424 Modern Food Microbiology Table 17–8 Currently Recognized Genera of Heterotrophic Sporeforming Bacteria that are Known to Occur in Foods Along with Some Whose Foodborne Status is Unknown Genus Gram Rx Morphology In Foods? Aerobes Alicyclobacillus Amphibacillus Aneurinibacillus Bacillus Brevibacillus Desulfotomaculum Geobacillus Gracilibacillus Halobacillus Paenibacillus Salibacillus Serratia marcescens subsp Sakuensis Sporolactobacillus Sporosarcina Virgibacillus Thermoactinomycetes Ureibacillus + positive + + + + +a + + + + + − + + + + + R rod R R R R R R R R R R R R Coccus R R R + yes + ? unknown + + + + ? + + ? ? + ? ? ? ? Anaerobes Anaerobacter Caloramater Clostridium Filifactor Moorella Oxobacter Oxolophagus Sporohalobacter Syntrophospora Thermoanaerobacter Thermoanaerobacterium + + + + + + + − − + + R R R R R R R R R R R ? ? + ? + ? ? ? ? + + a Often reported as Gram negative Thermal Death Time Thermal death time (TDT) is the time necessary to kill a given number of organisms at a specified temperature By this method, the temperature is kept constant and the time necessary to kill all cells is determined Of less importance is the thermal death point, which is the temperature necessary to kill a given number of microorganisms in a fixed time, usually 10 minutes Various means have been proposed for determining TDT: the tube, can, “tank,” flask, thermoresistometer, unsealed tube, and capillary tube methods The general procedure for determining TDT by these methods is to place a known number of cells or spores in a sufficient number of sealed containers in order to get the desired number of survivors for each test period The organisms are then placed in an oil bath and heated Food Protection with High Temperatures 425 Figure 17–2 Rate of destruction curve Spores of strain F.S heated at 240◦ F in canned pea brine pH 6.2 Source: From Gillespy,20 courtesy of Butterworths Publishers, London for the required time period At the end of the heating period, containers are removed and cooled quickly in cold water The organisms are then placed on a suitable growth medium, or the entire heated containers are incubated if the organisms are suspended in a suitable growth substrate The suspensions or containers are incubated at a temperature suitable for growth of the specific organisms Death is defined as the inability of the organisms to form visible colonies after extended incubations D Value This is the decimal reduction time, or the time required to destroy 90% of the organisms This value is numerically equal to the number of minutes required for the survivor curve to traverse one log cycle (Figure 17–2) Mathematically, it is equal to the reciprocal of the slope of the survivor curve and is a measure of the death rate of an organism When D is determined at 250◦ F, it is often expressed as Dr The effect of pH on the D value of C botulinum in various foods is presented in Table 17–9, and D values for S Senftenberg 775W under various conditions are presented in Table 17–4 D values of 0.20–2.20 minutes at 150◦ F have been reported for S aureus strains, D 150◦ F (65.5◦ C) of 0.50–0.60 minute for Coxiella burnetii, and D 150◦ F of 0.20–0.30 minute for Mycobacterium tuberculosis.54 For pH-elevating strains of Bacillus licheniformis spores in tomatoes, a D95◦ C of 5.1 minutes has been reported, whereas for B coagulans, a D95◦ C of 13.7 minutes has been found.39 426 Modern Food Microbiology Table 17–9 Effect of pH on D Values for Spores of C botulinum 62A Suspended in Three Food Products at 240◦ F (115◦ C) pH Spaghetti, Tomato Sauce, and Cheese D Value (minutes) Macaroni Creole Spanish Rice 4.0 4.2 4.4 4.6 4.8 5.0 6.0 7.0 0.128 0.143 0.163 0.223 0.226 0.260 0.491 0.515 0.127 0.148 0.170 0.223 0.261 0.306 0.535 0.568 0.117 0.124 0.149 0.210 0.256 0.266 0.469 0.550 Source: From Xezones and Hutchings,65 copyright c 1965 by Institute of Food Technologists D values of some yeasts and molds that cause fruit spoilage are listed in Table 17–10 along with D60◦ C values.50 It may be noted that Saccharomyces cerevisiae was the most heat resistant of the seven listed D70◦ C values of Listeria innocua and a mixture of six Salmonella serotypes determined in six meat and poultry products are noted in Table 17–11.40 Overall, the heat resistance of the salmonellae was only slightly higher than for L innocua Thermal D values of salmonellae in various products have been reviewed.16 The highest D values for salmonellae occurred in liquid egg white and yolks Using a pilot-scale pasteurized and five strains of Mycobacterium avium subsp paratuberculosis, the mean D72◦ C was determined to be 107 cell kill z Value The z value refers to the degrees Fahrenheit required for the thermal destruction curve to traverse one log cycle Mathematically, this value is equal to the reciprocal of the slope of the TDT