Abstract:
The invention relates to a process for removing at least a portion of salts contained in whey or a liquid based on whey by means of electrolysis while producing an acid mixture and a base mixture useful in food industries. In the process a four-compartment electrolytic cell is used in the electrolysis of the whey or the liquid based on whey. The four compartments between an anode and a cathode are separated from each other by means of ion exchange membranes and an independent liquid recirculation is connected with each one of the four compartments of the electrolytic cell.

Description:
BACKGROUND OF THE INVENTION 
     A process for the demineralization of whey or a liquid based on whey and for producing an acid mixture and a base mixture useful in food industries 
     The invention relates to a process for removing by electrolysis at least a portion of salts contained in whey or a liquid based on whey while producing an acid mixture and a base mixture useful in food industries. The invention is further concerned with products obtained by means of the process, particularly with an acid mixture and a base mixture useful in food industries. 
     The high salt content of whey obtained as a side product in the production of cheese and casein is the main reason why whey cannot be used in foodstuffs. However, demineralized whey powder has many uses in baby foods, ice-cream and bakery industries. Demineralization of whey has long been carried out by electrodialysis and ion exchange. Such techniques have also been combined for improving the efficiency and economy of the demineralization process (see e.g. British Patent Specificiation 1,583,814). It is typical of all the techniques mentioned above that the salts contained in whey as well as small amounts of the other whey solids, i.e. lactose and proteins, get into the waste water. Waste water treatment increases considerably the cost of the demineralization process and may even prevent the introduction of the process. Since waste waster regulations are becoming increasingly strict, attempts have been made to solve this problem. Recently, a new demineralization process based on ion exchange has been developed in which part of the regeneration chemicals can be reused (see e.g. U.S. Patent Specification 4,159,350). However, this process, to, produces considerable amounts of waste water, so there is still a need of new techniques by means of which waste water problems are avoided or at least substantially reduced. 
     On the other hand, it has long been known that a salt solution can be rendered to a corresponding acid and base in an electrolytic cell divided into four compartments by means of cation and anion exchange membranes (see e.g. French Patent Specification 1,324,549). This techique, however, has been applied only to the treatment of pure inorganic and organic salts. Electrolysis as such has also been used experimentally in the treatment of milk (see e.g. British Patent Specificiation 2,100,289 A); however, the cell structure described in the examples has thereby comprised two or three compartments and the aim has not been to demineralize milk but to adjust its pH. 
     SUMMARY OF THE INVENTION 
     It has now been unexpectedly discovered that problems caused by salt-containing waste waters formed in the demineralization of whey, such as cheese whey or acid whey, or a liquid based on whey, such as permeate or mother liquor from the production of lactose, are avoided when the demineralization is carried out by means of a four-compartment electrolytic cell. At the same time the anions and cations of salts contained in the whey or the liquid based on whey are recovered. The anions are recovered in an acid circulation solution which is thus composed of an acid mixture. Correspondingly, the cations are recovered in a base circulation solution which is thus composed of a base mixture. Said circulation solutions can be used as acids and bases in the food industries, for instance. In this way it is e.g. possible to produce acid and base from whey while it is demineralized. 
     As an electrolytic raw material, whey is in many respects more problematic than a pure salt solution. Whey contains proteins which precipitate easily especially close to their isoelectric point (pH 4 to 5). In addition, cheese whey always contains so called &#34;cheese dust&#34;, i.e., finely divided casein precipatate. Whey further contains residual fat and relatively plenty of calcium which precipitates easily at a high pH in particular. When demineralizing whey in an electrodialysis apparatus, for instance, it is necessary to continuously add hydrochloric acid to the salt solution side so as to prevent calcium precipitation. Therefore it is highly surprising that no precipitation problems could be observed in the whey electrolysis tests and that the demineralizing efficiency was not substantially dependent on the pH of the whey, as appears from the examples set forth below. 
     The process according to the invention, in which at least a portion of salts contained a liquid selected from whey and liquids based on whey is removed by electrolysis while producing an acid mixture and a base mixture useful in food industries, comprises using a four-compartment electrolytic cell in the electrolysis of the liquid selected from whey and liquids based on whey, the electrolytic cell comprising in succession (a) an anolyte compartment defined by an anode plate and a first cation exchange membrane, (b) an acid compartment defined by the first cation exchange membrane and an anion exchange membrane, (c) a whey compartment defined by the anion exchange membrane and a second cation exchange membrane and intended for the liquid to be treated which is selected from whey and liquids based on whey, and (d) a base compartment defined by the second cation exchange membrane and a cathode plate, independent liquid recirculations being connected to each one of the four compartments of the electrolytic cell; whereby the starting solution used in the liquid recirculation connected with the whey compartment is a liquid selected from cheese whey, acid whey, permeate and a mother liquor from the production of lactose, each as such, and from cheese whey, acid whey, permeate and a mother liquor from the production of lactose, each as concentrated to a dry matter content of no more than 40% by weight; the starting solution used in the acid recirculation connected with the acid compartment is a liquid selected from acid solutions having an anion composition correspondng to that of the liquid to be treated which is selected from whey and liquids based on whey, and lactic acid; the starting solution used in the base recirculation connected with the base compartment is a liquid selected from base solutions having a cation composition corresponding to that of the liquid to be treated which is selected from whey and liquids based on whey, and sodium hydroxide; and the starting solution used in the anolyte recirculation connected with the anolyte compartment is an aqueous solution of an acid. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING. 
     The drawing shows a schematic view of the use of a four-compartment electrolytic cell according to the invention in the demineralization of whey and simultaneous production of acid and base. 
    
