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Heat Resistance of Salmonella Typhimurium and Salmonella Senftenberg 775 W in Chicken Meat
Heat Resistance of Salmonella Typhimurium and Salmonella Senftenberg 775 W in Chicken Meat HENRY G. BAYNE, JOHN A. GARIBALDI AND HANS LINEWEAVER Western Regional Research Laboratory? Albany, California (Received for publication March 25, 1965)
X A laboratory of the Western Utilization Research and Development division, Agricultural Research Service, U. S. Department of Agriculture. Reference to a company or product nam: does not imply approval or recommendation of the product by the U. S. Department of Agriculture to the exclusion of others that may be suitable.
been cultured in the laboratory and its heat resistance studied extensively. A second exception to destruction by mild heat treatments is observed when heating is not uniform. Such is the case in scrambled eggs, omelets, or in turkey stuffing, where some survivors have been reported from temperatures of 70 to 80°C. (Gibbons and Moore, 1944; Castellani, 1953). Such survival is readily understood since all areas of the food do not reach the cooking temperature recorded. A third and rather puzzling exception is the work by Hussemann and Buyske (1954). They reported that S. typhimurium in ground chicken muscle heated in thermal death time tubes, where heating should have been uniform, survived for 10 minutes at 75, 80, 85, and 90°C. (Figure 1). That the suspending medium can markedly influence the heat sensitivity of bacteria is well documented (Hansen and Riemann, 1963). However the effect reported in their study (Hussemann and Buyske, 1954) is so great that we have reinvestigated the survival of Salmonella in heated chicken muscle. MATERIALS AND METHODS
Our procedures were essentially those of Hussemann and Buyske (1954). In general, known numbers of bacterial cells were mixed with ground pectoral muscle, placed in sterile glass tubes which were then sealed and heated for selected periods of time in either an oil or water bath. The tubes were cooled and the contents were dropped into an enrichment medium, which was then incubated at 37°C. Aliquots of this
1281
Downloaded from http://ps.oxfordjournals.org/ at Gazi Universitesi on May 2, 2015
E L I M I N A T I O N from foods of patho*—' gens of the genus Salmonella has been the object of many investigations in the past 60 years. Generally, temperatures of 60 to 65°C. for several minutes are sufficient to destroy Salmonella even when they are present in counts as high as a million per g. Rosenau (1912) reported the destruction of B. typhosus in milk in 2 minutes at 60°C. Tanner and Dubois (1925) confirmed the destruction of the paratyphoid group in 10 to 20 minutes at 60°C. Winter et al. (1946) reported that most of the 164 cultures they studied were destroyed in less than 3 minutes at 60°C. Osborne et al. (19S4) and Anellis et al.(1954) reported similar data from a somewhat more detailed study with a smaller number of strains. Nevot et al. (1958) observed that 5. typhimurium were destroyed in 3.3 minutes at 60°C. Whereas the foregoing citations seem to establish that Salmonellae are destroyed very rapidly at temperatures of 60 to 65°C, exceptions have been reported. One is the heat resistant strain of S. senftenberg 775 W. This strain, isolated by Winter et al. (1946), is 10 to 20 times as heat resistant as other senftenbergs or other species. Although its reinsolation from food has never been reported, the strain has
1282
H. G. BAYNE, J. A. GARIBALDI AND H. LINEWEAVEE
;»—To
(0
3 x 10* at t=0
0.1
10
Calculated from HussemannfA Buyske J ' I ' I 40 20 30 Minutes heated
enrichment medium were then plated on various differential media. Specific procedures were as follows: Preparation of Chicken Muscle. The pectoral muscle of frozen, thawed, commercially processed frying or stewing hens were removed aseptically. The muscles were ground three times in a sterile Hamilton Beach meat grinder. Fifty- and 100-g. portions of the ground muscle were placed in sterile jars and kept frozen until used. Ten g. samples of meat were used to determine the initial bacterial population of the muscle. Bacteriological examination of the prepared but uninoculated ground meat showed it contained no Salmonella and less than 100 bacteria per g. The pH of the meat was approximately 5.8. Cell Suspensions. Two species of Salmonella were used in these studies; Salmonella typhimurium Tm-1 (Hussemann's strain # 84) and Salmonella senjtenberg 775 W, the well known heat resistant strain. The cells were grown on tryptone-glucose-extract agar (Difco) in Roux flasks (200 ml.) at 37°C. for 18 hours. The cells were har-
Downloaded from http://ps.oxfordjournals.org/ at Gazi Universitesi on May 2, 2015
Frc. 1. Most probable number (MPN) of survivors calculated from the data of Hussemann and Buyske. The formula e"x = n/q was used where x = MPN, n = number of sterile tubes, q = number of tubes tested. In this case n/q = 1 — % positives/100, % positives were obtained from Table 1 of Hussemann and Buyske.
