Agriculture Reference
In-Depth Information
diced tomatoes showed a D
10
of 0.26-0.39 kGy when combined with a 1% calcium
chloride dip (Prakash and others 2007).
Goularte and others (2004) obtained D
10
values of
∼
0.11 kGy for
E. coli
O157:H7
and
0.2 kGy for
Salmonella
on shredded Iceberg lettuce. Niemira and others (2002)
determined that D
10
for
E. coli
O157:H7 inoculated on iceberg, Boston, red leaf, or
green leaf lettuce was dependent on which type of lettuce was examined. D
10
in that
study ranged from 0.12-0.14 kGy. In a subsequent study, the D
10
for
Salmonella
was
also dependent on the variety of the suspending lettuce and ranged from 0.23-0.35 kGy
on the same four varieties (Niemira 2003). In contrast,
L. monocytogenes
did not show
the same type of alteration on irradiation sensitivity. D
10
values were invariant on the
four lettuce types (0.19-0.20 kGy) (Niemira 2003). A combination of chlorination and
irradiation at doses of 0.15-0.5 kGy produced fresh-cut lettuce with a reduced micro-
bial population (Hagenmaier and Baker 1997). Although 0.81 kGy reduced the fi rm-
ness of lettuce, resulting in lower shear force, 0.5 kGy or less did not affect shear
force, and irradiated samples had similar shelf life as the control samples. A later study
(Foley and others 2002) found that chlorination plus irradiation (5.5 kGy) reduced
TAPC, yeasts, molds, and
E. coli
O157:H7 by 5.4 logs in shredded iceberg lettuce
without softening of tissues.
Pathogen regrowth in storage following irradiation is a known phenomenon.
Irradiation protocols must therefore be optimized within the context of GHP to
ensure lasting suppression of the target pathogens throughout the storage period.
Romaine lettuce, inoculated with
L. monocytogenes
, gave D
10
values of 0.16 - 0.25 kGy
and presented no indication of regrowth in refrigerated storage (Mintier and Foley
2006 ).
L. monocytogenes
was observed to regrow in refrigerated storage on endive
leaves following 0.42 kGy, a dose equivalent to effecting a 2-log reduction (Niemira
and others 2003). However, 0.84 kGy, equivalent to effecting a 4-log reduction,
suppressed
L. monocytogenes
throughout the 19-day storage period. Combining
irradiation with passive MAP was insuffi cient to suppress the regrowth of
L. mono-
cytogenes
to regrow after exposure to doses of irradiation suffi cient to achieve 1 - 3 - log
reductions (Niemira and others 2004). However, an active MAP using reduced-O
2
,
enhanced - CO
2
effectively prevented the pathogen from regrowing after these low
irradiation doses.
∼
Protected Pathogens
Pathogens that are hidden within natural anatomical openings are often protected from
chemical sanitizers and other conventional antimicrobial processes (Takeuchi and
Frank 2000). Similarly, pathogens in biofi lms on produce surfaces are protected from
chemical antimicrobial treatments (Stewart and others 2004; Robbins and others
2005). Free-living (planktonic) cells of
L. monocytogenes
were reduced by 8 logs after
a 0.5 min exposure to 10 ppm sodium hypochlorite; in the biofi lm habitat, 1,000 ppm
sodium hypochlorite for 20 min yielded only a 2-log reduction (Norwood and Gilmour
2000). Compared with their planktonic counterparts, biofi lm - associated cells of
E. coli
O157:H7 (Ryu and Beuchat 2005),
Staphylococcus aureus
(Luppens and others 2002),
and
Salmonella
(Joseph and others 2001) required orders-of-magnitude increases in
treatment severity to effect adequate kill.
