Electrophoretic analysis of whey proteins was used to determine the presence and amount of cow's milk in ewe's milk cheeses. Various combinations of types of whey proteins were compared; using the ratio of cow's β-lactoglobulin A and B to bovine serum albumin was most accurate. The influence of various technological parameters was studied. Vegetable rennet exerted greater proteolytic action on whey proteins than did animal rennet but did not influence the level of detection. Type of pressing had no effect, There was no influence of aging on the level of detection in either Manchego cheese or Roquefort cheese. When silver nitrate was used for staining, quantities of cow's milk at levels of 1% or more were detectable in various types of cheese. Heating of the cow's milk to 90°C for 30 min denatured the whey proteins and gave negative results when cow's milk was added to sheep's milk for cheese making. This method has the advantage that whey proteins are less susceptible to proteolysis than caseins and is a simple procedure and of low cost.

This work has been supported by Comisión Interministerial de Ciencia y Tecnologia (Project Number ALI-88-0299).