FEATURE
INCORPORATING COLD CHAIN
Continued from page 41 COOLING OPTIONS
by a need to regulate respiration rates (still
important) and instead more driven towards
preventing microbial decay, disorders, and
meeting phytosanitary requirements.”
For subtropical fruit, refrigeration is simply
non-negotiable. “Proper refrigeration slows
down the metabolism of fruit, therefore
slowing down the ripening process and
increasing the shelf life of the produce,”
explains Kaluwa. Now that we have established that cooling
is not optional, we take a look at what
refrigeration systems are commonly used to
cool products.
For table grapes, evaporative cooling is
used on harvested grapes prior to packing,
while forced-air cooling is used during initial
cooling of pallets. Refrigerated transport
is needed for long-distance movement
between cooling facilities, and in terms of
export, refrigerated containers are used for
shipment to markets.
Forced-air cooling is also used in citrus
export. In this case, cold airflow is extracted
(using fans) through the pallet stacks to
remove excess heat from the fruit more
rapidly, explains Dr Berry. “This approach is
useful, as the packaging creates a thermal
barrier around the fruit, which greatly
reduces fruit cooling rates compared to if
the fruit were still in bins.” This takes about
one or two days, but three days is usually
used (regulated).
Static cooling is also an option on citrus,
but this takes much longer than forced-air
cooling. Packaged or unpackaged (in
bins) fruit can also be placed directly in a
cold store.
Ambient loading is also popular for
citrus. Packaged fruit are loaded warm (or
only partially cooled) into a refrigerated
container. The refrigerated container then
gradually reduces the fruit temperatures over
three or four days. “This approach is highly
valued, as it allows fruit to bypass pre-cooling
facilities, particularly important during peak
seasons,” explains Dr Berry.
Regarding subtropical fruit, your options
are: room cooling, forced-air cooling, and
hydro-cooling, with refrigerated containers
when transporting produce abroad.
Freezing damage: To successfully maintain
pulp temperature at the optimum level,
the delivery air must be lower than the pulp
target temperature, typically between
-0.5°C and -1.5°C. Therefore, there is a
narrow temperature range that must
be maintained to avoid freezing injury,
which is reported to occur at -2.7°C in
ripe table grape berries and -2°C in the
stems. Although it becomes particularly
challenging to avoid freezing temperature
during the preparation of specific ‘cold
treatment’ consignments (where the target
pulp temperature is a maximum of -0.55°C),
a recent survey of the currently approved
facilities indicated that the cold store
operators are indeed able to avoid freezing
temperatures, within a safety margin, during
the pre-loading phase. Still, each individual
consignment requires diligent monitoring.
Post-harvest decay: There are several
potential post-harvest pathogens, such as
Botrytis, Penicillium, and Rhizopus, which
are able to develop and spread faster
on table grapes at storage temperatures
above 0°C, even with the inclusion of SO2
sheets. Therefore, maintaining the storage
temperature at -0.5°C will reduce the risk for
these pathogens spreading during transport
and storage. Conversely, transportation and
storage between 2°C and 5°C or above, will
increase the risk of decay development.
For subtropical fruit, the most common mistake producers make is that they begin
refrigeration too late, when the ripening process has already begun.
during the handling of grape pallets within
the export chain. The combination of
the various packaging components acts
as a mild form of insulation and specific
components are designed to buffer and
absorb a limited amount of condensation.
However, severe temperature breaks at
any stage of the export chain, including the
on-farm pack house, will result in excessive
condensation, which aggravates berry splits
and SO2 bleaching.
Phytosanitary risk management: The
management of phytosanitary risk and
therefore the requirement to avoid the
importation of live phytosanitary pests into
pest-free markets, are agreed by means
Condensation: It is unavoidable that short
and minor temperature breaks will occur
The motivation to apply proper cooling in citrus is driven towards preventing microbial decay
and disorders, and meeting phytosanitary requirements.
There are great risks involved in not keeping
fresh produce at the right temperature —
whether that be too hot or too cold. Getting
the temperature exactly right is vital.
When it comes to table grapes for
example, the optimum temperature for
transportation and storage is between -0.5°C
and +0.5°C, Moelich confirms. Outside of this
range, you run the following risks:
THE STAKES ARE HIGH
The maintenance of post-harvest quality starts when the table grapes bunches are harvested.
of bilateral technical protocols. Low-
temperature management techniques
are frequently prescribed to manage the
risk of specific ‘pests of concern’. In-transit
exceedance of the maximum allowed pulp
temperature during a specified period will
lead to rejection on arrival, which means the
consignment needs to be returned without
opening of the container, or diverted.
Fortunately, table grapes can withstand low-
temperature storage without the risk of the
development of ‘chilling injury’. The routine
shipment of pre-cooled grape consignments
at a set-point of -0.5°C is therefore inherently
suitable as an important measure to the
table grape export markets, to manage the
risk of phytosanitary pests.
When it comes to citrus, the risks are very
similar. When the temperature is too high, the
fruit will lose quality and decay will develop,
Dr Berry explains. Additionally, at too low
temperatures, chilling injury and even freeze
damage will negatively affect the fruit
market value. Chilling/freezing damage
is particularly challenging in the case of
phytosanitary/steri markets (for example the
US and China). These markets have very
strict temperature protocols, for example
temperatures should not exceed -0.6°C.
Whereas, generally, 4.5°C would actually be
a much more preferable storage/shipping
temperature.
Simply put, if temperatures are not cool
enough, then the fruit will ripen normally,
leaving it susceptible to post-harvest diseases.
If temperatures are too low, the fruit will
experience ‘chilling injury’, explains Kaluwa.
COLD LINK AFRICA • January/February 2019
A WORD OF ADVICE
Here are some tips from the experts on how
to get the most out of fresh produce and
refrigeration systems.
Moelich warns against incomplete
table grape pallets. At farm level, ‘excess’
or incomplete pallets, which cannot
be placed under forced-air cooling
conditions immediately or soon after
packing and are then kept under static
refrigerated conditions until the next
packing shift, are particularly prone to
excess condensation. “Avoid this practice
as far as possible,” he says.
“Know your refrigeration system well,”
Moelich advises. “Install proper monitoring
equipment.” Rather measure too much than
too little, on both the centre and sides of the
stacked grape pallets, using a combination
of thermocouples and probes. Make sure
the thermocouple wires are correctly placed
in the centre of the pallet. If not, you will not
know your pallet’s actual temperature status,
and this will create problems later in the
export chain.
All cold stores should be profiled to know
the location of both the warmer and colder
spots, Moelich advises. Such profiling is also
essential for the optimum placement of
temperature sensors. The control system
of the refrigeration unit relies on accurate
inputs/feedback from the system. If these
sensors are not optimally positioned, the
accuracy of the refrigeration system can be
Continued on page 45
www.coldlinkafrica.co.za
43