curve (Figure 17–3) Whereas D reflects the resistance of an organism to a specific temperature, z provides Table 17–10 D Values of Seven Fruit Spoilage Fungi Determined in 0.1 M Citric Acid Buffer at 60◦ C and pH 4.0 (Extracted from Shearer et al.50 ) Penicillium citrinum Torulaspora delbrueckii Rhodotorula mucilaginosa Zygosaccharomyces rouxii Penicillium roquefortii Aspergillus niger Saccharomyces cerevisiae 0.28 0.009 0.018 0.158 0.008 0.290 0.449 Food Protection with High Temperatures 427 Table 17–11 D70◦ C Values of a Six-Serotype∗ Mixture of Salmonella enterica Serotypes and Listeria innocua Determined in Five Food Products (Extracted from Murphy et al.40 ) Food Specimen Chicken patties Chicken tenders Frankfurters Beef patties Beef-turkey patties ∗ S Salmonella L innocua 0.32 0.32 0.39 0.25 0.37 0.21 0.29 0.36 0.29 0.18 California, Heidelberg, Mission, Montevideo, Senftenberg, and Typhimurium information on the relative resistance of an organism to different destructive temperatures; it allows for the calculation of equivalent thermal processes at different temperatures If, for example, 3.5 minutes at 140◦ F is considered to be an adequate process and z = 8.0, either 0.35 minute at 148◦ F or 35 minutes at 132◦ F would be considered equivalent processes D and z values of the beer-spoilage bacteria, Pectinatus spp., are noted in Table 17–12 where P frisingensis had the highest D60◦ C and z values of the three cultures tested.59 These investigators noted how the thermal properties of the tested organisms differed from strain to strain, and from one heating menstruum to another The mean D60◦ C values for an eight-strain cocktail of salmonellae were found to be 1.30 and 5.48 in beef (containing 12.5% fat), and 5.70 in chicken (containing 7% fat), all determined by linear regression methods.25 Figure 17–3 Thermal death time curve Spores of strain F.S heated in canned pea brine pH 6.2 Source: From Gillespy,20 courtesy of Butterworths Publishers, London 428 Modern Food Microbiology Table 17–12 D and z Values of Three Pectinatus spp Determined at 60◦ C in pH 5.2 Fresh Wort (Extracted from Watier et al.59 ) Organisms Pectinatus cerevisiiphilus P frisingensis Pectinatus sp D60 Values Z(◦ C) 0.12 1.69 1.17 3.53 8.49 6.13 F Value This value is the equivalent time, in minutes, at 250◦ F (121.1◦ C) of all heat considered, with respect to its capacity to destroy spores or vegetative cells of a particular organism The integrated lethal value of heat received by all points in a container during processing is designated Fs or F0 This represents a measure of the capacity of a heat process to reduce the number of spores or vegetative cells of a given organism per container When we assume instant heating and cooling throughout the container of spores, vegetative cells, or food, F0 may be derived as follows: F0 = Dr (log a − log b), where a is the number of cells in the initial population and b is the number of cells in the final population Thermal Death Time Curve For the purpose of illustrating a thermal destruction curve and D value, data are employed from Gillespy20 on the killing of flat sour spores at 240◦ F (115◦ C) in canned pea brine at pH 6.2 Counts were determined at intervals of minutes with the mean viable numbers indicated as follows: Time (minutes) 10 15 20 25 Mean Viable Count 340.0 65.0 19.0 4.5 1.3 The time of heating in minutes is plotted on semi-log paper along the linear axis, and the number of survivors is plotted along the log scale to produce the TDT curve presented in Figure 17–2 The curve is essentially linear, indicating that the destruction of bacteria by heat is logarithmic and obeys a first-order reaction Although difficulty is encountered at times at either end of the TDT curve, process calculations in the canning industry are based on a logarithmic order of death From the data presented in Figure 17–2, the D value is calculated to be minutes, or D240 = 8.0 D values may be used to reflect the relative resistance of spores or vegetative cells to heat The most heat-resistant strains of C botulinum types A and B spores have a Dr value of 0.21, whereas the most heat-resistant thermophilic spores have Dr values of around 4.0–5.0 Putrefactive anaerobe (PA) 3679 ... required time period At the end of the heating period, containers are removed and cooled quickly in cold water The organisms are then placed on a suitable growth medium, or the entire heated containers... reported, whereas for B coagulans, a D95◦ C of 13.7 minutes has been found.39 426 Modern Food Microbiology Table 17–9 Effect of pH on D Values for Spores of C botulinum 62A Suspended in Three Food. .. noted in Table 17–12 where P frisingensis had the highest D60◦ C and z values of the three cultures tested.59 These investigators noted how the thermal properties of the tested organisms differed