2000 Poultry Science Association, Inc.
INCIDENCE OF LISTERIA
MONOCYTOGENES ON PRE-SCALD AND
M. E. BERRANG,1 C. E. LYON, and D. P. SMITH
USDA-ARS, Poultry Processing and Meat Quality Research Unit
Russell Research Center, P.O. Box 5677, Athens, GA 30605-5677
Phone: (706) 546-3551
FAX: (706) 546-3633
J. K. NORTHCUTT
University of Georgia, Department of Poultry Science, Athens, GA 30602-4356
Primary Audience: Plant Managers, Quality Assurance Personnel, Researchers
Listeria monocytogenes is a human pathogen that can cause serious foodborne disease. For this
reason, this pathogen is of great interest to all food manufacturing companies. Recent outbreaks
of listeriosis in which poultry products were implicated have focused the attention of the poultry
industry on L. monocytogenes. Historically, this pathogen has been isolated from poultry processing
plants, especially during the latter stages of processing (post evisceration). This study was designed
to survey several plants and determine the incidence of L. monocytogenes on poultry carcasses.
Three commercial chicken processing plants were examined for the presence of L. monocytogenes
on carcasses from early (pre-scald) and late (post-chill) in the process. Overall, the percentage of
positive post-chill samples was lower than that previously reported in earlier studies. Carcasses
from two plants maintained a low incidence at both sample sites. Carcasses from the third plant
accumulated a signiﬁcant increase in L. monocytogenes from pre-scald to post-chill. This third
plant later replaced the evisceration equipment. A second evaluation, one month after placement
of the equipment, revealed much lower incidence on processed carcasses.
Key words: Chicken, Listeria monocytogenes, processing plant
2000 J. Appl. Poultry Res. 9:546–550
industry has recognized the importance of this
DESCRIPTION OF PROBLEM organism and has recently been pro-active in
Listeria monocytogenes is a human patho- efforts to prevent or reduce product contami-
gen that can cause foodborne illness. Unlike nation.
many bacterial diseases associated with food, Listeria, including L. monocytogenes may
listeriosis can cause alarming fatality rates be isolated from raw poultry products pur-
(20% or higher) among susceptible populations chased at retail outlets [2, 3, 4]. L. monocyto-
. In recent years, fully cooked, ready-to-eat genes has also been isolated from raw poultry
products containing poultry meat have been im- carcasses and meat in processing plants [4, 5,
plicated in outbreaks of listeriosis. The poultry 6]. Aside from the possibility of illness due to
To whom correspondence should be addressed
BERRANG ET AL.: LISTERIA ON CHICKEN DURING PROCESSING 547
consumer mishandling of raw poultry, another
concern with this pathogen is that it may be MATERIALS AND METHODS
carried into a further processing facility where PROCESSING PLANTS AND SAMPLES
raw product is cooked, making subsequent
cross contamination possible. This situation Three commercial chicken processing
was demonstrated by a study which reported plants (Plants A, B, C, and D) were used in
raw turkey necks and breasts among the princi- this survey. Plant A processes 60,000 birds a
day with a mean live weight ranging from 5 to
pal sources of listeriae for a further processing
8 lb. Quality assurance personnel in Plant A
reported use of 20 ppm chlorine in washers and
The sources of listeriae in poultry slaughter
sprayers and 50 ppm chlorine in the chill tank.
establishments are not entirely clear. A low
Plant B processes 134,400 birds a day with a
prevalence or complete lack of L. monocyto-
mean live weight of approximately 7 lb. Quality
genes has been noted in samples drawn from
assurance personnel in Plant B reported use of
early stages of slaughter and processing [3, 8,
20 ppm chlorine in washers, sprays, and the
9, 10]. However, it has been reported that a
chill tank. Plant C processes 250,000 birds a
small percentage (0–1.3%) of birds can carry
day with a mean live weight of approximately
this organism into the plant . Moreover, L.
4 lb. Quality assurance personnel in Plant C
monocytogenes has been isolated from a low
reported use of 20 ppm chlorine in sprayers and
percentage (1–6%) of samples in broiler hatch-
washers, and 50 ppm chlorine in the chill tank.
ing facilities, a situation which could lead to
Plant D is the same facility as Plant C after a
contaminated broilers . In reality, the sources
major renovation, including replacement of the
of listeriae in a poultry processing plant are
evisceration line with a high speed system.
probably numerous and varied. Once in the fa- Plant D used the same operational levels of
cility, this pathogen can become resident and chlorine as before the renovation. Each plant
may be able to survive sanitation procedures was sampled on two replicate processing days.
