8. Storage of fresh produce
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8.1. The need for storage
In temperate countries much of the production of fruits and vegetables is confined to relatively short growing seasons and thus storage becomes essential for provision of fresh produce out of the harvest season. In tropical countries production is often extended but storage may still be necessary or desirable for extended supply to the consumer. As consumer purchasing power increases, the reasons for storage may cease to be ones of traditional necessity but of satisfying consumer demand. Consumer demand is likely to include improved quality as well as improved availability and pressure is increasing, and will continue to do so, for improvements in storage techniques.
Produce may be stored for a few days or weeks as part of the normal marketing process but some temperate produce may also be stored for periods up to 12 weeks. The reasons for storage are:
There are various different forms of storage, the choice of which will depend on its cost and the produce to be stored. However, before contemplating storage of fresh produce there are other factors which must also be taken into account. The maximum storage life of a harvested crop depends on its production history and quality and maturity at harvest. The actual storage life which can be achieved in practice may be quite different and depends upon harvesting and handling procedures and the storage environment. Not all fresh produce is amenable to storage and some produce may require specific post-harvest treatments such as "curing" or "waxing" prior to successful storage (see Section 9. Some Special Post-Harvest Treatments). There may be features of the market structure or supply which create constraints whereby stored produce may compete at a disadvantage with freshly harvested produce. Encompassing all of these interactions is the question of storage economics.
8.2. Basic pre-treatments before storage and/or marketing
Section 9 describes various "special" post-harvest treatments necessary for specific crops, but there are some basic pre-treatments which must be performed before storage and/or marketing of any fresh produce.
8.3. Temperature, humidity and commodity considerations
Most rapidly maturing tropical fruits, soft fruits of all kinds, and leafy vegetables with a large surface area tend to have high respiration rates and normally have short storage lives. In contrast, most temperate fruits, cured potatoes and onions, and vegetable root crops often have lower respiration rates and consequently longer storage lives. Respiration of all produce increases with temperature which is why all storage techniques aim for a reduction in temperature of the produce.
Lower storage temperatures offer the additional advantage of greatly reduced water loss from the produce with reduced transpiration. High relative humidity slows down water loss and enhances storage life of the produce. Stores should ideally be maintained at the highest relative humidity (RH) that the crop can tolerate. Humidifiers of various types are generally available, and although 100% RH would totally prevent water loss, this can rarely be maintained because:
It is important to retain adequate circulation of the air within a store and around the produce to ensure efficient cooling. However, over-rapid air movement can drastically increase water loss by the produce. This is an important consideration when using forceddraught pre-cooling, as described in Section 8.5.3. below.
In conclusion, the choice of the correct storage technique is governed by:
8.4. Ventilated storage
Before the advent of refrigeration, ventilated storage was the only means available for storage of fresh produce and today is still in wide use all over the world for a variety of crops. Ventilated storage is ambient air storage which makes use of controlled ventilation for cooling of the produce and maintenance of lower temperatures. It requires much lower capital investment and operating costs than refrigerated storage and is perfectly adequate for some crops and conditions where:
Figure 8.1. (see Figure 8.1. A ventilated store for onions and sweet potatoes) illustrates a ventilated store for use by small farmers and built from readily available local materials. The store may be used for onions, garlic, yams and sweet potato. However, ambient or ventilated storage for most other commodities is not a practical nor an economic proposition because spoilage rates are simply too high. Some ventilated storage at the retail point may be an everyday reality for small shop keepers but larger shops and supermarkets, and most importers and wholesalers use refrigerated stores.
In the tropical climate of the Eastern Caribbean there is little opportunity for ventilated storage of most commodities beyond a few days. There are a few exceptions in that properly cured onions, garlic, sweet potato and yam may keep for up to two months if all the aspects of harvesting, curing, drying and handling are properly addressed and the store itself is thoroughly clean, well ventilated and protected from the rain, and located in a cool and shaded spot - preferably with some altitude to assist with lower temperature maintenance.
Although there are many islands with altitudes above 1000 metres, these are either too far away from the production zones or are not accessible by a good road, and topography prevents the building of large stores on steeply sloping terrain.
DO NOT FORGET RODENT CONTROL!!
8.5. Refrigerated storage
8.5.1. General Principles ant Considerations
It is now more than 130 years since the Australian James Harrison designed and built the first effective refrigeration equipment and the first icemaking plant in the world. Over 100 years ago regular shipments from Australia to England were commenced for the transport of frozen beef, an event soon followed by the operation of the first mechanically refrigerated cool stores for apple and pear. The most modern refrigeration plants available have changed very little in basic design since those times and accordingly consideration will be restricted to the mechanical functions and nomenclature of the equipment.
