Food Protection Trends - May/June 2014 - 138

O157:H7 was significantly more acid resistant than
S. enterica or L. monocytogenes at 10°C (50°F) in cold-fillhold acidified food products at pH 3.3 to 3.8 (3, 5). Data
on previous thermal processing at pH 4.1 showed that E. coli
O157:H7 and L. monocytogenes had similar heat and acid
resistance (3). By analysis of the estimated z and F values
from the current study, it was found that E. coli O157:H7
was more heat resistant than S. enterica and L. monocytogenes
at temperatures above 74°C (166°F), but less resistant
than L. monocytogenes below that temperature. Below 74°C
(166°F), L. monocytogenes was more heat resistant than cells
of the other two species. The physiological basis for this
difference is unclear. To derive a single set of parameters that
will ensure safety under all processing conditions, the F160
for L. monocytogenes (which had a z-value of 16.7°F) was
increased from 4.8 min (as shown in Table 3) to 5.6 min. The
resulting thermal processing recommendation, a z-value of
16.7°F and a F160 of 5.6 min, allowed achievement of a 5-log
reduction of all strains tested for any temperature in the range
tested and used for the recommended processing times and
temperatures. A linear model, corresponding to a z-value of

16.7°F and an F160 of 5.6 min is shown as the solid line
in Fig. 2. Based on these z- and F-values, a table of
recommended processing conditions was generated in
the range of temperatures typically used for commercial
processing of acidified vegetables (Table 4).
Previously, five times the standard error estimate was added
to the 5-log reduction times as an arbitrary safety factor for
recommended thermal processes (3). For recommended
thermal processing times and temperatures for acidified
foods using the data in Table 4, the authors suggest that
a competent process authority should be consulted to
determine if any additional safety factor is needed.
ACKNOWLEDGMENTS
he authors acknowledge Ms. Sandra Parker for excellent
editorial assistance. This work was supported in part by a
U.S. Department of Agriculture (USDA) National Integrated
Food Safety Initiative grant (2011-51110-31019), Pickle
Packers International, Inc. (Washington, D.C.), and USDA
Cooperative State Research, Education, and Extension
Service project 2006-01240.

T

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