A model has been produced for predicting the temperature of liquid foods during vacuum cooling. If includes a mathematical description of the ingress of ambient air into process equipment, which leads to reduced cooling rates and the potential for contamination of the foodstuff. The model considers the air flow as an adiabatic compressible flow through a nozzle. The model was tested on a full-scale processing vessel containing 500 litres of water or a fruit/sugar preparation used in the production of yoghurts. The temporal trends of total vessel pressure, liquid temperature and the mass of condensed I,vapour were predicted well. Throughout the period of cooling, the difference between measured and predicted values of pressure did not exceed 19 kPa, the liquid temperature was predicted to within 7 degrees C and the mass of condensed vaponr was predicted to within 5 kg.