vested with sterile saline (0.85%) and adjusted to a population density of 2.5-4 X 109 cells per ml. of suspension. Filling the Experimental Tubes. On day of use, the ground muscle was thawed and inoculated with 1.0 ml. of cell suspension per 10 g. of meat. The meat was placed aseptically in a 10 ml. sterile Tomac plastic syringe fitted with a 150 mm. sterile cannula and 1.2 ± 0.2 g. quantities were delivered into sterile 13 X 100 mm. Corning pyrex tubes. The tubes were sealed and placed in a test tube rack especially adapted to permit complete immersion in the heating bath. In some cases the tubes were clamped at different distances from the position of the meat. They were then immersed in an ice bath for 10 minutes. Exposure to Heat. The tubes were removed from the ice bath and completely immersed into the test bath. After the desired treatment period had elapsed, the tubes were cooled for a least 5 minutes in an ice bath. The temperature of the meat was determined by copper-constantan thermocouples placed between the tube and the meat and at the center of the meat sample and recorded on a sensitive potentiometer. The lag time required to bring the meat samples to within 0.5°C. of the bath temperature was about 4 minutes. Detection of Survivors. After cooling the tubes were opened with a hot wire and the contents dropped aseptically into 9.0 ml. of selective enrichment medium (Tetrathionate Broth (Difco) and Selenite F Cystine (BBL)). A pre-enrichment step in a non-selective medium resulted in no additional survivors. After incubation for 24 hours at 37°C. aliquots (0.1-0.2 ml.) of this enrichment were spread plated on trypticase soy or tryptone-glucose extract, brilliant green and salmonella-shigella agar plates. After 18-24 hours at 37°C. colonies on the plates were counted. Negative results were
HEAT RESISTANCE OF
confirmed by replating the enrichment after further incubation. Presumptive positive colonies were confirmed with Salmonella diagnostic sera.
TABLE 2.—Sensitivity of S. senftenberg 775-W to various temperatures (Initial count 3X10 8 /g. of chicken meat) Temperature '5C. 65
time RESULTS
TABLE 1.—Sensitivity of S. typhimurium Tm-1 to various temperatures (Initial count 3X10 8 /g. of chicken meat) Temperature Heat- • ing time min. 5 10 15 22 25 28 31 40 1 2
°c.
55 A
B
C
4/4 1 4/4
0/4
4/4 4/4 0/4 0/4
65 A
75 A
0/46 0/40
0/40
10/10 10/10 0/10 (45%) 2
Positive samples/total samples. Data of Hussemann and Buyske (1954).
