# How many of each type of cell are present the

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Unformatted text preview: are present. the relevant death rate constants. FOOD-297 Because it is impractical to obtain this information on a routine basis, the heating conditions used are those necessary to kill the most heat-resistant biological structures formed by potential food-bourne pathogens: the endospores of the bacteria Clostridium and Bacillus sp. Further, a large margin of safety is built into the heating conditions. Heating is conducted to ensure the endospore population (if present) will be reduced by 12 factors of 10 (= 12 orders of magnitude = 12 logs). This standard was decided by empirical observation. i.e. what worked and what didn’t. The time to reduce the population size by 12 logs can be determined experimentally and is called the 12 D time. For example the 12 D time for C. botulinum endospores is 2.52 minutes at 121oC/15 psi. Lowering the number of viable endospores by 12 logs means, if there was initially a single endospore in every can of a food product for example, after heat treatment, one would have to eat one trillion (1012) cans of food to be sure of eating one viable endospore. Note the food industry never says there is “0” chance of getting sick from eating a food product. There is always some degree of risk. This is clear from the death equation. It is impossible to solve this equation for an Nt = 0. Therefore it is impossible to specify a time that will result in sterilization. “z” values The food industry uses several parameters to evaluate the efficiency of heat killing. The D-value and the 12D time have already been described. The z-value is an additional parameter. It is the increase in temperature needed to lower the D-value to 1/10th its previous value. If, for example, D100 (the D value at 100°C) and D110 (the D value at 110°C) for a given organism are 20 minutes and 2 minutes, respectively, then 12D100 equals 240 minutes (that is, 20 minutes x 12), and the z-value is 10°C (because a 10°C increase in temperature reduces the D-value from 20 minutes to 2 minutes). These measurements are extremely important to the canning industry, which must ensure that canned goods do not contain endospores. Figure 9-9: Growth (and death) of E. coli O157:H7 in Greek eggplant salad at 5oC and different pHs (CFU = colony forming unit) FOOD-298 Decimal Reduction Time: Practice The following data was obtained for the destruction of Bacillus cereus endospores at a temperature of 121oC under a pressure of 15 psi. Heating time (min) Dilution factor Mean # colonies per plate 5 10 15 20 25 10 1 1 1 1 34 65 19 4.5 1.3 # of replicate plates counted 20 10 25 50 100 A dilution factor = 1 means the sample was not diluted before plating. A dilution factor = 10 means the sample was diluted by a factor of 10 before plating. (see below). Using 3 cycle semi-log graph paper, estimate the decimal reduction time, D, for the endospore population. Answer: ~ 8 min. Additional question: Why can we use colony counts from plates that have less than 30 colonies? FOOD-299 pH Low pH Hig...
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## This note was uploaded on 10/25/2013 for the course MICB 201 taught by Professor Davidturner during the Fall '12 term at The University of British Columbia.

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