THERMAL STERILIZATION OF FOOD Historical Review (original) (raw)

Thermal sterilization has been used to achieve long-term shelf stability for canned foods and is now used for a broad range of products. The majority of shelf-stable foods are thermally processed after being placed in the final containers. A relatively small percentage of shelf-stable foods are processed before packaging, using aseptic filling (Heldman and Hartel, 1997). Thermal sterilization of canned foods has been one of the most widely used methods for food preservation during the twentieth century and has contributed significantly to the nutritional well-being of much of the world's population (Teixeira and Tucker, 1997). The objective of thermal sterilization is to produce safe and high-quality food at a price that the consumer is willing to pay. It is a function of several factors such as the product heating rate, surface heat transfer coefficient, initial food temperature, heating medium come-up time, Z value for the quality factor, and target F ref value (Silva et al., 1992). The sterilization process not only extends the shelf life of the food but also affects its nutritional quality such as vitamin content. Optimal thermal sterilization of food always requires a compromise between the beneficial and destructive influences of heat on the food. One of the challenges for the food canning industry is to minimize these quality losses, meanwhile providing an adequate process to achieve the desired degree of sterility. The optimization of such a process is possible because of the strong temperature dependence of bacteria inactivation as compared to the rate of quality destruction (Lund, 1977). For this reason an estimate for the heat transfer rate is required in order to obtain optimum processing conditions and to maximize product quality. Also, a better understanding of the mechanism of the heating process will lead to an improved performance in the process and perhaps to energy savings. Basic principles for determining the performance of different but related processes have been presented by May (1997) and Wilbur (1996). In thermal sterilization of food, the heating medium temperature (steam or hot water) can deviate significantly from the design value during the heating phases. Such deviations may seriously endanger public safety due to under-processing of food (under-sterilization), waste energy, or reduce quality because of overprocessing of food (Datta et al., 1986). For these reasons, online retort control in thermal sterilization has been well studied by Datta et al., 1986; Gianoni and Hayakawa, 1982; Teixeira and Manson, 1982; and Teixeira and Tucker, 1997, to assure safety, quality, and process efficiency of thermally processed canned foods. In the design of thermal food process operations, the temperature in the slowest heating zone (SHZ) and the thermal center of the food during the process must be known. Traditionally this temperature is measured using thermocouples. The placement of thermocouples to record the temperature at various positions in a container during heating disturbs the flow patterns, causing errors in the measurements (Stoforos and Merson, 1990). Also, it is difficult to measure the temperature at the SHZ because this is a nonstationary region, which keeps moving during the heating progress, as