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Figure 2. Graphical representation of thermal death time (TDT) for microorganisms.

absolute zero count theoretically cannot be reached. Each order of D minutes results in a reduction of 90%, which implies that 10% are left. Thus, this becomes the probability of survival and a number of D- values is used to define the order of the process. To specify a TDT curve, all that is needed is a slope fe-value) and a reference point providing a D-value at a reference temperature. For the sterilization of low-acid foods with highly resistant microorganisms, a reference temperature of 250°F (121°C) is generally specified and the order = 12D, or 12 decimal reductions. In the case of C. botulinum, in low-acid foods, the highest Z)2so value known for this organism is 0.21 min; therefore, the minimum process sterilizing value is F = 0.21 x 12 = 2.52 min. Lethal rate may be calculated as 1 It = 10(r_250)&. Actually, all low-acid foods are processed far beyond the minimum botulinum cook in order to deal with spoilage bacteria of much greater heat resistance. In the case of high-acid foods or pasteurization processes in which microorganisms have lower heat resistance, a reference temperature of 212° (100°C) or 150°F (65.6°C) is used. Factors affecting thermal resistance of microorganisms or spores are pH and buffer components (eg, salts), ionic environment, water activity (particularly concentrated-type products, such as condensed soups or evaporated milk), and overall medium composition.

This information in combination with the temperature history of the sample will provide the information required to design an appropriate thermal process. Foods processed in containers require that their temperature history be determined at the slowest heating point (8,9). Heat penetration depends on: (1) surface heat-transfer coefficient, (2) physical and thermal properties of the product, (3) AT between the steam and initial product temperature, (4) container size (overall dimensions), and (5) type of heat transfer within the container (ie, convection or conduc tion). A good review on determining heat penetration in canned foods, which is beyond the scope of this presentation, and the use of Ball's formula for calculating thermal process times is presented by Teixeira (10).

In optimizing a thermal process, it is not only important to inactivate microorganisms or spores but also to retain sensory and nutrient quality factors. Since reaction rate constants and dependency on temperature are very different for nutrient and sensory quality factors as compared with thermal destruction of microbes, an opportunity exists to optimize quality. Aseptic canning systems have become more and more prevalent in recent years to minimize quality losses that occur in slow heated foods processed in conventional retort systems and to allow more economical packaging systems. The basic concept allows sterilizing the product using either HTST or ultrahigh temperature (UHT) processing outside the container, followed by filling aseptically into separately sterilized containers. A good example of quality improvement with such aseptic systems has been presented by Josylyn and Heid (11) showing thermal inactivation of a bacteria (F0 = 6 min) compared with thiamin destruction curves for various levels of loss (Fig. 3). The only criteria is that a time/temperature relationship be chosen along the TDT curve. It is clear that higher processing temperatures allow greater retention of thiamin than at lower processing temperatures at longer processing times. Thiamin is one of the most thermally labile of all the vitamins and provides a good illustration of how quality can be maximized without sacrificing microbiological safety. It should be kept in mind, however, that this particular example also assumes that a first-order rate re-

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