Agriculture Reference
In-Depth Information
at the present time, irradiation has been shown to eliminate pathogenic bacteria inter-
nalized within leaf tissues as a result of root uptake. Lettuce plants grown in hydro-
ponic solutions inoculated with E. coli O157:H7 contained the pathogen in the leaf
tissue. Irradiation effectively killed the pathogen although a treatment with 200 ppm
aqueous chlorine was ineffective (Nthenge and others 2007).
As an alternative to irrigation with contaminated water, a direct inoculation method
that introduces inoculum into the intracellular spaces of leaves has been developed.
In these studies, irradiation was similarly effective in eliminating internalized E. coli
O157:H7 from baby spinach and various types of lettuce (romaine, iceberg, Boston,
green leaf, red leaf); 300 or 600 ppm sodium hypochlorite was generally ineffective
(Niemira 2007, 2008). The limited data available suggests that D 10 values for internal-
ized cells (0.30-0.45 kGy) are often two- to threefold higher than for surface associ-
ated cells (0.12-0.14 kGy) (Niemira 2007). Because pathogen populations within the
leaf are expected to be very low in a commercial setting, nearly absolute elimination
of internalized pathogens may be practically achieved using irradiation doses that do
not cause undue sensory damage. Additional research is needed to more fully under-
stand the infl uence of internalization on pathogens and on the effi cacy of irradiation
and other treatments.
Irradiation: Not a “Silver Bullet” but a “High Hurdle”
Irradiation has sometimes been mischaracterized as a universal cure-all for microbial
contamination. Although irradiation is demonstrably effective in killing bacterial
pathogens, this effi cacy must be practical when assessed within the context of real-
world fruit and vegetable processing. The economic and commercial factors that
govern other antimicrobial processes are equally relevant for irradiation of fresh and
fresh-cut produce. Factors such as cost, effi ciency, throughput, administrative and
marketing overhead, and other issues will infl uence how irradiation may ultimately
be used to improve the safety of fresh and fresh-cut produce. The most useful under-
standing of produce irradiation is to consider the commodities and products for which
it is appropriate as well as those for which it may be inappropriate. Therefore, although
claims to “silver bullet” status must be viewed as hyperbole, it is reasonable to regard
irradiation as a “high hurdle” that can be incorporated as one important step in an
overall processing plan.
Processing Considerations
A number of factors infl uence the antimicrobial effi cacy of irradiation. The pathogen
targeted, the commodity and its state of preparation (whole vs. cored, peeled, cut,
chopped, etc.), type of MAP, and other product-specifi c factors can all modify the
results of the irradiation process. Like any other industrial food-processing technology,
irradiation must undergo process validation for each product being treated. Specifi c
details such as time, temperature, handling, etc., will differ depending on commodity
and purpose. For example, an irradiation process designed to eliminate Salmonella
from tomatoes may yield unacceptable quality or microbiological results when applied,
without modifi cation or validation, for the elimination of E. coli O157:H7 from leafy
greens.
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