To determine if there were any probes that consistently gave lower values than the others during each phase, the average temperature for each was calculated and compared. Next the ranges (i.e., maximum minus minimum) of temperatures of all TDTs at each time interval were calculated as a measure of the time when temperature uniformity was attained.In addition to the location of the slowest heating or cold areas in the product zone, the canner is often interested in locating those areas in which the heating is most rapid and in which product would receive the most heat during the process. This aspect will be dealt with in more detail later during the discussion of the heat penetration studies.12.3. heat penetration studiesheat penetration studies involve placing specially prepared test containers that cotain a TMD located at the slowest heating point within the container and contents at the slowest heating area in the product zone in the retort. The temperature within the cotainer is measured and recorded at specific and equal time intervals along with the heating or cooling media temperatures. Thermocouples are commonly used as the TMD as they are relatively inexpensive, accurate, and very sensitive (i.e., very quick response time). However, they require leads from the container out through the retort to the recorder. Devices are now available (e.g., the Ball Datatrace) that combine temperature sensing and recording in a self-contained unit that can be placed inside the container. Upon completion of the test, the units are linked to a computer, and the time/temperature sequences are read. These devices are particularly advantageous in the case of rotary retorts and have shown an accuracy equal to that of the thermocouple under most conditions (CFPR, 1989). One disadvantage, however, is their size. While relatively small, they are considerably larger than the thermocouples commonly used. In addition to the space they occupy in the container, maintaining their position within the container and their size can significantly affect the heat penetration. In small containers, the size will significantly reduce the quantity of food product that can be placed in the test containers, and, hence, the heat penetration. Some workers have by-passed the temperature measurement method and have placed high numbers of the target mocroorgnism, usually spores, in capillary tubes in the slowest heating point in the container and extrapolated the survivals/destruction data.The guidelines for the calibration and use of thermocouples in the temperature distribution studies are applicable to heat penetration studies. However, there are critical factors that must be known and controlled if reliable data are to be obtained.12.3.1 factors that affect heating characteristicsThe manner by which heat penetrates into a product, or departs when cooled, has been reviewed in Chapter 11.
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