3 5 8 10 15 1 2
A
B
10/10 1 10/10
8/8 8/8
10/10 0/10
0/8 0/8
75 A 24/38 16/38 0/24, 0/28 2 0/41
Positive samples/total samples. Oil bath.
death times been precisely measured, the data indicate that S. typhimurium Tm-1 in chicken meat does not have a low temperature coefficient that might be associated with survival at high temperatures. Possibly there is some unexpected effect of heating the heterogeneous mixture of meat and Salmonella above 60°C. Results of experiments conducted at both 65 and 75°C. are shown in Table 1. No Salmonella survived in 126 tubes subjected to these temperatures. This is in contrast to the report of Hussemann and Buyske that 45% of the tubes treated 40 minutes at 75°C. contained viable organisms. Note, however, that the number of survivors per tube (calculated from the data of Hussemann and Buyske) was less than 2 per tube (Figure 1). Loose or firm packing of meat in the tubes did not affect the results. For comparison, several experiments were conducted with ground muscle inoculated with the heat resistant strain of S. senftenberg 775 W. This organism survived exposure to 65° C. for S minutes but was destroyed in 10 minutes in one experiment and in 15 minutes in another (Table 2). Results of exposure of S. senftenberg 775 W at 75°C. are also given in Table 2. Exposure of a large number of tubes for 8 minutes in either a water or an oil bath (in
Downloaded from http://ps.oxfordjournals.org/ at Gazi Universitesi on May 2, 2015
The sensitivity of S. typhimurium Tm-1 (Hussemann's # 8 4 ) suspended in ground chicken muscle exposed to various temperatures is given in Table 1. Twenty-eight minutes at 55°C. killed all cells in 10 one g. samples of chicken meat containing 3 X 108 cells per g. At 60°C, 5 minutes exposure killed all cells in a 15 tube test. Complete kills were also recorded in each of IS tube tests exposed for 10, IS, and 20 minutes at this temperature. Angelotti et al. (1960) and others reported z values (number of degrees F. necessary for the thermal death time curve to traverse one log cycle) of approximately 9 for most strains of Salmonella. If their value holds for this organism in ground chicken meat and if none survives at 55°C. after 28 minutes heating as reported in Table 1, we would not expect to find survivors after 5 minutes heating at 60°C. Thus, although no corrections have been made for come-up time nor have thermal
1283
SALMONELLA
1284
H. G. BAYNE, J. A. GARIBALDI AND H. LINEWEAVER
SUMMARY
The heat resistance of S. typhimurium Tm-1 and 5. senftenberg 775 W in ground chicken muscle was determined at four temperatures in the range of 55°C. (131°F.) to 75°C. (167°F.). Multiple one g. samples of meat containing 3 X 10s cells of 5. typhimurium Tm-1 ( # 8 4 ) after exposure for 5 minutes to 60°C. (140°F.) contained no viable cells. The more resistant S. senftenberg 775 W required an exposure of from 10 to 15 minutes at 65°C. to kill an equal number of cells. The data indicate that the heat resistance of Salmonella in ground chicken muscle is similar to its heat resistance in other foods. ACKNOWLEDGMENT
The authors wish to acknowledge the fruitful discussions with Dr. Hans Riemann. REFERENCES Anellis, A., J. Lubas and M. M. Rayman, 19S4.
