used, the more effective is such a system but it was found that with this system 5 stages constituted the limit of vhat was practicable and the efficiency of the installation was then not sufficient to ensure an acceptable quality of end product even when very large quantities of water were used.

It was in fact found that it was difficult with such a system to lower the free acid content to below 0.2%, based on the solids content, and the lactose content to below 0.3%, based on the solids content, in spite of the amount of water used, which varied according to the case from 0.4 to 1 litre of water per litre of milk.

Other methods of washing casein have therefore been developed, among which may be mentioned that described in New Zealand Patent No. 196 624 belonging to the Applicant Company. This method consists of washing the casein in counter current in a multi-stage vertical tower.

This process has enabled the consumption of water to be greatly reduced and it now amounts to only 0.2 to 0.3 litre of water per litre of milk even though the free acid content and lactose content can be reduced to below 0.1%, based on the solids content. However, the volumes of water which are immobilised are unfortunately very large, which entails great expenditure in particular in time and safety measures in the employment of this washing system.

It must also be pointed out that in the processes for washing proteins, in particular casein, the factor of bacterial contamination must be taken into account very seriously and every effort must therefore be made to prevent exposure of the proteins to air as much as possible and to reduce the time of stay of the proteins in the treatment installations. Care must also be taken not to modify the physical characteristics and texture of the proteins, as these properties play an important part in the ultimate functional properties of the product.

The present invention is capable of meeting the various requirements of the art and resolving many of the difficulties still encountered in the washing of insoluble or insolubilised proteins of the type of casein.

The process according to the invention for washing insoluble ' or insolubilised proteins is carried out continuously, requires only a very low consumption of washing fluid, puts very small volumes into use, is quick to carry out and nevertheless can give rise to products of a very high degree of purity compatible with their use for pharmaceutical or food purposes.

This process consists of continuously circulating a protein suspension, in particular a suspension of milk protein and more particularly of casein, in one or more tangential filtration units each having at least one axial channel equipped with or consisting of filtration elements in which the suspension circulates and which is traversed by a flow of washing fluid entering and leaving.

The said flow of washing fluid is most generally established perpendicularly to the direction of circulation of the protein suspension.

The protein suspension is most generally an aqueous suspension but may also contain solvents such as hydrocarbons, ethers or alcohols, the latter being preferred.

The washing fluid is preferably water or an aqueous alcoholic solution but may in certain cases consist of some other solvent capable of solubilising or carrying away the impurities present in the protein.

The protein to be purified is preferably in suspension in water and the washing fluid is preferably water.

The invention will be better understood with the aid of the description given below and the attached drawings but it must be understood that this part of the description and the drawings are given mainly by way of example.

Figure 1 represents a filtration unit according to the invention. This consists of a supporting structure 1 to which the filtration unit 2 is attached. The suspension of insoluble protein, in particular casein, is supplied at 3 at a particular flow rate W. This protein suspension is removed at 4 and conveyed by a pump 8 into the filtration unit where it is carried tangentially to the filtering medium inside the axial channel 9. Washing is carried out by diafiltration, the washing fluid entering the circulation loop of the protein suspension at 5 while the water charged with soluble impurities is removed at 6 by means of a pump 10.

Efficient diaf iltration is ensured by maintaining the circulation rate in the filtration unit at a very much higher value than the flow rate W at which the protein suspension enters. The protein suspension therefore makes several turns through the loop in the unit. Thus in the Figure shown, the protein suspension ascends through the axial channel of the filtration unit. 0 In one advantageous embodiment of the process according to the invention and in order to obtain the desired degree of purity, the protein suspension is circulated through a plurality of filtration units mounted in series. The kinetics of desorption of the soluble impurities contained in the protein which is to be purified in most cases requires a sufficient time of stay in the filtration units, which in the case of casein can generally only be achieved by using at least two filtration units.

The number of units depends on the ratio of concentration 0 of the contaminating elements in the protein between the inlet and outlet of the washing installation and on the ratio of the rate of flow of washing fluid to the rate of flow of the protein suspension.

Figure 2 shows schematically a continuous protein washing installation comprising six filtration units arranged in series. The protein to be purified enters the washing system from 11 to reach the first unit, where it is removed to enter the first loop while the washing fluid arrives in counter-current and the protein is thus purified at the filtering element 12 of this first unit.

The protein suspension is then conveyed to the second unit, where it undergoes further purification by diafiltration with washing fluid circulating in the second loop.

The protein suspension thus successively enters the six filtration units in which it is progressively purified in contact with the washing fluid circulating in counter current and it leaves at 13 to be conveyed to the drying stage.

The washing fluid is injected into the circuit at 14 and circulates in counter current through each unit to become progressively charged therein with soluble impurities and leaves at 15. It may then be filtered for recovery of the protein fines carried along in the course of the washing process.

