Bactericidal efficiency of a helically coiled tube turbulent flow processing system

Abstract

The recent advances in ultra-high temperature (URT) processing have been focused on improving the quality and shelf life of fluid dairy products. This process could be used, however, to process other fluid foods if more technical information were available on flow properties, heat exchange characteristics, and on lethality of selected microorganisms. The two general types of URT processing equipment are the direct and indirect heat exchange systems. In the direct system, heating of the product involves direct contact with culinary steam, whereas in the indirect system, heating of the product is accomplished through a metal barrier. The direct heating system dilutes the product with the condensate, and after a short holding time it is concentrated to its original volume by a vacuum process. The disadvantages of this process are physical damage and possible contamination from the steam. Even with these disadvantages, several commercial applications have been made in Europe, the United States, and Canada. Indirect systems do not subject the food to contamination or to chemical breakdown by dilution and are highly adaptable to aseptic processing systems. Present commercial units consist primarily of plate-type and tube-type indirect heat exchange systems. The plate exchangers are more widely used, because many units are already in operation for high temperature short time (HTST) processing of fluid foods. The principal disadvantage of plate-type systems for UHT processing is that they cannot withstand the extreme pressures that occur during heating and cooling. Helically coiled tube DRT heat exchange systems can withstand extreme changes in pressure and can be operated up to 5000 lb/sq in. product pressure. This permits the use of high flow rates that are necessary to maintain turbulent flow during processing highly viscous foods. Many fluid food products, including concentrated foods such as cream and ice cream mix, can be processed by heat treatments in helically coiled tube exchange systems. Before wider commercial applications can be made, additional data are needed on factors affecting bactericidal efficiency of helically coiled tube heat exchange systems. More basic information is needed on flow properties, and on heat exchange characteristics of these systems. In addition to this, information on bacterial destruction is required in these types of systems involving rapid heating and cooling treatments with short holding times. Application of this information would increase processing efficiency, reduce "burn-on," and improve the quality of foods pasteurized or sterilized in helically coiled tube heat exchange systems.