the present invention provides a process for obtaining a specific protein - enriched product by ultrafiltering a fluid through metallic membranes and using a ph shift to fractionate components of lower molecular weight or higher molecular weight than the desired protein on the basis of charge as well as size . the process of the present invention makes feasible the fractionation of immunoglobulins , particularly igg , from the majority of the other proteins contained in milk , cheese whey or other immunoglobulin containing fluids and involves two factors : ( 1 ) the metallic membrane produces a high rejection of igg at a ph of 5 . 8 or lower , but a much lower rejection of igg at a ph of 6 . 5 or greater , and ( 2 ) the metallic membrane allows continual passage of smaller molecular weight proteins at any ph level , even at very high concentrations . fig1 is a schematic diagram of an exemplary apparatus according to the present invention . an output from a reservoir 10 is connected to an input of a control valve 12 by a line 11 . an output from the control valve 12 is connected to an input of a t connection 14 by a line 13 . an output from the t connection 14 is connected to an input of a control valve 17 by a line 16 . an output from the control valve 17 is connected to an input of a retentate collection reservoir 19 by a line 18 . an output from the t connection 14 is connected to an input of a pump 20 by a line 15 . an output from the pump 20 is connected to an input of a t connection 22 by a line 21 . an output of the t connection 22 is connected to a pump 24 by a line 23 . an output from the pump 24 is connected to an input of a module 27 by a line 26 . a pressure gauge 25 is attached to the line 26 . a retentate output from the module 27 is connected to an input of a module 29 by a line 28 . a retentate output from the module 29 is connected to a t connection 32 by a line 31 . a pressure gauge 30 is attached to the line 31 . an output of the t connection 32 is connected to an input of a control valve 35 by a line 34 . an output of the t connection 32 is connected to an input of the t connection 22 by a line 33 . an output from the control valve 35 is connected to an input of the reservoir 10 by a line 36 . an addition of a fluid to the reservoir 10 is controlled by an entry port 37 . a permeate output from module 29 is connected to the outer shell of module 27 by a line 38 . a permeate output from the module 27 is connected to an input of a control valve 40 by a line 39 . an output from the control valve 40 is connected to a permeate collection reservoir 42 by a line 41 . the fluid containing the protein which is to be enriched and concentrated is added to the reservoir 10 through the entry port 37 . in general , the fluid is acidified to a ph suitably below the isoelectric point of the protein by the addition of any food grade acid , for example , phosphoric or acetic acid through the entry port 37 . after the acidification , the fluid is pumped out of the reservoir 10 through the line 11 , control valve 12 and lines 13 and 15 by the pump 20 , which is a diaphragm positive displacement pump . pump 20 creates the inlet pressure for the ultrafiltration . the fluid exits the pump 20 by the line 21 , passes through the line 23 , and enters the pump 24 which is a c - 114 centrifugal pump . the pump 24 propels the fluid into the ultrafiltration modules 27 and 29 by means of the line 26 . the inlet pressure of the fluid is measured by the pressure gauge 25 . the ultrafiltration modules 27 and 29 , which are connected by the line 28 , separate the fluid into a retentate and a permeate . the permeate from the module 29 flows to the outer shell of module 27 through the permeate line 38 . the permeate is drained from module 27 by means of line 39 through control valve 40 into the permeate collection reservoir 42 . the retentate exits the module 29 through the line 31 . the outlet pressure is measured by the pressure guage 30 . a portion of the retentate is recirculated directly back to the ultrafiltration modules by the line 33 . the remainder of the retentate flows through the line 34 , past the concentrate control valve 35 into the reservoir 10 to be recirculated again until the desired concentration of the retentate is achieved . after concentration , the retentate flows from the reservoir through the line 11 , past the control valve 12 , through line 13 to a t connection 14 . the retentate flows through this t connection to line 16 , through control valve 17 and then into the retentate collection reservoir 19 . the following example is directed to a specific embodiment of the present invention and is not to be construed as limiting the scope of the invention . the protein concentrations and protein ratios ( with igg as the basis ) of a chedder cheese whey starting material obtained from a cheese manufacturing plant are presented in table 1 . table 1______________________________________protein conc . ( mg / ml ) ratio______________________________________igg 0 . 29 1 . 0bsa 0 . 25 0 . 9beta - lactoglobulin 2 . 58 9 . 0alpha - lactalbumin 0 . 69 2 . 4______________________________________ fig2 shows the hplc profile of the whey starting material . the ph of the whey was adjusted to 5 . 0 by addition of food grade acid and 94 . 