    
     DETAILED DESCRIPTION 
     The electrolytic cell 1 comprises in succession an anolyte compartment A defined by an anode plate 2 and a first cation exchange membrane 3, an acid compartment B defined by the first cation exchange membrane and an anion exchange membrane 4, a whey compartment C defined by the anion exchange membrane and a second cation exchange membrane 5 and intended for the whey or the liquid based on whey which is to be treated, and a base compartment D defined by the second cation exchange membrane and a cathode plate 6. 
     The anode can be formed by conventional plate electrodes used in electrolytic cells, such as a platinated titan electrode. The cathode can correspondingly be formed by conventional plate electrodes used in electrolytic cells, e.g. of stainless steel. 
     The following net reactions occur at the anode and the cathode: ##STR1## 
     The ion exchange membranes can be conventional ion-selective semi-permeable cation exchange membranes and anion exchange membranes used in electrodialysis. In the cell the ion exchange membranes and the electrode plates are assembled into a stack in which the ion exchange membranes are separated from each other and/or from the electrode plates by means of conventional spacers. Individual cells can be connected in series and/or in parallel. 
     A liquid recirculation 7 is connected with the whey compartment C, an acid recirculation 8 is connected with the acid compartment B, a base recirculation 9 is connected with the base compartment D, and an anolyte compartment B. 
     The independent liquid recirculations connected with the different compartments of the cell are provided with separate pumps 11, containers 12 and outlets. The recirculations, particularly the acid, base and anolyte recirculations, can be connected with the respective recirculations of other cells. 
     The process according to the invention can be carried out in a batchwise manner, whereby the demineralized liquid and the resultant acid and base solutions are recovered. The process can also be carried out continuously, whereby the recirculation of the liquid to be demineralized can be continuously supplied with fresh liquid while demineralized liquid is being removed from the circulation. The acid and base recirculations can be correspondingly made continuous. 
     When several cells are used the recirculations can be connected with the respective recirculations of another cell or other cells. 
     In the anolyte recirculation connected with the anolyte compartment, the starting solution is an aqueous solution of an acid, preferably an aqueous solution of a 0.5 to 3% by weight acid. The most suitable anolyte solution is an aqueous solution of 1% sulphuric acid. 
     In the acid recirculation connected with the acid compartment, the starting solution is an acid solution having an anion composition corresponding to that of the liquid to be treated which is selected from whey and liquids based on whey, or lactic acid. Preferably, said acid solution is used as a starting liquid in the acid recirculation means. 
     When an acid solution has an anion composition corresponding to that of whey, its anion composition resembles that of whey and preferably is similar to that of whey. 
     In the base recirculation connected with the base compartment, the starting solution is a base solution with a cation composition corresponding to that of the liquid to be treated which is selected from whey and liquids based on whey, or sodium hydroxide. Preferably, said base solution is used as a starting solution in the base recirculation. 
     When a base solution has a cation composition corresponding to that of whey, its cation composition resembles that of whey and preferably is similar to that of whey. 
     In the liquid recirculation connected with the whey compartment, the starting solution is e.g. cheese whey, acid whey, permeate obtained from the ultrafiltration of whey, or mother liquor from the production of lactose as such or concentrated to a dry matter content of no more than 40% by weight. 
     A suitable process temperature is 0° to 60° C., preferably 20° to 50° C. 
     The process according to the invention is particularly suitable for use in the demineralization of whey. The composition of typical cheese whey is shown in the following Table 1; the composition of acid whey is nearly similar to that of cheese whey. 
     
