Food Protection Trends - May/June 2017 - 185

table 5. accumulated log reduction of Morganella morganii based on holding at a fixed
temperature after a 2°c/min heating rate
Minimum Core Temperatures

Elapsed Time

Reduction log (CFU)

60°C
59°C
58°C
57°C
56°C

1 min
2 min
4 min
7 min
12 min

5.68
5.41
5.59
5.30
5.06

Adapted from Table 1.
Based on a z = 4.1°C and D(60) = 0.26 min, using the Trapezoidal Rule.
Based on a 2°C/min heating rate and then holding at a fixed temperature.

application of more than a 5 log heat process is just a waste
of heat and resources if the goal is to achieve product safety,
although some processors may choose to do so to produce
different fish textures.
In the 4th scenario, shown in Table 1, the fish were heated
quickly and then held at a constant temperature. This process
is similar to a vacuum precooker cycle, in which the fish is
precooked in a step-down process. For example, if the fish
are heated until the cores are 57°C and then held for seven
minutes, or heated to 58°C and then held for four minutes,
or held at 59°C core temperature for two minutes, more
than a 5 log lethality reduction is achieved in all cases. The
automatic controller of VPCs can be programmed with
specific precooking regimes like these that ensure the
minimum CLs are achieved.
Using minimum core temperatures to monitor precooking
CLs will require adherence to a series of prerequisites
detailed in the Canned Tuna HACCP Guide (25). A key
concept for making preparations for precooking is to keep
things uniform: uniform sizing, uniform thawing, uniform
splitting, and uniform racking. Properly thawing uniformly
sized fish before precooking saves a tremendous amount of
steam because the fish are not being thawed in the precooker.
If all of the fish in a batch pass through the initial temperature
lag phase (Fig. 1) at the same rate and time, the end point
temperatures in all fish will be very close to one another,
provided there is good temperature distribution throughout
the precooker. It will also help processors to determine
whether the fish in the batch have been cooked enough to
have reached a 5 log reduction in lethality.
At the start of precooking, the increase in tuna core
temperature follows a certain pattern. There is an initial
lag of temperature increase as the heat moves towards the
geometric center of the fish or piece of fish (6); Fig. 1. Once
this lag phase has passed, the rate of temperature increase
is relatively uniform for the size of the fish, the type of
precooker, and ambient temperature. Note, from Fig. 1,

the variation in the core temperature when the steam is
turned off or at the maximum core temperature depends on
how long the core temperatures remain in the lag phase. A
practical monitoring procedure to measure this variation for
each type of precooking, CAP or VPC, the topic of a future
paper by the authors.
In a CAP, after the steam is turned off (6), the tuna core
temperatures continue to increase over a period of several
minutes (Fig. 1). Since in a tuna factory, most precooking
heating profiles do not achieve a heating rate of exactly 1°C
or 0.5°C per minute (6), another more practical approach
was developed to determine the accumulated log lethality
by simply sampling the core temperature from the same fish
at two different times. Smaller fish will start cooling faster
than larger fish, so the temperatures need to be collected and
analyzed promptly.
If the fish temperatures are measured during the time when
the core continues to heat after the steam has been turned
off, but the fish have not achieved a 5 log lethality when the
first core temperature measurement is collected, the core
temperatures can be remeasured after 5, 10, or 15 minutes.
So, for the simplest example, if all the fish have been removed
from the precookers and all of the fish in the sample have
core temperatures over 60°C, the operators can be certain
the 5 log reduction of M. morganii was reached. But if core
temperatures had not reached that goal but were over 56°C,
the operator can wait, since the core temperature continues
to increase for some time. If the core temperature is 56°C
and the operator waits 5 minutes before re-measuring, and
if the core temperature measures 60°C, the operator can be
confident that the log reduction is much greater than 5 log,
based on data in Table 2. If the core temperature starts at
56°C and after 5 minutes is only 58°C, the 5 log reduction
has not yet occurred, based on data in Table 2. If the operator
now waits another 5 minutes, and the temperature is still
58°C, then there is almost an 8.4 log reduction of lethality at
the core, based on Table 3.

May/June Food Protection Trends

185



Table of Contents for the Digital Edition of Food Protection Trends - May/June 2017

Contents
Food Protection Trends - May/June 2017 - Cover1
Food Protection Trends - May/June 2017 - Cover2
Food Protection Trends - May/June 2017 - Contents
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