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Indicators of bacterial lethality in semi-rigid plastic food containers during thermal processing Sandberg, Gary

Abstract

Thermocouple methods which have been used traditionally for gathering time-temperature data for agitated retort processes have some physical limitations in that thermocouples located in the centres of containers cannot accurately simulate free-moving particles. For this reason alternative approaches have been taken to measure, understand and model such thermal processes, and this was the purpose of the research carried out in these studies. Thermal diffusivity was determined for a potato/alginate gel in order to detemiine the thermal characteristics of the gel and thereby ascertain its ability to function as a model food particle. Thermal diffusivity was found to average 2.048 x 10⁻⁷ m²/s, which approximates values reported in the literature for potatoes (ranging from 1.64 x10⁻⁷ m²/s to 1.93 x 10⁻⁷ m²/s). Visualisation based on video recording of 307 diameter plastic OMNI® bowls containing four food-simulating particles were conducted in a Lagarde steam/air retort simulator to evaluate the effect of rotation on particle movement. The speed of rotation was 2.8 rpm and particles were found to remain in contact with the outer edge of the container near its lowest point, thus confirming that the principal force acting on the particles is gravitational and not centrifugal. As well, a thin-wire (flexible) thermocouple anchored in the centre of a spherical particle was evaluated as a method of gathering time-temperature data while simulating the movement of free particles. The tethered particle was found to be constrained to rotation in an arc from the top inner surface of the container to the bottom inner surface. Thermal processing studies were carried out during agitated processes in order to investigate heating of 307 diameter plastic OMNI® bowls within a production-scale Lagarde steam/air retort. at various rotational speeds (0, 5, 10, and 15 rpm). Since the goal was to understand processes under production conditions, and since evidence of radial positional effects had been described for the Lagarde steam/air retort simulator, the degree of positional effect was investigated. Bowls positioned at the axial centre and the outside edge heated faster than bowls positioned in the intermediate portion of the retort basket. The zone of slower heating was thought to be due to the annular disc in the door which served to increase heating medium velocity down the outside edges of the retort vessel and assist in its subsequent drawing back towards the fan through the centre. Experiments were then repeated in the Lagarde retort simulator at 0, 5, 10, 15, 20 and 25 rpm to determine whether equivalent patterns were observed. Studies in the Lagarde were carried out with rigid thermocouples and flexible thermocouples with their sensing junctions in potato/alginate gel particles to determine if the flexible thermocouple gave a better model of the temperature history within a freely moving particle in random motion during heating. Positional effects were determined under static and rotational modes. Flexible thermocouples were found to yield lower values for jh , fn and te than the rigid thermocouple. A further stage of this research was aimed at assessing the effects of rotational thermal processes on populations of microorganisms. Suspensions of Bacillus stearothermophilus ATCC 7953 were placed into DSC pans, hermetically sealed and incorporated into the centres of potato/alginate particles (18 mm in diameter). Five such particles were placed into a 307 diameter OMNI® bowl container and processed in the Lagarde steam/air retort simulator. Positional effects were determined as well as rotational effects at 0, 15 and 30 rpm. Studies were done with particles immobilised on rigid thermocouples positioned at the centres of containers as well as on flexible probes (to simulate a flexible thermocouple). Within each small volume of the DSC pans, integrated sterilisation (IS) values were determined, based on spore recovery from free particles, particles with centrally located thermocouples and flexible thermocouples, and the liquid portion of the product. With flexibly anchored and freely moving particles there was close agreement for IS values, while for the rigidly held particles and the liquid contents of the container there was close agreement for IS values. A final study made use of the thermal indicator concept with Clostridium pasteurianum, Bacillus coagulans, and lysozyme to evaluate pasteurisation of tomato-based sauces compared to traditional temperature history methods. Three container sizes were used (3.8 L and 1.9 L plastic jugs and a 750 mL glass jar). Measured lethalities were found to be greater than calculated lethalities for all containers.

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