Heat resistance in liquid eggs of some strains of the genus Salmonella. Food Research, 19: 377-395. Angelotti, R., M. J. Foter and K. Lewis. 1960. Time-temperature effects on Salmonellae and Staphylococci in foods. II. Behavior at warm holding temperatures. Thermal-death-time studies. U. S. Health, Education and Welfare, Robert A. Taft Sanitary Engineering Center Technical Report F60-5, 49 pp. Castellani, A. G., R. R. Clarke, M. I. Gibson and D. F. Meisner, 1953. Roasting time and temperature required to kill food poisoning microorganisms introduced experimentally into stuffings in turkeys. Food Research, 18: 131-138. Gibbons, N. E., and R. L. Moore, 1944. Dried whole egg powder. XII. The effect of drying, storage, and cooking on the Salmonella content. Can. J. Research, Sec. F. Technology, 22: 58-63. Goresline, H. E., K. M. Hayes, R. E. Moser, Jr., M. A. Howe, Jr. and E. E. Drewniak, 1951. Pasteurization of liquid whole egg under commercial conditions to eliminate Salmonella. U. S. Dept. Agr. Circ. 897. U. S. Govt. Printing Office, Washington. 15 pp. Hansen, N.-H., and H. Riemann, 1963. Factors affecting the heat resistance of nonsporing organisms. J. Appl. Bact, 26: 314-333. Hussemann, D. L., and J. K. Buyske, 1954. Thermal death time-temperature relationships of Salmonella typhimurium in chicken muscle. Food Research, 19: 3S1-356. Nevot, A., Ph. Lafont et Y. Lafont, 1958. De la destruction des bacteries par la chaleur; etude de I'efficacite de la pasteurisation du lait. Paris. Institut national d'hygiene (Monograph No. 18). Osborne, W. W., R. P. Straka and H. Lineweaver, 1954. Heat resistance of strains of Salmonella in liquid whole egg, egg yolk, and egg white. Food Research, 19: 451-463. Rosenau, M. J., 1912. Thermal death point of pathogenic microorganisms in milk. U. S. Hyg. Lab. Bull. 2nd ed., 56: 683-686. Tanner, F. W., and G. C. Dubois, 1925. Some notes on the effect of heat on members of the colontyphoid group in milk. J. Dairy Sci. 8: 47-53. Winter, A. R., G. F. Stewart, V. H. McFarlane and M. Solowey, 1946. Pasteurization of liquid egg products. III. Destruction of Salmonella in liquid whole egg. Am. J. Public Health 36: 451-460.
Downloaded from http://ps.oxfordjournals.org/ at Gazi Universitesi on May 2, 2015
the latter poorer heat transfer increased the come-up time) was sufficient to kill all of 3 X 10s cells per tube. In evaluating 3 to 5 minute exposures, it is important to remember that it required approximately 4 minutes for the meat to rise to within 0.5°C. of the test temperature. The results with S. typhimurium agree with the heat resistance studies of Winter et al. (1946), Goresline et al. (1951) and Angelotti et al. (1960). Similarly the results with 5. senftenberg 775 W parallel those of Angelotti et al. (1960) with chicken a la king. In our view, the data indicate that suspension of Salmonella in chicken muscle does not measurably alter their heat resistance.
X A laboratory of the Western Utilization Research and Development division, Agricultural Research Service, U. S. Department of Agriculture. Reference to a company or product nam: does not imply approval or recommendation of the product by the U. S. Department of Agriculture to the exclusion of others that may be suitable.
been cultured in the laboratory and its heat resistance studied extensively. A second exception to destruction by mild heat treatments is observed when heating is not uniform. Such is the case in scrambled eggs, omelets, or in turkey stuffing, where some survivors have been reported from temperatures of 70 to 80°C. (Gibbons and Moore, 1944; Castellani, 1953). Such survival is readily understood since all areas of the food do not reach the cooking temperature recorded. A third and rather puzzling exception is the work by Hussemann and Buyske (1954). They reported that S. typhimurium in ground chicken muscle heated in thermal death time tubes, where heating should have been uniform, survived for 10 minutes at 75, 80, 85, and 90°C. (Figure 1). That the suspending medium can markedly influence the heat sensitivity of bacteria is well documented (Hansen and Riemann, 1963). However the effect reported in their study (Hussemann and Buyske, 1954) is so great that we have reinvestigated the survival of Salmonella in heated chicken muscle. MATERIALS AND METHODS
Our procedures were essentially those of Hussemann and Buyske (1954). In general, known numbers of bacterial cells were mixed with ground pectoral muscle, placed in sterile glass tubes which were then sealed and heated for selected periods of time in either an oil or water bath. The tubes were cooled and the contents were dropped into an enrichment medium, which was then incubated at 37°C. Aliquots of this
1281
Downloaded from http://ps.oxfordjournals.org/ at Gazi Universitesi on May 2, 2015
