O

Instrument y panel

Container sterilizer

Instrument y panel

Container sterilizer

Filled can discharge

Empty can infeed

Cover

Figure 10. Dole Aseptic Canning Line. Source: Ref. 4, courtesy of CTI Publications, Inc.

Closing machine

Filled can discharge

Empty can infeed u^rr Controls

Cover

^",mug sterilizer chamber

Figure 10. Dole Aseptic Canning Line. Source: Ref. 4, courtesy of CTI Publications, Inc.

systems are usually based on the use of form-fill-seal operations. In these machines, the packaging material is fed from either precut blanks or directly from roll stock, passed through a chemical sterilant bath or spray treatment, formed into the final package shape while being filled with cool sterile product from the product sterilizing system, and then sealed and discharged, all within a controlled aseptic environment.

Another important commercial application of aseptic processing technology is in the storage and handling of large bulk quantities of sterilized food ingredients, such as tomato paste, fruit purees, and other liquid food concentrates that need to be purchased by food processors or institutional end users for use as ingredients in further processed prepared foods. The containers for such applications can range in size from the classic 55-gallon steel drum to railroad tank cars or stationary silo storage tanks. Specially designed aseptic transfer valves and related handling systems make it possible to transfer sterile product from one such container to another without compromising sterility (Fig. 11).

Pasteurization Processes Systems

Pasteurization can be carried out by either in-container or out-of-container processes. The main difference from sterilization is that the lower temperatures used for pasteurization do not require the need for operating under pressure. Thus, the equipment systems needed for pasteurization are much simpler in design and easier to operate and maintain.

Normally, liquid foods with delicate heat-sensitive quality attributes, such as milk and fruit juices, are pasteur ized out-of-container using HTST heat exchangers to pasteurize with minimum quality degradation before filling in clean packages. These HTST pasteurization systems are similar to the aseptic process systems used in sterilization except that they operate at lower temperatures and at atmospheric pressure, and they do not require rigid aseptic filling conditions. Some liquid dairy products, such as dairy cream and coffee whitener, are given a sterilization heat treatment by operating the heat exchanger under pressure to achieve sterilizing temperatures, but are filled into conventional sanitary cartons without aseptic filling systems. Such products are marketed as ultrapasteurized with markedly longer storage life under refrigeration.

Less heat-sensitive foods as well as most nonliquid foods are pasteurized in-container much like the retort process for sterilization, except that an open tank of hot or near-boiling water is sufficient, and there is no requirement to use pressure vessels like retorts or autoclaves. A third method of pasteurization, known as hot fill, makes use of the high pasteurizing temperature reached by the product in a batch tank or mixing kettle as part of the product preparation. The clean empty containers are filled with the hot product and sealed. They are held upright for a few minutes to transfer sufficient heat to the container walls and bottom, and then they are inverted for an additional few minutes to complete pasteurization of the container lid and seal area using heat transferred from the still hot product. Most canned fruits, fruit preserves, and acidified (pickled) products are pasteurized in this way.

Note that the food examples given for the hot fill method of pasteurization are nonrefrigerated foods that enjoy long-term storage at room temperature without the use of sterilization heat treatments. That is because they are high-

Figure 11. Aseptic filling system for 55-gallon drums. Source: Ref. 4, courtesy of Cherry-Burrell Corporation and CTI Publications.

acid foods (pH < 4.5) that cannot support the growth of heat-resistant spore forming pathogens. High-acid foods are subject to spoilage principally by yeasts and molds, which have low heat resistance and can be inactivated by pasteurization heat treatments alone. These are technically canned foods, but are essentially processed by the use of pasteurization technology. That is why it is important to distinguish between high-acid and low-acid canned foods in the context of thermal processing.

SCIENTIFIC PRINCIPLES OF THERMAL PROCESSING Important Interrelationships

An understanding of two distinct bodies of knowledge is required to appreciate the basic principles involved in thermal process calculation. The first of these is an understanding of the thermal inactivation kinetics (heat resistance) of food-spoilage-causing organisms. The second body of knowledge is an understanding of the heat transfer considerations that govern the temperature profiles achieved within the food container during the process, commonly referred to in the canning industry as heat penetration.

Figure 12 conceptually illustrates the interdependence between the thermal inactivation kinetics of bacterial spores and the heat transfer considerations in the food product. Thermal inactivation of bacteria generally follows first-order kinetics and can be described by a logarithmic reduction in the population or concentration of bacterial spores with time for any given lethal temperature, as shown in the upper family of curves in Figure 12. These

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