1 What are perishables?
1.1 A "perishable" may be described as something that is easily damaged or destroyed.
In the context of this informative material, perishables are usually, but not
always, foodstuffs. Without careful treatment, the time taken to deteriorate to a
condition which will either reduce the value or render it unsalable (shelf life) may
become unacceptably short.
1.2 Careful consideration of the factors affecting the "shelf life" of perishables
should be made and transport conditions during the "storage life" of the cargo
correctly applied.
1.3 Perishables include frozen produce, meats, seafood, dairy products, fruit and
vegetables, horticultural products such as flowering bulbs and fresh flowers plus
chemical compounds and photographic materials.
2 General issues
2.1 Shippers and consignees should be aware of the maturity indices for chilled
fruit, vegetable and horticultural produce. Whilst there are procedures for
retarding the ripening process, it is not possible to reverse it.
2.2 There are various makes and models of refrigerated containers in use. When
exporting temperature, atmospheric and time sensitive commodities, exporters should
liaise accordingly with the shipping company to ensure a container fit for purpose
is supplied that is capable of operating to desired and mutually agreed
requirements.
2.3 Maintaining proper conditions during shipment from the packing shed to the
overseas market is an important factor in minimising quality loss.
2.4 Problems could occur in the carriage of containerzsed perishable cargo due to the
lack of adequate and accurate carriage instructions issued by shippers. It is
extremely important that rational procedural precautions are routinely adopted and
instructions are always given in writing to all parties in the transport chain.
Shippers should ensure that all documentation shows the Set Point temperature and
atmospheric conditions settings. It is recommended that the information contained in
the electronic Pre-receival Advice should be made available to all parties in the
transport chain.
2.5 The shipper is in the best position to know the optimum temperature and container
vent settings (or Fresh Air Exchange rates) for the carriage of his product and his
reefer instructions should be followed unless they are obviously wrong or raise a
natural uncertainty. In that event, clarification should be sought. Carriage
instructions given to a shipping company should be complete, adequate and accurate
to avoid the risk of damage to the cargo.
2.6 The successful delivery of fruit, vegetable and horticultural produce from origin
to destination in refrigerated containers is also dependent on the maintenance of
suitable storage and packing conditions during transport.
2.7 The quality of the produce can be maintained only if each link in the chain
continuously maintains the integrity of the chain.
2.8 When packing refrigerated CTUs the perishable cargo should be pre cooled to the
required transport temperature (see also section 9).
3 Conditions which affect the commodity
3.1 General
3.1.1 There are several interrelated factors which affect each type of perishable
product during its useful life, either under refrigeration or not. These are briefly
dealt with in the ensuing sections.
3.1.2 The CTU owner may contribute to these conditions through equipment purchase and
operation. The consignee may be indirectly concerned, through the choice of wrapping
material, for the appearance of the product at the retail outlet.
3.1.3 Consignors should ensure that commodities leave their care in prime condition
and, in the case of fruit and vegetables, that harvesting was carried out at the
correct maturity. Fungicidal or similar treatments are often required for safe
carriage over long distances. Occasionally the type of package which the producer or
consignor consider to be economically acceptable may have a significant bearing on
the condition through the effect on air circulation and cooling.
3.2 Temperature
3.2.1 General
3.2.1.1 Temperature is particularly important both for long and short journeys. The
object of refrigeration is to prolong the storage life of a perishable food product
by lowering the temperature so that metabolic deterioration and decay caused by
microorganisms or enzymes are retarded.
3.2.1.2 For a commodity whose storage life is counted in weeks, transport within one
or two degrees of the optimum carrying temperature may be satisfactory when the
journey time is only a few days. When storage life is counted in days it is
essential to transport at the optimum temperature for the particular product.
However, for maintaining the goods in their best condition all goods should be
carried at their optimum temperature no matter the storage life or the transport
time.
3.2.1.3 There are regulations in various countries concerning the transport of
certain chilled and frozen produce which limit the maximum product temperature
within the transport chain.
3.2.1.4 It should be stressed that the only temperature, which can be controlled is
the "Set Point". The Set Point corresponds to air delivery temperature for chilled
cargo. The term "carriage temperature" therefore, cannot be used in carriage
instructions.
3.2.2 Air delivery temperature
3.2.2.1 This is the temperature at which air leaves the cooler to be delivered to the
interior of the vehicle or container by ducts or through a plenum chamber. The
required air delivery temperature is sometimes given in instructions from
consignors, generally with the intention of avoiding chilling or freezing injury of
the commodity.
3.2.2.2 Air delivery temperature is usually controlled in containers and various
machinery suppliers have set their temperature control point between -3°C and
-10°C.
3.2.2.3 Many designs of refrigerated road vehicles do not have a means of controlling
the delivery air temperature as a single thermometer, generally placed in the return
air, is used by the temperature controller. Air entering the cargo space can thus be
below the freezing point of the commodity in question.
3.2.3 Air return temperature
3.2.3.1 This is the temperature of the air leaving the interior of the CTU before
entering the cooler.
3.2.3.2 Air return temperature is generally accepted as representing the average
temperature of the commodity within the carriage space.
3.2.3.3 Many road vehicles use this temperature for controlling the operation of the
refrigeration plant. In general, containers with their sophisticated control
equipment use return air control only for frozen cargoes below -4°C.
3.2.4 Space temperature
3.2.4.1 Few, if any, road vehicles monitor the temperature of the commodity, or the
air space within the vehicle. In container transport, where in-transit sterilisation
(cold treatment) may be required by regulations covering particular destinations, up
to four sensors may be placed at locations within the commodity.
3.2.4.2 It is impossible to define a single position within a vehicle or container
which is representative of the average commodity temperature. Even with
comparatively well designed equipment the maximum commodity temperature is usually
greater than the return air temperature.
3.2.5 Temperature range
3.2.5.1 The temperature range defines the limits within which all temperatures in the
cargo should fall. If a carrying temperature is suggested which is likely to cause
the temperature of any part of the cargo to fall outside these limits, it should be
a subject of careful enquiry and possible rejection of responsibility.