The tangential or axial velocity of circulation of the protein suspension in the axial channel is advantageously above 1.0 m/s, preferably from 1.1 to 10.0 m/s and more preferably from 2 to 3 m/s. If the velocity is too low, problems of sedimentation of the protein suspension may arise, whereas if the imposed velocity is too high there is a risk of increased loss of proteins passing through the filtering medium. The velocity of the suspension must, moreover, be sufficient to produce shearing forces to limit the amount of clogging of the filtering medium at the interface between itself and the protein suspension.

The axial channel or channels provided in each tangential filtration unit has or have a predetermined size so that the washing system can operate under satsifactory conditions. The dimension of the passage through this channel or these channels is preferably from 1 to 20 mm, more preferably from 7 to 13 mm. The form of this channel may be cylindrical or parallelepipedic in the former case, the dimension in question is obviously the diameter of the channel, and in the second case it is the distance separating the two surfaces carrying or consisting of the filtration elements.

The mesh size of the filtration elements which are provided on or, preferably, constituted by the axial channel or channels present in each unit is from 5 to 100 pm, preferably from 10 to 80 pm, more preferably from 30 to 70 pm. If the mesh size is too small, there is a risk of the filtering elements becoming clogged and a risk of increased transverse loss of charge. The mesh size must, however, be less than the particle size of the protein in suspension in order to prevent clogging. Conversely, if the mesh is too large, there is an increased loss of fines.

The filtration elements may, for example, consist of JOHNSON (Trade Mark) grids having the chosen mesh size and cut to the required dimensions and then arranged inside the units.

The rate of replenishment of washing fluid entering the washing system is generally from 5 to 100% and preferably from 10 to 80% of the rate of flow of the protein suspension.

In the case of casein, the rate of supply of water 5 is from 2 to 20%, preferably from 5 to 15% and more preferably of the order of 10% of the rate of flow of milk entering the coagulation installation.

The solids content of the protein suspension introduced into the washing installation is from 1 to 35%, preferably from 5 to 20%. In the case of casein, the solids content in question is preferably from 10 to 15%.

An increase in temperature increases the percolation but overheating should be avoided to prevent denaturing of the protein. The various stages of the washing installation may all be maintained at the same temperature or a temperature gradient may b?e established so that the temperature increases or decreases from the first unit to the last. The temperature gradient is preferably an increasing gradient, the protein suspension being raised to a temperature from 35 to 55°C, preferably from to 45 to 50°C, before entering the first washing unit and being then introduced into the last unit at a temperature above or equal to 65 *C, preferably 70*C, to bring about pasteurisation. This may easily be carried out by heating the washing fluid circulating in the last filtration unit, for example by steam injection.

In order to prevent solubilisation of the protein, the pH of the protein suspension and/or of the washing fluid is adjusted to a value close to the isoelectric point of the said protein, generally by the addition of acid. For acid casein, for example, hydrochloric acid is added to the wash water to adjust the pH from 4-4 to 5.0.

The loss of protein fines is evaluated from the concentration of proteins in the washing fluid leaving the installation. The loss increases with the ratio of the quantity of washing fluid to the quantity of protein suspension. Since this level is very low in the process according to the invention, the loss of fines is also very low. Thus for casein, the ratio of the quantity of water to the quantity of milk being generally close to 0.1, the loss of fines is only of the order of 4 to 5% before clarification.

In general, the trans-medium pressures required to ensure the correct flows inside the circulation are from 3.103 to 12.103 pascals, preferably from 5.103 to 1.104 pascals.

Typical operating pressures of an installation according to the ivention are as follows: - input pressure: from 1.104 to 2.5.104 pascals, - pressure at entrance to units: 1.104 to 4.104 pascals, - pressure at exit from units: 1.104 to 2.104 pascals, - entry pressure of clean washing fluid: approximately 1.104 pascals, - pressure of washing fluid between loops: identical to the input pressure, - pressure of extraction of charged washing fluid: approximately 0 to 1.104 pascals.

The continuous protein washing installation according to the invention may equally well be used for concentrating the protein suspension. In that case, no washing fluid is injected into the circulation. The protein suspension is concentrated by tangential filtration in the unit or units, part of the fluid, which is generally water, in which the protein is in suspension, being removed through the filtering medium and withdrawn from the circulation by pumps. The concentration of the protein suspension may in this way be increased by a factor of 1.2/1 to 15/1. Thus in the case of casein, for example, the concentration of casein can be increased from 2.7% to 14% when three units arranged in series are used. This stage of concentration is preferably carried out at a temperature from 20 to 50*C, more preferably from 20 to 40’C.

Figure 3 shows a complete installation for the preparation of purified casein from milk, this installation comprising a coagulation stage, a stage for concentration of the curds obtained by the passage through the three filtration units without the injection of water, and a washing stage using six filtration units, water being injected into the said washing stage and the casein obtained being then concentrated and dried.

