This paper defines the separations and chemical reactions in the cheesemaking process as a system of equations. Mass balances are formally defined for milk separation and cheesemaking in terms of input and output streams, each with a number of components. Pasteurisation and the coagulation of milk during curd production are discussed as chemical reactions. Front the model, expressions are derived for cheese yield. The modelling process reveals a number of assumptions which require testing and verification by experimentation. The model provides a means by which process engineers can more easily understand and analyse the cheesemaking process and it also provides a framework which enables comparison of research on cheese yields. The sensitivities of cheese yield to some of the assumptions are calculated and discussed.