Browsing by Subject "cooling"
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Item Heat resistance and outgrowth of clostridium perfringens spores as affected by the type of heating medium, and heating and cooling rates in ground pork(2009-05-15) Marquez Gonzalez, MayraThe survival and germination of Clostridium perfringens spores in different heating media and at different heating rates was studied to determine the fate of C. perfringens spores during abusive cooking and cooling of pork products. The heat resistance (HR) of C. perfringens spores from three strains that were either previously heat shocked (HS) or non-heat shocked (NHS) was determined individually and as a cocktail in phosphate buffer solution (pH 7.4) (PBS), beef gravy (BG), ground pork (GP) and cured ground pork (CGP) at 75?C. The effect of the heating rate on HR, germination and outgrowth of C. perfringens spores in CGP was determined by increasing the temperature from 20 to 75?C at a rate of 4, 8, and 12?C/h prior to heating and holding at 75?C for 48 h. Heating rates at 4?C/h in GP and CGP were repeated with additional cooling from 54.4 to 7.2?C within 20 h (temperature abuse). Linear survival curves were observed on NHS spores in the four heating media, whereas HS spores showed linear curves when heated in PBS and BG, and biphasic curves when heated in GP and CGP. In general, HS spores were more heat sensitive than NHS spores. NHS spores heated in GP had greater HR than spores heated in CGP, BG or PBS. There were no significant differences (P>0.05) on the HR of C. perfringens spores in CGP heated from 20 to 75?C at 4, 8, or 12?C/h. Heating rates of 8 and 12?C/h showed no difference in germination and outgrowth of inoculated spores, whereas at 4?C/h, growth of C. perfringens occurred between 44 and 56?C. Temperature abuse during cooling of GP resulted in 2.8 log CFU/g increase of C. perfringens counts. In CGP, C. perfringens counts decreased by 1.1 log CFU/g during cooling from 54.4 to 36.3?C and then increased by 1 log CFU/g until the product reached 7.2?C. However, with an initial inoculum in raw CGP of 5 log CFU C. perfringens spores/g, C. perfringens counts did not exceed 3.4 log CFU/g during a 20 h abusive cooling. These results suggest there is no risk associated with C. perfringens in cured pork products under the conditions tested. Results from the present study indicate that different behavior may be expected with different meat products.Item Optimization of the configuration and working fluid for a micro heat pipe thermal control device(Texas A&M University, 2006-04-12) Coughlin, Scott JosephContinued development of highly compact and powerful electronic components has led to the need for a simple and effective method for controlling the thermal characteristics of these devices. One proposed method for thermal control involves the use of a micro heat pipe system containing a working fluid with physical properties having been speciffcally selected such that the heat pipes, as a whole, vary in effective thermal conductance, thereby providing a level of temperature regulation. To further explore this possibility, a design scenario with appropriate constraints was established and a model developed to solve for the effective thermal conductance of individual heat pipes as a function of evaporator-end temperature. From the results of this analysis, several working fluids were identified and selected from a list over thirteen hundred that were initially analyzed. Next, a thermal circuit model was developed that translated the individual heat pipe operating characteristics into the system as a whole to determine the system level effects. It was found that none of the prospective fluids could completely satisfy the established design requirements to regulate the device temperature over the entire range of operating conditions. This failure to fully satisfy design requirements was due, in large part, to the highly constrained nature of problem definition. Several fluids, however, did provide for an improved level of thermal control when compared to the unmodified design. Suggestions for improvements that may lead to enhanced levels of thermal control are offered as well as areas that are in need of further research.Item Thermal protection of high temperature polymer-material-carbon fiber composites(Texas A&M University, 2006-04-12) O'Neal, Justin EarlTwo evaporative-cooling materials were studied which are (i) salt hydrates and (ii) polyacrylic acid for the purpose showing proof of concept of being able to put evaporative-cooling materials into a composite with the Air Force polyimide AFR-PEPAN. The salt hydrates were observed to absorb water and then evaporate water, but due to having a collapsible lattice, made them incapable of reabsorbing water. Polyacrylic acid was mixed into an epoxy sheet at polacrylic acid weight percentages of 5, 10, 12.5. For each weight percentage there was a hydrated epoxy specimen and a dry epoxy specimen. All specimens were individually shot with a hot air stream (temperature approximately 1300C). Temperature readings were taken for each sheet. The hydrated specimen exhibited greater evaporative cooling over its dry counterpart. 12.5 wt% was shown to have the best evaporative cooling mechanism. Experiments were repeated to show that the polyacrylic could reabsorb water. This study illustrates proof of concept utilizing polyacrylic acid as an evaporative cooling material.