Heat transfer in two-phase solid-liquid food flows: A review

Higher quality food can be produced by continuous aseptic processes rather than by the essentially batch process of in-container sterilization of particulate foods. In continuous aseptic processing, a food mixture passes continuously through a heat-hold-cool system, and is then packaged in presterilized containers. This results in shorter processing times, higher production rates, superior product quality, reduced power requirements, and improved process control. The design of such a plant requires knowledge of the rates of heat exchange (both fluid-particle and wall-fluid) which take place within the process. At present there is a severe lack of understanding of two-phase solid-liquid food flows, and suitable commercial sterilization schedules must be determined experimentally for every food product processed. Until enough knowledge of real flows has been gained, food manufacturers will adhere to conservative approaches in the design of each stage in a heat-cold-cool system. Following the recent review on the fluid mechanics of solid-liquid food flows(6), this paper reviews the current state-of-the-art in the area of heat transfer. The fluid-particle and the wall-fluid heat transfer coefficients are crucial design parameters. Measurement techniques and mathematical models for estimating them are reviewed, the application of existing knowledge to the design of continuous aseptic processes is discussed, and research needs are highlighted.