                       TABLE 1______________________________________Composition of typical cheese whey______________________________________Dry content  6.0%         by weightLactose      4.5%         &#34;Protein      0.7%         &#34;Fat          0.1%         &#34;Ash          0.5%         &#34;Calcium      350          mg/1Magnesium    90           &#34;Sodium       450          &#34;Potassium    1400         &#34;Phosphor     450          &#34;Chloride     1000         &#34;Lactic acid  900          &#34;Citric acid  1400         &#34;______________________________________ 
    
     The cation and anion composition of whey appears from Table 1. 
     In the electrolytic cell shown in the drawing, the cations contained in the whey recirculation, such as K +   and Na +   ions, migrate through the cation exchange membrane into the base recirculation, where they form hydroxides with OH -   ions formed at the cathode, so that the base recirculations is concentrated and the amount of hydroxides corresponding to these particular ions is increased in the base recirculation. If an aqueous solution of sodium hydroxide is used as a starting solution in the base recirculation, it is converted to a mixture of the aqueous solutions of potassium, sodium, calcium och magnesium hydroxides during the process. At the same time anions contained in the whey recirculation, such as Cl -   and PO 4   3-   ions, migrate through the anion exchange membrane into the acid recirculation, where they form corresponding acids with H +   ions migrated into the acid recirculation from the anolyte recirculation through the cation exchange membrane, so that the acid recirculation is concentrated. If an aqueous solution of lactic acid is used as a starting solution in the acid recirculation, it is converted into a mixture of the aqueous solutions of citric acid, lactic acid, hydrochloric acid and phosphoric acid during the process. 
     Being physically separated from a foodstuff, the base and acid mixtures forming the base recirculation and the acid recirculation can obviously be regarded as suitable for use in foodstuffs as such. Said acid and base mixtures could thus be used e.g. in washings and pH adjustments at the same plant or they could be sold to other food manufacturers after concentration, if necessary. 
     Demineralized liquid, such as whey, is recovered as such or is concentrated and dried into a powder for further use. 
     Whey is preferably concentrated before electrolysis for improving conductivity and the demineralizing efficiency dependent thereon. 
     The process according to the invention can also be used in the treatment of liquids based on whey and having the same salt composition as whey. Such liquids include the ultrafiltration permeate of whey and the mother liquor formed in the production of lactose. Whey proteins are recovered in the ultrafiltration of whey while permeate is obtained as a side product containing the ingredients which have passed through the ultrafiltration membrane, i.e., lactose, whey salts and low-molecular nitrogen compounds. In the lactose manufacture, lactose is removed from whey by crystallization, thus obtaining a mother liquor containing the whey salts, for instance. These possibilities are also described in the examples, where the percentages are percentages by weight. 
     EXAMPLE 1 
     The tests were carried out with a Stackpack electrolysis apparatus of Ionics Inc., comprising 18 four-part electrolytic cells. The effective membrane area of each cell was 232 cm 2 , the effective membrane area of the whole stack being 0.42 m 2 . The tests were carried out in a batchwise manner so that the whey, acid and base compartments formed separate recirculations with their pumps and containers; in addition, the anolyte compartment comprises separate recirculation in which 1% sulphuric acid was circulated. 3% lactic acid was used as a starting solution in the acid recirculation and 3% sodium hydroxide in the base recirculation. 20% cheese whey concentrate having a pH of 5.3 was used as a raw material. Table 2 shows the parameters of the test and Table 3 the results obtained from the test. 
     