3.2.5.2 In many cases the lower limit will be the product freezing point. In the case
of fresh fruit and vegetables the freezing point is an absolute limit, which if
passed, will almost certainly result in irreversible damage. For many tropical and
sub-tropical fruit the lower storage temperature is that minimum below which
chilling injury can occur and this temperature may be substantially higher than the
freezing point.
3.2.5.3 The upper temperature limit is less rigidly defined except in cargoes of
fruit that are being subjected to in-transit sterilisation where the upper limit
should not be exceeded by any part of the cargo at any time within the stated
quarantine period.
3.2.5.4 There are distinct differences between the range of air temperature as
indicated by the delivery and return air thermometers, the range of air temperature
within the vehicle and the range of the commodity temperature.
3.2.5.5 All three can be kept to a minimum and can be made to converge, by limiting
the heat inflow from the outside of the vehicle or by increasing the refrigerated
air flow or by a combination of both.
3.2.5.6 The general relationship between the various temperatures is illustrated
below.
Figure 8.1 Relationship of cargo
and air temperature
|
3.3 Relative humidity
3.3.1 The relative humidity of the air around the produce is of particular importance
both in long and short term storage.
3.3.2 Dry air may cause desiccation of the product which can affect the appearance
and will certainly reduce the weight at the point of sale.
3.3.3 Very damp air, with high relative humidity, will encourage the growth of moulds
and bacteria on chilled carcass meat and also lead to the development of various
fungal disorders on many fruits and vegetables.
3.3.4 When chilled meat is transported, there are significant changes in relative
humidity when the refrigeration unit is turned off for any reason.
3.3.5 Typically the relative humidity increases from 85% to nearly 100% and prolonged
periods at these levels can have a significant effect on the microbiological
spoilage.
3.3.6 Generally levels between 90% and 95% are recommended for fresh vegetables and
up to 98% for root crops. For fresh fruit levels vary but are generally between 85%
and 95% depending on the fruit and variety.
3.3.7 Relative humidity of the air around the produce is dependent on the water
activity at the surface of the product, the rate of fresh air ventilation, the
relative humidity of the fresh air and the temperature of the refrigerant coil
relative to the dew point of the air in the cargo space. Thus any problems which
arise may be related to any of several factors.
3.4 Loss of mass
3.4.1 This is one of the least understood effects of perishable cargo. Produce loses
mass by the transfer of water vapour to the surrounding air. If this air is very dry
then the rate of transfer will be increased and hence the rate of mass loss will
also be increased.
3.4.2 When warm unwrapped perishables are packed into a refrigerated CTU there is a
loss of mass during cooling due to evaporation. In this situation the refrigeration
plant may be operating at full rate. The evaporator coil will be at a much lower
temperature than the dew point of the air passing over it, from which water will
then condense drying the air which will cause further evaporation from the product.
For example, carcass quarter beef can lose 2% of its initial mass in cooling from
20°C to 6°C. Under these circumstances the cooling loss will be more significant
than the transport loss.
3.4.3 Similar effects apply to fruit and vegetables particularly when loaded above
the transport temperature and cooled in transit. Loss of mass can be reduced by
effective design of packaging, notably by the use of plastic films, but this can
result in condensation on the inside of the film.
3.4.4 The design of refrigeration equipment, particularly the air cooler or
evaporator coil, is important as is the need to ensure that the coil temperature
does not fall to very low temperatures thus promoting rapid air drying.
3.5 Air circulation and distribution
3.5.1 The need for adequate air circulation and particularly for even distribution is
paramount. Poor air distribution can adversely affect localized product temperatures
and result in a wide spread of temperature through the load. This together with the
effect on localized humidity and loss of mass combine to reduce the quality, storage
life and shelf life.
3.5.2 If warm perishables are packed then a good distribution of air is essential for
even cooling and a satisfactory product temperature range in the vehicle or
container. An adequate volume of air should be circulated to cool it quickly and to
maintain the desired range of air temperature (this practice is not recommended
except in special circumstances).
3.5.3 Air distribution depends on equipment, packaging design and the way the cargo
is packed.
4 Packing
4.1 General
Packing is one of the more important factors in all types of transport and is
particularly affected by the packaging of the commodity, whether it be carton,
pallet, net bag or hanging meat. The stow should be stable to avoid damage during
handling and in transit yet it should permit air to circulate freely through and
around the commodity.
4.2 Frozen products
4.2.1 Frozen products should only be accepted for transport when precooled to the
correct transport temperature. It is then only necessary for air to circulate around
the periphery of the load and a block stow, i.e. one that has no deliberate spacing
between any of the packages or pallets, is all that is required. It is of course
necessary to ensure that air can circulate under, over and to each side and end of
the stow.
4.2.2 The air space between the vehicle wall and the product is often maintained by
permanent spacers or battens which are built into the walls. There has been an
increasing trend for side walls to be smooth and concern has been expressed about
the possibility of elevated temperatures in these areas. Several trials with frozen
product in smooth sided containers have failed to demonstrate a significant problem
as there is invariably space for air to flow as a result of slightly loose stowage.
Problems would arise where boxes fit tightly across the space.
4.3 Chilled product
4.3.1 Chilled products such as fruit and vegetables are living organisms and produce
heat as they respire (or breathe). The quantity of heat generated depends on the
variety of fruit or vegetable and usually varies with the product temperature. To
ensure that this heat is removed it is essential that a large proportion of the
circulating air passes through, rather than around the stow, to give good contact
with all parts of the load.
4.4 Cartons for fruit
4.4.1 If the dimensions of the package are suitable, a block stow can be used with
cartons stowed one on top of the other preferably aligned vertically. Brick stows,
whilst giving good stability, do not allow free passage of air between the cartons
and may give rise to local hot spots. Ventilated cartons generally give better
results than enclosed cartons and are used, for example, for bananas which have high
respiration rates and are accepted for carriage within a few hours of cutting to be
cooled in transit.
4.4.2 Deciduous fruit such as apples and pears, when precooled to storage
temperatures, can be transported satisfactorily in closed cartons of either the tray
pack or cell pack types.
