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Survivor Curves of Selected Salmonella enteritidis Serotypes in Liquid Whole Egg Homogenates at 60°C.
Survivor Curves of Selected Salmonella enteritidis Serotypes in Liquid Whole Egg Homogenates at 60°C. ROGER DABBAH 1 , W. A. MOATS AND V. M. EDWARDS Market Quality Research Division, Agricultural Research Service, United States Department of Agriculture, Beltsville, Maryland 20705 (Received for publication April 2, 1971)
INTRODUCTION
T
The time-temperature requirements for pasteurization of liquid whole eggs are based mainly on extensive studies by Anellis et al. (1954), Osborne et al. (1954), and Sugihara et al. (1966), among others. However, Anellis et al. (1954), indicated that 3.5 min. at 60°C. for a 106/ml. concentration is at best a marginal heat treatment and could conceivably yield inadequately pasteurized liquid whole egg products. Cotterill (1968) using 106/ml. concentrations reached the same conclusion for liquid egg white, and Cotterill and Glauert (1968) suggested that "pasteurization" of egg yolk products could be accomplished at 73°C. for 3.75 min. In England, Heller et al. (1962) recommended pasteurization of egg products at 64.4°C. for 2.5 min. to eliminate Salmonella. 1 Present Address: Ross Laboratories, Cleveland Avenue, Columbus, Ohio 43216.
625
Cotterill and Glauert (1968) also reported tailing of Salmonella survivor curves at 60°C. which were probably induced by salt. Dabbah et al. (1971 a, b) obtained extensive tailings of survivor curves for food-borne bacteria suspended in commercially sterilized whole milk. This paper, presents survivor curves at 60°C. of selected Salmonella enteritidis serotypes suspended in liquid whole egg homogenates. MATERIALS AND METHODS
Most of the typed cultures were from the laboratory collection of the Poultry Quality Investigations, M.Q.R.D., A.R.S., U.S.D.A., Beltsville, Maryland. The following cultures were used: Salmonella enteritidis serotype Typhimurium (var. Copenhagen), S. enteritidis serotype Newport, S. enteritidis serotype Derby, 5. enteritidis serotype Anatum (ATCC 9270), S. enteritidis serotype Senftenberg (775 W), and Streptococcus faecalis. Twentyfour-hour cultures of test bacteria grown at 35°C. in BBL Trypticase Soy Broth2 (TSB) were centrifuged at 2°C, washed twice with pH 7.0, 0.1 M phosphate buffer, and resuspended in 1.5 ml. of buffer at 4°C. just prior to the heating experiment. Grade A large eggs, purchased in a retail outlet in Beltsville, Md., were washed in warm soapy water, rinsed, and 2 Mention of specific instruments or trade names is made for identification purposes only and does not imply any endorsement by the U. S. Government.
1772
Downloaded from http://ps.oxfordjournals.org/ at NERL on May 23, 2015
HE U. S. Department of Agriculture, Consumer and Marketing Service Regulations (1970) specify that pasteurization requirements for liquid whole egg by 3.5 min. at 60°C. (140°F.). However, the National Academy of Sciences report on the Salmonella problem (1969) indicates that Salmonella have been found in pasteurized egg products. The required pasteurization treatment is generally thought to destroy most Salmonella serotypes with the exception of S. enteritidis serotype Senftenberg (775 W) without damaging the functional properties of the egg.
1773
SALMONELLA SURVIVOR C U R V E S
coccus Confirmatory Agar (ECA) in addition to plating in TSA. At each time interval, 1 ml. of each heated sample was also added to 9 ml. TSB and incubated at 35°C. for at least 24 hours. After incubation, the samples were streaked on either Difco Brilliant Green Agar (BGA) or E C A (for Str. faecalis). Heated and unheated egg homogenates without added bacteria were plated as controls on either TSA, LIA, or ECA. T h e sterility of the egg homogenates prior to inoculation by various serotypes was thus monitored and no bacteria were recovered on either TSA, LIA, or ECA. RESULTS AND DISCUSSION
Results for Salmonella serotypes illustrated in Figures 1-5 indicate t h a t survivor curves at 60°C. are non-logarithmic. All had extensive tailings with the exception of serotype Anatum. A small number of all Salmonella tested, with the exception of serotype A n a t u m survived a h e a t 10
-
9 •st*
8 7
A\ -|'\
!• -
\
u. 5
- \ \ 34
s 3 2
t'l -
\
1
I
2
3
4
5
6
7
8
9
io~
TIME-MIN.
