Food Protection Trends - May/June 2017 - 178
inhibited in all cases for at least 18 hours after this precooking
process ended. The work of Vogl et al. (35) validated and
supported reported results from Enache et al. (7) and
validates the values proposed later in this paper.
All the HDC enzyme that had been active before Vogl et
al. (35) precooked the fish must have been inactivated (i.e.,
denatured) by precooking because, within 6 hours after they
stopped precooking, even though the fish core temperatures
were again in the optimal histamine-formation zone, no
histamine formed until at least 18 hours. By then, the bacteria
populations appear to have regrown (from a depleted state),
leading to the production of new HDC enzyme and formation
of new histamine. The work of Vogl et al. (35) demonstrated
the effectiveness of a heat treatment in delaying histamine
formation by precooking, a delay allowing sufficient time for
completion of processing of the larger tuna.
Commercial tuna processors ultimately achieve control of
bacterial pathogens through canning and retorting at the end
of the process. However, fish are exposed to conditions that
allow histamine to form during processing, prior to being
retorted or frozen. Therefore, to prevent histamine formation,
the entire process must be controlled. Some facilities have
temperature-controlled processing areas where the room
temperature is kept low, or the product may be actively
cooled at certain steps in the process to help protect it from
spoilage; however, even with these controls, processing
conditions may allow for histamine formation. FDA
guidelines recommend that the time of exposure to unfrozen
conditions during processing, including precooking, should
be less than 12 hours if the fish is exposed to temperatures
exceeding 21°C (70°F) at any point in the process (9).
Since the precooking utilizes temperatures that far exceed
21°C (70°F), the 12-hour guideline is in effect and must be
adhered to at present, unless special precautions are taken.
Most commercial tuna processing facilities are in tropical
regions, so areas around the precookers and the areas for
post-precooker fish cooling also tend to exceed 21°C, even
outside of the actual precooking process. Smaller tuna can
be processed in under 12 hours; however, more than 12
hours are typically needed for processing larger fish. For
example, tuna weighing more than 10 kg commonly require
at least 24 hours to process from start of thawing through
start of retorting (5). Thus, processes such as thawing and
precooking just take more time for larger fish.
Based on research by Enache et al. (7), Nolte et al. (27)
proposed that precooking tuna to 60°C at the cold-point
would provide sufficient reduction in histamine-forming
potential to allow at least 12 hours of additional safe
processing time after the end of the precooking step.
Precooking tuna
The art and science of precooking tuna and the equipment
and practices used by commercial processors are described
by DeBeer et al. (6). During precooking, the two most
178
Food Protection Trends May/June
important factors that determine the rate of temperature
increase are the thickness of the fish, i.e., the distance from
the surface of the fish to its core or geometric center, and
the temperature differential, or delta T (∆T), between the
surface of the fish and the core. When the core temperature
is approaching the bacterium's lethal temperature zone
(starting at 50°C) during the precooking cycle, the smallest
fish (~0.5 kg) can have a fast heating rate of ~2°C/min at the
core of the fish, while larger (thicker) pieces of fish will heat
more slowly (6). To optimize processing, the fish are sorted
by weight, and the larger fish, e.g., > 10 to 15 kg, are usually
cut into smaller portions prior to precooking to reduce yield
(moisture) loss due to "overcooking" of the external layers of
the fish before the inner portion is fully precooked (5).
Another significant factor impacting precooking time is the
initial temperature (IT) of the fish (6) when the precooking
starts. If the fish has any frozen meat in the core, a substantial
amount of extra time and energy is required to thaw the
portions that are still frozen before core heating can even
start. This is a separate phenomenon from moisture loss
through cooking, as moisture loss is more closely correlated
with the maximum backbone temperature reached (2).
Figure 1 displays a typical temperature profile of steam and
fish in a conventional atmospheric precooker (CAP). The
time it takes for the heat to pass through the flesh to the core,
before the core temperature starts to increase, is called the
lag time. Note the different lag times of the different fish and
how the core temperatures continue to increase (change),
even after the steam is turned off. This variation is expected
and normal, even if the fish are properly sized (6).
Precooking equipment
Processors precook the tuna in batches on multi-layered
oven racks that are rolled into the precookers. The vast
majority of precookers currently used in the world are
either CAPs or vacuum precookers (VPCs) (24, 26). CAPs
generally precook with saturated steam at an ambient
temperature of ~100°C, and the precooking ambient
temperature is held at 100°C by adding steam, as needed,
during the process. Since the precooker steam bleeders and
drains remain open during processing, the internal pressure
remains close to atmospheric pressure (26).
VPCs control the precooking temperature by controlling
both the ambient pressure and ambient steam temperature in
a special vacuum chamber through the use of steam, vacuum
pumps, and water sprays on the precooker shell to cool the
shell; see Fig. 2 for an ambient steam profile for a VPC. The
lag phase in a VPC is at least as long as the lag phase in a CAP
(24), depending on fish size and initial core temperatures.
The heating rates are generally slower for the same size
fish, and, toward the end of the heating cycle, the core
temperatures may be more uniform than in a CAP (24).
Most precooking cycles in a VPC feature a step-down
steam temperature profile in which the steam temperature
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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Food Protection Trends - May/June 2017 - Cover3
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