Food Protection Trends - November/December 2017 - 417

and L. monocytogenes (P < 0.05) (Table 2). Breidt et al. (4)
found that thermal inactivation rates of E. coli O157:H7 and
L. monocytogenes in pickle brine (pH 4.1) were identical,
while S. enterica was significantly less heat tolerant. Similarly,
Mazzotta (16) found that, in heated fruit juices (pH 3.9),
S. enterica was heat sensitive while E. coli O157:H7 was
heat resistant. Acidification of a product reduces the heat
resistance of vegetative cells and reduces the possibility of
recovery from sub-lethal injury (24). In acidified tomato
purée samples, all three pathogens were significantly more
heat tolerant when purée was acidified with citric acid rather
than with acetic acid (P < 0.05; Table 2). When tomato
purée was acidified to pH 3.8 with acetic acid rather than
with citric acid, the average D54°C -values were 7.34 min, 8.53
min, and 5.19 min lower for E. coli O157:H7, S. enterica,
and L. monocytogenes, respectively. In heated cucumber
purée, Bae et al. also found that E. coli O157:H7, S. enterica
and L. monocytogenes were more resistant to citric acid than
to acetic acid (3). The antimicrobial effects of citric and
acetic acid are dependent on their pKa values (23). Acetic
acid (pKa 4.76) is a stronger acid than citric acid, which is a
triprotic acid (pKa1 3.13, pKa2 4.77, pKa3 6.39). In acidified
tomato-based foods with pH 4.5 and below, acetic acid has
a greater proportion of undissociated acid than citric acid
has, and acetic acid is more likely than citric acid to diffuse
into the cell, lower cytoplasmic pH, and inhibit metabolic
reactions (1, 17). While this phenomenon may at least
partially explain our findings, our experimental design
was aimed only at achieving a target pH-value and did not
take into account either buffering capacity of the system
or actual quantity of acid added. For industrial food safety
applications, it is advantageous to use organic acids with
a pK a near the pH of the food environment. However, the
amount and concentration of acid required to achieve the
target pH value, the associated flavor characteristics, and
the labeling provision of an acidulant are also important
factors to consider when selecting acidulants for product
formulation (12).
In previous work from our laboratory, pathogen inactivation in unacidified tomato purée (pH 4.5) was modeled over
a range of temperatures (52-58°C) using the re-parameterized Weibull method and the shape of the inactivation curve
(β) was evaluated, with nonlinearity frequently observed
(10). In the current study, when pathogen inactivation was
modeled in acidified tomato purée (pH 4.2 and 3.8) as
compared to unacidified purée, nonlinearity in heat inactivation curves increased from 31% to 59% of trials (data not
shown), suggesting that a nonlinear curve-fit model, such
as the Weibull method, may be of even greater utility when
establishing accurate thermal processing parameters in low
pH food systems.
Using data derived from a cucumber juice medium,
Breidt et al. (6) calculated thermal processing parameters for pathogen destruction in acidified cucumber juice

across a wide range of potential processing temperatures
(60.6-82.2°C/141-180°F) and showed that L. monocytogenes
was more heat resistant than E. coli O157:H7 and S. enterica
at temperatures below 74°C (166°F), but E. coli O157:H7
was the most heat resistant at temperatures above 74°C. The
present work found that when 5-log reduction values were
plotted against a wide range of typical processing temperatures, data lines for E. coli O157:H7 and L. monocytogenes
intersected at 65°C; E. coli O157:H7 was the most heatresistant pathogen at temperatures below 65°C (149°F), but
L. monocytogenes was the most heat resistant above 65°C
(Fig. 2). While the physiological explanation for this difference in z-values between the two pathogens is not completely
understood, the difference in z-values is important to consider when calculating processing time/temperature combinations. Therefore, to establish a single set of parameters that
ensure lethality of vegetative pathogens across a wide range
of relevant temperatures, a straight line was created that
ensured at least a 5-log pathogen reduction across all temperatures. The linear equation that defined the straight line was
used to generate a table of calculated time-temperature pairs
(Table 5) that would ensure at least a 5-log reduction for all
pathogens tested, similar to the method of Breidt et al. (6).
Processors of tomato-based acidified foods should consult a
competent process authority to determine additional heating
requirements beyond the minimal recommended here, to
account for formulation effects on thermal processing, to address spoilage, and/or to address destruction of acid-resistant
microbiota and other relevant factors in the establishment of
the scheduled process.
RECOMMENDATIONS
Using the re-parameterized Weibull model, we have
calculated time/temperature conditions that ensure at least
a 5-log reduction of vegetative pathogens of concern in
tomato-based acidified canned foods (pH ≤ 4.5).
* Processing time and temperature combinations were
calculated that can be applied to acidified products
with tomato as the primary ingredient/formulation
base. These recommendations support safety while
avoiding over-processing. Processors will wish to take
into account product formulation and the potential
presence of spoilage organisms in using these minimal to
establish operational processing parameters.
* Acidification of tomato-based canned foods using
citric or acetic acid will increase pathogen lethality
across all heating temperatures, with acetic acid having
greater antimicrobial activity than citric acid.
* Processing times and temperatures achieve at least
a 5-log pathogen reduction across a wide range
of anticipated processing conditions. Modeling
pathogen survival across typical commercial processing
temperatures indicated that the target organism depends
on temperature; E. coli O157:H7 had the greatest

November/December Food Protection Trends

417

Table of Contents for the Digital Edition of Food Protection Trends - November/December 2017