4.4.3 Stone fruits are susceptible to problems arising from respiratory heat and
without good air circulation have been found to rise in temperature, particularly
when block stowed on pallets.
4.4.4 Where fruit is not properly precooled, spacing between packages will facilitate
air distribution which can be achieved by the use of dunnage where this is found to
be practicable. To achieve adequate cooling rates the whole of the floor area should
be covered without leaving any large gaps between adjacent cartons, preferably not
greater than 10mm, so that a uniform distribution of the air flow between the
cartons will occur.
4.4.5 It should be recognized that most refrigerated CTUs are designed to maintain
perishables at the transport temperature, their use for cooling should only take
place after careful consideration of all the factors involved. It is a recognized
practice to cool bananas in containers but in-transit cooling is an accepted part of
the banana delivery chain from cutting to point of sale.
4.4.6 For most products, a CTU is unlikely to cool cargo from ambient levels of 20 to
25°C down to carrying temperatures close to 0°C in much less than 5 to 7 days.
4.4.7 Cooling rates are dictated by the need to avoid over cooling the cargo and by
the rate of heat transfer from the cargo in addition to any limitations in the
refrigeration capacity of the equipment.
4.5 Vegetables
4.5.1 The heat of respiration of many vegetables is higher than for fruit and for
journeys under refrigeration these commodities should be precooled to the carriage /
set point temperature.
4.5.2 Certain leafy vegetables, salad crops etc. are precooled by vacuum coolers or
hydrocoolers, wrapped in polyethylene bags and then placed in cardboard cartons. At
storage temperatures these commodities can be carried safely with a block stow,
preferably with the cartons in vertical alignment.
4.5.3 For commodities stowed in net bags, for example onions, potatoes, carrots and
melons, whether carried under refrigeration or forced ventilation, it is advisable
to break the stow with dunnage when the size of the commodity is particularly small.
For example, onions for pickling present a much higher resistance to air flow than
those used for other culinary purposes.
4.5.4 Carrots are a further example where product density under some circumstances
can impede air flow. With commodities in nets or sacks, the bottom tier should be
vertical with alternate layers stowed horizontally.
4.5.5 When commodities are carried without refrigeration it is essential to break the
stow by using pallets turned on end, particularly in periods of hot weather. All
fruit and vegetables produce heat which will, unless vented to the atmosphere, raise
the product temperature as will the ventilation fans.
4.6 Chilled meat
4.6.1 Hanging meat carcasses should be arranged to allow adequate air circulation to
all parts of the load. Care should be taken with stowage to minimize possible
product damage. It is prudent to load meat to meat and bone to bone always placing
bone against the side walls of the vehicle or container.
4.6.2 Effect of stowage on air and temperature distributions
4.6.3 In order to ensure good temperature distribution it is essential to have air
uniformly distributed throughout the load. This can be brought about by having the
cargo uniformly stowed over the floor of the vehicle or container. Poor stowage
results in poor air distribution which gives rise to slow cooling when produce is
not fully precooled. A large spread of temperature throughout the load may also
result.
4.6.4 The major principles to adopt are:
4.6.4.1 Stow as uniformly as the product will allow. Do not leave large gaps between
pallets or at the ends of the vehicle. Avoid alternating areas of very tight and
loose stowage which may lead to local hot spots building up over a period of
time.
Figure 8.2 Ideal packing pattern
for pallets
|
Figure 8.3 Irregular packing
pattern
|
4.6.4.2 With break bulk stows, empty cartons or timber should be used to fill the gap
between the end of the load and the doors. If the cargo is on pallets the floor
should be covered wherever there are blank spaces.
4.6.4.3 Always leave an air gap between the top of the load and the roof of the
vehicle. This is usually 10 cm on long vehicles and 7.5 cm on 20 ft. containers.
Good air circulation is not possible if there is no gap. Some vehicles have canvas
ducts to distribute air – these should not be distorted with too high a load.
4.6.5 With loose cartons it is possible to have a load uniformly spaced over the
floor area when the dimensions of the cartons are compatible with the internal
dimensions of the container or vehicle.
4.6.6 Vertical separations (dunnage) are useful with cartons, particularly with warm
or respiring cargoes, but it is better to use ventilated cartons to allow a through
flow of air. Some cargoes have a higher resistance to air flow than others and this
will have an effect on both the volume of air circulated by the fan and as a
consequence the temperature distribution.
Figure 8.4 block stacked to side
wall
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Figure 8.5 Blocked stacked with
air passage
|
4.6.7 Direct sunlight on the exterior of a refrigerated CTU may, over time, cause
parts of the side wall to heat up locally and without the cooling effect of moving
air over the inner face, penetrate into the cargo. This is caused by the cargo being
stacked directly against the side wall of the CTU.
5 Packaging
5.1 Temperature considerations
Temperature is considered to be measured and stated in Degrees Celsius [°C], while
Fresh Air Exchange rates should be stated in cubic metres per hour (CMH) for the
purpose of this informative material. Any variance from this practice should be
highlighted to all parties in the chain to ensure that there is no misunderstanding.
5.2 Carton design
5.2.1 Many perishable commodities are transported in some form of carton. The quality
of the carton tends to depend on the value of the product and occasionally on the
length of the journey. Practically all fibreboard has a poor wet strength so there
is a limit to the height to which cartons of fruit can be stowed without the load
gradually compressing. A good quality tray pack carton can be stowed about nine high
for a period of six weeks without collapsing. The effect of carton collapse, apart
from possible bruising of the contents, is to reduce the air gaps, making dunnage
battens ineffective and leading to an increase in the pressure drop through the load
with a reduction in the volume of air being circulated.
5.2.2 Package designs facilitating good cooling rates and the maintenance of small
temperature gradients in the load usually have perforations to allow air to move
freely through the cartons.
Figure 8.6 Ventilated
carton
|
5.3 Packaging Design and Heat Transfer
5.3.1 Package design plays an important part in transferring heat from the product to
the cooling air and the two examples given below typify two extremes.
