Survival of Salmonella kentucky in Frozen Minced Pork, Beef and Lamb

Survival of Salmonella kentucky in Frozen Minced Pork, Beef and LambAuthor(s): J. J. SheridanSource: Irish Journal of Food Science and Technology, Vol. 6, No. 2 (1982), pp. 177-181Published by: TEAGASC-Agriculture and Food Development AuthorityStable URL: http://www.jstor.org/stable/25558059 .

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Ir. J. Fd Sci. Technol. 6: 177-181, 1982

COMMUNICATIONS TO THE EDITOR

SURVIVAL OF SALMONELLA KENTUCKY IN FROZEN MINCED PORK, BEEF AND LAMB

J. J. Sheridan

An Foras Taluntais, Dunsinea Research Centre, Castleknock, Co. Dublin

ABSTRACT

The decrease in survival of Salmonella kentucky in minced pork, beef and lamb frozen to ?35?C was determined over a ten-week period. Survival of the organism was similar in pork and beef and lower in lamb. This difference reflects the greater cryoprotective properties of pork and beef. Survival as a function of time declined more rapidly in lamb than in pork or beef.

INTRODUCTION

The survival of pathogenic bacteria in frozen meat is well known and Georgala and Hurst suggest that in minced beef stored at ?20?C Salmonella typhimurium can survive for long periods (1). These authors showed that both S. typhimurium and

Escherichia coli survive best at ?20?C, compared to ?2?C and that after an initial

rapid decrease in viability from freezing, bacterial numbers decreased gradually to a level of 50% after 3 months. This initial decrease over the first few days has also been noted for species of Salmonella by Gunderson and Rose (2) in chicken chow mein frozen to ?25?C. In their work Farrell and Upton (3) showed a steady decline in the survival of S. typhimurium on bacon pieces and after 32 days storage at

?22?C only 10.4% of the original inoculum survived. Differences in survival using a variety of substrates have been observed (4, 5). In

their work Mackey et al. (6) have shown that different meat types vary in their

ability to protect bacteria from the lethal effects of freezing. In consequence, sterile meat discs of pork, beef and lamb inoculated with pathogenic and non-pathogenic

177



178 IR. J. FD SCI. TECHNOL., VOL. 6, NO. 2

strains of E. coli showed different levels of survival when stored at ?20?C. Survival varied with the meat type and the strain of the organism.

The present study was undertaken to establish the effect of freezing and storage on the survival of a species of Salmonella using different types of minced meat.

MATERIAL AND METHODS

Pork and beef were obtained from carcasses of normal pH (5.4 ?

5.8) slaughtered in the abattoir of the Industrial Development Unit (IDU) at Dunsinea and lamb from Dublin Meat Packers, Cloghran, Swords, Co. Dublin. The meat was procured in approximately 1000 g amounts and trimmed of as much fat as possible. After

grinding through a Bauknecht, Model AL2/1 mincer with a 5mm plate, the

following fat values were obtained using the Foss Let technique (7), beef 9.1%, pork 8.7% and lamb 4.4%.

The isolate of Salmonella kentucky was obtained from frozen pig head meat by Dr. J. F. Dempster, Meat Research Department, Dunsinea. After incubation at

37 ?C for 24 h in nutrient broth (NB) bacterial numbers in solution were estimated

using a haemocytometer. The solutions used to inoculate the pork and lamb contained log10 7.23/ml and for beef log10 7.19/ml. The mince was inoculated by pouring 10 ml of the culture onto each meat type with constant mixing by hand, using sterile gloves. The inoculated minced meats were then passed through the

mincer a further three times to ensure the thorough dispersal of the inoculum

throughout the sample. The inoculated mince was placed in polythene bags in 100 g lots, frozen and stored in a blast freezer in the IDU at a mean air temperature of

?35?C.

At weekly intervals a 100 g sample of each meat type was withdrawn from the freezer and thawed for 5 min in a water-bath at 37?C. To each bag 200 ml of 0.1%

peptone water was added and the contents macerated in a Colworth stomacher for 1 min. Pre-dried plates of Oxoid Brilliant Green Agar (BGA) were surface inoculated in duplicate with 0.025 ml of the meat solutions or successive tenfold dilutions of these. The plates were incubated at 37?C and examined for the presence of S.

kentucky after 24 hr. The initial mesophylic flora on the three minced meats was established after

inoculating plates of Oxoid Plate Count Agar (PCA) and incubation at 25 ?C for three days.

A regression of the numbers of S. kentucky (log10/g) on time was fitted to the data for each of the three meat types.

RESULTS AND DISCUSSION

After 10 weeks storage at ?35?C there was a reduction of log101.60 in survival of S.

kentucky when inoculated on pork mince (Fig. 1). With beef mince there was a small further decrease in survival to log^ 1.81. The greatest reduction in numbers was with



SHERIDAN: S. KENTUCKIS IN FROZEN MEAT 179

lamb where the viability of the test organism was reduced by log10 2.5. The results of these experiments are considered to represent the differences in the cryoprotective properties of the three meat types. In their work Mackey et al (6) inoculated sterile

meat discs with pathogenic and non-pathogenic strains of E. coli and assessed the survival after storage for 220 days at ?20?C. For the enteropathogenic strains the decrease in survival using pork was 73% followed by lamb at 53% and beef at 35%.