[3, 5, 8, 9, 11]. In general, L. monocytogenes Samples were taken after a plant had com-
becomes more prevalent on processing equip- pleted from 4 to 5 hr of continual operation
ment and carcasses as they progress through (approximately mid-shift). Samples were col-
the plant [3, 4, 5, 8, 10]. In 1997, Cox et al. lected at two points in the ﬂow of processing
reported an increase in incidence from pre- at each plant. One sample was taken early to
scald carcasses to carcasses after immersion show what, if any, level of L. monocytogenes
chilling . arrived at the plant on the birds. A second sam-
The initial objective of the current study ple was taken late in processing to measure
was to partition poultry slaughter and proc- what, if any, levels of L. monocytogenes accu-
essing to determine the procedures that lead mulated on the product during initial proc-
to increased incidence of L. monocytogenes. essing. The ﬁrst sample consisted of 25 car-
However, analysis of preliminary data revealed casses removed from the shackle line after
that this pathogen may not be as prevalent on bleed out, prior to scalding. Each carcass was
carcasses now as it was several years ago. handled with new latex gloves and placed indi-
Changes in operational sanitation procedures vidually into a sterile plastic bag.
due to the implementation of pathogen-reduc- The second sample was as late in processing
tion strategies, including increased usage of as could be arranged in order to follow the same
chlorine and water, may have affected the re- group of carcasses from the ﬁrst site. In Plants
covery rate of L. monocytogenes from proc- A, C and D, the second sample site was after
essed raw chicken. The design of the study was carcass chilling, where 25 carcasses were col-
altered to reﬂect this possibility by including lected as they emerged from the chill tank be-
more samples at only two locations. The ﬁnal fore re-hang. Each carcass was removed from
objective was to determine if the incidence of the table with new latex gloves and individually
L. monocytogenes in chicken carcasses changed placed into a sterile plastic bag. In Plant B it
as the carcasses progressed through processing. was possible to follow carcasses through post-
548 JAPR: Research Report
chill re-hang and manual debone, allowing ex- and examined for beta hydrolysis, characteristic
amination of product that had undergone sub- of L. monocytogenes. Two positive colonies
stantial handling. Therefore, in Plant B the sec- were picked from each positive plate and used
ond sample was taken after manual cut up and for biochemical tests. Isolates were conﬁrmed
deboning from the same operator. The second as L. monocytogenes by observation of charac-
sample consisted of 25 samples of one skin-on teristic tumbling motility on a wet mount, a
boneless thigh, each individually placed into a positive beta lysin disc  CAMP reaction,
sealable bag. In all plants, sample collection positive catalase test, acid production from
was timed such that the second site was sampled rhamnose, and lack of acid production from
when carcasses removed at the ﬁrst site would xylose and mannitol.
have moved through. This procedure made it
possible to sample from the same group of car- RESULTS AND DISCUSSION
casses at both locations. The time necessary for
carcasses to reach the second sample site was The results of L. monocytogenes isolation
estimated from production line speeds and op- from raw chicken carcasses appear in Table 1.
erations at each plant. On the ﬁrst of two replicate visits to Plant A,
4 of 25 pre-scald samples were found to be
MICROBIOLOGICAL positive for L. monocytogenes. After passing
CULTURE METHODS through processing (post-chill), only 1 of 25
Carcasses and thighs were subjected to a chilled carcasses was found positive, a ﬁnding
vigorous one min hand rinse for recovery of which did not represent a signiﬁcant difference
L. monocytogenes. Feathered carcasses were . On the second visit, no L. monocytogenes
rinsed in 500 mL, while chilled carcasses and was detected at either sample site. In Plant B,
thighs (Plant B) were rinsed in 100 mL. All no L. monocytogenes was recovered from any
samples were either handled in the plant or carcasses prior to scald. After passing through
transported to the laboratory according to the the plant, including the additional step of imme-
preference of plant management. In Plant A, diate post-chill whole carcass deboning, 1 out
each carcass was rinsed on-site in sterile dis- of 25 samples was found to be positive on each
tilled water. These rinses were collected, replicate visit. This did not represent a signiﬁ-
poured into sterile specimen cups, placed on cant increase in recovery . In Plant C, no
ice, and transported to the lab. The volume of L. monocytogenes was detected in the pre-scald
rinse recovered was noted. Listeria enrichment rinses on either visit. However, after processing
broth (LEB) at 10 times strength (10 × LEB) there was a signiﬁcant increase  in the inci-
was added (1:9 10 × LEB:rinse) to provide each dence of L. monocytogenes to 6 and 5 positives
sample with single strength LEB within 3 hr out of 25 chilled carcasses on the ﬁrst and sec-
of sample collection . In Plants B, C and ond visit, respectively. After a major renovation
D, all samples were collected, placed on ice, effort that included new equipment on the evis-
and removed from the plant. These samples ceration line and minor changes on the slaugh-
were rinsed in single-strength LEB within 2 hr ter line, this pathogen was detected in 1 of 25
of collection. pre-scald carcasses on the ﬁrst visit and 0 of
The USDA-FSIS method for Listeria recov- 25 on the second visit. What separates Plant D
ery and identiﬁcation was used . Brieﬂy, from Plant C is that no L. monocytogenes was
LEB was incubated at 30 °C for 24 hr; 0.1 mL recovered from any of the chilled carcasses on
was transferred to 10 mL of Fraser broth . either of the two replicate sample days in
Fraser broth was incubated at 35 °C for 24–48 Plant D.
