0 10 15 20 0 10 15 20 Rotational speed (rpm)

Figure 7. Overall heat transfer coefficient between a canned liquid and the heating medium during end-over-end rotation as influenced by the rotational speed of the can and the liquid viscosity. Source: Ref. 21.

terized by the slope — 1/f and intercept j. This enables the f and j values to be obtained experimentally from measured T\t) data. These / and j values are used to calculate sterilization time for a process as discussed in books such as Ball and Olson (1).

Sterilization Values and Distribution

Because the detailed temperature and flow patterns have not been solved or experimentally obtained for the various complex heating situations, knowledge of distribution of sterilization (TTD) inside the agitated containers is not available. Due to the presence of agitation, liquid would generally have a smaller spatial variation of temperature within the container and therefore a smaller spread in sterilization values. The precise cold point in the can is generally unavailable for specific heating situations due to unavailability of detailed spatial variation in temperature. In axially rotating cans under flame sterilization (15), the cold points were on the axis near the ends of the can, although temperatures at other locations varied very little except near the wall. The center of the can is generally assumed to be the cold point for agitated heating, although for small variations in temperature within the container this is somewhat irrelevant.

Effect of Non-Newtonian Liquid Behavior

Several non-Newtonian liquids in the steritort have been studied, and correlations similar to Nusselt-Prandtl number correlations for Newtonian liquids have been provided. Several other studies have also included non-Newtonian liquids, and these studies have been summarized (11). Studies in nonfood applications indicate that the convec-tive heat transfer characteristics of non-Newtonian liquids are generally similar to those for Newtonian liquids (22,23)

Effect of Presence of Particulates

Several authors have studied agitated canned liquids in the presence of particulates, although rigorous engineering studies providing detailed temperature and velocity profiles in the liquid in presence of particulates are generally lacking (24,25). The additional critical parameter due to the presence of particles is, of course, the convective surface heat transfer coefficient between the fluid and the particles. It was noted that for agitations in a steritort, this surface heat transfer coefficient depended on the ratio of particle to container diameter, rotational speed of steritort, and liquid properties (26). In axial rotation of horizontal cylindrical containers, liquid to particle heat transfer coefficient for a physically restrained particle was found to be nearly invariant at 160 ± 30W/m2K (27).

In EOE agitation, the overall heat transfer coefficient is shown (Fig. 8) to increase initially as particle concentration is increased from a single particle. This is attributed to the additional particles aiding the fluid mixing as compared to a single particle. After some concentration value, further increase in concentration reduces the overall heat transfer coefficient. Such decrease was also observed by others (26,27) and is attributed to the decreased ability of the fluid to move inside the can (28). The overall heat transfer coefficient decreased with increase in particle size (28), as shown in Figure 9. This trend is supported by reference 29 but is opposite that reported by reference 26. Spherical-shaped particles lead to a higher overall heat transfer coefficient, followed by cylindrical shape, with the cubic shape producing the smallest heat transfer coefficient (30).


Processing Equipment and Methods of Heating

Nonagitating or still retorts are used when agitators are to be avoided to keep the product or package integrity or

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