UBC Theses and Dissertations
Residual gas effects on heat transfer in overpressure processing of flexible packages Weintraub, Sara Elisabeth
Experiments have been performed to test effects of entrapped air on heating rate indices (f[sub h]) of test pouches subjected to variable conditions in overpressure retort processes. Teflon bricks with centrally located thermocouples were employed as a test material. Water was added within the pouch to provide vapor pressure which would simulate the water activity of a food. Air was injected by means of a hypodermic syringe through a silicone rubber droplet on the surface of the pouch which formed a septum. With repeated injections, air volumes were varied from 0 to 35 mL. Processing temperatures of 115, 120 and 125°C were studied under pressure levels corresponding to 65, 75 and 85% steam in a steam/air mixture. Three retorts were employed: (1) an industrial-scale horizontal Lagarde steam/air retort, (2) a pilot-scale vertical positive flow steam/air retort and (3) the same pilot-scale retort using superheated water with air overpressure. The heating rates of teflon bricks without retort pouch packaging were also studied at the different temperatures and pressures; analysis of variance revealed no significant differences (p>0.05) among conditions of temperature and overpressure level. There were, however, significant differences among the retort systems used. In general, bricks tested in the Lagarde forced circulation steam/air retort demonstrated more rapid heating than in the pilot-scale retort with both media types. Relationships of heating rate indices as functions of included air volume consisted of either one or two essentially linear segments. One segment formed a plateau region in which there was no change in f[sub h] with increasing volumes of included air. The second section, apparent with some conditions only, exhibited an increase in f[sub h] as a function of increasing air volumes. A moving regression frame consisting of ten consecutive points was compared against a baseline by covariance analysis. Critical volumes of air were considered to be those volumes above which the f[sub h] vs. air volume function was significantly different (p<0.05) from the baseline. Covariance analysis was performed to compare slopes of the heating rate vs. air volume functions with appreciable data in the second section. Results of the experiments imply that with an increase in overpressure, larger amounts of air may be entrapped in the pouch with heat transfer remaining unchanged. Air overpressure above 80 kPa allowed up to 35 mL of air to be included with no detrimental effect on heating rates. Increasing overpressure and temperature was shown to display decreasing slopes in the second linear section. Critical air volumes for water/air overpressure processes were substantially higher than those with steam/air mixtures. Steeper slopes were evident on the increasing section with steam/air media indicating a more severe detrimental effect of entrapped air on heat transfer. Studies with retort types demonstrated that processing in the Lagarde retort allowed for larger amounts of air to be included without affecting heat transfer. Predictions of volumes of non-condensible gases which would prevail at retort conditions were made by determining expansion factors from ideal gas relationships. Comparisons of slopes of the log f[sub h] vs. adjusted air function showed significant differences. Water/air processes in the positive flow retort had the least slope, the Lagarde and positive flow steam/air processes had increasing slopes, respectively.
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