5.3.2 Maximum cooling (and heating) rates are achieved with unwrapped fruit in
ventilated cartons, e.g. citrus fruit (these are sometimes individually tissue
wrapped). At the other extreme, wrapped pears in telescopic cartons with
polyethylene liners have a very slow rate of cooling.
5.3.3 The rate of air circulation within the CTU also has an effect on the heat
transfer from the package. It is possible to obtain improvements in cooling of
cartons up to a maximum rate of air circulation of 90 times the empty volume of the
storage space per hour. Above this level returns are small as the increase in heat
transfer coefficient between the surface and the air is offset by the insulating
effect of the carton material.
5.3.4 Cooling rates decrease with lower air circulation rates and at very low rates,
probably less than around 10 changes per hour, the air volume flowing past the
individual packages may be insufficient to remove respiratory heat with a resulting
rise in product temperature.
5.3.5 Some figures for cooling at different rates of air circulation are as
follows:
| Average ½ cooling times
|
60 air changes
|
90 air changes
|
| Non ventilated cartons
|
69.1 hours
|
54.6 hours
|
| Ventilated cartons
|
26.6 hours
|
24.5 hours
|
5.3.6 However, when stacking ventilated cartons, it is important to ensure that
ventilation holes line up. If using an interlocked stack, the ventilation holes may
not align when the carton is designed for vertical stacking. Where the air passage
through the cartons is blocked there is a risk of the contents deteriorating.
Figure 8.7 Free passage of
air
|
Figure 8.8 Blocked air
passage
|
5.3.7 Generally, fruit and vegetables which have a high metabolic heat production
rate should always be carried in packages which have a high rate of heat transfer to
the surrounding air.
6 Ventilation
6.1 Many cargoes, particularly fruit and vegetables carried in the chilled condition,
require some form of fresh air ventilation. This can be indicated by the measurement
of the concentration of carbon dioxide in the cargo air. Outside marine operations
little if anything is done to monitor this gas.
6.2 With CTUs, which are independent of a central monitoring system, it is usual to
ventilate continuously even though the amount of ventilation may exceed
requirements. Commodities that are known to be sensitive to the effects of ethylene
are generally ventilated at a high rate.
6.3 Several manufacturers of transport refrigeration equipment are now fitting
adjustable venting ports which allow the operator to set the vent to allow fresh air
exchanges in accordance with the requirements of the commodity being carried and
with reference to the ambient conditions in the operational area. For a typical 40
ft CTU air exchange rates in the range 30-250 m3/hr equivalent would be equivalent
to 0.5-4.5 changes/hr.footnote
Figure 8.9 Ventilation
port
|
7 Atmospheres – effects on quality and storage
7.1 The gases which affect the storage life of fruit and vegetables are oxygen,
carbon dioxide, and ethylene. Carbon dioxide is a product of the normal metabolism
where oxygen is absorbed from the atmosphere and carbon dioxide is given back to the
atmosphere.
7.2 Uncontrolled levels of carbon dioxide can be harmful to fruit and vegetables
during transport and storage. It can normally be replaced by ventilating the storage
space with fresh air. Approximately one air change of the empty space (CTU) per hour
is sufficient to maintain carbon dioxide at tolerable levels for most fruit. Higher
rates of ventilation may be specified for other reasons e.g. ethylene removal.
7.3 Low levels of oxygen, usually brought about by the use of liquid nitrogen as the
refrigerant, may have an undesirable effect on product quality. Consequently liquid
nitrogen should only be used with caution, as a total loss refrigerant for chilled
produce.
7.4 All fruit and vegetables produce ethylene, at varying rates depending on
commodity. Ethylene stimulates ripening and accelerates senescence to a varying
degree in all fruit and vegetables but the effects are sufficiently severe to cause
problems in only a proportion of commodities. It is also a by-product from internal
combustion engines and may be present in the atmosphere where these are operated in
enclosed spaces. For example, diesel or LPG powered fork lift trucks should never be
used for packing CTUs with fruit, cut flowers or shrubs.
7.5 As with carbon dioxide the effects of ethylene can be reduced by ventilation with
fresh air or absorbing material. Concentrations of ethylene gas at or below one part
per million can cause problems and measurement of such small amounts can prove
difficult. The use of sophisticated and expensive equipment such as a gas
chromatograph can only be carried out for test purposes rather than regular
monitoring. Consignors of commodities known to be sensitive to ethylene should
ensure that the packer is aware and that ventilation of the CTU is between two and
three air changes, of the empty volume, per hour. For less sensitive commodities
about one air change per hour is usually sufficient.
7.6 Various methods of absorbing ethylene from the atmosphere are available. These
include:
- potassium permanganate, sometimes used as a coating or with silica gel
(absorbent pads);
-
activated charcoal filters;
-
brominated charcoal filters;
-
catalytic filters;
-
combination with ozone. Ozone generators are available but are probably
better suited to use in large storage spaces. However, some CTU
refrigeration units do now have this facility.
7.7 In the transport field fresh air provides the most convenient and reliable method
of maintaining low ethylene levels.
8 Controlled atmosphere (CA) and modified atmosphere (MA)
8.1 The principles of atmosphere control have been known for many years and have been
applied successfully to long term storage, in cold stores, of apples and pears. The
techniques are now being applied to transport and packaging, not as a replacement,
but as an enhancement of good temperature control.
8.2 CA or MA does not eliminate the need for good temperature control. CA or MA with
reduced oxygen content and increased carbon dioxide content, with appropriate
temperature control, can retard deterioration and maintain the quality or increase
the storage life of various fruit and vegetables.
8.3 The beneficial effects of CA and MA include:
- retarding fruit ripening;
-
retarding leaf senescence (ageing);
-
control of fungal and bacterial spoilage and insects;
-
control of physiological disorders e.g. spotting in leaf crops and bitter pit
in apples;
-
reduction of ethylene production;
-
reduction of sensitivity to ethylene;
8.4 MA in CTUs
A packed CTU is purged with a tailored gaseous nitrogen mix immediately after packing
and just before final sealing.
