The amount of rennet casein that can be replaced with MPC/MPI depends on the type of processed cheese application and the expertise of the manufacturer. In general, almost every manufacturer should be able to replace one-third of their rennet casein requirement with MPC/MPI without any noticeable changes in product quality.
If one replaces too much rennet casein, the most noticeable effect would be a softening of product texture or a significant change in melt rate and spread of the cheese product. MPI-85 would be the most logical product to use when replacing rennet casein. The protein content of MPI-85 is almost identical to the protein content of rennet casein and, therefore, would be a direct substitute for rennet casein without the need of adjusting the formula to meet required protein/fat ratios.