A refrigeration plant, as seen in the accompanying figure 8.2. (see Figure 8.2. Typical refrigeration plant for fresh produce) consists of three basic components:
Fans are usually necessary to circulate air over the cooling coils of the evaporator and through the stacks of produce in the store. The compressor and the condenser are always outside the cold store and usually mounted in tandem. The link between the three units is completed by insulated copper-piping. To increase efficiency the evaporator is fitted with metal fins to improve the heat exchange properties. Air-cooled condensors are fitted with similar fins and air is forced through by an electric fan. More detailed information can easily be obtained from any one of a large number of textbooks on the subject.
MANAGEMENT - MAINTENANCE - MOTIVATION
Refrigerated storage rooms are in common use for many types of fresh produce. Although the technology for designing and installing refrigeration facilities is well established it is a sad fact that many refrigerated stores operate unprofitably because of a number of common problems. The most significant of these problems are:
Each of these problems could be directly attributed to inadequate planning and management performance. Successful operation of cold storage facilities is dependent on some knowledge of costing, specific commodity requirements, refrigeration technology, and produce marketing for the successful operation of cold storage facilities. The construction and operation costs of refrigerated stores are high and so investment should not be considered until a thorough feasibility study has been carried out.
Too often stores remain in operation at great cost when they are almost empty of produce or when no increase in the selling price of the commodity is expected. A clear appraisal must be made of the commodities planned for storage, which are compatible in storage at specific temperatures and humidities and which commodities are not compatible, the expected storage life of the various commodities and the applicability to the anticipated market situation. Managers should have the confidence and authority to shut-down stores, even, if it means storing some produce at ambient temperature, rather than incur operating losses.
8.5.2. Storage Expectations for Fresh Produce
Table 8.1. lists the approximate storage life expected of different types of fresh produce and the specific temperatures and humidities which must be maintained in order to realise these expectations. It is assumed that the produce is:
Failure to meet these demands will result in a shortening of the expected storage life. To avoid confusion it is assumed that the storage life is the maximum period for which the produce will remain fit for marketing, and not the period to total senescence or spoilage. The list given is for example only and is by no means comprehensive. More extensive information can be found in the texts listed in Section 12.
8.5.3. Pre-Cooling of Fresh Produce
Once produce is placed in the cold-store it will radiate heat to the room by virtue of fieldheat and heat of respiration. The sooner the produce is brought to its optimum storage temperature then the sooner will respiration be brought under control and the maximum storage life of the produce be realized.
The heat from the produce is transmitted to the air which transfers this heat to the evaporator which removes it in the normal mechanical refrigeration cycle. The cooling, of the air and hence the produce is speeded up by the presence of electric fans mounted across the evaporator coils and may be supplemented by circulatory fans placed in the room and directed across the produce.
The time it takes for the produce to reach the optimum storage temperature (sometimes called the pull-down time) will be limited by the overall refrigeration capacity of the equipment and the speed of the air passing over the evaporator and over the produce assuming there are no barriers to air circulation around the produce.
Rapid air movement over produce enhances water loss and so in most refrigerated stores for long-term storage, air circulation is moderated to keep water loss to a minimum over the storage period. Produce temperature reduction under these conditions will be slow and the rate of respiration will only be slowly reduced.
To overcome these problems various pre-cooling methods have been devised for the rapid cooling of produce prior to its placement into long-term cool storage.
Although the rapid air movement creates more water loss from the produce, cooling is much more rapid than otherwise and the respiration rate is reduced very quickly. As soon as the produce has been cooled down to close to the optimum storage temperature it can be transferred to an ordinary cold-store for the rest of its storage life. There are many different types of forced-draught cooling and most depend upon the produce being in appropriate containers - often fibreboard cartons. Ships and containers adapted specially for refrigeration and carriage of fresh produce use a variation of this system.
Forced-draught cooling has the advantages of being a relatively cheap pre-cooling method which is easily operated and maintained and is widely used for many different kinds of produce. Heat transfer from produce to air is less efficient than that from produce to water, but many fruits, especially soft fruits, and some vegetables will spoil rapidly in store following water contact.
Efficiency of the cooling technique depends upon rapid movement of the cold water over the produce. Rate of cooling is dependent on the surface to volume ratio of the produce. Hydrocooling is only suitable for fruits which can withstand the excess of water but is widely used for rapid cooling of many vegetables.
Vacuum cooling is only of benefit however for produce with a high surface area to volume ratio, such as cabbage, lettuce, celery and other leafy vegetables, which allow for evaporation and thorough cooling in about 20 minutes.
Full-scale plants are expensive to install, but portable units are available which can be powered by a farm tractor. In recent years a system known as 'Hydrovac' cooling has been initiated which is identical to ordinary vacuum cooling but water is added in a controlled manner before cooling commences. In this way water loss from the produce is restricted and has been shown to be of benefit for some crops in allowing longer treatment time and more intense cooling.
8.5.4. Long-Term Refrigerated Storage
Once the produce has been cooled down to the required temperature it should be transferred as rapidly as possible to a store designed specifically for long term storage. Occasionally this is the same store as was used for pre-cooling but normally is an adjacent and often much larger storeroom.