hr. All Fraser tubes that turned black were used When the current data is compared to a
to streak modiﬁed oxford agar (MOX) plates similar data set collected in Plants B and C and
. Characteristic small black colonies were reported in 1997, some differences are noted.
picked from MOX and streaked for isolation Like the current data, the 1997 study  re-
on horse blood overlay agar (HBO) plates. HBO ported low percentages of L. monocytogenes
plates were incubated at 35 °C for 24–48 hr on pre-scald carcasses (0 and 1.3%). However,
BERRANG ET AL.: LISTERIA ON CHICKEN DURING PROCESSING 549
TABLE 1. Recovery of Listeria monocytogenes from samples collected at commercial chicken processing plants
(number positive/number collected)
COLLECTION DAY 1 COLLECTION DAY 2 TOTAL
PLANT Pre-scaldA Post-chillB Pre-scald Post-chill Pre-scald Post-chill
A 4+/25 1+/25 0+/25 0+/25 4+/50 (8%) 1+/50a (2%)
B 0+/25 1+/25 0+/25 1+/25 0+/50a (<2%) 2+/50a (4%)
C 0+/25 6+/25 0+/25 5+/25 0+/50b (<2%) 11+/50a (22%)
D 1+/25 0+/25 0+/25 0+/25 1+/50a (2%) 0+/50a (<2%)
Samples were feathered carcasses.
Plants A, C and D samples were chilled carcasses; Plant B samples were deboned thighs.
Values in the same row with no like superscripts are signiﬁcantly different (p < 0.01) by Chi square test for independence.
the authors found a higher percentage of L. piece (or pieces) of equipment that had become
monocytogenes positive chilled carcasses (20 colonized with the organism. Such colonization
and 40%) than the percentage found in Plants has been previously suggested as an important
A, B and D of the current study. It is unclear source of cross contamination [5, 11, 16].
what has led to the apparent decrease in L. Despite the possibility of effective decon-
monocytogenes incidence; perhaps some ac- tamination, vigilance must be maintained as
listeriae can re-enter the plant and potentially
tions or interventions implemented by proc-
re-colonize the processing environment. Future
essing plant personnel could be responsible. For work will be directed at determining the most
example, after Plant C underwent renovation, important sources of L. monocytogenes in com-
chilled carcasses from Plant D had a lower inci- mercial processing and further processing facil-
dence of L. monocytogenes. This reduction may ities to elucidate potential intervention ap-
result from the removal or replacement of a proaches.
CONCLUSIONS AND APPLICATIONS
1. When compared to data published 4 years ago, Listeria monocytogenes was found to be less
prevalent on eviscerated post-chill chicken carcasses.
2. It may be possible to stop cross contamination of poultry products with Listeria monocytogenes
resident in the plant by removal or replacement of processing equipment.
3. Recent pathogen reduction efforts by the poultry processing industry may have led to a lower
incidence of Listeria monocytogenes on raw chicken carcasses.
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monocytogenes from meat, poultry, and egg products. Pages 8-1
to 8-18 in: USDA-FSIS Microbiology Laboratory Guidebook, 3rd The authors gratefully acknowledge expert technical assistance
Edition. USDA-FSIS, Washington, DC. by Mark N. Freeman and Lauren G. Pittenger.