Comparison of heat transfer rates during thermal processing under end-over-end and axial modes of rotation

Abstract

Thermal processing under conditions enabling container agitation are attractive to food processors since they provide higher rates of heat transfer and more uniform temperature due to product mixing. Two common types of agitations are end-over-end and axial agitation. While end-over-end agitation is common in batch retorts, axial agitation is predominant in continuous cookers in which the cans roll in a helical path along the retort shell. In this study, heat transfer rates to canned Newtonian fluids in end-over-end and axial modes of rotation were evaluated in the same retort. A single basket rotary retort was retrofitted to accommodate simultaneously both end-over-end and two types of axial modes of rotation, free axial and fixed axial. In the free axial rotation, the cans were allowed to rotate on their own axis as they rolled along the retort shell during the bottom third of the rotation, while in the fixed axial mode, the cans in the axial direction were held stationary in the rotating basket. The overall heat transfer coefficient U, heating rate index, lag factor and cook values associated with cans filled with Newtonian fluids were evaluated using a central composite rotatable design of experiments with glycerin at five concentration level (no glycerin to all glycerin), heating medium temperature (111.6–128.4 °C) and five basket-rotational speeds (4–20 rpm). Higher U, and lower heating rate index, lag factor and cook values were associated with higher rotational speed and retort temperatures for all modes of agitation. The heat transfer parameters associated with free axial rotation were significantly higher than in the end-over-end mode which was higher than in the fixed axial mode.