E L I M I N A T I O N from foods of patho*—' gens of the genus Salmonella has been the object of many investigations in the past 60 years. Generally, temperatures of 60 to 65°C. for several minutes are sufficient to destroy Salmonella even when they are present in counts as high as a million per g. Rosenau (1912) reported the destruction of B. typhosus in milk in 2 minutes at 60°C. Tanner and Dubois (1925) confirmed the destruction of the paratyphoid group in 10 to 20 minutes at 60°C. Winter et al. (1946) reported that most of the 164 cultures they studied were destroyed in less than 3 minutes at 60°C. Osborne et al. (19S4) and Anellis et al.(1954) reported similar data from a somewhat more detailed study with a smaller number of strains. Nevot et al. (1958) observed that 5. typhimurium were destroyed in 3.3 minutes at 60°C. Whereas the foregoing citations seem to establish that Salmonellae are destroyed very rapidly at temperatures of 60 to 65°C, exceptions have been reported. One is the heat resistant strain of S. senftenberg 775 W. This strain, isolated by Winter et al. (1946), is 10 to 20 times as heat resistant as other senftenbergs or other species. Although its reinsolation from food has never been reported, the strain has
1282
H. G. BAYNE, J. A. GARIBALDI AND H. LINEWEAVEE
;»—To
(0
3 x 10* at t=0
0.1
10
Calculated from HussemannfA Buyske J ' I ' I 40 20 30 Minutes heated
enrichment medium were then plated on various differential media. Specific procedures were as follows: Preparation of Chicken Muscle. The pectoral muscle of frozen, thawed, commercially processed frying or stewing hens were removed aseptically. The muscles were ground three times in a sterile Hamilton Beach meat grinder. Fifty- and 100-g. portions of the ground muscle were placed in sterile jars and kept frozen until used. Ten g. samples of meat were used to determine the initial bacterial population of the muscle. Bacteriological examination of the prepared but uninoculated ground meat showed it contained no Salmonella and less than 100 bacteria per g. The pH of the meat was approximately 5.8. Cell Suspensions. Two species of Salmonella were used in these studies; Salmonella typhimurium Tm-1 (Hussemann's strain # 84) and Salmonella senjtenberg 775 W, the well known heat resistant strain. The cells were grown on tryptone-glucose-extract agar (Difco) in Roux flasks (200 ml.) at 37°C. for 18 hours. The cells were har-
Downloaded from http://ps.oxfordjournals.org/ at Gazi Universitesi on May 2, 2015
Frc. 1. Most probable number (MPN) of survivors calculated from the data of Hussemann and Buyske. The formula e"x = n/q was used where x = MPN, n = number of sterile tubes, q = number of tubes tested. In this case n/q = 1 — % positives/100, % positives were obtained from Table 1 of Hussemann and Buyske.
vested with sterile saline (0.85%) and adjusted to a population density of 2.5-4 X 109 cells per ml. of suspension. Filling the Experimental Tubes. On day of use, the ground muscle was thawed and inoculated with 1.0 ml. of cell suspension per 10 g. of meat. The meat was placed aseptically in a 10 ml. sterile Tomac plastic syringe fitted with a 150 mm. sterile cannula and 1.2 ± 0.2 g. quantities were delivered into sterile 13 X 100 mm. Corning pyrex tubes. The tubes were sealed and placed in a test tube rack especially adapted to permit complete immersion in the heating bath. In some cases the tubes were clamped at different distances from the position of the meat. They were then immersed in an ice bath for 10 minutes. Exposure to Heat. The tubes were removed from the ice bath and completely immersed into the test bath. After the desired treatment period had elapsed, the tubes were cooled for a least 5 minutes in an ice bath. The temperature of the meat was determined by copper-constantan thermocouples placed between the tube and the meat and at the center of the meat sample and recorded on a sensitive potentiometer. The lag time required to bring the meat samples to within 0.5°C. of the bath temperature was about 4 minutes. Detection of Survivors. After cooling the tubes were opened with a hot wire and the contents dropped aseptically into 9.0 ml. of selective enrichment medium (Tetrathionate Broth (Difco) and Selenite F Cystine (BBL)). A pre-enrichment step in a non-selective medium resulted in no additional survivors. After incubation for 24 hours at 37°C. aliquots (0.1-0.2 ml.) of this enrichment were spread plated on trypticase soy or tryptone-glucose extract, brilliant green and salmonella-shigella agar plates. After 18-24 hours at 37°C. colonies on the plates were counted. Negative results were
HEAT RESISTANCE OF
confirmed by replating the enrichment after further incubation. Presumptive positive colonies were confirmed with Salmonella diagnostic sera.