FIG. 1. Heat survivor curves at 60°C. of Salmonella enterilidis serotype Typhimurium (var. Copenhagen) in liquid whole egg homogenates. Enumeration of survivors in Trypticase Soy Agar (upper curve) and in Lysine Iron Agar (lower curve).
Downloaded from http://ps.oxfordjournals.org/ at NERL on May 23, 2015
air dried. Following rapid flaming of the shell with alcohol, the eggs were cut around the air sac area with sterilized surgical scissors. T h e contents of a number of eggs were pooled in a sterile glass Waring Blendor container and mixed one minute at low speed. After the foaming subsided, 150 ml. of the homogenate were aseptically introduced into a 300ml. sterile three-necked Pyrex flask. T h e flask was placed in a thermostatically c o n t r o l l e d w a t e r b a t h a t 6 0 o C . ( + 0.01°C.). T h e temperature of the whole egg homogenate was determined using a Thermistor thermometer (Yellow Springs I n s t r u m e n t Co.), calibrated before every experiment. T h e contents of the flask were stirred a t low speed (to prevent foaming) with a magnetic stirrer to maintain uniform temperature. After equilibrating the temperature of the egg homogenate in the flask for 1 hour a t 6 0 ° C , the washed bacterial suspension in 1.5 ml. of buffer was injected into the flask. The decrease in temperature following injection of the suspension was negligible, on the order of 0.1 to 0.2°C. Final concentration of bacteria in the heating flask was approximately 10 9 /ml. At pre-set intervals, 5 ml. of heated inoculated whole egg was removed from the flask with a sterile disposable pipette (without touching the lip or the wall of the flask) and placed in a sterile capped test tube and held in an ice-water bath for no more than 10 min. Plating was done by the pour plate technique, either directly or after serial dilutions in sterile 0 . 1 % peptone water, in duplicate, in BBL-Trypticase Soy Agar (TSA) and in BBL-Lysine Iron Agar (LIA). The plates were incubated a t 35°C. for 48 hours. When no growth was present, the plates were reincubated for an additional 24 hours. When Streptococcus faecalis was used as a test organism, the samples were plated in BBL-Entero-
1774
R. DABBAH, W. A. MOATS AND V. M. EDWARDS
I0_ 9
Ml
8 7
l\ '
' ' .i
i
i i
.\
i '
S 4| 2
=
2!
0.25
treatment of 60°C. for longer than 3.5 min. Differences in bacterial counts between plating in TSA and plating in LIA can be used as an index of sublethal injury to heated Salmonella inasmuch as the unheated Salmonella test organisms are not inhibited by LIA. It appears that sub10
-
O.S
J_
0.7S
I
1.25
U
T55
2 ~
TIME-MIN.
FIG. 4. Heat survivor curves at 60°C. of Salmonella enteritidis serotype Anatum in liquid whole egg homogenates. Enumeration of survivors in Trypticase Soy Agar (upper curve) and in Lysine Iron Agar (lower curve).
lethally heat injured Salmonella are more exacting in their nutritional requirements than unheated Salmonella. The use of D values based on the log-
^> -\\ .i
i: S 4
s S3
V\
w v-
1
2
3
4
5
6
7
FIG. 3. Heat survivor curves at 60°C. of Salmonella enteritidis serotype Derby in liquid whole egg homogenates. Enumeration of survivors in Trypticase Soy Agar (upper curve) and in Lysine Iron Agar (lower curve).
FIG. 5. Heat survivor curves at 60°C. of Salmonella enteritidis serotype Senftenberg (775 W) in liquid whole egg homogenates. Enumeration of survivors in Trypticase Soy Agar (upper curve) and in Lysine Iron Agar (lower curve).
Downloaded from http://ps.oxfordjournals.org/ at NERL on May 23, 2015
FIG. 2. Heat survivor curves at 60°C. of Salmonella enteritidis serotype Newport in liquid whole egg homogenates. Enumeration of survivors in Trypticase Soy Agar (upper curve) and in Lysine Iron Agar (lower curve).
