Computer simulation programs used by the food industry today are not very well developed, partly because few food processes ar easily described by mathematical equations. The heat treatment of food, however, is one group of processes for which mathematical modelling exists. Programs used for prediction of the time-temperature relationship for various food products during heat treatment have been developed at SIK. A specially designed program calculates the temperature in viscous liquid foods with discrete solid particles. This program facilitates study of the influence of processing conditions on the product quality in an aseptic processing system. The F value is a measure of the safety and a so-called C value (cook value) is a measure of the quality of the product. The simulation program has been further developed to take into account product flow rate, residence time distribution, particle size and geometry, equipment specifications, particle load and the thermal properties of the product. The simulation program makes it possible to analyse the entire sterilisation system, including cooling, and determine the required holding time to reach a desired lethality value in the centre of the fastest-moving particle. The design of the program is described in this paper, and some applications are treated in a following paper. © 1993.