Effects of Temperatures

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Temperature is the element in the post-harvest environment that has the greatest impact on the storage life of fruits and vegetables. In some regions of the globe, mostly tropical and subtropical climates, post-harvest losses of horticultural crops are estimated to be more than 50% of the production due to poor handling techniques such as improper temperature management.

Proper and adequate temperature management is the most important and simplest procedure for delaying product deterioration. Optimum temperature storage delays the ageing of fruit and vegetables, softening, textural and colour changes, as well as slowing undesirable metabolic changes, moisture loss and loss due to pathogen invasion.

Temperature is also the factor that can easily and promptly be controlled. Preservation of fruits and vegetables’ quality can only be achieved when the produce is promptly cooled and maintained under its optimum temperature as soon as possible after harvest.

In general, the lower the storage temperature, the longer the storage life. For example: in the case of asparagus – a highly perishable vegetable – scientific research has demonstrated that the higher the storage or shipping temperature is, the higher the losses in quality are. When asparagus are held under simulated air transport temperatures, 1.7 days shelf-life reduction occur between storage temperatures of 0°C and 15°C.

Furthermore after simulated transport at 20°C and 25°C, asparagus shelf-life may be reduced to 2 days. When transported at temperatures above 15°C, spears of asparagus can easily show symptoms of wilting within a short period of time. Therefore, asparagus quality can be better maintained with a 0°C holding temperature throughout the supply chain.

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Effects of high temperature on snap beans (decay, browning) and cucumbers (shrivelling, loss of water) during storage at high temperatures (20°C). 

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Effects of high temperature on cauliflowers (opening of the florets, wilting) and mushrooms (dryness and browning) during storage at high temperatures (20°C).

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Effects of high temperature on green bell peppers (shrivelling, colour changes) and lettuce (wilting and yellowing) during storage at high temperatures (20°C).

AIR TEMPERATURE CONTROL DURING AIR SHIPMENT

Like any means of transportation, air transportation can encounter significant changes in temperature. For example, about 47% of air-shipped perishables come from countries with a warm or tropical climate. Containers, when waiting to be loaded on the aircraft, are exposed to adverse environmental conditions and become sensitive to temperature increase. However, since air transportation is characterized by its speed, swift measures can be taken to minimize the effects of such detrimental conditions.

The most vulnerable phase in an airport-to-airport shipment is the aerial operations section. Perishables are often transported from a warm climate to a cold or moderate one, causing a load to be exposed to both high and low extreme temperatures in a matter of a few hours. Mostly shippers will try to protect their goods from either high or low temperature, yet rarely will a load be packed to deal with both extremes. Furthermore, it is almost impossible to predict the temperature at departure and arrival at the airport, since many factors affect it.

When exposed to the sun for example, a typical load’s temperature will increase substantially within a few hours. For this reason, good collaboration between all parties involved throughout the airport operations is critical to ensure success.

Another important part, is the flight phase.

Contrary to the common myth in air transportation, abnormal temperatures in the cargo holds are not necessarily cold. Often in many cases temperatures are quite high despite the fact that at 10 000m altitude the aircraft structure is exposed to extremely cold air of about -55°C. During the flight, temperature of cargo holds of certain aircraft can be controlled, but in many cases the capacity of the refrigeration system does not allow for the cooling down of the temperature of warm perishables.

Each type of aircraft, hold area and packaging has its own temperature pattern. So it is important to know the type of aircraft, its configuration as well as the available locations in the aircraft (i.e forward, aft, main or lower deck compartments) beforehand. The following figures show temperature patterns recorded on different flights with a wide body, narrow body and a commuter type aircraft.

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Air Temperature in a 747-300 AFT Compartment near Thermostats During a 7 Hours Trip

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Air Temperature in a 727F AFT Compartment During a 3.15 Hours Trip

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Temperature in a Twin Otter during a 3.25 Hours Trip

Currently, the worldwide demand for fresh fish and seafood products is increasing. Air transportation of these kinds of perishables represents an effective and speedy means of transport, but it requires specific conditions to guarantee the freshness and quality consumers are looking for. When transporting fresh fish and seafood, temperature is the most critical factor that affects quality if it is not effectively controlled. It is therefore essential to maintain the product temperature at or near 0°C throughout the entire shipping period.

It is important that before packaging takes place the temperature of the product, whether seafood or fish, reaches a temperature of -1°C to 1°C. Cooling agents, such as wet ice or gel packs, used within the packaging should not be used to cool down the product but rather to maintain the inside temperature as low as possible. Cooling the fish down before packing will slow spoilage and reduce the melting of the refrigerant used in shipping containers. A pre-chill of shipping containers before packaging will prevent fish from absorbing heat from the package.

The produce should preferably be kept buried in finely ground ice. Crushed ice is not recommended because of possible sharp edges. It may also damage the skin of sensitive species and puncture the polyethylene bags, causing undesirable leaks. These leaks may furthermore prevent the proper functioning of the containment system and spoil both content and packaging used. Flake ice, which is produced by freezing water on the surface of a drum and scrapping the ice off with a blade, is a valid alternative. Gel refrigerants may be another option to maintain a cold temperature within the packaging.

Ice not only keeps the product at a low temperature but it also prevents the loss of moisture from the surface of the product, which causes the flesh to dry and toughen. In addition, ice will slow down chemical processes, such as rancidity produced by the reaction of fats with oxygen (which causes objectionable odour and flavour) by limiting the exposure of the product to oxygen. Re-icing should be employed as frequently as necessary.