The protein washing installation according to the invention is characterised in that it comprises at least one filtration unit having at least one axi al channel equipped with or consisting of filtration elements, means for carrying the protein suspension into the said axial channel and causing it to circulate therein, means for introducing a washing fluid into and circulating it in the said axial channel, and means for producing a circulation loop between the entrance and exit of the said filtration unit.

Vilien the washing installation according to the invention 0 comprises a plurality of filtration units, these units are arranged in series, circulating means are established between each of them, and the protein suspension is introduced into the first unit and the washing fluid into the last unit.

The invention also relates to an installation for the concentration of a protein suspension, the said installation being characterised by the fact that it comprises at least one filtration unit having at least one axial channel equipped with or consisting of filtration elements, means for introducing the protein suspension into the said axial channel and circulating it therein, and means for producing a circulation loop between the entrance and exit of the said filtration unit.

When the installation for concentration according to the invention comprises a plurality of filtration units, these are arranged in series.

Lastly, the invention relates to an installation for the manufacture of casein, characterised by the fact that it comprises a milk coagulating stage, a stage for concentration of the curds thus obtained, consisting of a concentrating installation according to the invention, and a stage for washing the casein, consisting of a washing installation according to the invention.

The invention may be better understood with the aid of the following Examples which relate to advantageous embodiments.

EXAMPLE 1 This Example illustrates the influence of the ratio of washing water to the rate of flow of milk on the efficiency of casein washing.

Four tests were carried out on an installation comprising two filtration units arranged in series. The filtering element mounted in each unit is a JOHNSON (Trade Mark) sieve having a mesh size of 50 μη. In each unit, two sieves 10 cm in width and 1 m in length are mounted parallel to one another on the framework with a space between them of 10 mm. The surface for percolation is thus 0.2 m2 per unit.

The operating conditions of the tests are summarised in Table 1 below.

TABLE 1 TEST 1 TEST 2 TEST 3 TEST 4 Overall flew rate of suspension 2760 kg/h 2700 kg/h 1080 kg/h 510 kg/h casein cancentratian 78 q/tog 76 g/kg 88 g/kg 104 g/kg Flew rate of casein (M.S:100%) 215 kg/h 205 kg/h 95 kg/h 53 kg/h Flow rate of milk equivalent to flew rate of casein 7700 1/h 7700 1/h 3450 1/h 1890 1/h Flow rate of wash water 1000 1/h 1000 1/h 1000 1/h 1000 1/h Ratio of wash water to flew rate of milk 0.13 0.13 0.29 0.53 Flew rate in loep 12000 1/h 32000 1/h 12000 1/h 12000 1/h Velocity in loeps 2.5 n/s 2.5 m/s 2.5 m/s 2 .5 m/s Transfer flew rate 5000 1/h/m2 5000 1/h/m2 5000 l/lVin2 5000 1/h/m2 Tenperature 46*C 46’C 48 *C 47’C PH 4.6 4.6 4.72 4.8 Table 2 belcw shews the results obtained in the course of these tests concerning the efficiency of washing TABLE 2 TEST 1 TEST 2 TEST 3 TEST 4 LACTOSE IN CASEIN (in terms of 100% of M.S.) Inlet washing cancentratian Cin 0.065 kg/kg 0.063 kg/kg 0.056 kg/kg 0.057 kg/kg CXitlet washing concentraticn Cf 0.037 kg/kg 0.040 kg/kg 0.017 kg/kg 0.009 kg/kg Cin Cf 1.7 1.57 3.3 6.3 LACTOSE IN SUPERNATANT UQUID PHASE Inlet washing concentration Cin 15.0g/kg 14.7 g/kg 14.7 g/kg 13.7 g/kg Outlet washing concentration Cf 10.4 g/kg 10.5 g/kg 5.1 g/kg 2.8 g/kg £in Cf 1.49 1.4 2.88 4.89 ASH CONTENT IN SUPERNATANT LIQUID PHASE Inlet washing 3.3 g/kg 3.2 g/kg 3.0 g/kg 2.9 g/kg Outlet washing concentration Cf 2.1 g/kg 2.2 g/kg 1.2 g/kg 0.7 g/kg Cin Cf 1.57 1.5 2.5 4.14 less of fines (% casein produced at 10O% M.S) 5.5% 4.5% 9.0% 14.0% As shown in Table 2, the casein washing process according to the invention provides for very substantial removal of impurities from the protein, particularly in the case of tests 3 and 4. The loss of fines observed in tests 1 and 2 is very slight, and this obviously constitutes a main feature of interest of the process according to the invention. The loss of fines is higher in tests 3 and 4 but is still not representative of the kind of losses which would be observed in an industrial installation comprising a larger number of filtration units. In the present case, the losses are due to increases in the flow rate of wash water, which were fixed for obtaining greater purity of the protein in this installation comprising only two units.