                                           TABLE 2__________________________________________________________________________Parameters in the demineralization of cheese whey concentrate (pH 5.3)when using electrolysis                            FlowTime    Temperature      Current/cell             Voltage/cell                     Current density                            whey                                acid                                   base                                      anolyte(min)    (°C.)      (A)    (V)    (A/m.sup.2)                            (l/min)__________________________________________________________________________0   24     4.2    5.7    180     1.6 2.5                                   5.3                                      5.310  28     3.7    5.8    160     0.7 1.3                                   3.1                                      3.020  31     3.4    6.0    150     0.8 1.3                                   3.3                                      3.130  34     3.0    6.2    130     0.8 1.4                                   3.3                                      3.140  36     2.5    6.3    110     0.9 1.4                                   3.4                                      3.250  38     1.8    6.5    80      1.0 1.4                                   3.5                                      3.3__________________________________________________________________________ 
    
     
                       TABLE 3______________________________________Results from the demineralization of cheesewhey concentrate (pH 5.3) when using electrolysis       Time (min)       0    10     20     30    40   50______________________________________Whey recirculationDry matter (°Brix)         19.0   18.5   18.2 18.1  17.9 17.6Conductivity (mS/cm)         9.5    4.3    3.0  2.0   1.2  0.7Phosphor, soluble(mg/kg)       830    550    390  280   200  140Chloride (g/kg)         3.7    1.1    0.65 0.35  0.18 0.10pH            5.3    5.2    5.0  4.8   4.6  4.3Ash (%)       1.5                           0.24Calcium (mg/kg)         1100                          290Magnesium (mg/kg)         230                           74Sodium (mg/kg)         1600                          320Potassium (mg/kg)         4400                          430Protein (%)   2.5                           2.6Lactose (%)   13.7                          13.8Solution volume (1)         7.3                           6.7Acid recirculationTitratable acidity(% lactic acid)         2.2    2.8    3.1  3.3   3.4  3.5Conductivity (mS/cm)         3.3    22     28   31    33   34pH            2.2    1.4    1.3  1.2   1.2  1.2Phosphor, soluble(mg/kg)       10     150    260  380   470  540Chloride (g/kg)         0.12   2.2    2.9  3.2   3.3  3.3Solution volume (1)         6.1                           8.1Base recirculationTitratable acidity(% sodium hydroxide)         2.4    2.6    2.7  2.9   3.1  3.2Conductivity (mS/cm)         74     78     83   88    90   93Solution volume (1)         4.6                           4.6______________________________________ 
    
     The current density was adjusted at the beginning of the run to 4.2 A/cell and it was not adjusted during the run. The apparatus did not comprise temperature adjusting means, wherefore the temperature of the solutions rose during the run. As appears from Table 3, the whey salts are removed similarly as in conventional demineralization techniques, simultaneously obtaining acid and base. Lactose and protein do not migrate through the membranes to any greater extent. 
     EXAMPLE 2 
     Example 1 was repeated except that the pH of the whey concentrate was adjusted for run (a) prior to the run to 6.3 and for run (b) prior to the run to 4.3. These changes did not substantially affect the results. 
     EXAMPLE 3 
     Concentrated acid whey obtained from the production of casein was used as raw material. Table 4 shows the parameters and Table 5 the analysis results. With the exception of the ray material, the conditions were the same as in Example 1. 
     
