Preventing Introduction,
Growth and
Cross-Contamination of
Listeria monocytogenes
©2006 Department of Food Science - College of Agricultural Sciences at Penn State University
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Contamination

Introduction
– LM is brought into the facility from an outside
source

Growth
– LM present in the facility/food has the ability
to multiply

Cross-contamination
– LM present in the facility/food is transferred
to another food or surface
Introduction of LM
Non-food items
Vendors
Rodents/Pests
Retail
Facility
Contaminated Food
Employees
Customers

Retail establishments
are open to the public

Restrictions &
controls should be
designed to
complement normal
operations
Introduction of LM

Employees
– Sources
 Ill employees
 Unclean hands, clothing
– Prevention
 Good personal hygiene
 Restriction of ill employees
 Training
Introduction of LM

Vendors
– Sources
 Ill vendors
 Unclean hands or clothing
– Prevention
 Restriction from high risk areas
 Training/vendor agreements
Introduction of LM

Customers
– Sources
 Ill customer
 Unclean hands
– Prevention
 Facilitate proper handing of food
– Tongs, single use paper
– Instructions for proper handing
Introductions of LM

Rodents/Pests
– Sources
 Feces
 Rodents/pest carrying LM on feet and body
– Prevention
 Pest control program
 Maintenance of indoor and outdoor facilities
Introduction of LM

Contaminated Food
– Sources
 Raw products (meat, poultry, fruits, vegetable)
 Ready-to-eat foods
– Prevention
 Vendor assurances (HACCP plan, testing)
 Vendor audits
Introduction of LM

Non-food Items
– Sources
 Contaminated packaging,
other supplies
– Prevention
 Receiving standards
Growth of LM

Since it is not possible to eliminate all
sources of LM in the retail environment,
preventing growth is essential
– On Food
– On Equipment
– In the Environment
Controls to Prevent Growth
1.
Adequate time and temperature controls
2.
Proper cleaning and sanitation
3.
Good personal hygiene
Time/Temperature Controls

Cold Holding (Refrigeration)
41ºF for 7 days
OR
45ºF for 4 days
– LM can grow at refrigeration temperatures, so
holding time is important
– Control of LM growth is the basis for 2005
Food Code cold holding temperature/time
combinations
9
log10 cfu/ml
8
7
47.7F
6
38.3F
34.7F
5
33.4F
4
0
10
20
30
40
50
60
Time (days)
Growth of Listeria monocytogenes CFA 433 in chicken broth when
o
incubated at 47.7. 38.3, 34.7, and 33.4 F. Adapted from S.J. Walker et al.
(1990).
Time/Temperature
Control

Date Marking
– Requires active managerial control
– Applies to:
 Certain high risk foods
 Foods held for greater than 24 hours
– After prepared by a food establishment
– After opened, if a processed food
Estimated Reduction of Cases of Listeriosis
from Limits on Refrigeration Temperatures
Cases of Listeriosisa
Maximum Refrigerator
Temperature
Media
n
5th
Percentile
Baseline
2105
¾
7 °C (45 °F) maximum
656
331
761
5 °C (41 °F) maximum
28
1
126
b
95th
Percentile
c
a
c
¾
Values for the median, upper and lower uncertainty levels.
The baseline uses the full empirical distribution of refrigerator temperatures from the Audits
International (1999) survey.
c
The baseline number of cases of listeriosis is fixed based on CDC surveillance data.
b
Source: FDA/CFSAN and USDA. 2003. Quantitative Assessment of Relative Risk to Public Health
from Foodborne Listeria monocytogenes among Selected Categories of Ready-to-Eat Foods
Time/Temperature Controls

Hot holding
– Minimum of 135ºF

No temperature control
– Can be held for no longer than 4 hours at
ambient temperatures
Cleaning & Sanitation

Prevention of LM growth requires a
sanitation program that is:
– Properly designed
– Properly carried out

Target areas
– Food contact equipment
at room temperature
– Refrigerated storage &
display cases
Controls to Prevent
Cross-Contamination
Cleaning & Sanitation
 Good Personal
Hygiene
 Proper Flow of Food
& Employees

Cleaning & Sanitation

To prevent cross-contamination
– Always clean between working with raw and
ready-to-eat products
– Regularly clean to reduce chance that
contaminated product transfers to
uncontaminated product
Personal Hygiene

Handwashing stations
– Easily accessible to
promote frequent
handwashing
– Properly supplied

Appropriate training
– Employees should
understand concept of
cross-contamination
Proper Flow of Food

Facility design
– Separation of raw and ready-to-eat areas
– Adequate storage and work space
 Refrigeration
 Preparation tables
 Display cases
Flow of Employees

Facility design
– Location of hand washing facilities
– Locations of locker & rest rooms
– Separation of raw and ready-to-eat areas
Designated tasks for employees
 Designated work areas

Flow of Employees
Refuse
Storage
Receiving
Locker / Rest Rooms
Dishwashing Area
Refrigerated
Storage
Cooler
Dry Storage
Employee Entrance
Prep Table
Prep Table
Handwashing
Sink
Service Area / Cooler
Customer Entrance
Prevention of LM
 Prevent
– Introduction
– Growth
– Cross-contamination