The build-up of unwanted deposits on processing-equipment surfaces. The fouling layer conducts heat poorly, raising resistance to heat transfer and cutting heat-exchanger effectiveness. Chemical-reaction fouling involves deposits formed by reactions at the heat-transfer surface; biological fouling is the deposition of organic films of microorganisms and their products. Keeping fouling under control — through CIP regimes and equipment design — is essential in dairy plants and other food factories. In dairy plants, milk-protein and mineral deposits (milk stone) build up on pasteuriser plates, cutting heat-transfer efficiency and harbouring thermoduric bacteria; in evaporators, scaling limits concentration ratios. Monitoring pressure drop and heat-transfer coefficients triggers cleaning before performance or hygiene is compromised.