5 kg of the whey was concentrated 15 - fold by passing the whey through a carre bio - 1 ci - 200 membrane , available from carre , inc ., seneca , s . c ., with an initial permeability to water of 2 . 5 at 36 . 7 ° c . the permeate flux was 50 . 9 1 / m 2 / hr initially and was 34 1 / m 2 / hr after the 15 - fold concentration . the inlet pressure was 207 kp and the circulating velocity of the fluid was 6 . 1 m / s . fig3 shows the hplc profile of the whey after the 15 - fold concentration . the protein concentrations and ratios after the 15 - fold concentration are provided in table 2 . table 2______________________________________protein conc . ( mg / ml ) ratio______________________________________igg 4 . 65 1 . 0bsa 4 . 05 0 . 9beta - lactoglobulin 23 . 72 5 . 1alpha - lactalbumin 4 . 35 0 . 9______________________________________ the first concentration significantly improved the protein ratios in favor of igg with close to 100 % recovery of the igg into the retentate . the igg to beta - lactoglobulin ratio shifted from 1 : 9 . 0 to 1 : 5 . 1 . the profile also showed a near elimination of peptides and a very large reduction in the proportion of alphalactalbumin to igg . further enrichment of the igg was next accomplished by diafiltration of the concentrate at ph 5 . 0 with water in the amount of one - fifth the weight of the original whey or 18 . 9 kg of water through the carre bio - 1 ci - 200 membrane with all the parameters being kept the same . the protein concentrations and ratios after diafiltration and dilution to a 2 . 2 fold concentration compared to the original whey are shown in table 3 . table 3______________________________________protein conc . ( mg / ml ) ratio______________________________________igg 0 . 61 1 . 0bsa 0 . 53 0 . 9beta - lactoglobulin 1 . 99 3 . 2alpha - lactalbumin 0 . 27 0 . 4______________________________________ the hplc of fig4 shows that after diafiltration the igg concentration represented 96 % recovery of the total igg from the starting whey ( i . e ., 0 . 61 mg / ml divided by 2 . 2 equals a concentration of 0 . 28 mg / ml ). there was a large increase in the void volume peak from fig2 to fig4 because the compounds of larger molecular weight than igg were being concentrated as well . removal of the larger molecular weight compounds remaining after the fractionation of igg from the other whey proteins was accomplished by adjusting the ph of the whey concentrate to 7 . 0 by addition of food grade base , such as naoh , and passing it through the same carre bio - 1 ci - 200 membrane used in the initial step , at all the same parameters , after cleaning the tubes with , for example , an alkaline hypochlorite solution at ph 11 followed by a nitric / phosphoric acid solution at ph 2 and a water rinse . this ultrafiltration removed large particulate matter , such as casein micelles , fat globules and bacteria , but passed 90 % of the igg into the permeate . the resulting permeate was a water - clear , near sterile solution enriched in igg . the protein concentrations and ratios of the second permeate are shown in table 4 . table 4______________________________________protein conc . ( mg / ml ) ratio______________________________________igg 0 . 26 1 . 0bsa 0 . 19 0 . 7beta - lactoglobulin 1 . 17 4 . 5alpha - lactalbumin 0 . 20 0 . 7______________________________________ fig5 shows the hplc profile of the final permeate . the bacterial cell population was reduced by 7 . 3 logs , enabling the solution to be easily and efficiently polished with standard bacterial filters for a final sterile product . the weight concentration ratio ( wcr ) is a method of calculating yield . it is determined by calculating the ratio of the weight of the starting material to the weight of the material after a single pass through the ultrafiltration membrane . protein concentration factor ( pcf ) is calculated by determining the concentration of a particular protein and dividing that figure by the original concentration of that protein in the feed whey . 100 % retention of a protein is obtained when the protein concentration factor equals the weight concentration ratio . fig8 , and 10 are graphs which show the protein concentration factors of 4 proteins in whey ( pcf values ) versus the weight concentration ratio ( wcr values ) at various ph &# 39 ; s . alpha - lactalbumin is represented by circles ; beta - lactoglobulin is represented by squares ; bovine serum albumin ( bsa ) is represented by diamonds ; and igg is represented by triangles . analysis of fig8 and 10 indicates that at ph 5 . 0 and ph 6 . 5 , respectively , igg and bsa have the same pcf value . this illustrates that at these ph values , igg cannot be separated from bsa in a single pass through the ultrafiltration method disclosed by the present invention . analysis of fig9 indicates that at ph 5 . 5 , however , igg has a pcf equal to about 7 . 5 whereas bsa has a pcf equal to about 6 . 5 . this illustrates that igg can be separated from bsa in a single pass through the ultrafiltration method disclosed by the present invention at ph = 5 . 