Transcript Salmonella - York College of Pennsylvania

Resistance of Salmonella in Frozen Chicken
Nuggets to Microwave Cooking
Ryan C. Shaw and Carolyn F. Mathur, Department of Biological Sciences, York College of PA
Results
Abstract
The presence of potentially pathogenic Salmonella sp. in raw
chicken is recognized as a possible health hazard. It is
commonly believed that frozen chicken nuggets are precooked,
when in fact most frozen nuggets and strips are raw. The
objective of this study was to measure survival of Salmonella in
chicken nuggets cooked in a microwave. Frozen nuggets were
injected with 107cells of Salmonella, an amount which has
been associated with previous salmonellosis outbreaks from
eating undercooked chicken. A 1300 watt microwave was used,
and injected-nuggets were heated (3 at a time) at the highest
power for 30, 60 and 120 seconds. Salmonella were recovered
from individual nuggets by homogenization with sterile buffer,
followed by plate counts using nutrient agar. Controls included
unheated, injected nuggets, as well as un-injected nuggets.
Bacteria were detected at 103/g in un-injected, unheated
nuggets, but none were Salmonella sp. Cell recovery
exceeded over 95% in unheated controls, and values differed
less than 1% among replicates. While Salmonella levels were
reduced in injected-nuggets at each heating time, the reduction
from the initial 107 level ranged from 106 to 102 Salmonella cells
recovered. The highly variable results from the microwaveheated samples could be the result of uneven heating
frequently observed when foods are cooked in the microwave.
Even low concentrations of Salmonella in foods can be
problematic, and the suggested preparation protocols for
chicken nuggets need to be reviewed to assure food safety.
Introduction
•Salmonellosis is caused by the bacterium Salmonella.
•This gram negative rod is very common in poultry products.
•Improper preparation of foods that are potential carriers of this
bacterium can lead to infection.
•There are over 40,000 cases of Salmonellosis and 600 of
these lead to death annually in the United States.
•The main symptoms are diarrhea and fever. In fact, it may
take some infected individuals several months to fully recover
(Center for Disease Control, 2004).
•Not all chicken nugget products are precooked. This public
misconception has lead to a series of Salmonellosis outbreaks
in British Columbia, Canada (MacDougall et AL. 2004).
Objective: To determine if Salmonella survives in
uncooked breaded chicken nuggets prepared in a
microwave
Methods
Salmonella preparation
►
Salmonella (ATCC 14028) was grown ON in nutrient broth at 370C
and quantified by a standard plate count method. The amount
injected in each nugget contained 107 bacteria in 0.4 ml
suspension.
•Bacteria were detected at 103 cells/g in un-injected controls, but
none were Salmonella sp.
•Salmonella recovery from injected controls exceeded 95%, and
varied <1%
•Recovery from the microwaved nuggets ranged from 102 to 106
cells/gm, and the values were highly variable compared to the
unheated controls.
•Heated recoveries of Salmonella from nuggets are given in Fig. 1.
Sample treatments and processing
►
The experimental treatments are given in Table 1. Each nugget (3
per treatment) was injected with Salmonella, arranged equidistantly
in a sterile glass beaker in the microwave, and heated (1300 watts)
for either 30, 60 or 120 seconds. After heating, nuggets were
analyzed individually for the presence of viable Salmonella. Each
nugget was cut into 6 smaller pieces, homogenized with 40mL of
sterile buffer for 5 minutes (to achieve a relatively homogeneous
suspension), and quantified by serial dilutions and plate counts.
Table 1.
Experimental Design________________________________________________
107 cells
Nugget Groups b
Salmonella Injected
0(s) a
30(s) 60(s)
120(s)
1
+
+
2
+
+
3
+
+
4
+
+
5
+
6
+
_________________________________________________________________
a seconds heated at maximum setting
b n = 3 per group
►
Controls
•Salmonella-injected, unheated nuggets (group 1) were evaluated to determine
the sensitivity of the recovery method.
•Both heated and unheated, un-injected nuggets (groups 5 and 6) were
processed to detect any bacteria present, and suspicious isolates were
checked to determine if they were Salmonella.
.
Fig. 1. Recovered Salmonella cells/nugget injected with
107 cells/nugget. Average recoveries ranged from 10^6 to
10^2 cells Salmonella when heated at 30, 60 and 120
seconds (see groups 2, 3 and 4 respectively in Table 1).
Discussion
•The highly variable results from the microwave-heated samples could be the
result of uneven heating frequently observed when foods are cooked in the
microwave.
Conclusions
•Microwave cooking of frozen, raw, breaded chicken nuggets will not totally
remove viable Salmonella that may be present in the raw food.
•This is important because a small amount of Salmonella can cause serious
sickness.
Literature
•Baron, E.J., Peterson, C.R and Finegold S.M. 1994 Barely and Scotts Diagnostic Microbiology Mosby.
St. Louis Missouri
•Benson, Harold J. Microbiological applications : laboratory manual in general microbiology. 8th ed.,
short version. Boston : McGraw-Hill, c2002.
•Center For Disease Control Division of Bacterial and Mycotic Diseases. Salmonella 2004
<http://www.cdc.gov/ncidod/dbmd/diseaseinfo/salmonellosis_g.htm#What%20is%20salmonellosis>
•Doyle, M. Beuchat, L and Montville. Food Microbiology. (Herndon VA: ASM Press, 2001), 91-169.
•MacDougall et AL. 2004. Frozen Chicken Nuggets and Strips-A Newly Identified Risk Factor for
Salmonella Heidelberg Infection in British Columbia, Canada. Journal of Food Protection 67: 1111-1115