Milk Protein Isolate (MPI) is the white powder obtained when all the non-protein components such as lactose and minerals are removed from skimmed milk. The finished product contains at least 90%w/w protein if not more and less than 3% w/w fat, carbohydrates, lactose and any other component.
It is produced by filtering milk to retain large molecular weight fragments, mainly proteins and to remove all lactose where feasible. The majority of proteins are casein (including micellar casein), lactalbumen, lactoferrin, immunoglobulins and beta-lactoglobulin. The proportions of each protein are the same as in milk because none of the protein is lost during filtration. It may be considered a concentrated form of casein and whey combined.
Milk Protein Isolate (MPI) is used as a regular supplement by those not consuming enough of the recommended amount of protein in their diet or who suffer with lactose intolerance. Milk Protein Isolate has very high and almost balanced amino acid composition that makes it ideal for use in protein bars and meal replacement powders. It is also combined with other protein sources such as casein, whey protein isolate and egg albumen to offer the complete amino-acid profile or simply to bolster the protein content of various food products. It is commonly found in sports supplement powders e.g. such as in the branded products from PRO JYM, various meal replacement powders, shakes and bars.
The American Dairy Products Institute has some very readable articles about various ingredients including milk protein isolate.
I like these simple articles but there are other brands out there which use MPI and I think you need to discuss them too.