The presence of fat in milking machine liners is probably the chief cause of their deterioration. The primary effect of the fat is to cause softening and swelling of the rubber and greasiness of its surface; the secondary effect is to increase greatly the rate of oxidation. The primary effect may be reversed by removing the fat by solvent extraction or prolonged treatment with cold caustic soda; the secondary effect is irreversible.

Although the fat is derived chiefly from milk, rubber absorbs fat only slowly from liquid milk. Thus it is likely that most of the absorption is from milk residues remaining on liners between milkings, under conditions which favour the breaking of the natural emulsion. Such conditions would include drying, souring or other bacterial activity, and the presence of traces of detergents which act as protein denaturing agents.

Thus, if the widely recommended technique of cold or tepid rinsing is properly practised (8), fat absorption is kept to a minimum, and the life of liners will be prolonged. Under such conditions the absorption of fat from the cow's skin will be more important and methods of removal, or of prevention, such as the use of fat-resistant rubber may be advantageous.