In the fractionation of liquid proteins, an increased rate of producton without denaturing is obtained by use of apparatus including (A) A HEAT-EXCHANGER FOR THROUGHFLOW OF A COOLING FLUID WHEREBY THE HEAT-EXCHANGER PRESENTS COOLING SURFACING FOR CONTACT BY A THROUGHFLOWING LIQUID; AND (B) A PAIR OF NOZZLES FOR DISCHARGING RESPECTIVELY JETS OF PROTEINACEOUS SOLUTION AND LIQUID PRECIPITANT, THE NOZZLES BEING SO DISPOSED THAT THE JET LIQUIDS CONVERGINGLY MERGE TO FORM A COMPOSITE STREAM WHICH IMPINGES ON THE COOLING SURFACING OF THE HEAT EXCHANGER AND INSTANTLY ON IMPINGEMENT BECOMES A REACTION MIXTURE CONTAINING THE PRECIPITATED PROTEIN FRACTION WHICH REACTION MIXTURE THEN FLOWS OVER THE COOLING SURFACING AND FROM THE HEAT EXCHANGER.