When the non-pathogenic strains were used the decrease in survival with pork was

67%, followed by beef with 60% and lamb 56%. Differences in survival such as these have also been observed by Gunderson and Rose (2) for a number of species of Salmonella on frozen chicken meat.

.amb

^ 2'51 P^ o

t ? Beef

- I H

x I i * ol B ii?I_

Fig. J: Reduction in survival o/S. kentucky on minced pork, beef and lamb after frozen storage at ?35?Cfor ten weeks

According to Thatcher et al (8) the competitive action of spoilage bacteria may repress the multiplication of specific pathogens. In the present study the initial numbers of mesophiles on the mince meats were log10 5.3 and 4.9/g for pork and beef and log,0 7.6 for lamb. The higher numbers on the lamb mince were not considered to be the cause of the reduced survival of S. kentucky because the

mesophilic flora would not be capable of exerting any effect on the pathogens present in the very short thawing time of 6 min.

In order to assess the relationship between the decrease in the numbers of S.

kentucky and time, regression lines were fitted to the data for the three meat types.



180 IR. J. FDSCI. TECHNOL., VOL. 6, NO. 2

The results show (Fig. 2) that the slopes for pork and beef were similar and that both were significantly different to lamb (p<,01). The linear relationship of survival with time was different to that shown by Gunderson and Rose (2) for a number of Salmonella species inoculated into chicken chow mein and stored frozen at ?25.5?C and by Georgala and Hurst (1) with S. typhimurium in minced beef at ?20?C. These authors showed an initial rapid decline over the first few days with a more

gradual fall in numbers over the remaining storage time. The results of Farrell and

Upton (3) with bacon pieces, surface inoculated with S. typhimurium and stored at

?22?C, do not show a sudden decline but a gradual decrease over time as reported in the present study. These differences are considered to reflect the speed of the freezing process. In the present work blast freezing ensured a rapid reduction in

temperature with consequent preservation of larger numbers of bacteria, compared with slow freezing (9). Bacteria on the surface of meat would also freeze rapidly, compared to those protected by mincing.

70-|

^"'^ O-OPork

k '^-^ A-a Beef

5> ^^""^^w ^'*>'**>_^ *-* Lamb

2 60- ^^^^^^^^SC*"^'^

g 5-0- ? ^-^^-_

? % </Sl

4-0- Meat type fntercept Slope S.Eofstope* r2 Pork 6.37 -0.17 ?0.02 0.91

Beef 6.22 -0.18 ?0.02 0.92 Lamb 6.89 -0.26 ?0.02 0.96

*S.E. = standard error.

3Or-1-1-1-1-r i-1-1-1-?i 0 1 23456789 10

Storage time (weeks)

Fig. 2: Relationship between survival and time for S. kentucky on frozen minced

pork, beef and lamb

ACKNOWLEDGEMENTS

The skilled technical assistance of Mr. B. Lynch is gratefully acknowledged.



SHERIDAN: 5. KENTUCKIS IN FROZEN MEAT 181

REFERENCES

1. Georgala, D. L. and Hurst, A. 1963. The survival of food poisoning bacteria in frozen foods. J.

appl. Bad. 26: 346. 2. Gunderson, M. F. and Rose, K. D. 1948. Survival of bacteria in a pre-cooked fresh-frozen food. Fd

Res. 13: 254.

3. Farrell, G. M. and Upton, M. E. 1978. The effect of low temperature on the growth and survival of

Staphylococcus aureus and Salmonella typhimurium when inoculated onto bacon. /. Fd. Techol. 13: 15.

4. Woodburn, M. J. and Strong, D. H. 1960. Survival of Salmonella typhimurium, Staphylococcus aureus and Streptococcus faecalis frozen in simplified food substrates. Appl. Microbiol. 8: 109.

5. McFarlane, V. H. 1942. Behaviour of micro-organisms in fruit juice sucrose solutions stored at ?17,8?C (0?F). Fd Res. 7: 509.

6. Mackey, B. M., Derrick, C. M. and Thomas, J. A. 1980. The recovery of sublethally injured Escherichia coli from frozen meat. J. appl. Bad. 48: 315.

7. Pettinati, J. D. and Swift, C. E. 1975, Rapid determination of fat in meat and meat products by Foss Let solvent extraction and density measurement. J. Ass. off.fanalyt.J Chem. 58: 1182.

8. Thatcher, F. S., Robinson, J. and Erdman, I. 1962. The 'vacuum pack* method for packaging foods in relation to the formation of the botulinum and staphylococcal toxins. /. appl. Bact. 25: 120.

9. Meryman, H. T. 1966. Review of biological freezing. In: "Cryobiology" Ed. H. T. Meryman, Academic Press, London and New York.

Received April 23, 1982.



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Survival of Salmonella kentucky in Frozen Minced Pork, Beef and Lamb