3
1775
SALMONELLA SURVIVOR CURVES
TABLE 1.—Maximum survival times of Salmonella enteritidis serotypes and of Streptococcus faecalis at 60°C. in liquid whole egg1 as determined by various methods Survival time (min.) Bacteria
Plated after heating After resuscitation for 24 hrs. at 35°C. TSA* LIA in T S B . Plating on TSA
Salmonella enteritidis serotype Anatum 1 Salmonella enteritidis serotype Newport >60 Salmonella enteritidis serotype Derby 6 Salmonella enteritidis serotype Senftenberg (775 W) >180 Salmonella enteritidis serotype Typhimurium (var. Copenhagen) 10 Streptococcus faecalis >180 1
Initial inoculum ca. 10 9 /ml. TSA =Trypticase Soy Agar; LIA =Lysine Agar; Iron TSB =Trypticase Soy Broth. 2
IO
&
^5
*S 55
«o 5o
«S 35
K5T
Teo"
TIME-MIH.
FIG. 6. Heat survivor curves at 60°C. of Streptococcus faecalis in liquid whole egg homogenates. Enumeration of survivors in Trypticase Soy Agar (upper curve) and Enterococcus Confirmatory Agar (lower curve).
Confirmatory Agar) and a non-selective agar (TSA). Small numbers of cells capable of forming colonies in agar plates remained after 180 min. at 60°C. The extensive tailings of survivor curves of bacteria suspended in whole egg homogenates are similar to curves obtained by Dabbah et al. (1971 a, b) for bacteria in commercially sterilized whole milk and heated at 60°C. However, heat resistance of the Salmonella enteritidis serotypes suspended in whole egg is less than their heat resistance in whole milk suggesting that egg solids have less protective effect than milk solids. A bacterial concentration of 109 cells/ ml. is much higher than would ever be encountered in commercial egg products (Garibaldi et al., 1969). However, occasional gross contamination is possible and the recommended 3.5 min. at 60°C. for pasteurization of untreated whole egg would provide a thin margin of safety with most Salmonella serotypes. The use of higher temperature—shorter time "pasteurization", i.e. 64.4°C. for 2.5 min.—suggested by Heller et al. (1962) in England should be studied in the USA
Downloaded from http://ps.oxfordjournals.org/ at NERL on May 23, 2015
arithmic portion of the survivor curves will not completely describe the heat resistance of S. enteritidis serotypes in liquid whole egg for survivor curves generally tailed extensively. A small but definite number of serotype Senftenberg (775 W) survived an exposure of 180 min. at 60°C. (Fig. 5). Serotype Newport survived 60 min. (Fig. 2), serotype Typhimurium (var. Copenhagen) survived 10 min. (Fig. 1), and serotype Derby survived 6 min. (Fig. 3). Serotype Anatum did not survive 3.5 min. at 60°C. (Fig. 4). However even for serotype Anatum, cells heated for as long as 15 min. recovered following incubation for 24 hours at 35°C. in TSB (Table 1). Likewise, survival time of serotype Typhimurium (var. Copenhagen) was increased from 10 to 40 min. and survival of serotype Derby was increased from 6 min. to 30 min. by resuscitation in TSB (Table 1). For comparison, survivor curve of Streptococcus faecalis in liquid whole egg homogenates was also determined. As shown in Fig. 6, the curve was non-logarithmic. Large differences in survivor counts were noted between a selective agar (Enterococcus
1776
R. DABBAH, W. A. MOATS AND V. M. EDWARDS
under large scale commercial conditions to provide an added margin of safety to the presence of Salmonella enteritidis serotypes in liquid whole eggs. Furthermore, the possibility of "resuscitation" of "dead" bacteria shown in this paper and in other studies (Dabbah et al., 1969; Cotterill and Glauert, 1968) should be taken into consideration in the determination of time-temperature values for heat processing of liquid whole eggs. SUMMARY
REFERENCES Anellis, A., J. Lubas and M. M. Rayman, 1954. Heat resistance in liquid eggs of some strains of the genus Salmonella. Food Res. 19: 377-395.
