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High pressure processing (HPP; 100-700 MPa at temperatures < 45°C) and pressure-assisted thermal processing (PATP) (500-700 MPa; 90-120°C) have been used to inactivate pathogenic and spoilage bacteria and produce high quality foods. The objectives of this dissertation were to evaluate the influence of various pressure-temperature combinations on quality, microbial lethality and thermophysical properties of selected foods. Experiments were conducted to investigate the influence of process temperature (95-121°C) at different pressures (0.1, 500-700 MPa) on carrot quality. Results indicated that under comparable process temperatures (up to 105°C), pressure-assisted thermal processing (PATP) retained the carrot quality attributes such as color and carotene content better than thermal processing (TP). However, process and preprocess thermal history greatly influenced carrot textural change. Pressure protective effects on product hardness at elevated temperatures (110-121°C) were less pronounced. Subsequently, experiments were conducted to evaluate the role of pressure during sequential (pressure pre-treatment at ambient temperature followed by TP) or simultaneous (PATP) treatment in preserving product quality attributes. To learn how different food matrices are influenced by various pressure-heat combinations, experiments were also carried out using carrot, jicama, red radish, zucchini, and apricot. Results showed that TP degraded product texture severely but HPP followed by TP improved texture retention. In comparison to TP alone, PATP better retained texture and color. The beneficial effects of PATP may come from the densification of the tissue due to pressurization or biochemical changes of the pectic substances. Texture retention was product dependent, with jicama being the least influenced among the foods tested. An instrumental based crunchiness index (CI) was developed and validated using sensory data. CI was able to describe textural transformation of various processed products. In-situ thermal conductivity (k), diffusivity (α), volumetric specific heat (ρCp) and isobaric specific heat (Cp) of tomato puree, soy protein isolate (10% W:V), soybean oil, guacamole, honey, cream cheese and sucrose solution (10% W:V) were measured up to 600 MPa at 25°C by a dual needle probe adapted to elevated pressures. Increasing pressure linearly increased k values. Among the food materials tested, the maximum increase in k under pressure was observed for soybean oil (0.173 -0.256 W/m°C), while k of honey had the least change (0.324 – 0.396 W/m°C). Thermal diffusivity of the tested materials showed a positive pressure dependence and can be expressed as a second order polynomial function of pressure. Isobaric specific heat of food materials decreased with increase in pressure. The maximum combined uncertainty in the measurement of k, α, ρCp and Cp were 3.1, 6.8, 6.6 and 6.9%, respectively.A model was developed to calculate accumulated lethality during PATP. A differential equation that considered momentary inactivation rates as a function of pressure and temperature was formulated and numerically solved using Rung-Kutta method. The model was experimentally validated under different process conditions. Predicted log-reduction computed by the model using nth order kinetics was found to be in good agreement with experimental values. The ability of the model to predict microbial reduction was found to be satisfactory when evaluated under various process scenarios. |