                                           TABLE 4__________________________________________________________________________Parameters in the demineralization of an acid whey concentrate (pH 4.1)when using electrolysis                            FlowTime    Temperature      Current/cell             Voltage/cell                    Current density                            whey                                acid                                   base                                      anolyte(min)    (°C.)      (A)    (V)    (A/m.sup.2)                            (l/min)__________________________________________________________________________0   27     4.2    5.3    180     1.6 2.5                                   5.4                                      5.410  32     4.0    5.5    170     0.7 1.3                                   3.0                                      3.020  35     3.4    5.5    150     0.8 1.4                                   3.0                                      3.030  37     2.7    6.0    120     0.9 1.4                                   3.1                                      3.040  39     1.8    6.3    80      1.0 1.4                                   3.0                                      3.1__________________________________________________________________________ 
    
     
                       TABLE 5______________________________________Results from the demineralization of acid wheyconcentrate (pH 4.1) when electrolysis was used       Time (min)       0     10      20     30    40______________________________________Whey recirculationDry matter (°Brix)         15.8    14.9    14.6 14.1  13.9Conductivity (mS/cm)         12.3    6.7     3.6  1.4   0.5Phosphor, soluble(mg/kg)       1200    1090    870  550   200Chloride (g/kg)         9.1     4.1     1.9  0.55  0.12pH            4.1     4.1     4.0  4.2   4.3Solution volume (1)         3.2                        2.8Acid recirculationTitratable acidity(% lactic acid)2.2         3.0     3.5     3.7  3.9Conductivity (mS/cm)         2.2     20      28   34    35pH            2.3     1.4     1.2  1.1   1.1Phosphor, soluble(mg/kg)       9       43      100  220   390Chloride (g/kg)         0.1     3.1     4.5  5.4   5.8Solution volume (1)         5.5                        5.6Base recirculationTitratable alkalinity(% sodium hydroxide)         2.5     2.7     2.8  2.9   3.0Conductivity (mS/cm)         63      66      68   70    71Solution volume (1)         4.1                        4.0______________________________________ 
    
     EXAMPLE 4 
     Example 3 was repeated except that the raw material used was concentrated acid whey. The pH of the concentrate, however, was adjusted to 6.1 before electrolysis. This change had no greater effect on the test results. 
     EXAMPLE 5 
     Concentrated permeate obtained from the ultrafiltration of whey was used as a raw material. Table 6 shows the parameters and Table 7 the test results. Otherwise the conditions were the same as in Example 1. 
     
                                           TABLE 6__________________________________________________________________________Parameters in the demineralization of permeate concentrate when usingelectrolysis                            FlowTime    Temperature      Current/cell             Voltage/cell                    Current density                            whey                                acid                                   base                                      anolyte(min)    (°C.)      (A)    (V)    (A/m.sup.2)                            (l/min)__________________________________________________________________________0   27     4.2    5.5    180     1.8 2.5                                   5.2                                      5.210  31     3.6    5.7    160     0.9 1.3                                   3.1                                      3.020  35     2.5    6.2    110     0.9 1.4                                   3.3                                      3.230  36     1.5    6.5    60      1.0 1.4                                   3.5                                      3.3__________________________________________________________________________ 
    
     
                       TABLE 7______________________________________Results from the demineralization of a permeateconcentrate when using electrolysis         Time (min)         0    10        20     30______________________________________Whey recirculationDry matter (°Brix)           17.2   16.5      16.2 16.1Conductivity (mS/cm)           9.1    3.0       1.0  0.3Phosphor, soluble(mg/kg)         750    440       180  60Chloride (g/kg) 3.5                   0.1pH              5.2    5.1       4.3  3.4Solution volume (1)           3.8                   3.6Acid recirculationTitratable acidity(% lactic acid) 2.2    2.8       3.1  3.2Conductivity (mS/cm)           3.7    21        24   25pH              2.1    1.5       1.4  1.3Solution volume (1)           6.0                   6.6Base recirculationTitratable alkalinity(% sodium hydroxide)           2.3    2.5       2.6  2.7Conductivity (mS/cm)           75     81        84   87Solution volume (1)           4.8                   4.8______________________________________ 
    
     EXAMPLE 6 
     The demineralization test was carried out as described in Example 1 but the raw material used was unconcentrated whey (dry matter 6%). Table 8 shows the most important parameters and results. 
     