NEWS AND NOTES (continued from page 1771) The faculty of the new Department will be brought together in the Animal Science-Nutrition Building. Dr. W. D. Morrison, formerly Director, Nutrition and Research, Agricultural Division, Maple Leaf Mills Limited, Toronto, has been appointed Chairman of the new Department, effective September 1st.
He was born in Provost, Alberta, in 1927, and received a B.S.A. degree at the University of Toronto (Ontario Agricultural College) in 1949. From 1949 to 1952 he was Territory Manager, Master Feeds, Toronto Elevators Limited, Toronto, and from 1952 to 1955 was a Graduate Student at the University of Illinois, obtaining a M.Sc. degree in 1954, and a Ph.D. degree in 1955. In 1955 he was
{continued on page 1823)
Downloaded from http://ps.oxfordjournals.org/ at NERL on May 23, 2015
Heat treatment of 3.5 min. at 60°C. for liquid whole egg homogenates inoculated with a heavy concentration (109/ml.) of Salmonella enteritidis serotypes resulted in either survival of a small number of bacteria as shown by the occurrence of extensive tailings of survivor curves, or by recovery of "dead" bacteria after incubation in Trypticase Soy Broth for 24 hours. Exposure at 60°C. for as long as 6, 10, 60, and 180 min. for serotype Derby, Typhimurium (var. Copenhagen), Newport, and Senftenberg (775 W), respectively, yielded a small number of survivors. No survivors were detected for serotype Anatum following 1.50 min. at 60°C. The importance of "recovery" tests is also emphasized.
Cotterill, O. J., 1968. Equivalent pasteurization temperatures to kill salmonellae in liquid egg white at various pH levels. Poultry Sci. 47: 354-365. Cotterill, O. J., and J. Glauert, 1968. Thermal resistance of Salmonella in egg yolk products containing sugar and salt. Poultry Sci. 47: 1156-1166. Dabbah, R., W. A. Moats and V. M. Edwards. 1971a. The heat resistance of food-borne bacteria in commercially sterilized whole milk. I. Salmonella. J. Dairy Sci. in press. Dabbah, R., W. A. Moats and V. M. Edwards, 1971b. The heat resistance of food-borne bacteria in commercially sterilized whole milk. II. Bacteria other than Salmonella. J. Dairy Sci. in press. Dabbah, R., W. A. Moats and J. F. Mattick, 1969. Factors affecting resistance to heat and recovery of heat injured bacteria. J. Dairy Sci. 52: 608-614. Garibaldi, J. A., H. Lineweaver and K. Ijichi, 1969. Number of Salmonellae in commercially broken eggs before pasteurization. Poultry Sci. 48: 10961101. Heller, C. L., B. C. Roberts, A. J. Amos, M. E. Smith and B . C . Hobbs, 1962. The pasteurization of liquid whole egg and the evaluation of the baking properties of frozen whole egg. J. Hyg. Camb. 60:135-143. National Academy of Science-Natioral Research Council, 1969. An Evaluation of the Salmonella Problem. Washington, D. C. 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 Res. 19: 451-463 Sugihara, T. F., K. Ijichi and L. Kline. 1966. Heat pasteurization of liquid whole egg. Food Technol. 20: 100-107. United States Department of Agriculture, 1970. Regulations governing the grading and inspection of egg products. 7CFR, Part 55. Consumer and Marketing Services, Washington, D. C.
INTRODUCTION
T
The time-temperature requirements for pasteurization of liquid whole eggs are based mainly on extensive studies by Anellis et al. (1954), Osborne et al. (1954), and Sugihara et al. (1966), among others. However, Anellis et al. (1954), indicated that 3.5 min. at 60°C. for a 106/ml. concentration is at best a marginal heat treatment and could conceivably yield inadequately pasteurized liquid whole egg products. Cotterill (1968) using 106/ml. concentrations reached the same conclusion for liquid egg white, and Cotterill and Glauert (1968) suggested that "pasteurization" of egg yolk products could be accomplished at 73°C. for 3.75 min. In England, Heller et al. (1962) recommended pasteurization of egg products at 64.4°C. for 2.5 min. to eliminate Salmonella. 1 Present Address: Ross Laboratories, Cleveland Avenue, Columbus, Ohio 43216.
