Best Environment for Crop Storage and Complete Guide
For proper crop storage, temperature management is important throughout the period between harvest and consumption in order to maintain good produce quality.
Cooling practices provide marketing flexibility by making it possible to market produce at the optimum time and over longer distances.
Presents a variety of packing methods and packaging materials that can help to maintain product quality and reduce mechanical damage during handling, transport and storage.
Conditions for Crop Storage
Storage may be defined as the act of preserving and keeping agricultural produce or any commodity for future use without necessarily losing its quality. There should be little or no change in chemical or physical condition as to reduce its quality.
Thus, the main objectives of storage is to preserve the produce such that it will still be valuable and useful to the ultimate consumer.
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Produce can be stored for both short-term and long-term purposes. Short-term storage is mainly used to provide flexibility in marketing (e.g. when awaiting transport), or because buyers are not immediately available.
Most horticultural crops are perishable and can only be stored for a few days. Only rarely is it worthwhile storing perishable crops to await higher prices, as storage will reduce quality and shelf life whilst adding to costs.
Storage is costly and, in most instances, when the produce is withdrawn from storage it has to compete in the market against much fresher produce.
A few crops are adapted for long-term storage. These can be held in store well beyond the normal harvesting period.
When they are eventually sold higher prices can usually be obtained and, by extending the marketing season, a larger volume of produce can be marketed. Often, the most successful stores are located in urban areas because
If produce is to be stored, it is important to begin with a high quality product. The lot of 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.
Commodities stored together should be capable of tolerating the same temperature, relative humidity and level of ethylene in the storage environment.
High ethylene producers (such as ripe bananas, apples, cantaloupe) can stimulate physiological changes in ethylene sensitive commodities (such as lettuce, cucumbers, carrots, potatoes, sweet potatoes) leading to often undesirable color, flavor and texture changes.
Cooling and Storage
Temperature management is important throughout the period between harvest and consumption in order to maintain good produce quality.
Cooling practices provide marketing flexibility by making it possible to market produce at the optimum time and over longer distances. In order to select the best cooling method, it is necessary to understand the basic principles of cooling.
Importance of Pre-cooling
Pre-cooling prior to shipment, storage or processing is essential for the removal of field heat from many perishable crops.
Proper pre-cooling can:
- Prevent quality loss due to softening by suppressing enzymatic degradation and respiratory activity;
- Prevent wilting by slowing or inhibiting water loss;
- Slow the rate of decay of produce by slowing or inhibiting the growth of decay-producing micro-organisms (moulds and bacteria);
- Reduce the rate of ethylene production; and
- Minimize the impact of ethylene on ethylene sensitive produce items.
Factors that Govern the Selection of a Pre-cooling Technology
The choice of cooling methods is dependent on a number of considerations:
Nature of the produce, e.g. fruit or vegetable – different types of produce have different cooling requirements. Strawberries and broccoli, for example, require near-freezing temperatures, whereas similarly low temperatures would damage bananas, mangoes or tomatoes.
Package design – the level of package ventilation (i.e. number and size of ventilation holes) as well as palletisation design can greatly impact on the rate of product cooling.
Product flow capacity – some methods of cooling are more efficient than others. Rapid cooling methods, in general, are required for the efficient cooling of large product volumes.
Economic factors – construction and operating costs vary among cooling methods. The selection of a cooling procedure must be justified by the volume and selling price of the produce item.
In cases where small volumes of produce are available and where electricity costs are high, higher-cost methods of cooling cannot be used since the cost incurred cannot be justified by the end profit margins.
Social factors – in low-income areas and in areas that lack electricity or cooling infrastructure, the use of simple and appropriate, inexpensive cooling methods makes sense.
Pre-Cooling Technologies
1. Cooling with Cold Air
Room cooling: Room cooling involves exposing the produce to cold air in a refrigerated room to start the cooling process and to remove field heat.
Room cooling may be used with most commodities, but may be too slow for produce that requires rapid cooling. If properly designed, a room cooling system can be relatively energy efficient.
Room cooling is very often inadequate for produce stored in large containers, such as bulk boxes or pallet loads. During the room cooling of such large containers of produce, heat is slowly removed from produce positioned near the periphery of the container.
Meanwhile at the centre of the container, heat is often generated by natural respiration more rapidly than it can be removed, causing the temperature to rise.
Room cooling is also inadequate for produce requiring rapid and immediate cooling. Strawberries, for example, must be cooled as quickly as possible after harvesting if their quality is to be preserved.
Even a delay of several hours may be enough to reduce their quality considerably. Room cooling is not rapid enough to prevent serious damage.
2. Forced-air Cooling
Forced-air cooling makes use of fans that increase the rate of cooling in a refrigerated room by pulling cool air through packaged produce, thereby picking up heat and greatly increasing the rate of heat transfer.
Although the cooling rate is dependent on the air temperature and the rate of airflow through the packages, this method is usually 75 to 90 per cent more efficient than room cooling
Produce items that can be cooled by forced air include; Coconut, Mango, Prickly pear, Avocado, Cucumber, Melons, Pumpkin, Banana, Eggplant, Okra, Rhubarb, Breadfruit, Grape, Orange, Strawberry, Brussels sprouts, Grapefruit, Papaya, Summer squash, Guava Passion fruit Tangerine Cassava, Kiwifruit, Pepper (Bell), Tomato, Pineapple, Pomegranate, Litchi.
