Cross-Contamination Concepts:
Sources & Vectors
Pathogenic microorganisms can enter food processing areas
from several main routes: the external environment, raw materi-
als, people, equipment and in-plant microbiology laboratories.
Once inside, pathogens can be temporary or sporadic visitors
(present until they lose viability or are removed via cleaning
and disinfection procedures) or they may persist for long peri-
ods. When pathogens persist in the environment, they gener-
ally survive in harborage sites, which can be defined as physical
areas in which pathogens can lodge and be protected from
hands through interaction with a source, subsequently transfer
the contamination to a tool and then contaminate a product
contact surface with the tool while performing simple mainte-
nance on the line. In this example, the mechanic’s hands may
be considered an environmental vector while the mechanic’s
tool may be considered a product vector. In other circum-
stances, a cross-contamination event may have only a single
vector, for example, contaminated water droplets from a com-
pressed air line entering a product stream.
“Microbiological cross-contamination has been a contributing factor
to several well-documented outbreaks of foodborne illness.”
cleaning and disinfection actions, for example, poor hygienic
design features of processing equipment or damaged areas of
the plant’s building structure. When a harborage site also provides an environment suitable for growth, it can be considered
a growth niche. Both harborage sites and growth niches are potential sources of contamination within the processing environment.
For a pathogen to move from a source within the processing
environment to other locations (and perhaps even into product), a vector is required. A vector can be defined as anything
(air and other gases, water and other liquids, physical objects or people)
that carries or transfers a pathogen from one place to another. Vectors
may be further described as those that carry a pathogen from a
source to another location within the processing environment,
that is, an environmental vector, or those that carry a pathogen
from a source to the product or product ingredients, that is, a
product vector (Figure 1). It should be noted that cross-contamination usually occurs as an event in which a number of vectors
may be involved. For example, collecting a product sample
from an enclosed process line for quality control analysis by inserting a sampling bag into the product stream by hand may
have potential product vectors of the operator’s hand (or
glove), the operator’s sleeve, the sampling bag and the air. In
another example, a line mechanic may contaminate his or her
Figure 1: Transfer of pathogens from likely sources directly to food
products or their ingredients via product vectors or indirectly to
secondary or temporary sources.
carried in a liquid vector, the liquid may be absorbed into another surface or food completely, which would increase the potential for transfer to be nearly absolute. Conversely, if a
pathogen is being carried on a solid vector, such as a mechanic’s tool, the potential for transfer to a secondary surface,
including a food product, depends on the physical properties
and interaction between the pathogen and the surface as well as
the interaction of the vector with the surface. Smith12
demonstrated that the transfer of microorganisms from one surface to
another on contact can be approximated to 50 percent for practical purposes. For stationary air, transfer of microorganisms
from the air via sedimentation, which has defined rates for particles of a given size and buoyancy according to Stokes’ law,
13
and the number of microorganisms transferred depends on the
microbiological loading of the air and the exposure time. When
product is transported via air, or when air is blown over a product for cooling or drying, microorganisms can enter the product via impingement in addition to sedimentation, and the
number of microorganisms transferred may be related to the
volume of air to which the product is exposed.
Identifying Potential Sources and Vectors of
Contamination
Potential pathogen sources and cross-contamination vectors
in a processing plant can be determined by a physical examination of the processing environment and may include microbiological sampling. Sources and vectors may be associated with a
specific process step or may affect the processing line in general. For example, contaminated air in the production environment might affect many processing steps within a production
line, whereas vectors associated with a specific line procedure
may be associated solely with a specific process step.
In an exercise similar to determining the product process
flow within the HACCP plan, a cross-functional team (
comprising personnel knowledgeable about plant operations, sanitation, hygienic design, microbiology and engineering) can be
assembled to identify potential sources by walking the line and
examining the processing equipment and environment. Poten-