625
Cotterill and Glauert (1968) also reported tailing of Salmonella survivor curves at 60°C. which were probably induced by salt. Dabbah et al. (1971 a, b) obtained extensive tailings of survivor curves for food-borne bacteria suspended in commercially sterilized whole milk. This paper, presents survivor curves at 60°C. of selected Salmonella enteritidis serotypes suspended in liquid whole egg homogenates. MATERIALS AND METHODS
Most of the typed cultures were from the laboratory collection of the Poultry Quality Investigations, M.Q.R.D., A.R.S., U.S.D.A., Beltsville, Maryland. The following cultures were used: Salmonella enteritidis serotype Typhimurium (var. Copenhagen), S. enteritidis serotype Newport, S. enteritidis serotype Derby, 5. enteritidis serotype Anatum (ATCC 9270), S. enteritidis serotype Senftenberg (775 W), and Streptococcus faecalis. Twentyfour-hour cultures of test bacteria grown at 35°C. in BBL Trypticase Soy Broth2 (TSB) were centrifuged at 2°C, washed twice with pH 7.0, 0.1 M phosphate buffer, and resuspended in 1.5 ml. of buffer at 4°C. just prior to the heating experiment. Grade A large eggs, purchased in a retail outlet in Beltsville, Md., were washed in warm soapy water, rinsed, and 2 Mention of specific instruments or trade names is made for identification purposes only and does not imply any endorsement by the U. S. Government.
1772
Downloaded from http://ps.oxfordjournals.org/ at NERL on May 23, 2015
HE U. S. Department of Agriculture, Consumer and Marketing Service Regulations (1970) specify that pasteurization requirements for liquid whole egg by 3.5 min. at 60°C. (140°F.). However, the National Academy of Sciences report on the Salmonella problem (1969) indicates that Salmonella have been found in pasteurized egg products. The required pasteurization treatment is generally thought to destroy most Salmonella serotypes with the exception of S. enteritidis serotype Senftenberg (775 W) without damaging the functional properties of the egg.
1773
SALMONELLA SURVIVOR C U R V E S
coccus Confirmatory Agar (ECA) in addition to plating in TSA. At each time interval, 1 ml. of each heated sample was also added to 9 ml. TSB and incubated at 35°C. for at least 24 hours. After incubation, the samples were streaked on either Difco Brilliant Green Agar (BGA) or E C A (for Str. faecalis). Heated and unheated egg homogenates without added bacteria were plated as controls on either TSA, LIA, or ECA. T h e sterility of the egg homogenates prior to inoculation by various serotypes was thus monitored and no bacteria were recovered on either TSA, LIA, or ECA. RESULTS AND DISCUSSION
Results for Salmonella serotypes illustrated in Figures 1-5 indicate t h a t survivor curves at 60°C. are non-logarithmic. All had extensive tailings with the exception of serotype Anatum. A small number of all Salmonella tested, with the exception of serotype A n a t u m survived a h e a t 10
-
9 •st*
8 7
A\ -|'\
!• -
\
u. 5
- \ \ 34
s 3 2
t'l -
\
1
I
2
3
4
5
6
7
8
9
io~
TIME-MIN.
FIG. 1. Heat survivor curves at 60°C. of Salmonella enterilidis serotype Typhimurium (var. Copenhagen) in liquid whole egg homogenates. Enumeration of survivors in Trypticase Soy Agar (upper curve) and in Lysine Iron Agar (lower curve).
