Roughly 30% of food that is consumed in developing countries is perishable. Cold storage facilities are crucial to minimize post-harvest losses; however, losses occur at every step in the post harvest cycle, and therefore cold storages cannot be considered as independent solutions to prevent post harvest spoilage but as one component that needs to be integrated in a cold chain network from the point of harvest to the point of purchase by the end consumer.
Cold Storage Management
Deterioration of fruits and vegetables during storage depends largely on temperature. One way to slow down this change and so increase the length of time fruits and vegetables can be stored, is by lowering the temperature to an appropriate level. It must be remembered that if the temperature is too low the produce will be damaged and also that as soon as the produce leaves the cold store, deterioration starts again and often at a faster rate.
It is essential that fruits and vegetables are not damaged during harvest and that they are kept clean. Damaged and bruised produce have much shorter storage lives and very poor appearance after storage. Dirty produce can introduce pests and moulds into the store.
The produce should be harvested carefully using a sharp stainless steel knife. The fruits and vegetables should not be placed on the ground where they could pick up dirt. Either a clean harvesting basket or clean mats should be used.
It is essential that the fruits and vegetables are harvested at the correct harvesting time.
It is important that the produce does not get dirty or damaged during handling. Careful handling should be the rule. The best option is for the produce to be prepared for storage in the field and placed carefully in the storage containers used in the cold store. This considerably reduces the amount of handling and will keep damage to a minimum. It is essential that the produce is handled and placed in the store as quickly as possible as delays between harvesting and cooling can substantially reduce storage life.
If the produce is dirty it should be cleaned before storage. The water used has to be kept clean or fungus spores will be spread throughout the produce.
Some fruit and vegetables need their outer leaves removed before storage. However, usually it is better to leave the leaves on during storage to reduce moisture loss, and then remove them before sale.
Preliminary cooling (Precooling)
Dipping the produce in cool water to remove field heat can reduce the energy requirements of the store. However, this can spread fungus spores throughout the produce. A suitable alternative is to pick the produce either early in the morning when it is cool or late in the evening and leave it overnight to cool down.
All fruits and vegetables have a ‘critical temperature’ below which undesirable and irreversible reactions or ‘chill damage’ takes place. Carrots for example blacken and become soft, and the cell structure of potatoes is destroyed. The storage temperature always has to be above this critical temperature. One has to be careful that even though the thermostat is set at a temperature above the critical temperature, the thermostatic oscillation in temperature does not result in storage temperature falling below the critical temperature. Even 0.5°C below the critical temperature can result in chill damage. Table 1 gives the critical temperatures for various fruits and vegetables.
|Temperature °C||Relative humidity %||Maximum storage time recommended (ASHRAE handbook 1982)||Storage time in cold stores for vegetables in tropical countries|
It can be seen from the table that there are basically three groups of fruit and vegetables: those stored at 0 – 4°C; those stored at 4 – 8°C; and those that require a storage temperature above 8°C. It is often more convenient to concentrate on one of these groups.
For most produce, a high but not saturated, relative humidity is required, eg 85 – 95%. Table 1 shows specific relative humidities for fruits and vegetables. There is always some moisture loss during cold storage but excessive moisture loss is a problem. It is essential that the relative humidity is kept above 85%. This can be done by:
• allowing the produce to reach storage temperature and then covering in plastic
• sprinkling the produce with water, this should be done before storage since if the vegetables are sprinkled during storage condensation occurs at the refrigeration unit.
A packing system is required so that the produce can be loaded and unloaded easily and safely. This can be a system using crates that can be stacked or a more complicated system using racks and trays. It is important that the crates are small enough to be moved easily when full of vegetables; are able to be easily and safely stacked; and are strong enough to hold the weight of the other crates. It is essential that the airflow is not restricted, therefore a two foot gap should be left between the walls and the crates and three foot gap left between the crates, the ceiling and refrigeration units. Figure 1 shows the design of a crate that is suitable for cold stores. For small-scale cold storage systems, stackable crates are the most appropriate packing system.
The layout plan for the crates needs to be simple and clear so that produce can be removed easily and quickly.
Length of storage
Table 1 gives the maximum storage times recommended by various organisations. These recommendations are based on figures from Europe and America and are often for too long for small-scale stores in tropical countries for the following reasons:
• It is often difficult to prevent delays between harvest and storage.
• The produce is often bruised during harvest and loading of the store.
• Transport can be bumpy and result in damage.
