Temperature and heat transfer studies in a water immersion retort
Creators: name: Morello, Gerry F
Publisher: name: University of British Columbia
Year: 2010

description: Temperature and heat transfer studies in a pilot-scale water immersion retort were performed. The temperature study investigated the temperature distribution and stability of the retort during the cook period. The investigation of heat transfer uniformity within the retort was based on heating and cooling parameters calculated from the heat penetration curves of food-simulating teflon transducers. The uniformity of sterilizing conditions within the retort was determined from process lethalities calculated for the transducers. Variable retort operating conditions consisted of two retort temperatures (115 and 125°C) and three weir heights (29.2, 31.2 and 34.6 cm). Mean standard deviations of thermocouple readings indicating temperature distribution during the cook period ranged from 0.19 to 0.22 C°. Slight temperature gradients were found between the upper and lower water channels and between the entrance and exit regions of water channels. The coldest locations (the exit regions of water channels 1 and 2) averaged approximately 0.6 C° lower than the hottest locations (the entrance and exit region of water channel 11 and the exit region of water channel 10). Mean standard deviations of thermocouple readings indicating temperature stability during the cook period ranged from 0.10 to 0.20 C°. Temperature stability was uniform between all water channels, except channel 11, which was less stable. The entrance and exit regions of water channels displayed similar stability. The existence of heat transfer variability within the water immersion retort was indicated. A retort temperature of 125°C produced smaller fh and fc values than 115°C. Variations in weir height influenced the distribution of fh values between trays. Weir height 2 (31.2 cm) exhibited uniform values between all trays. Weir height 1 (29.2 cm) exhibited uniform fh values between all trays, except for a significantly larger value for the very top tray. Although weir height 3 (34.6 cm) created the most variability between tray levels, weir height 1 displayed the widest range of fh values. More variability in fc values between trays was shown during the cool period. Weir height 2 displayed the most uniform fc values between trays, however, the range of fc values between trays was similar for all three weir heights. Within trays, a gradient of fh and fc values was found between the entrance, exit and middle positions, with the smallest values found in the entrance positions. In comparison, the largest fh values were found in the exit and middle positions of trays 1 and 10. The largest fc values were found in the middle positions of trays 1 and 3 and the middle and exit positions of tray 10. Weir heights 1 and 2 produced smaller jh values than weir height 3, variations in weir height had no influence on jc values. A gradient of jh values between tray levels was shown, with smaller values associated with upper trays and larger values with lower trays. Smaller jh and jc values were associated with the entrance positions of trays than with the middle and exit positions. A comparison with steam processing indicated larger fh values for the water immersion process and larger fc values for the cooling method used with the steam process. Calculation of process lethalities indicated variability of sterilizing conditions within the retort. Larger Fo values were associated with upper trays than with lower trays. Within trays, larger Fo values were found in the tray entrance positions than the middle and exit positions. The largest Fo values were exhibited in the entrance positions of the middle to upper trays, while the smallest values were found in the middle and exit positions of the bottom trays. Retort pressure studies indicated pressure stability during the cook period, however, during the initial minutes of the cool period, a significant pressure drop occurred, which the retort corrected. Pressure stability was maintained once the target pressure was re-established.