A continuous process for protein production that uses high-pressure carbon dioxide (CO2) instead of organic acids as precipitant has been used to isolate casein rom milk. Central to the process is a pressure reduction stage that follows a reaction/precipitation stage, in which protein produced in the reactor may be removed without depressurizing the reactor and subjecting the products to excessive shear. Two reactor/precipitators were designed and tested. One was a spray reactor in which milk and CO2 were contacted by spraying milk into CO2. The other was a tubular reactor fed a mixture of liquid CO2 and milk. Both reactors gave casein products of good quality, but casein yield from the tubular reactor was greater than that from the spray reactor The methods presented here may be extended to other processes that currently use organic acids for precipitation or supercritical extraction systems at pressures less than 15000 kPa for continuous transport of solid-liquid mixtures from high-pressure units to atmospheric pressure.