EXAMPLE 2 Tests for the concentration of a casein suspension are carried out in the same installation as that described in Example 1. The speed of circulation in the loops is 2.5 metres per second. The speed of inflow of casein is 1980 litres per hour, which is the equivalent of about 6300 litres of milk per hour. The rates of extraction of serum'’ in a first test 5 are 400 litres per hour in the first loop and 600 litres per hour in the second loop, and in a second test 6 they are 500 litres per hour in the first loop and 450 litres per hour in the second loop.

The input pressure of the installation is from 1.6.104 to 2.1.104 pascals. The temperature for the two tests is 44*C and the pH is 4.65. The results and concentration balances for these two tests are indicated in Table 3 below.

TABLE 3 TEST 5 Entrance concentration Unit 1 9.15% Exit concentration Unit 1 9.95% (-entrance concentration of unit 2) Exit concentration Unit 2 14.60% TEST 6 8.45% 9.81% .60% This Table shows that the concentration of the protein suspension can be very considerably increased by means of the concentration installation according to the invention.

The same final concentration may be obtained by starting with a casein suspension, taken at the outlet of the coagulation tube and having a concentration of about 2.7 %.

Claims (14)

1. continuous process for washing protein, in particular milk protein and more particularly casein, characterised by the fact that it consists of circulating a suspension of 5 the said protein through one or more tangential filtration units each having at least one axial channel equipped with or consisting of filtration elements In which the suspension circulates and which is traversed by a flow of washing fluid entering and leaving substantially 10 perpendicularly to the direction of circulation of the said suspension.

2. Continuous protein washing process according to claim 1, characterised by the fact that the tangential velocity of circulation of the protein suspension in the axial 15 channel is above than 1.0 m/s, preferably from 1.1 m/s to 10.0 m/s and more preferably from 2 to 3 m/s.

3. Process according to one of the claims l or 2, characterised by the fact that dimension of the passage through the axial channel is from 1 to 20 mm, preferably 20 from 7 to 13 mm.

4. Process according to any one of claims 1 to 3, characterised by the fact that the mesh size of the filtration elements is from 5 to 100 gm, preferably from 10 to 80 gm and even more preferably from 30 to 70 gm. 25 5. Process according to any one of claim 1 to 4, characterised by the fact that the flow rate of addition of washing fluid entering the washing system is from 5 to 100%, preferably from 10 to 80% of the flow rate of the protein suspension. 30 6. protein washing installation, characterised by the fact that it comprises at least one filtration unit having at least one axial channel equipped with or consisting of filtration elements, means for conducting a protein suspension to the interior of the said axial channel and

5. Circulating it therein, means for conveying a washing fluid into the said axial channel and circulating it therein, and means for producing a circulation loop between the entrance and exit of the said filtration unit.

6. 7. Washing installation according to claim 6, 10 characterised by the fact that it comprises several filtration units arranged in series with circulating means established between each of them, the protein suspension being conveyed to the inside of the first unit and the washing fluid into the last unit, these two fluids 15 circulating in counter-current.

7. 8. Installation for the concentration of a protein suspension, characterised by the fact that it comprises at least one filtration unit having at least one axial channel equipped with or consisting of filtration elements, means 2 0 for conducting the protein suspension to the inside of the said axial channel and circulating it therein, and means for producing a circulation loop between the entrance and exit of the said filtration unit.

8. 9. Installation for the concentration of a protein 25 suspension according to claim 8, characterised by the fact that it comprises several filtration units arranged in series.

9. 10. Installation for the washing or concentration of a protein suspension according to any one of claims 6 to 9, 3 0 characterised by the fact that the largest dimension of the axial channel is from 5 to 15 mm and more preferably from 7 to 13 nun and that the mesh size of the filtration elements provided on or constituting the said axial channel is from 5 to 100 /xm, preferably from 10 to 80 μιη, even more preferably from 30 to 70 pm. 5 ll. Installation for the manufacture of casein, characterised by the fact that it comprises a milk coagulating stage, a stage for the concentration of the curds obtained, consisting of an installation for concentration according to any one of claims 8 to 10, and a 10 casein washing stage consisting of a washing installation according to any one of claims 6, 7 or 10.

10. 12. A continuous process according to claim 1 for washing protein, substantially as hereinbefore described and exemplified.

11. 13. A washed protein whenever obtained by a process claimed in any one of claims 1-5 or 12.

12. 14. A protein washing installation according to claim 6, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

13. 15. An installation according to claim 8 for the concentration of a protein suspension, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

14. 16. An installation according to claim 11 for the manufacture of casein, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.