8.5 CA in CTUs
CA CTUs for marine applications control the oxygen level either using liquid nitrogen
or by use of a continuous nitrogen generator in which air is pumped through a
membrane to produce a gas mixture of 98% nitrogen and 2% oxygen. For some
applications the commodity produces carbon dioxide at a sufficient rate to maintain
the required level which can then be limited by scrubbing. Higher levels for the
carriage of meat require a supply from either a cylinder or from blocks of dry ice.
8.6 CTUs where gases are introduced for conditioning purposes may be subject to
additional provisions relevant to the transport of dangerous goods. However, gases
which are used in refrigeration equipment are not regulated by the aforementioned
provisions.
9 Precooling
9.1 Why is it necessary?
9.1.1 In the first place to maintain the quality of products. Prompt cooling of fruit
and vegetables, immediately after harvesting, will lengthen the potential storage
life.
9.1.2 Secondly and more importantly, CTUs are not designed to cool products as they
are designed only to maintain the product at the transport temperature. CTUs, in
general, do not have sufficient capacity to cool the product quickly to maintain its
condition, whereas cold stores, cooling tunnels and pressure cooling systems are
designed for this task.
9.1.3 Fruit and vegetables are living organisms, consuming oxygen from the atmosphere
and giving off carbon dioxide and water vapour and heat. This heat of respiration
can add a significant load to the cooling system. The higher the temperature of the
product, the greater the heat of respiration.
9.1.4 The level of heat of respiration can have a very significant effect on the time
taken to cool the product to the transport temperature.
9.1.5 Tight stows of cartons on pallets are prone to slow cooling when warm product
is packed (see figure 8.10).
Figure 8.10 – Cooling on a
pallet
|
9.2 Vacuum damage
9.2.1 The consequence of cooling air is that the volume decreases in proportion to
its temperature. Therefore, a CTU opened and left with the doors open so that the
inside temperature is the same as ambient can cause problems when pre-cooling. If
the ambient temperature is high and the internal temperature is permitted to rise
towards that temperature, then the doors are closed and the machinery activated with
a low set point the volume of air inside will substantially decrease.
9.2.2 Refrigerated CTUs are designed with low air leakage so that the cold air cannot
escape and air drawn in by the ventilation port can be properly controlled, the
consequence of which is that when the doors and ventilation port are closed there
can be very little air movement between the exterior and the interior. In such
circumstances cooling the internal air will result in the internal pressure of the
cargo space dropping. This can result in a vacuum that prevents the doors from being
opened and in severe cases can result in the CTU imploding.
9.2.3 It is essential therefore that the ventilation port is opened when pre-cooling
and set once the interior has been cooled to the required temperature. Thereafter
packers should endeavour to keep the internal temperature as low as possible.
10 Equipment
10.1 Types of refrigerated CTUs
10.1.1 Descriptions of refrigerated CTUs can be found in informative material 3,
section 1.3.
10.1.2 For land transport, the refrigerated semi-trailer is the most popular form of
vehicle although for local deliveries and short haul operations rigid vehicles are
also used. The external dimensions of European semi-trailers can be as large as
13.6m (long) x 2.6m (wide) x 2.7m (high) although in other countries they may be
larger.
10.1.3 For marine use the most common type of container is the 40ft high cube
integral refrigerated container, which has an inbuilt refrigeration unit similar to
the refrigerated semi-trailer. The smaller 20 foot version is available but only
constitutes 7% of the world's refrigerated fleet.
10.1.4 As with all types of transport equipment, there are mass restrictions which
may limit the volume of the more dense product which can be carried. This is more
often found with frozen cargo.
10.2 How does a mechanically refrigerated CTU work?
The refrigeration unit fans cause temperature controlled air to circulate around the
inside of the vehicle floor, walls, doors and roof to remove heat which is conducted
from the outside. Some of the air should also flow through and between the cargo,
particularly when carrying fruit and vegetables, where heat of respiration may be a
significant proportion of the heat load. The various components of the heat load of
a refrigerated CTU are given in figure 8.11.
Figure 8.11 Heat load of a
refrigerated CTU
|
10.3 Top air delivery systems
Top air delivery is used predominately on refrigerated semi-trailers. Air is ducted
from the refrigeration unit to the end of the vehicle or passes through and around
the load returning via the floor or space under pallets. For chilled cargoes
horizontal channels are required between rows of cartons to allow good return
airflow through the load, whereas block stows are recommended for hard frozen
cargoes that have been fully precooled. Some trailers are fitted with a false
bulkhead wall with metal grill or holes in the lower part for return air passage.
The cargo may be stacked against this bulkhead. Where return air bulkheads are not
used it is a common practice to set wooden pallets on end between the front wall and
the front of the load thus creating a return air channel.
Figure 8.12 Top air delivery
reefer
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10.4 Bottom air delivery systems
10.4.1 Bottom air delivery is generally used in marine containers. Air is blown
through the evaporator into a plenum chamber, which distributes the flow evenly
across the width of the floor. Depending on the stowage pattern the air passes along
the floor to be circulated up through and around the stow returning via the roof
space. With respiring cargoes, the most even temperature distribution is attained if
the load completely covers the floor and the packaging or dunnage has been designed
to allow a high proportion of the air to circulate through the load as well as
around it. Where precooled frozen cargoes are concerned, a block stow is acceptable
as only the heat from the container body has to be removed.
Figure 8.13 Bottom air delivery
reefer
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10.4.2 The heat, gained by the air as it circulates around the CTU, is removed in the
evaporator section. The air also picks up moisture from the produce and also from
air from the refreshing vents when in use in ambient conditions with high humidity.
This is deposited on the evaporator as water or ice, depending on the coil
temperature. When ice is formed the air flow through the evaporator becomes
restricted and defrosting becomes necessary when the flow falls to 75% of the frost
free rate.
10.4.3 The rate of air circulation within the CTU is equivalent to 60 to 90 air
changes per hour of the empty volume. Some container operators are increasing the
rate to 120 for chilled cargoes. Under maximum summer temperatures of 30°C and 0°C
set point, for example, the range of air temperatures would be about 1.5°C at full
speed and 2.5°C at half speed on 40ft semi-trailers. Tighter tolerances are achieved
on marine containers where a 1°C spread would not generally be exceeded.