Downloaded from http://ps.oxfordjournals.org/ at NERL on May 23, 2015
air dried. Following rapid flaming of the shell with alcohol, the eggs were cut around the air sac area with sterilized surgical scissors. T h e contents of a number of eggs were pooled in a sterile glass Waring Blendor container and mixed one minute at low speed. After the foaming subsided, 150 ml. of the homogenate were aseptically introduced into a 300ml. sterile three-necked Pyrex flask. T h e flask was placed in a thermostatically c o n t r o l l e d w a t e r b a t h a t 6 0 o C . ( + 0.01°C.). T h e temperature of the whole egg homogenate was determined using a Thermistor thermometer (Yellow Springs I n s t r u m e n t Co.), calibrated before every experiment. T h e contents of the flask were stirred a t low speed (to prevent foaming) with a magnetic stirrer to maintain uniform temperature. After equilibrating the temperature of the egg homogenate in the flask for 1 hour a t 6 0 ° C , the washed bacterial suspension in 1.5 ml. of buffer was injected into the flask. The decrease in temperature following injection of the suspension was negligible, on the order of 0.1 to 0.2°C. Final concentration of bacteria in the heating flask was approximately 10 9 /ml. At pre-set intervals, 5 ml. of heated inoculated whole egg was removed from the flask with a sterile disposable pipette (without touching the lip or the wall of the flask) and placed in a sterile capped test tube and held in an ice-water bath for no more than 10 min. Plating was done by the pour plate technique, either directly or after serial dilutions in sterile 0 . 1 % peptone water, in duplicate, in BBL-Trypticase Soy Agar (TSA) and in BBL-Lysine Iron Agar (LIA). The plates were incubated a t 35°C. for 48 hours. When no growth was present, the plates were reincubated for an additional 24 hours. When Streptococcus faecalis was used as a test organism, the samples were plated in BBL-Entero-
1774
R. DABBAH, W. A. MOATS AND V. M. EDWARDS
I0_ 9
Ml
8 7
l\ '
' ' .i
i
i i
.\
i '
S 4| 2
=
2!
0.25
treatment of 60°C. for longer than 3.5 min. Differences in bacterial counts between plating in TSA and plating in LIA can be used as an index of sublethal injury to heated Salmonella inasmuch as the unheated Salmonella test organisms are not inhibited by LIA. It appears that sub10
-
O.S
J_
0.7S
I
1.25
U
T55
2 ~
TIME-MIN.
FIG. 4. Heat survivor curves at 60°C. of Salmonella enteritidis serotype Anatum in liquid whole egg homogenates. Enumeration of survivors in Trypticase Soy Agar (upper curve) and in Lysine Iron Agar (lower curve).
lethally heat injured Salmonella are more exacting in their nutritional requirements than unheated Salmonella. The use of D values based on the log-
^> -\\ .i
i: S 4
s S3
V\
w v-
1
2
3
4
5
6
7
FIG. 3. Heat survivor curves at 60°C. of Salmonella enteritidis serotype Derby in liquid whole egg homogenates. Enumeration of survivors in Trypticase Soy Agar (upper curve) and in Lysine Iron Agar (lower curve).
FIG. 5. Heat survivor curves at 60°C. of Salmonella enteritidis serotype Senftenberg (775 W) in liquid whole egg homogenates. Enumeration of survivors in Trypticase Soy Agar (upper curve) and in Lysine Iron Agar (lower curve).
Downloaded from http://ps.oxfordjournals.org/ at NERL on May 23, 2015
FIG. 2. Heat survivor curves at 60°C. of Salmonella enteritidis serotype Newport in liquid whole egg homogenates. Enumeration of survivors in Trypticase Soy Agar (upper curve) and in Lysine Iron Agar (lower curve).
3
1775
SALMONELLA SURVIVOR CURVES
TABLE 1.—Maximum survival times of Salmonella enteritidis serotypes and of Streptococcus faecalis at 60°C. in liquid whole egg1 as determined by various methods Survival time (min.) Bacteria
Plated after heating After resuscitation for 24 hrs. at 35°C. TSA* LIA in T S B . Plating on TSA
Salmonella enteritidis serotype Anatum 1 Salmonella enteritidis serotype Newport >60 Salmonella enteritidis serotype Derby 6 Salmonella enteritidis serotype Senftenberg (775 W) >180 Salmonella enteritidis serotype Typhimurium (var. Copenhagen) 10 Streptococcus faecalis >180 1
Initial inoculum ca. 10 9 /ml. TSA =Trypticase Soy Agar; LIA =Lysine Agar; Iron TSB =Trypticase Soy Broth. 2
IO
&
^5
*S 55
«o 5o
«S 35
K5T
Teo"
TIME-MIH.
FIG. 6. Heat survivor curves at 60°C. of Streptococcus faecalis in liquid whole egg homogenates. Enumeration of survivors in Trypticase Soy Agar (upper curve) and Enterococcus Confirmatory Agar (lower curve).