For long-term storage it is important that the room air is well circulated but at a low velocity so that transpiration and water loss from the produce is kept to a minimum. Temperature of the store, and hence the produce within it, should be carefully monitored and maintained and the humidity should be carefully checked and elevated if necessary. Some form of ventilation is vital to prevent accumulation of carbon dioxide and ethylene gases and depletion of oxygen to harmful levels.
Produce should be stacked so as not to hinder circulation and thus permit the creation of localized "hot-spots" and subsequent premature spoilage. Packed produce should not be stacked against the side-walls nor directly against the evaporators. Stacking produce in regularly spaced 'corridors' will permit inspection of the produce at intervals during storage and hence removal, if necessary, of infected, over-ripe or otherwise spoiled produce.
8.6. Design, construction and management of refrigerated stores
Refrigerated stores are important components of the marketing process for fresh fruits and vegetables. They also demand careful planning in their design, construction, management and day-to-day operation if the substantial capital invested in them is to be protected and if they are to serve their function in the marketing structure.
8.6.1. Design and Construction
Before anything else it is important to determine the needs of the cold store and the environmental conditions. The products to be stored, their types, quantities and periods of production have to be weighed against the storage conditions demanded by the produce and the market place. Inherent factors, such as the local environment, the availability of labour and its skills and experiences, also have to be considered.
The size of the store will be determined by economic and technical factors. Small rooms are more expensive per unit volume for building and operation than large ones, but stock control and management of large cold stores is more complex and difficult. The volume of the cold store will depend upon the stacking patterns necessary for air circulation and heat dissipation, and the height of the rooms will depend on the handling and stacking methods to be used, 2.5 to 3 meters for manual handling, and 6 or even 9 meters if mechanical handling and pallet boxes are used.
Once all the above factors are accounted for then calculation of the refrigeration load can be made and hence the required refrigeration capacity and room insulation determined. These calculations are based on assessment of:
Finally, the type of refrigeration machinery, with a power rating sufficient for all refrigeration needs plus a safety margin, can be selected. The usual frigorific power for long-term cold storage, as opposed to pre-cooling, of fruit and vegetables is of the order of 30 to 50 watts per cubic meter.
With regards to construction, many types of building can be used for cold stores. Where the produce will be handled manually the height of the building will allow the use of relatively cheap local materials. For large stores however, specific systems must be designed with metal structures to support insulation or prefabricated walls and ceilings. The limitations are principally economic. The insulation properties must be sufficient to prevent excessive heat leakage and moisture transmission through the walls and ceiling and must constitute on effective vapour barrier.
The elements of refrigeration capacity and insulation of the store room can have considerable bearing on its construction and operating costs. It is recommended that an expert in refrigeration of fruits and vegetables is consulted before investment in the facilities begins.
8.6.2. Management and Maintenance of the Store
Good management of cold stores needs knowledge and experience of:
The loading of a room should be as rapid as possible if there is no pre-cooling process but should be monitored carefully so as not to over-load the refrigeration plant, or cooling of the produce will take much longer leading to reduced storage life.
POOR MANAGEMENT AND POOR MAINTENANCE - POOR STORAGE!
The stacking of the produce must allow for quick removal of some products, especially with mixed commodity storage, and also not impede air circulation. Opening of doors is an important heat leakage point and must be controlled by disciplined management. If doors need to be beekept open for extended periods, doorways can be fitted with a curtain of wide transparent plastic strips to prevent excessive heat leakage. Store rooms need regular disinfecting to prevent contamination and spoilage of healthy produce and this should be properly supervised.
When loaded, the store room temperature must be checked daily and the thermostat regularly examined to ensure it has not been tampered with. Recording thermometers should be used in large commercial cold stores. The relative humidity of the store room should also be checked regularly to prevent undue water loss by the produce. Evaporator coils should be checked daily for icing and defrosted when necessary. Maintenance and repair of the refrigeration equipment must be done by specialized, well-trained technicians.
One of the most important aspects of store management is the careful and accurate keeping of records. Records of produce type and volume, daily temperatures and humidities, produce losses during storage and when they were first observed and removed, are all essential historical documents assisting in overall store management and auditing of operating costs and profits, but they are also frequently the first indicators of faults and troubles.
CONSEQUENCES OF POOR STORE MANAGEMENT - SPOILAGE AND LOSSES !!
8.7. Economics of storage
Storage adds to the cost of a product and the more elaborate the storage method, the higher the added cost. It is not usually worthwhile storing fresh produce if the price increase resulting from storage does not exceed the costs of storage and show a profit on the operation. Sometimes it is acceptable to break-even on the cost/return ratio if this means that a greater volume of produce is sold overall or that the storage facility is being used more efficiently.
In certain marketing avenues the pre-cooling and/or storage of fresh produce is a standard requirement, and its cost is assumed as an accepted part of the production and marketing strategy. Provided that the storage is accomplished successfully then the price increase resulting from storage can be forecast from previous seasons and in any case will be known exactly in retrospect.
The costs of storage of fresh produce are often difficult to assess precisely and must take into account:
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