10.5 Floor designs
10.5.1 There are generally four alternatives available, a T-bar section floor, a
castellated section floor, a perforated floor or the pallet.
10.5.2 T-bar section floors – cause minimum obstruction to air flow, but can be
damaged by fork lift trucks and are difficult to keep clean.
10.5.3 Castellated floors – some obstruction to flow of air and increased pressure
drop, very strong and easy to clean.
10.5.4 Perforated floors – used traditionally in refrigerated ships and have been
modified for use in containers. Give less obstruction to air flow and better
distribution in the container than castellated. Difficult to clean unless removable.
10.5.5 Pallets – may be used with flat floors which are easily cleaned.
10.5.6 Road vehicles generally use flat checker plate or glass reinforced plastic
(GRP) floors and marine containers are fitted with T-bar section floors.
11 Capacity of the refrigeration unit
11.1 Most vehicle refrigeration units are fitted with compressors which will maintain
internal temperatures of -20°C in ambient temperatures of up to 40°C. When running
in the chill mode at maximum speed the cooling capacity is approximately double that
at low temperature. Reducing compressor speed to 50% will reduce the cooling
capacity by 35% to 40% but the net capacity may still exceed the refrigeration load.
11.1.1 All marine containers are capable of maintaining at least -25°C internal
temperature in ambient temperatures of up to 40°C. Requirements for trade in desert
regions have led to the development of units that will hold -25°C in 50°C ambient.
Cooling capacities on marine containers and other units are reduced by various
methods to give precise temperature control and heating is available for higher
temperature products during carriage in cold ambient conditions.
11.2 Temperature control
11.2.1 This is a function of refrigeration plant capacity and the load demand on the
refrigeration unit. Systems vary from simple ON/OFF which is used on many road
vehicles at all temperatures and for frozen control on marine containers, to
sophisticated capacity regulation using electronic control of chill temperatures on
marine containers.
11.2.2 Road vehicle control
11.2.2.1 The typical road vehicle temperature control for a unit on diesel drive
would be:
- Return Air > (Set Point + 2°C) High Speed Cool
-
Return Air < (Set Point + 1°C) Low Speed Cool
-
Return Air > (Set Point – 1°C) Low Speed Heat
-
Return Air < (Set Point – 2°C) High Speed Heat
In practice these tolerances may vary or be subject to
proportional-integral-derivative (PID) control.
11.2.2.2 On many diesel driven units, the compressor, condenser fan and evaporator
fan are connected to a common drive train, consequently the evaporator fan speed is
reduced when the compressor goes on to low speed and the reduced air flow allows the
temperature gradient across the load to increase.
11.2.2.3 Typical air temperature variations under on/off control and two speed
control are as follows:
Figure 8.14 Variations of air
temperature under thermostatic control
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11.2.2.4 Control cycles of this type are known to cause chilling and freezing damage
to sensitive fruits and vegetables. The main problem is the practice of controlling
the return air temperature combined with relatively wide control swings.
11.2.2.5 Where parts of a load are several degrees above set point the thermostat may
cause the compressor to run on full cool and thus freeze other parts of the load
near to the air delivery location. This problem can be eliminated by controlling the
delivery air temperature.
11.2.2.6 The variation between delivery and return air temperatures will tend to
increase when the fan runs at low speed.
11.2.3 Continuous temperature control
11.2.3.1 The marine container industry has made significant improvements in
temperature control which are of particular importance for the carriage of chilled
product over long distances involving total time spans of 6 to 8 weeks.
11.2.3.2 Temperatures are controlled to within -0.25°C of set point whilst the
differential between supply and return air temperatures is minimized by high
continuous rates of air circulation.
11.2.3.3 Precise control has been achieved by running the compressor continuously and
reducing the cooling capacity to exactly balance the heat load at the required
carriage temperature. The cooling capacity can be reduced in a variety of ways
including the following:
11.2.3.3.1 Discharge gas bypass – hot gas from the compressor discharge is redirected
to the evaporator. The flow rate is controlled either by a diverting valve or a
combination of solenoid valves. This system has the advantage of precise temperature
control over a very wide range of carriage temperatures, regardless of the ambient
temperature, with stepless change between heating and cooling. However, the system
is not energy efficient and uses more power to hold a load at say +5°C in an ambient
of +5°C than to hold the same load at -20°C using on/off control.
11.2.3.3.2 Reduction of Refrigerant Flow – the volume of gas pumped by the compressor
may be reduced by either unloading compressor cylinders (by lifting valves), by
increasing the cylinder head space volume or by throttling the flow with a valve
placed in the suction line. These systems reduce power draw and work well in fairly
high ambient temperatures but may give too much cooling power in low ambient
temperatures leading to compressor cycling.
11.2.3.4 CTU temperature can be controlled using sophisticated energy saving
software. With this software the compressor is not running all the time and allows
the temperature of the delivered air to be lower than the set point temperature
(during short periods of time). During the compressor stop periods the air
circulation fans are running at low/half speed.
12 Factors affecting the relative humidity of air in the refrigerated space
12.1 The level of humidity in the air circulating in a temperature controlled CTU
largely depends on the following:
- surface area of the cooler;
-
minimum temperature of the cooler;
-
rate of moisture transfer between the air and the commodity;
-
fresh air ventilation rate; and
-
relative humidity of the fresh air.
12.2 Container refrigeration units that offer some degree of dehumidification control
as an option are now available. The relative humidity may be controlled in the range
50% to 95%, with the refrigeration unit operating in the chill temperature range.
12.3 The circulation of dry air causes water loss from the product with consequent
loss of mass and reduction in quality. Modern packaging, particularly films, has
reduced the rate of moisture transfer from the commodity to the circulating air.
Vacuum packaging is used for the transport of fresh and chilled meats.
12.4 Films are increasingly being used for most fruits and vegetables, often with
perforations or of permeable quality to limit moisture build up and avoid
condensation within the package.
12.5 Some films are specifically designed to maintain a specific atmosphere mix
within the package. The technique has been applied commercially and is dealt with in
the section on controlled and modified atmospheres.
