Heat transfer studies of steam/air mixtures for food processing in retort pouches
Creators: name: Ramaswamy, Hosahalli Subrayasastry
Publisher: name: University of British Columbia
Year: 2010

description: Heat transfer characteristics associated with steam/air mixtures were studied in two pilot scale batch type steam/air retorts: a vertical positive flow retort and a horizontal forced circulation Lagarde retort. A method employing transient heat conduction into rectangular bricks of aluminum and stainless steel was developed to evaluate the surface heat transfer coefficient (h) of steam/air mixtures. A system was designed to facilitate an instantaneous drop of the test brick, from an insulated box inside the retort, into a specified steam/air medium after the come-up period. The influences of steam content, temperature, flow rate and flow direction of the heating medium and orientation of test bricks on the associated h values, temperature distribution and pressure stability in the retorts were studied. In addition, thermal processing efficacy was evaluated by measuring the rate of heat penetration into bricks of silicone rubber and rigid nylon which have thermal diffusivities in the range common for foods. In both retorts, steam content (S) of the mixture was found to be the major factor influencing h (p&lt;0.05); however, temperature had no significant effect (p&gt;0.05). Further, the flow direction and flow rate of the heating media in the positive flow retort, and brick orientation in the Lagarde retort also influenced h (p&lt;0.05). The general relationship between h and S was exponential: h = a exp(bS). In the positive flow retort with the test brick in the vertical orientation, the values of a and b were 153 W/m²C and 0.0421 respectively, for steam/air media flowing in an upward direction, and were 337 W/m²C and 0.0355 respectively, for the media flowing downward. The surface heat transfer coefficient was also found to increase linearly with the medium flow rate. With the Lagarde retort, steam/air flow was always horizontal and flow rate was not adjustable. In this case, h was influenced by the test brick orientation. For bricks in the vertical orientation, the exponential parameters, a and b, were 1011 W/m²c and 0.0226 respectively, whereas in the horizontal orientation, these were 1669 W/m²C and 0.0132 Temperature distribution studies in the positive flow retort indicated that the overall standard deviation of the medium temperature at several locations during the cook period (excluding come-up) increased (p&lt;0.05) with a decrease in the steam content and flow rate of the heating media. The effects of temperature and flow direction were nonsignificant (p&gt;0.05). In the Lagarde retort, the temperature distribution was not influenced either by steam content or temperature of the steam/air medium. Pressure stability studies indicated that the air content and temperature of the medium increased (p&lt;0.05) the standard deviations of retort pressure during the cook period. Based on the temperature and pressure deviations in the two retorts, steam/air mixtures with 86-90% steam contents were considered to provide satisfactory overriding air pressures for processing of retort pouches at 105-120°C. Heat penetration studies in the positive flow retort using nonpackaged test bricks of silicone rubber and nylon revealed an increase of up to 11% (p&lt;0.05) in the heating rate index (f) of test bricks when the steam content of the media decreased from 100% to 50%. Heating of bricks at 120°C resulted in f values that were 5.5% larger (p&lt;0.05) than those for bricks heated at 105°C. In the Lagarde retort, the effects of temperature and steam content of the media on f values were not significant. Heating bricks in the vertical orientation resulted in higher f values than in horizontal orientation in some tests, while a reverse trend was observed in others. The influence of entrapped air (15-30 mL per pouch) in retort pouches containing the bricks on f values was small when using a vertical rack that tightly constrained the bricks, whereas up to 260% higher values of f resulted when using an unconstraining horizontal rack while processing at 105-120°C in media of steam contents above 65%. These increases in f value could be prevented by using overriding air pressures of 70-100 kPa during the retort operation. The lag factor, j, was generally in the range of 0.5-1.0 for test bricks, with or without packaging, in the positive flow retort, and 0.8-1.1 in the Lagarde retort, when evaluated at 42% effectiveness for the come-up time. It was observed that in order for the j values to match the theoretical value of 1.27 for an infinite plate, the effectiveness was in the range of 60-90%.