Mechanisms of fouling in dairy processing

During heating of milk in a heat exchanger in the range of 70-100 degrees C an undesirable deposit, mainly consisting of protein and mineral, is formed on the heating surface. Even at room temperature a monolayer of protein is immediately adsorbed. Further deposition of protein on top of this monolayer occurs if serum proteins undergo heat denaturation. Then so-called activated beta-lactoglobulin (beta-lg) molecules are formed in the bulk. This rate-determining step is followed by the transport of this activated molecule to the surface and its subsequent deposition through a reaction with an already deposited molecule. During its transport to the surface the activated molecule may be inactivated, i.e. no longer be prone to deposition, through a reaction with another activated molecule. On the other hand, a reaction of activated beta-lg with another milk component, for example with casein micelles or fat globules, may lead to the incorporation of such components in the deposit layer. Since other serum proteins also undergo heat denaturation they may be involved in the deposition process in a way comparable to beta-lg. A second major cause of fouling is the decreased solubility of calcium phosphate salts upon heating. The precipitation of these minerals is partly driven by the temperature difference between the bulk and the surface. However, part of the minerals will precipitate together with proteins. In its ionic form calcium influences the protein deposition by its effect on the aggregation of whey proteins and on the stability of casein micelles. Components other than serum proteins and minerals in milk play a minor role in fouling. Only if the colloidal stability of the casein micelles in the milk is decreased, e.g. by lowering the pH, does severe fouling by casein micelles occur. Also the presence of air bubbles at the surface enhances fouling. For the process of cleaning a mechanism for the removal of a milk deposit is presented. From the results obtained we derive recommendations on how to control fouling and on how effective cleaning can be achieved.