13 Ventilated transport
13.1 Ventilated CTUs were developed for the carriage of respiring cargoes that do not
require refrigeration and goods that may suffer condensation damage when carried in
dry freight units. Ventilation removes the products of respiration and allows the
product and container interior temperatures to closely follow the ambient
temperature thus minimizing condensation which will occur where the product is
several degrees colder than the ambient air.
13.2 Passive ventilated CTUs rely on thermal convection within the units achieved by
the natural convection of the atmosphere within the container by non-mechanical
vents at both the upper and lower parts of their cargo space.
13.3 A mechanically ventilated CTU is fitted with an exhaust fan mounted either in a
door or on the front bulkhead. Fresh air exchange rates of between 30 to 40 volumes
per hour are attained.
13.4 Passive ventilated CTUs are used for the carriage of coffee and cocoa beans,
chemicals and canned product where even temperatures are necessary to limit
condensation. Respiring products might be carried in mechanically ventilated CTUs.
14 Commodities
14.1 Chilled products
14.1.1 Compatibility of cargoes in store
14.1.1.1 The mixing of several commodities in a single load, a common cold store
practice, often appears to be economically advantageous where a common transport
temperature is to be used.
14.1.1.2 To a long distance shipper a mixed load may mean two or more fruits or
vegetables, to a meat shipper mixed carcasses and boxes of cuts or cryovac packs and
to a grocer or ship's chandler a mixture of meats, dairy products, fruit, vegetables
and non-food products.
14.1.1.3 It is essential not to mix any commodity in a mixed load that will impair
the quality of any other product within the load. With this aim in view the
following factors must be studied to discern the compatibility of products:
- carriage temperature;
-
transit time;
-
packaging and stowage patterns. -ethylene production rate. -sensitivity to
ethylene;
-
emission of objectionable odours; and
-
sensitivity to odours of other product, e.g. odours given off by apples,
citrus fruits, onions, pineapples and fish are absorbed by dairy products,
eggs, meats and nuts;
14.1.1.4 Film packaging of products can reduce the risk of taint but too much
reliance should not be placed on the method.
14.1.1.5 The problems of ethylene have been mentioned in the section on atmospheres
and solutions suggested. There are obvious combinations where it is inadvisable to
mix cargoes: as a general rule, bananas, avocado pears and kiwi fruit are among
those fruit which should not be stored with other fruit producing ethylene.
14.1.2 Fruit
14.1.2.1 Transport temperatures for fruit fall into two groups. Fruit which are
essentially tolerant of low temperatures are carried at temperatures in the range
-0.5 to 0°C. The aim is to carry at or as near to the freezing point of the
particular fruit as possible, taking into account control temperature variations to
avoid freezing any of the cargo.
14.1.2.2 More sensitive fruit are carried at higher temperatures which are a
compromise between the harmful effects of low temperature, which may result in
chilling damage and the benefit from low temperatures of slow ripening and retarded
development of rots. C hilling damage is the physiological damage which results from
exposure of fruit and vegetables to temperatures below a critical level for each
variety and causes most problems with fruit and vegetables from tropical and
sub-tropical areas.
14.1.3 Vegetables
14.1.3.1 Most temperate vegetables are tolerant of low temperatures and are carried
close to 0°C, but as most tend to have a higher freezing point than fruit the
delivery air temperature should not go below 0°C.
14.1.3.2 A higher range of temperatures are specified for certain vegetables which
would otherwise suffer from chilling damage (see section on fruit). These include
aubergines, cucumbers, marrows and most tropical vegetables.
14.1.3.3 Transport temperatures are given for some vegetables, which may be carried
using fresh air ventilation without refrigeration. The method used would depend on
the distances involved, ambient conditions and required storage / shelf life. Two
good examples are onions and potatoes.
14.1.4 Meat and dairy products
14.1.4.1 Chilled foods must be carried at temperatures between about -1.5°C and +5°C.
For some products an upper maximum temperature of not more than +2°C may be
specified, e.g. for chilled beef an upper limit of 0°C is recommended.
14.1.4.2 Difficulties may arise when transporting chilled meat with a specified
return air temperature of between -1 and 0°C in high ambient temperatures. To
maintain this level the delivery air temperature may have to fall to below the
temperature at which the meat starts to freeze. For short journeys the problem
should not arise as carriage temperatures of +1°C are usual.
14.1.4.3 High levels of carbon dioxide may be used for the carriage of chilled meat
when the transport time is about 28 days and some figures are given below:
|
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10%-20% CO2
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RH 90% +/-5%
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|
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20% CO2
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RH 90% +/-5%
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|
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25%-30% CO2
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RH 90% +/-5%
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14.1.4.4 Most beef and lamb for transport over long distances is either vacuum
packaged or modified atmosphere packaging is employed. A gas mixture of 50/50 carbon
dioxide and nitrogen is sometimes used, although as few films are totally
impermeable the mixture is likely to change after sealing.
14.1.4.5 Vacuum packaging, which is difficult to apply to whole carcasses, is
generally used for individual cuts of meat. Similar packaging containing a high
carbon dioxide content rather than a vacuum is sometimes used for lamb
carcasses.
14.2 Frozen product
14.2.1 There are several important levels of temperature in the carriage of frozen
product:
14.2.1.1 Final thaw temperature around -1.5°C which should never be encountered
during transport and storage.
14.2.1.2 Softening temperature at about -4.5°C. Surface temperatures may occasionally
reach this whilst loading carcass meat. Surfaces of outer packages or carcasses in
CTUs moving without refrigeration may also reach this figure.
14.2.1.3 The lower limit for mould development is -8.5°C. Considerable time is needed
for mould to grow at these temperatures.
14.2.1.4 Additional constraints, such as temperature, may be contained in legislation
of the exporting, transiting or importing countries.