Confirmatory Agar) and a non-selective agar (TSA). Small numbers of cells capable of forming colonies in agar plates remained after 180 min. at 60°C. The extensive tailings of survivor curves of bacteria suspended in whole egg homogenates are similar to curves obtained by Dabbah et al. (1971 a, b) for bacteria in commercially sterilized whole milk and heated at 60°C. However, heat resistance of the Salmonella enteritidis serotypes suspended in whole egg is less than their heat resistance in whole milk suggesting that egg solids have less protective effect than milk solids. A bacterial concentration of 109 cells/ ml. is much higher than would ever be encountered in commercial egg products (Garibaldi et al., 1969). However, occasional gross contamination is possible and the recommended 3.5 min. at 60°C. for pasteurization of untreated whole egg would provide a thin margin of safety with most Salmonella serotypes. The use of higher temperature—shorter time "pasteurization", i.e. 64.4°C. for 2.5 min.—suggested by Heller et al. (1962) in England should be studied in the USA
Downloaded from http://ps.oxfordjournals.org/ at NERL on May 23, 2015
arithmic portion of the survivor curves will not completely describe the heat resistance of S. enteritidis serotypes in liquid whole egg for survivor curves generally tailed extensively. A small but definite number of serotype Senftenberg (775 W) survived an exposure of 180 min. at 60°C. (Fig. 5). Serotype Newport survived 60 min. (Fig. 2), serotype Typhimurium (var. Copenhagen) survived 10 min. (Fig. 1), and serotype Derby survived 6 min. (Fig. 3). Serotype Anatum did not survive 3.5 min. at 60°C. (Fig. 4). However even for serotype Anatum, cells heated for as long as 15 min. recovered following incubation for 24 hours at 35°C. in TSB (Table 1). Likewise, survival time of serotype Typhimurium (var. Copenhagen) was increased from 10 to 40 min. and survival of serotype Derby was increased from 6 min. to 30 min. by resuscitation in TSB (Table 1). For comparison, survivor curve of Streptococcus faecalis in liquid whole egg homogenates was also determined. As shown in Fig. 6, the curve was non-logarithmic. Large differences in survivor counts were noted between a selective agar (Enterococcus
1776
R. DABBAH, W. A. MOATS AND V. M. EDWARDS
under large scale commercial conditions to provide an added margin of safety to the presence of Salmonella enteritidis serotypes in liquid whole eggs. Furthermore, the possibility of "resuscitation" of "dead" bacteria shown in this paper and in other studies (Dabbah et al., 1969; Cotterill and Glauert, 1968) should be taken into consideration in the determination of time-temperature values for heat processing of liquid whole eggs. SUMMARY
REFERENCES Anellis, A., J. Lubas and M. M. Rayman, 1954. Heat resistance in liquid eggs of some strains of the genus Salmonella. Food Res. 19: 377-395.
NEWS AND NOTES (continued from page 1771) The faculty of the new Department will be brought together in the Animal Science-Nutrition Building. Dr. W. D. Morrison, formerly Director, Nutrition and Research, Agricultural Division, Maple Leaf Mills Limited, Toronto, has been appointed Chairman of the new Department, effective September 1st.
He was born in Provost, Alberta, in 1927, and received a B.S.A. degree at the University of Toronto (Ontario Agricultural College) in 1949. From 1949 to 1952 he was Territory Manager, Master Feeds, Toronto Elevators Limited, Toronto, and from 1952 to 1955 was a Graduate Student at the University of Illinois, obtaining a M.Sc. degree in 1954, and a Ph.D. degree in 1955. In 1955 he was
{continued on page 1823)
Downloaded from http://ps.oxfordjournals.org/ at NERL on May 23, 2015
Heat treatment of 3.5 min. at 60°C. for liquid whole egg homogenates inoculated with a heavy concentration (109/ml.) of Salmonella enteritidis serotypes resulted in either survival of a small number of bacteria as shown by the occurrence of extensive tailings of survivor curves, or by recovery of "dead" bacteria after incubation in Trypticase Soy Broth for 24 hours. Exposure at 60°C. for as long as 6, 10, 60, and 180 min. for serotype Derby, Typhimurium (var. Copenhagen), Newport, and Senftenberg (775 W), respectively, yielded a small number of survivors. No survivors were detected for serotype Anatum following 1.50 min. at 60°C. The importance of "recovery" tests is also emphasized.
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