                       TABLE 8______________________________________The most important parameters and results ofthe demineralization of unconcentrated cheese whey  Tempera- Current/ Voltage/                           Current                                  WheyTime   ture     cell     cell   density                                  conduct.(min)  (°C.)           (A)      (V)    (A/m.sup.2)                                  (mS/cm)______________________________________0      21       4.2      6.7    180    3.15      24       1.9      7.7    82     1.210     26       1.2      8.0    52     0.6______________________________________ 
    
     It appears from the table that unconcentrated whey, too, demineralizes well whereas a rapid decrease occurs in the current and current density due to the low conductivity. Therefore it is preferable to demineralize concentrated whey. 
     EXAMPLE 7 
     The demineralization test was carried out as described in Example 1 except that the raw material used was a mother liquor formed in the production of lactose and having a dry matter of about 20%. The following Table 9 shows the most important parameters and results. 
     
                       TABLE 9______________________________________The most important parameters and results inthe demineralization of a mother liquor formedin the production of lactose                   Conduct. Conduct.                                   Conduct. Tempera- Current- of mother                            of acid                                   of baseTime  ture     density  liquor   solution                                   solution(min) (°C.)          (A/m.sup.2)                   (mS/cm)  (mS/cm)                                   (mS/cm)______________________________________0     24       180      17       2.8    7910    29       180      9.5      25     8420    32       170      6.7      35     9030    35       150      4.3      40     9540    36       110      2.8      42     9850    39       86       1.8      43     10060    41       68       1.2      43     102______________________________________ 
    
     It appears from the table that the process is also well suited for a mother liquor formed in the production of lactose. 
     EXAMPLE 8 
     The raw material used was the same cheese whey concentrate as in Example 1 and the run conditions were the same with the exception that the starting solution used in the acid recirculation was not 3% lactic acid but a 1% acid solution having an anion composition corresponding to that of whey. Correspondingly, the startng solution used in the base recirculation was not 3% sodium hydroxide but a 1% base solution having a cation composition corresponding to that of whey. The compositions of these solutions are given in Tables 10 and 11. The acid and base recirculation solution after the first run was used as a starting solution in the next run. The acid and base recirculation solutions after the second run were used as starting solutions in the third run. After the third run the compositions of the solutions were analyzed and they are shown in Tables 10 and 11. 
     
                       TABLE 10______________________________________Composition of the acid solution before thefirst run and after the third run    Hydrochl.             Phosphoric                       Lactic   Citric    acid     acid      acid     acid______________________________________Acid before run I      0.25%      0.25%     0.25%  0.25%Acid after run III      0.83%      0.60%     0.78%  0.67%______________________________________ 
    
     
                       TABLE 11______________________________________Composition of the base solution before the first runand after the third run    KOH    NaOH    Ca(OH).sub.2                              Mg(OH).sub.2______________________________________Base before run I      0.65%    0.22%   0.11%    0.02%Base after run III      2.1%     0.68%   0.25%    0.04%______________________________________ 
    
     EXAMPLE 9 
     The acid produced in Example 8 was concentrated by evaporating to a concentration of 30%. The solution was completely clear and slightly yellowish in colour. The pH of the whey drink was adjusted from 4.5 to 3.8 with said acid. Normally, the pH adjustment is carried out by means of citric acid. A test panel observed no difference when comparing a drink adjusted with said acid solution with a normal product. 
     EXAMPLE 10 
     The base produced in Example 8 was used for adjusting the pH of curd whey from 4.5 to 6.0 before drying. Normally, the pH is adjusted with sodium hydroxide. The drying of a curd whey neutralized with said base solution was as successful as that of a normal product and the powders had an equally good taste.