5 . it is important in any membrane process to clean the membranes back to their original permeability to water so as to maintain maximum performance with respect to permeate flux . in the process described herein , the cleaning is equally important for maintaining separation capabilities . this is due in large part to the dependence on charge for separations . both organic foulants , which are primarily proteins , and mineral foulants abound in milk and whey and can significantly alter membrane permeability . the operational parameters of high velocity and low pressure minimize protein fouling as evidenced by steady fluxes and continued protein passage , even at very high volume recoveries or concentrations . however , separations could suffer if the membranes were not properly cleaned . acid washing has been found to maintain separation capability as well as recovering membrane permeability . a preferred commercial cleaning fluid is uracil 75 , available from the henkel corp ., burlington , iowa . this acid cleaner is to be exposed to the membrane at the operating parameters of the ultrafiltration membrane . another significant cleaning procedure is the use of back flow during both base and acid cleaning . in cleaning studies , the use of back flow during both acid and base cleaning after a typical whey run was found to increase permeability to water between 8 and 69 per cent . in other cleaning studies , back flow gave immediate improvement in water permeability even after several previous cycles of base / acid cleaning without backflow . therefore , it is recommended that backflow be used in both base and acid cleaning steps . egg yolks were diluted with water to obtain a solution containing 5 % yolk as compared to water on a weight to weight basis . fig6 shows the hplc profile of the egg yolk starting material . peak 3 was identified as igg based on retention time . peak 2 may also be igg because peaks 2 and 3 were not well separated . the ph of the solution was adjusted to 5 . 0 by addition of phosphoric acid and 75 . 5 liters of this solution was concentrated 10 - fold by passing the egg yolk solution through the carre ci - 200 membrane with an initial permeability to water of 2 . 5 at 36 . 7 ° c . the permeate flux was 50 . 9 1 / m 2 / hr at 37 . 8 ° c . the inlet pressure was 207 kp and the circulating velocity of the fluid was 6 . 1 m / s . fig7 shows the hplc profile of the egg yolk solution after the 10 - fold concentration . peaks 1 , 2 , and 3 increased in proporation to peaks 4 - 8 , indicating that igg was concentrated , whereas smaller molecular weight proteins were drastically reduced in concentration . this example suggests that other fluids can be concentrated and enriched by the process disclosed by the present invention . blood serum was diluted with water to obtain a 2x dilution . the ph of the solution was adjusted to 5 . 0 by addition of phosphoric acid and 75 . 5 liters of this solution was concentrated 20 - fold by passing the blood serum solution through the carre ci - 200 membrane with an initial permeability to water of 2 . 5 at 36 . 7 ° c . the permeate flux was 30 . 5 at 32 . 2 ° c , the inlet pressure was 207 kp and the circulating velocity of the fluid was 6 . 1 m / s . although bsa was not separated from igg using the ultrafiltration apparatus described above , at ph 7 . 0 some fractionation of igg and bsa was observed . additionally , the results depicted in fig8 , and 10 illustrate that separation of bsa and igg may be obtained by ultrafiltration at a ph of 5 . 5 . the ultrafiltration system that has been found to provide the surprising results noted above is generally described in u . s . pat . no . 4 , 200 , 533 to gaddis et al and more specifically described in u . s . pat . no . 4 , 762 , 619 to gaddis et al . certain particular parameters of the system per se , for purposes of the present invention , are not particularly set forth therein . particularly , according to the present invention , the support upon which the food grade titanium dioxide is deposited is preferably a rigid porous surface , such as sintered stainless steel , so as to alleviate any concern for certain process conditions , such as temperature and pressure which might otherwise normally adversely affect the support or not be suitable at all therefor , or for certain cleaning conditions , such as acid or base washing . the titanium dioxide coated ci - 200 membrane and the bio - 1 ci - 200 membrane used in the present invention are available from carre , inc ., seneca , s . c . a modified bio - 1 ci - 200 membrane which has not yet been tested may also prove useful in the enrichment and concentration of a selected protein from other proteins in a solution . future modifications of this membrane , as available from carre , are intended to be within the scope of the present invention . it is expected that a rigid porous surface made of ceramic materials also could serve as a support for the food grade metallic oxide membrane . it is anticipated and expected that proteins from other biological fluids than those described above , such as skim milk , whole milk , colostrum , colostral whey , whole blood , tissue culture fluids , or fermentation broths may be enriched and concentrated by employing the method of the present invention . the foregoing detailed description is given for clearness of understanding only , and no unnecessary limitations should be understood therefrom , as aspects and advantages within the scope of the present invention will be apparent to those skilled in the art .