ASEPTIC PROCESSING OF PARTICULATE FOODS The concept of aseptic processing originated to solve problems associated with conventional ‘in- container’ sterilization of foods such as low rate of heat penetration to the slowest heating point in the container, the long processing times required to deliver the required lethality, destruction of the nutritional and sensory characteristics of the food, low productivity, and high energy costs (Smith et al., 1990). Aseptic processing was initiated in 1927 at the American Can Company Research Department in Maywood, IL under the direction of C. Olin Ball (Mitchell, 1988). The result was the development of the HCF (heat, cool, Jill) process for which Ball was granted a patent in 1936. The HCF process was targeted at pumpable liquid and semi-liquid foods. In 1938, two HCF units were installed for commercial production of chocolate-flavoured milk beverage. Product was heated to 300°F in less than 15 s, immediately cooled (Ball & Olson, 1957) and filled into sterile cans. The process was not a commercial success because of the associated high cost of the equipment, inflexibility with respect to can size and the frequent blockages of the can-closing machine. Despite these problems, Ball is considered the pioneer of aseptic processing (Mitchell, 1988). The Avoset process was another development in the field of aseptic processing and this process is unique in that the filling and closing area was treated to eliminate bacteria and further protection was accomplished by ultraviolet (UV) lamps (Mitchell, 1988). The area was also enclosed by a wall with an opening for conveying the finished product. Sterilization was achieved by direct steam injection to a temperature of 260-280°F. The Avoset process is no longer in operation, but nevertheless it was another milestone in the development of aseptic processing. Real progress in the commercial development of aseptic processing technology of foods began with the invention of the Martin-Dole process in the late 1940s in California by the Dole Engineering Company (Lopez, 1987). The process could be used for the sterilization of any low or high acid fluid. The technical success was not, however, accompanied by the expected high level of exploitation, the reason being the package, a metal can, had nothing to differentiate from the conventional ‘in-container’ sterilized foods (Dennis, 1992). Product heating and cooling was achieved by heat exchangers based on the principles of high temperature/ short time sterilization, while the containers were sterilized by superheated steam (Mitchell, 1988). The first commercial Dole system was installed in the early 1950s in California for the production of soups (split pea soup) and sauces (white, cheese and Hollandaise). Aseptic processing involves processing involves sterilizing the products and package separately, and filling under sterile conditions. This is in contrast to conventional canning where the product is sterilized in the can. Aseptic processing is the shorthand name for the food production system where product moves in continuous flow through a heat-hold-cool thermal process and is then filled into a sterile package. The package is sterilized, filled and sealed in a sterile environment. Fig: 1 Schematic diagram of an aseptic processing system for product sterilization. Fluids and small particle suspensions can be sterilized by heating in heat exchangers. Figure: 1 is a schematic diagram of an aseptic processing system. The liquid phase reaches the processing temperature very rapidly, therefore the small sterilization value of the heating phase of the process is generally neglected. The specified process for sterilization in continuously flowing systems is a time of residence in a holding tube, an unheated section of the piping system that leads the fluid from the heat exchangers for heating to the heat exchangers for cooling. A back pressure valve or a positive displacement timing pump is positioned after the cooler to maintain the pressure within the system at a level needed to keep the product boiling temperature higher than the processing temperature. After cooling, the sterile product must be handled in a sterile atmosphere, therefore the process is called aseptic processing. The time of residence is set by the volume of the holding tube and the rate of fluid flow delivered by a positive displacement pump. Packaging materials used in aseptic packaging systems and surfaces of equipment may be sterilized using moist heat, dry heat, hydrogen peroxide, high-intensity ultraviolet, and ionizing radiation from either gamma rays or high-energy electron beams. The latter three methods have not been adopted in commercial food packaging, but various forms of heat and hydrogen peroxide combined with heat are commercially utilized. Discreet particulates within a flowing fluid will be heated by heat transfer from the suspending fluid. Thus, heat transfer coefficients between the particle and the fluid play a significant role in the rate of heating. Simplified equations for heat transfer will not be applicable because the fluid temperature is not constant as the mixture passes through the heater, and temperature of fluid in an unheated holding tube may not be constant because of heat exchange between the fluid and the suspended particles. Taking a conservative approach of ignoring the heat absorbed by the particles in the heaters can result in a significant over-processing, particularly if the suspended particles have less than 0.5 cm as the thickness of the dimension with the largest area for heat transfer. Finite element or finite difference methods for solving the heat transfer equations with appropriate substitutions for changes in the boundary conditions when they occur is the only correct method to determine the lethal effect of heat in the holding tube. Residence time distribution of particles must also be considered, and as in the case of fluids in turbulent flow, the use of a probability distribution function for the residence time in the finite difference or finite element methods will allow calculation of an integrated sterility. Aseptic processing technique has been successfully applied to liquid foods and acid foods containing discrete particulates. However, the extension of aseptic processing to low-acid heterogeneous liquid foods containing discrete particulates has been difficult due to lack of data on critical factors such as interfacial heat transfer coefficient between the liquid and the particle (hfp) as well as the residence time distribution of particles in the holding tube of the aseptic system. Conventional thermal processing calculation methodology cannot be employed for the establishment of these processes, because of the difficulties associated with gathering experimental time-temperature data at the particle centre as it travels through the aseptic system. Mathematical modelling followed by biological verification has been attempted as a possible alternative. These models require accurate data on the thermo-physical properties of the particles, associated fluid to particle heat transfer coefficient (hfp) as well as residence time distribution (RTD), especially in the holding section of the system. Both hfp and RTD depend on several factors which may also be interdependent: Theological properties, flow rate, temperature, and density of the carrier fluid, shape, density and concentration of the solid particles, as well as holding tube diameter and length. Rheological properties of the carrier fluids have been shown to influence fluid velocity profiles in the holding tube, and intuitively the food particle RTD in both the heat exchangers and the holding tube. Aseptic processing involves sterilizing the product (most meat products being low-acid foods containing particulates) and package separately, and filling under sterile conditions. Advantages include better product quality compared with canned products, lower transport and storage costs compared with frozen products, and virtually no restriction on package size. Problems include sterility, preventing separation of particles from the carrier liquid, and retention of particle structure and shape. Particulate foods can be sterilized in scraped-surface heat exchangers. Other methods involve heating the particles separately, and combining them during filling. Current aseptic processing and packaging technology is primarily limited to liquid foods, but there is considerable interest for the extension of the technology to low acid liquid foods containing large particulates. The early problems facing the establishment of a process for continuous heat-hold- cool sterilization of low-acid liquid foods containing particulates were: a mechanical means of physical handling in order to maintain proper distribution and particle integrity; and the assurance of commercial sterility with minimal quality loss. The first problem was successfully handled with equipment such as scraped surface heat exchangers (SSHEs) or tubular heat exchangers with a displacement pump. A typical aseptic process involves sterilization of the product by a heat-hold-cool approach followed by filling and sealing into pre-sterilized containers under aseptic conditions. Product heating in aseptic systems can be performed directly (steam infusion and steam injection) or indirectly (plate, tubular or scraped surface heat exchangers) for pasteurization and sterilization of foods, whereas the containers are sterilized by superheated steam and/or H2O. ASEPTIC PROCESSING SYSTEMS