Food Protection Trends - November/December 2017 - 413

TABLE 2. Decimal reduction times (D-value) for pathogen cocktails in tomato purée at 54°C
Pathogen

E. coli O157:H7

L. monocytogenes

S. enterica

pH

Acidulanta

nb

D* (SD) minc

n

D* (SD) min

n

D* (SD) min

4.5d

None

4

20.85 (1.67)A,a

6

10.63 (0.71)A,b

4

6.97 (0.81)B,c

4.2

Citric

4

16.39 (2.23)B,a

7

10.63 (1.20)A,b

4

6.12 (0.26)B,c

4.2

Acetic

4

9.40 (0.76)C,a

4

6.73 (1.10)B,b

14

3.79 (1.49)C,c

3.8

Citric

4

10.05 (0.87)C,a

4

8.68 (0.90)B,a

4

9.16 (2.24)A,a

3.8

Acetic

4

2.71 (0.32)D,b

4

3.49 (0.19)C,a

7

0.63 (0.21)D,c

Acidulant: None = unacidified, Citric = citric acid monohydrate, Acetic = glacial acetic acid.
n = Number of independent experiments.
c
Mean (standard deviation) of the D-value (min). Mean values within a column with different uppercase superscripts (A-D) are
significantly different (P < 0.05). Mean values within a row with different lowercase superscripts (a-c) are significantly different (P < 0.05).
d
Data at pH 4.5 (unacidified) from Dufort et al. (10).

a

b

in 38 of 64 (59%) thermal inactivation trials in acidified
tomato purée (this study), the shape of the pathogen
inactivation curve differed significantly from linearity
(P < 0.05; data not shown).
β values averaged 0.93 (± 0.20), 0.55 (± 0.15) and 0.91 (±
0.14) across all pH/acid combinations for E. coli O157:H7,
S. enterica, and L. monocytogenes, respectively. Inactivation
kinetics for E. coli O157:H7 in tomato purée acidified to pH
3.8 with citric acid are depicted in Fig. 1. β values across the
four trials ranged from 1.20 to 1.38; β = 1.30 for the average
curve fit with the Weibull model (Fig. 1). Post-processing
pH measurements indicated that pH of heated tomato purée
deviated only slightly from the initial pH value, fluctuating
-0.06 to 0.05 pH units across all heated purée samples and all
target pH values (n = 116; data not shown).
Results from a 3*2*2 factorial statistical analysis indicated a
high level of interaction between the acidulant, pH level, and
pathogen (data not shown; P < 0.01). Pathogen inactivation
trends were not uniform across acidulant and pH level
combinations. Subsequently, using a 3*5 factorial design, all
D-values at 54°C were statistically compared as a one-way
ANOVA with 15 treatments: three pathogens processed in
five different tomato purée conditions (unacidified at pH 4.5,
citrate-acidified at pH 4.2 and 3.8, and acetate-acidified at
pH 4.2 and 3.8). D54°C-values were not significantly different
at several combinations of pathogens and treatments (P >
0.05) (data not shown). Meaningful and relevant statistical
comparisons of pathogen inactivation across treatments are
shown in Table 2. Under experimental conditions, E. coli
O157:H7 was significantly more heat- and acid-resistant
than S. enterica and L. monocytogenes in tomato purée at pH
4.5 (unacidified) and in purée acidified to pH 4.2 using citric

or acetic acid (P < 0.05). L. monocytogenes was the most
heat- and acid-resistant in purée acidified to pH 3.8 using
acetic acid (P < 0.05), but in purée acidified to pH 3.8 using
citric acid, there was no difference in pathogen inactivation
(P > 0.05). Thermal tolerance in heated tomato purée
at pH 4.5 or 4.2 followed the trend E. coli O157:H7 > L.
monocytogenes > S. enterica (P < 0.05); at pH 3.8 with acetic
acid as the acidulant, thermal tolerance followed the trend L.
monocytogenes > E. coli O157:H7 > S. enterica (P < 0.05).
Data from previous work (10) were used to calculate
5-log reduction times for E. coli O157:H7, S. enterica, and
L. monocytogenes in tomato purée, pH 4.5 (Table 3). From
data on each pathogen, z-values were estimated based on the
D-values from nonlinear modeling of microbial inactivation
as described previously (10, Table 4). The z-value for L.
monocytogenes, 7.95°C (14.3°F), was significantly higher than
the z-value for E. coli O157:H7 and S. enterica (P < 0.05),
which were not significantly different (P > 0.05). Using a
reference temperature of 71.1°C (160°F), the F-160 value
was estimated for each pathogen using the respective z-values
in °C (Table 4). Using pathogen z-values and D*ref , the
data were extrapolated to the range of common processing
temperatures (141-181°F/60.6-82.2°C); data for E. coli
O157:H7 and L. monocytogenes are shown in Fig. 2. E. coli
O157:H7 was the most heat resistant of the three pathogens
at temperatures below 65°C (149°F), but L. monocytogenes
was the most heat resistant at temperatures above 65°C
(Fig. 2). To establish a single set of parameters that ensure
lethality of vegetative pathogens in the range of temperatures
tested as well as those relevant to commercial processors,
the two endpoints for the plotted 5 log-reduction times for
the most heat tolerant organism at 52°C and 82.2°C, E. coli

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