20 40

Process time (min)

Figure 14. Center temperature profile in cylindrical container of conduction-heating food product in response to constant retort temperature process during heating and cooling.

steam retort could be expressed as follows for an element of food volume facing the can wall of surface area A and thickness L:

where T is product temperature, Tr is retort temperature, and p, Cp, and k are density, specific heat, and thermal conductivity of the product, respectively. Because of the high surface heat transfer coefficient of condensing steam at the can wall and high thermal conductivity of the metal can, overall surface resistance to heat transfer can be assumed negligible in contrast to the product's resistance to heat transfer. After rearranging terms, expression 4 can be written in the form of an ordinary differential equation:

By letting the thermal diffusivity (a) represent the combination of thermal and physical properties (k/pCp) and letting T0 represent the initial product temperature, the solution to equation 5 becomes

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