TABLE 2.—Sensitivity of S. senftenberg 775-W to various temperatures (Initial count 3X10 8 /g. of chicken meat) Temperature '5C. 65
time RESULTS
TABLE 1.—Sensitivity of S. typhimurium Tm-1 to various temperatures (Initial count 3X10 8 /g. of chicken meat) Temperature Heat- • ing time min. 5 10 15 22 25 28 31 40 1 2
°c.
55 A
B
C
4/4 1 4/4
0/4
4/4 4/4 0/4 0/4
65 A
75 A
0/46 0/40
0/40
10/10 10/10 0/10 (45%) 2
Positive samples/total samples. Data of Hussemann and Buyske (1954).
3 5 8 10 15 1 2
A
B
10/10 1 10/10
8/8 8/8
10/10 0/10
0/8 0/8
75 A 24/38 16/38 0/24, 0/28 2 0/41
Positive samples/total samples. Oil bath.
death times been precisely measured, the data indicate that S. typhimurium Tm-1 in chicken meat does not have a low temperature coefficient that might be associated with survival at high temperatures. Possibly there is some unexpected effect of heating the heterogeneous mixture of meat and Salmonella above 60°C. Results of experiments conducted at both 65 and 75°C. are shown in Table 1. No Salmonella survived in 126 tubes subjected to these temperatures. This is in contrast to the report of Hussemann and Buyske that 45% of the tubes treated 40 minutes at 75°C. contained viable organisms. Note, however, that the number of survivors per tube (calculated from the data of Hussemann and Buyske) was less than 2 per tube (Figure 1). Loose or firm packing of meat in the tubes did not affect the results. For comparison, several experiments were conducted with ground muscle inoculated with the heat resistant strain of S. senftenberg 775 W. This organism survived exposure to 65° C. for S minutes but was destroyed in 10 minutes in one experiment and in 15 minutes in another (Table 2). Results of exposure of S. senftenberg 775 W at 75°C. are also given in Table 2. Exposure of a large number of tubes for 8 minutes in either a water or an oil bath (in
Downloaded from http://ps.oxfordjournals.org/ at Gazi Universitesi on May 2, 2015
The sensitivity of S. typhimurium Tm-1 (Hussemann's # 8 4 ) suspended in ground chicken muscle exposed to various temperatures is given in Table 1. Twenty-eight minutes at 55°C. killed all cells in 10 one g. samples of chicken meat containing 3 X 108 cells per g. At 60°C, 5 minutes exposure killed all cells in a 15 tube test. Complete kills were also recorded in each of IS tube tests exposed for 10, IS, and 20 minutes at this temperature. Angelotti et al. (1960) and others reported z values (number of degrees F. necessary for the thermal death time curve to traverse one log cycle) of approximately 9 for most strains of Salmonella. If their value holds for this organism in ground chicken meat and if none survives at 55°C. after 28 minutes heating as reported in Table 1, we would not expect to find survivors after 5 minutes heating at 60°C. Thus, although no corrections have been made for come-up time nor have thermal
1283
SALMONELLA
1284
H. G. BAYNE, J. A. GARIBALDI AND H. LINEWEAVER
SUMMARY
The heat resistance of S. typhimurium Tm-1 and 5. senftenberg 775 W in ground chicken muscle was determined at four temperatures in the range of 55°C. (131°F.) to 75°C. (167°F.). Multiple one g. samples of meat containing 3 X 10s cells of 5. typhimurium Tm-1 ( # 8 4 ) after exposure for 5 minutes to 60°C. (140°F.) contained no viable cells. The more resistant S. senftenberg 775 W required an exposure of from 10 to 15 minutes at 65°C. to kill an equal number of cells. The data indicate that the heat resistance of Salmonella in ground chicken muscle is similar to its heat resistance in other foods. ACKNOWLEDGMENT
The authors wish to acknowledge the fruitful discussions with Dr. Hans Riemann. REFERENCES Anellis, A., J. Lubas and M. M. Rayman, 19S4.