3. Cooling with Water
Hydro-cooling
Hydro-cooling is appropriate for commodities that are not sensitive to wetting. This cooling process involves the flow of chilled water over the produce, rapidly removing heat.
At typical flow rates and temperature differences, water removes heat about 15 times faster than air.
Hydro- cooling is only about 20-40 per cent energy efficient, as compared to 70 or 80 per cent for room and forced-air cooling, respectively.
During hydro-cooling, the produce comes into contact with water. Good water sanitation practices must, therefore, be observed during the hydro-cooling process in order to minimize contamination. Once cooled, the produce must be kept cold.
Produce packaged in wire-bound wooden crates, waxed fibre board cartons, mesh poly bags and bulk bins can be hydro-cooled. Palletised packages can be hydro-cooled if they are carefully stacked to allow water to enter the packages.
If the water flows around and not through the packages, little cooling will occur. Produce in waxed cardboard cartons with solid tops is particularly difficult to cool since the tops preclude the entry of water.
Produce that can be hydro-cooled are; Artichoke, Celery, Peas, Asparagus, Chinese cabbage, Pomegranate, Beet, Cucumber, Rhubarb, Broccoli, Eggplant, Radish, Brussels sprouts, Green onions, Spinach, Cantaloupe, Kiwifruit, Summer squash, Carrot, Leek, Sweet corn, Cassava, Orange, Swiss chard, Cauliflower, Parsley.
4. Cooling by Contact with Ice
Icing is particularly effective on dense packages that cannot be cooled with forced air. Ice removes heat rapidly when first applied to produce but, unlike other cooling methods, continues to absorb heat as it melts.
Because of this residual effect, icing works well with commodities such as broccoli that have high respiration rates. Icing is relatively energy efficient. One pound of ice will cool about three pounds of produce from 29.4oC to 4.4oC. Ice must, however, be free of chemical, physical and biological hazards.
Top Icing
Top icing is used to cool a variety of commodities. In the top-icing process, crushed ice is added either by hand or machine over the top of the produce.
Crops that can be cooled by top icing: Broccoli. Green onions, Brussels sprouts, Leek, Cantaloupe, Parsley, Carrot, Peas, Chinese cabbage.
Liquid Icing
Liquid icing involves injecting a slurry of water and ice into produce packages through vents or hand-holds without de-palletising the packages or removing their tops. Growers with both small and large operations can use crushed and liquid ice cooling methods effectively.
Liquid icing is an excellent cooling method, despite the fact that the produce is wet during the process. The surface of warm, wet produce, however, provides an excellent site for the development of post- harvest diseases.
5. Individual Package Icing
The simplest method of icing is to manually add a measured amount of crushed ice to the top of each carton filled with produce.
This method is sufficient in many instances, but can result in uneven cooling since the ice generally remains in the location where it was placed until it has melted.
The process is also slow and labour intensive since each carton must be opened, iced and re-closed. Individual package icing has been automated to some extent by ice-dispensing devices and the use of package conveyors and roller benches. This method of icing is not usually recommended for high-volume production.
Packaging Containers Suited for Icing
Many types and sizes of fresh produce containers can be used successfully for package icing. Popular types include waxed fibre board cartons; wooden wire-bound crates, baskets and hampers; and perforated plastic liners.
Any container that will retain its strength after wetting can be used satisfactorily for icing. Waxed fibre board cartons are particularly well suited for icing operations.
They have minimal openings, offer some insulation to help reduce the rate of melting and their strength is unaffected by wetting.
Vacuum Cooling
Vacuum cooling is effective on produce having a high ratio of surface area to volume. This includes produce items such as leafy greens and lettuce, which would be very difficult to cool with forced air or hydro-cooling.
During the vacuum cooling process, the produce is placed inside a large metal cylinder and much of the air is evacuated.
The vacuum so created causes water to evaporate rapidly from the surface of the produce, lowering its temperature. The process may cause wilting from water loss if overdone.
Crops that can be vacuum- cooled: Brussels sprouts, Lettuce, Carrot, Peas, Cauliflower, Snap beans, Celery, Spinach, Chinese cabbage, Sweet corn, Leek, Swiss chard.
Evaporative Cooling
Evaporative cooling is an appropriate, effective and inexpensive means of providing low temperature and high relative humidity conditions for cooling produce. The process involves misting or wetting produce in the presence of a stream of dry air.
Evaporative cooling works best when the relative humidity of the air is less than 65 per cent. Produce should be picked during the coolest parts of the day and kept in the shade away from direct sunlight.
Appropriate Cooling Technologies
A Solar Assisted Cooling Chamber
A solar assisted cooling chamber can be used for temporary storage of fresh fruits at the farm levels. The hollow walls of the chamber, which are made of porous clay bricks, are kept moist by a water source.
Evaporation of moisture from the outer surfaces of the walls due to solar energy carried by the wind, results in the drop of the temperature within the chamber by 4-5oC below ambient temperature.
The moist walls of the cooler maintain a relative humidity of 85-90 per cent within the chamber. The storage life of fresh fruit stored within the chamber can be prolonged by two to three weeks.
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