CLASIFICADOR POR OBJETO DEL GASTO
50000 ACTIVOS FINANCIEROS
To balance fluctuations in product supply and market demand, fresh vegetables often require short- or long-term storage. In some cases, long-term storage is necessary to extend the supply of these vegetables beyond the end of the harvest season. The storage potential of vegetables is very much dependent on their physiology as well on the storage conditions. Understanding the interaction between the vegetables and the environment is crucial in obtaining the most suitable conditions for extending shelf life. Good management of storage conditions is essential to slow down and delay the loss of quality.
A. Precooling Systems
The design of a cooling system depends largely on the specific requirements for each particular situation. In order to design efficient and effective cooling systems, one should be aware of the many factors that affect the cooling rate and cooling cost. The vegetable’s physical properties (i.e., shape and size), thermal properties, configuration (i.e., bulk, cartons, unpackaged), initial temperature, and the desired final temperature influence the rate of cooling. A number of additional factors need to be considered when designing a cost-effective cooling system. These include the costs for cooling space, refrigeration equipment, labor, and electricity. A number of possible tradeoffs can occur when selecting design variables for a cooling system (81). For instance, while rapid cooling in forced-air cooling systems can be achieved by applying higher air velocities, the fan power requirement is also increased. Furthermore, lowering the temperature
Figure 12 Forced draft cooling of harvested vegetables in ventilated packaging.
accelerates the cooling process with the added expense of higher energy consumption. The selection of the cooling system should therefore be tailored on several considerations, such as the vegetable’s limitations and temperature requirements, and the costs of operation.
B. Storage Facilities
Close control of the storage conditions is essential to optimize the storage life of vegetables. The maintenance of optimal storage conditions within the prescribed range depends on several design factors. The refrigeration system should be designed to handle maximum heat load. The air temperature leaving the refrigeration coils should be as close as possible to the desired temperature to prevent large fluctuations as the refrigeration system cycles on and off (79). Air circulation as well as adequate wall and ceiling insulations should also be provided to minimize temperature variation. The refrigeration equipment should also be designed to allow control of high humidity conditions. The advancement of computer technology and improvement of electronic and sensing components enable continuous monitoring and precise control of storage conditions of large refrigeration systems.
VII. TRANSPORT
It is important that the required conditions for storage of vegetables are maintained throughout transport. The most crucial factors (temperature, relative humidity, airflow, and atmospheric composition) have already been mentioned. Different commodities require different transport conditions and should be transported separately (e.g., respiring product, nonrespiring product, product producing high levels of ethylene, products requiring different temperatures). Optimal storage conditions for various vegetables are shown inTable 2.
Modern transportation systems provide avenues for a wide distribution of fresh vegetables to areas where they cannot be grown. There are number of important factors that need to be considered to ensure a high level of quality maintenance in the distribution chain. These include minimization of mechanical damage, maintenance of proper temperatures, and ensuring compatibility of the produce. These considerations are influenced by the type of transport system employed (e.g., air, road, or sea). The choice of a particular type of transportation is usually not quite simple, as the transport requirements between vegetables and the cost of transportation vary in postharvest systems.
A. Air
For air transport, vegetables are usually packed into cartons and placed in closed containers or net covered pallets specifically designed to fit into the aircraft (80). This mode of transport is used for quickly moving highly perishable vegetables over long distances. Airfreight cargo is usually exposed to pressurized compartments with facilities to provide control of temperature and other gases. Normally, airfreight charges are much higher than other transport systems (e.g., sea freight), hence the transport cost is a major issue to consider. Special care is also necessary for perishable vegetables transported by air, as no power is available during flights. Good insulation is essential, as is adequate precooling of product. In-transit cooling can be provided by water ice or dry ice where necessary.
B. Road
Transportation by road is appropriate for quickly moving fresh vegetables over short distances, for example from the field to the market or directly to the consumer. This mode of transport appears to be simple and relatively cheap, but it may result in losses due to physical damage if appropriate measures are not taken.
Extra hazards to be avoided during road transport include
Physical damage due to vibration of the vegetables. Suitable packaging should be used to prevent this by providing adequate cushioning.
Interruption to refrigeration. Truck refrigeration units should remain on power continuously until delivery of the vegetables.
Mixed loads.
Loading vegetables that are inadequately precooled. Truck refrigeration systems are unable to cool warm product efficiently.
C. Sea
This mode of transport is not generally used for vegetables owing to their relatively short storage life and their need for rapid transit to discerning markets. In cases where this is utilized (where the distance between markets is short or the product has a longer storage life, e.g., onions, Allium cepa L., and potatoes, Solanum tuberosum L.), care must be taken. Refrigeration and ventilation (where required) must be uninterrupted. Most important is that vegetables be adequately precooled prior to loading, as refrigerated containers are not designed to remove field heat. Palletized vegetables should be tightly stowed to ensure both even distribution of cool air throughout the stow and reduced movement of the vegetables, since movement can cause physical damage. Dunnage should be used where necessary to prevent the short-circuiting of airflow through the stow and maintain a ‘‘tight’’ stow.
VIII. CONCLUSION
Postharvest handling and storage systems influence the quality of harvested vegetables by affecting the physiology of harvested vegetables. Changes in the physiology of harvested vegetables are mediated through the influence of the environmental conditions created by the handling and storage system. The physiology of the harvested vegetables, environmental conditions, and handling and storage systems all interact and therefore cannot be considered in isolation when successfully preserving and storing high quality harvested vegetables.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the assistance of Dr. Bruce MacKay and Dr. Maarten Hertog (Massey University) for manuscript review and Ms. Vicki Eggleston (Food Science Australia) for formatting of data and manuscript review.
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