Heat resistance in liquid eggs of some strains of the genus Salmonella. Food Research, 19: 377-395. Angelotti, R., M. J. Foter and K. Lewis. 1960. Time-temperature effects on Salmonellae and Staphylococci in foods. II. Behavior at warm holding temperatures. Thermal-death-time studies. U. S. Health, Education and Welfare, Robert A. Taft Sanitary Engineering Center Technical Report F60-5, 49 pp. Castellani, A. G., R. R. Clarke, M. I. Gibson and D. F. Meisner, 1953. Roasting time and temperature required to kill food poisoning microorganisms introduced experimentally into stuffings in turkeys. Food Research, 18: 131-138. Gibbons, N. E., and R. L. Moore, 1944. Dried whole egg powder. XII. The effect of drying, storage, and cooking on the Salmonella content. Can. J. Research, Sec. F. Technology, 22: 58-63. Goresline, H. E., K. M. Hayes, R. E. Moser, Jr., M. A. Howe, Jr. and E. E. Drewniak, 1951. Pasteurization of liquid whole egg under commercial conditions to eliminate Salmonella. U. S. Dept. Agr. Circ. 897. U. S. Govt. Printing Office, Washington. 15 pp. Hansen, N.-H., and H. Riemann, 1963. Factors affecting the heat resistance of nonsporing organisms. J. Appl. Bact, 26: 314-333. Hussemann, D. L., and J. K. Buyske, 1954. Thermal death time-temperature relationships of Salmonella typhimurium in chicken muscle. Food Research, 19: 3S1-356. Nevot, A., Ph. Lafont et Y. Lafont, 1958. De la destruction des bacteries par la chaleur; etude de I'efficacite de la pasteurisation du lait. Paris. Institut national d'hygiene (Monograph No. 18). Osborne, W. W., R. P. Straka and H. Lineweaver, 1954. Heat resistance of strains of Salmonella in liquid whole egg, egg yolk, and egg white. Food Research, 19: 451-463. Rosenau, M. J., 1912. Thermal death point of pathogenic microorganisms in milk. U. S. Hyg. Lab. Bull. 2nd ed., 56: 683-686. Tanner, F. W., and G. C. Dubois, 1925. Some notes on the effect of heat on members of the colontyphoid group in milk. J. Dairy Sci. 8: 47-53. Winter, A. R., G. F. Stewart, V. H. McFarlane and M. Solowey, 1946. Pasteurization of liquid egg products. III. Destruction of Salmonella in liquid whole egg. Am. J. Public Health 36: 451-460.
Downloaded from http://ps.oxfordjournals.org/ at Gazi Universitesi on May 2, 2015
the latter poorer heat transfer increased the come-up time) was sufficient to kill all of 3 X 10s cells per tube. In evaluating 3 to 5 minute exposures, it is important to remember that it required approximately 4 minutes for the meat to rise to within 0.5°C. of the test temperature. The results with S. typhimurium agree with the heat resistance studies of Winter et al. (1946), Goresline et al. (1951) and Angelotti et al. (1960). Similarly the results with 5. senftenberg 775 W parallel those of Angelotti et al. (1960) with chicken a la king. In our view, the data indicate that suspension of Salmonella in chicken muscle does not measurably alter their heat resistance.