13 - Heat and Mass Transfer in Food Processing

Abstract

Heat transfer in food processing is complicated by the occurrence of simultaneous heat and mass transfer in drying and frying, free convection heat transfer in thermal sterilization of cans, and phase change in freezing and thawing. Air drying, freeze drying, spray drying, and steam drying are accompanied by moisture transfer, which also undergoes phase change (evaporation). Heat transfer, together with moisture and vapor transfer, controls these processes. Other phase change operations, such as evaporation and condensation, are commonly used in the food industry, mainly to concentrate liquid foods such as milk and juices. Food is dried with hot air or superheated steam, fried in hot oil and usually sterilized in cans or pouches with steam. Heat is transferred from the heating fluid to the surface of the material by free or forced convection. With solid food, heat is transferred by conduction while vapor is transported by diffusion or other mechanisms through the pores of the food. For liquid food in containers, natural convection rather than conduction dominates heat transfer. Thawing of products such as meat is usually done in a hot air environment or by using hot water. However, microwave and radio frequency may also be used to assist in thawing of meat. Microwaves and radio frequency waves have the ability to penetrate deep inside the food, producing heat within the material. Vacuum microwave drying is sometimes applied for heat sensitive food so that evaporation occurs at temperatures well below 100° C thus minimizing serious damage caused by high temperature applications.