14.2.2 Frozen foods continue to deteriorate, very slowly, and the lower the
temperature the lower the rate of deterioration and consequent increase in storage /
shelf life. Deterioration appears as a loss of quality rather than any dramatic
change and is the result of chemical activity such as oxidation and physical changes
resulting from evaporation and the growth of ice crystals. The rate of change is
also influenced by the exposed surface area of the cargo in relation to its mass and
by the presence and nature of any packaging which can limit loss of mass. For the
small unit such as frozen fish, fruit and vegetables, packaging is essential.
14.2.3 Dried Products
Milk powder and similar products, having been dried during manufacture, tend to
absorb moisture / water and taint odours. These are best transported in sealed
insulated CTUs and should be kept dry.
14.2.4 Coffee and cocoa beans
See section 13 (Ventilated transport).
14.2.5 Chemicals
Many chemicals, films, industrial and biological non–food products are shipped in
refrigerated or ventilated CTUs. Specific instructions, including dangerous goods
regulations, as regards handling, packaging, packing and temperature for each
product should be strictly observed.
15 Condensation
15.1 Condensation damage is a collective term for damage to cargo in a CTU from
internal humidity especially in freight containers on long voyages. This damage may
materialize in form of corrosion, mildew, rot, fermentation, breakdown of cardboard
packaging, leakage, staining, chemical reaction including self–heating, gassing and
auto-ignition. The source of this humidity is generally the cargo itself and to some
extent timber bracings, pallets, porous packaging and moisture introduced by packing
the CTU during rain or snow or packing in an atmospheric condition of high humidity
and high temperature. It is, therefore, of utmost importance to control the moisture
content of cargo to be packed and of any dunnage used, taking into consideration the
foreseeable climatic impacts of the intended transport.
15.2 There may be instances where the ingress of humid air could result in internal
condensation.
15.3 The nature of perishable cargoes makes them particularly susceptible to the risk
of condensation. The CTU operator should be consulted regarding feasible measures to
eliminate or reduce the effect of condensation.
15.4 Many condensation problems can be avoided by ensuring packaging materials are
dry at loading. Film wraps can also be of benefit.
15.5 For many products the use of ventilated CTUs has proved to be a solution to
condensation problems (see section 13 (Ventilated transport)).
16 Miscellaneous
16.1 Taint
16.1.1 Care should be taken to avoid mixing incompatible cargoes and with packaging
to protect the product from odour problems.
16.1.2 Some sources of taint are:
-
materials, generally sulphur compounds or of petrochemical origin, used in
the manufacture of plastics, paint and sealants;
-
previous cargoes which have persistent odours, e.g. citrus fruit, potatoes,
various chemicals; particular care should be taken when transporting
chemicals inCTUs that are used for foodstuffs;
-
odours absorbed by the insulation of the CTU;
16.1.3 Taint can be removed by:
- CTU cleansing to remove odours;
-
washing with detergent, rinsing with clean water, then ventilating;
-
with particularly severe or persistent odours steam cleaning may be
necessary, again followed by ventilation;
-
some odours can be eliminated by alternate heating and ventilation.
16.2 Hygiene
16.2.1 Washing, as outlined above, should be carried out prior to carrying food.
Fumigation may be necessary before loading such cargoes as chilled meat. A number of
proprietary sprays are available for this purpose.
16.2.2 The use of fumigants, such as methyl bromide may be restricted by national or
regional legislation.
17 Points to consider when packing perishable products in CTUs
17.1 Before packing
17.1.1 Ensure that the refrigeration unit is set correctly for the load, functioning
properly and controlling the temperature at the required level.
17.1.2 A pre-trip service inspection procedure is strongly recommended for all
transport refrigeration equipment.
17.1.3 The CTU should be clean, dry and free from odour particularly before packing
products that are susceptible to taint.
17.2 Packing
17.2.1 Precooling of CTUs should not be undertaken unless the CTU is tightly sealed
against a temperature controlled warehouse. When this is possible the internal
temperature of the CTU should be equalized with that of the warehouse before
packing.
17.2.2 Where it is not possible to connect the CTU to the warehouse, the CTU should
not be precooled or the refrigeration unit run during packing. Only precooled cargo
should be packed. If packing is interrupted, the doors should be closed and the
refrigeration unit run.
17.2.3 Check the temperature of the product with a thermometer of an accuracy
complying with any relevant standards. Take several product temperatures at random
and write them down on the loading sheet.
17.2.4 Take note of any defects: broken cartons or cases or other mechanical damage
to the product. Any peculiar odours or moulds on product or packages should be
noted.
17.2.5 Stow the commodity uniformly in accordance with the shippers instructions
remembering that air should flow between the packages when respiring products are
carried. A space of not less than 10 cm (4 in) between the top of the load and the
roof should always be left. With top air delivery using canvas ducts, avoid
distorting the ducts. Do not stow cartons tight up against the side walls. If they
do not fit across the width, stagger from one side to another, e.g. row 1 to left
hand side and row 2 to right hand side.
17.2.6 During the usual practise of loading pallets unavoidable voids may remain.
This may be useful for ventilation. Large voids should be avoided.
17.2.7 Whenever possible, the cargo should be evenly distributed across the entire
floor of the CTU. When this is not possible, additional cargo securing measures
should be applied to prevent movement during transport.
17.2.8 When bottom air delivery is used and there are only sufficient goods to
partially cover the floor, the exposed floor should be covered with flattened
cartons or similar so that air is forced through the load instead of bypassing it.
17.2.9 When carrying a mixed load of fruit or vegetables, the higher of the
temperatures recommended for the transport of each of the products should be chosen.
17.2.10 Cargoes should not be permitted to cool down in transit without specific
clearance from the consignor and consignee.
17.3 In transit
17.3.1 Run the refrigeration unit continuously unless restrictions apply as on a
ferry or in a noise abatement area. Where switching off is unavoidable try to park
in the shade.
17.3.2 Check the thermostat setting at the start and after any lengthy interruptions
in the journey.
17.3.3 Keep an eye on the indicated temperature, alarm lamps and defrost operation.
17.4 Unpacking
17.4.1 Run the unit until the doors are about to be opened.
17.4.2 If there is any damaged cargo, make sure that the position of the goods is
noted as this may help identify the cause of the damage.
17.4.3 Check temperatures of packages from various sections of the load.