Design of Cold Storage for Fruits & Vegetables

By Mahesh Kumar, BVC Mahajan

Cold storage is the one widely practised method for bulk handling of the perishables between production and marketing process. 

cold storage 1The optimal storage temperature must be continuously maintained to obtain the full benefit of cold storage. Image credit: Burgess Rawson

It is one of the methods of preserving perishable commodities in a fresh and wholesome state for a longer period by controlling temperature and humidity within the storage system. Maintaining adequately low temperature is critical, as otherwise it will cause chilling injury to the produce. Also, relative humidity of the storeroom should be kept as high as 85-90% for most of the perishables.

Most fruits and vegetables have a very limited life after harvest if held at ambient harvesting temperatures. Post-harvest cooling rapidly removes field heat, allowing longer storage periods.

Proper post-harvest cooling can:

  • Reduce respiratory activity and degradation by enzymes;
  • Reduce internal water loss and wilting;
  • Slow or inhibit the growth of decay-producing microorganisms;
  • Reduce the production of the natural ripening agent, ethylene.

In addition to helping maintain quality, post-harvest cooling also provides marketing flexibility by allowing the grower to sell produce at the most appropriate time. Having cooling and storage facilities makes it unnecessary to market the produce immediately after harvest. This can be an advantage to growers who supply restaurants and grocery stores or to small growers who want to assemble truckload lots for shipment. Post-harvest cooling is essential to delivering produce of the highest possible quality to the consumer. Cold storage can be combined with storage in an environment with addition of carbon dioxide, sulphur dioxide (in case of grapes) nitrogen, according to the nature of product to be preserved. The cold storage of dried/dehydrated vegetables can be successfully carried out for a storage time of more than one year, at 0°-10°C with a relative humidity of 80-95%.

The cold storage of perishables has advanced noticeably in recent years, leading to better maintenance of organoleptic qualities, reduced spoilage, and longer shelf lives.

Also read: How to improve cold-storage supply-chain management

Care should be taken to store only those things which do not show incompatibility of storage when storing multi-produce in the same room. For example, apples can be stored with grapes, oranges, peaches, and plums and not with bananas. However, with potato and cabbage, slight danger of cross actions can occur. Contrary to this, grapes are incompatible with all other vegetables except cabbage. To resolve the incompatibility during cold storage, foodstuffs are grouped into three temperature ranges.

  • Most perishable products, not sensitive to cold (0-4°C)
    e.g. Apple, grape, carrot and onion
  • Vegetable produce moderately sensitive to cold (4-8°C)
    e.g. Mango, orange, potato and tomato (ripened)
  • Vegetable produce sensitive to cold (>8°C)
    e.g. Pineapple, banana, pumpkin and okra

Based on the purpose, the present-day cold stores are classified into following groups:

  • Bulk cold stores: Generally, for storage of a single commodity which mostly operates on a seasonal basis, for example, stores for potatoes, chillies and apples.
  • Multi-purpose cold stores: It is designed for storage of variety of commodities, which operate practically throughout the year.
  • Small cold stores: It is designed with pre-cooling facilities. For fresh fruits and vegetables, mainly for export-oriented items, like grapes.
  • Frozen food stores: It is designed for with or without processing and freezing facilities for fish, meat, poultry, dairy products and processed fruits and vegetables.
  • Mini units or walk in cold stores: It is located at distribution centre.
  • Controlled atmosphere (CA) stores: It is mainly designed for certain fruits and vegetables.

What to consider

If produce is to be stored, it is important to begin with a high-quality product. The produce must not contain damaged or diseased units, and containers must be well ventilated and strong enough to withstand stacking. In general, proper storage practices include temperature control, relative humidity control, air circulation and maintenance of space between containers for adequate ventilation and avoiding incompatible product mixes.

The general features of a cold store operational programme (products, chilling and chilled storage and freezing) include total capacity, number and size of rooms, refrigeration system, storage and handling equipment and access facilities. The relative positioning of the different parts will condition the refrigeration system chosen.

Also read: Refrigerated Warehousing And Storage – Refrigerants Matter

There is a general trend to construct single-storey cold stores. The single storey has many advantages:

  • Lighter construction;
  • Span and pillar height can be increased;
  • Building on lower resistance soils is possible;
  • Internal mechanical transport is easier.

Several parameters must be defined within a cold store. The total volume is the space comprised within the floor, roof and walls of the building. The gross volume is the total volume in which produce can be stored that is excluding other spaces not for storage. The net volume represents the space where produce is stacked, excluding those spaces occupied by pillars, coolers, ducts, air circulation and traffic passages inside the chambers that are included in the gross volume. Storage density referred to as net volume is expressed in kg/m3 but is the most commonly referred to as gross volume. About 3.4m3 of volume is required per ton of potato to be preserved while for onions this value is about 5.7 m3/t.

An index of how reasonably and economically the cold store has been designed is the gross volume divided by the total volume. It must be in the range of 0.50 to 0.80. Similarly, gross volume is about 50% greater than net volume, and gross area (same concept as volume) is about 25% greater than net area. The extent of occupation is the ratio between the actual quantity of produce in storage at a given moment and that which can be stored. Equally the extent of utilisation is the average of the extent of occupation during a given period — usually a year, but it can also be per month.

Managing temperature

Temperature management during storage can be aided by constructing square rather than rectangular buildings. Rectangular buildings have more wall area per square meter of storage space, so more heat is conducted across the walls, making them more expensive to cool.

Temperature management can also be aided by shading buildings, painting storehouses white or silver to help reflect the sun’s rays, or by using sprinkler systems on the roof of a building for evaporative cooling. Facilities located at higher altitudes can be effective, since air temperature decreases as altitude increases. Increased altitude, therefore, can make evaporative cooling, night cooling and radiant cooling more feasible.

Heat load calculations

The optimal storage temperature must be continuously maintained to obtain the full benefit of cold storage. To make sure the storage room can be kept at the desired temperature, calculation of the required refrigeration capacity should be done using the most severe conditions expected during operation. These conditions include the mean maximum outside temperature, the maximum amount of produce cooled each day, and the maximum temperature of the produce to be cooled. The total amount of heat that the refrigeration system must remove from the cooling room is called the heat load. If the refrigeration system can be thought of as a heat pump, the refrigerated room can be thought of as a boat leaking in several places with an occasional wave splashing over the side.

Keys to designing a cold storage facility

The design of cold storage facilities is usually directed to provide for the storage of perishable commodities at selected temperature with consideration being given to a proper balance between initial, operating, maintenance, and depreciation costs.


 

 


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