There are multiple factors to consider in the technology and site selection of a cold storage facility:
- Location: the location chosen should be easily https://diazepamshops.com accessible and close to market hubs and/or produce collection points
- Electricity: the access to and the supply of electricity to the cold storage facility should be given. Electricity supply has an impact on the size and equipment feasible of the cold storage
- Drainage and waste disposal: good drainage options and waste management equipment needs to be readily available
- Water: supply of water is important for matter of sanitation. Also the removal of wastewater generated has to be given
- Economic viability: the value generated though cold storage needs to justify investment and running costs.
In general, choosing an appropriate cold storage technology depends on the temperatures and level of reliability required, resources available – including skilled labor, capital, electricity and water supply.
Operation and maintenance
Cold storage is a very costly and energy intensive undertaking and therefore the door to the cold storage should be opened as few times as possible and that also in the early morning or evening hours. Also the usage of lighting and fans should be kept at a minimum. Further points should further be considered:
- Logbook: operators should keep track of incoming and outgoing, as well as produced stored, including anticipated arrival time, quantity, quality, anticipated storage time, storage conditions required, sales price on day stored, actual sales price, energy consumed, etc. of new produce.
- Inventory: an up-to-date inventory should be kept including information about location within cold storage, type, quantity and harvest date of produce, use of pre-cooling method, date produce entered the facility, etc.
- Loading: in general, loading rates should be 4-5% of total capacity. The cold storage should be divided in areas for short-term and longer-term storage space
According to information of the FAO, cold storage facilities should be cooled to the storage temperature required at least three days before produce is being entered.
Some produce, especially of tropical origin, is sensitive to chilling, which means that it will incur physiological damages if stored at a certain time period below a certain temperature but above their freezing points. In general, the longer the time period that produce is exposed to temperature below their level of chilling sensitivity and the lower the temperature, the faster damages will occur. It should also be noted that effects can be of a cumulative nature, i.e. the time periods of storage below the level of chilling sensitivity add up even if produce is stored at optimal conditions in between. Several factors, such as the level of maturity and level of ripeness at the point of harvest can affect chilling sensitivity.
Common visual symptoms:
- Surface lesions (pitting, large sunken areas and discoloration)
- Water soaking (disruption of cell structure and accompanying release of substrate favors the growth of microorganisms)
- Internal discoloration of the pulp
- Failure to ripen in the expected pattern
- Accelerated rate of senescence
- Increased susceptibility to decay
- Compositional changes
Below 0°C all type of produce freezes due to dissolved soluble solids which are present in cell saps. Often, damages incurred through freezing only become visible once the produce is returned to temperatures above 0°C.
- Glassy areas in the flesh
In order to prevent pathogen built-up within the facility, strict hygiene measures are pivotal. All hygiene measures have to take necessary safety measures for staff into consideration.
- Clean thoroughly at the end of each season: wash walls and floor with sanitizers, such as hypochlorite solution
- Use of ozone generators to prevent molds and fungi
- Regular ventilation to avoid built-up of ethylene, carbon dioxide and odors
Adequate airflow and the even distribution of cold air need to be ensured. It is important to note that air always takes the path where resistance is lowest, and hence partly or unevenly filled storage facilities will have poor cooling rates. The following points should be considered:
- A gap of at least 8 cm between walls and the floor, and the stacks of produce should be kept in order to ensure air flow
- Well-ventilated storage boxes/containers/crates will improve cooling speed, such as PVC crates or ventilated boxes made of cardboard.
- adequate space in between storage pallets should be about 4-6 inches
Different produce has different storage demands with regards to factors, such as optimum temperature, level of relative humidity, levels of ethylene production and sensitivity, and chilling sensitivity. Hence, the storage of single commodities is less complex than the storage of multiple commodities; however, the latter can still be the more viable option depending on external circumstances. Besides damages due to mismatches in temperature, also odors can be transmitted and ripening and decay can be affected through the exposure to ethylene, which can result in changes in color, flavor and texture. Further, only high quality produce should be ‘allowed’ in the cold storage and produce should be sorted accordingly.
Examples of odor transfers which should be avoided:
- apples/pears with celery, cabbage, carrots, potatoes or onions
- celery with onions or carrots
- citrus with strongly scented vegetables
- pears/apples with potatoes à former acquire unpleasant taste
- green pepper will taint pineapples
- onions, nuts, citrus, potatoes should be stored separately
Examples of ethylene producing and ethylene sensitive products:
- Ethylene producing: e.g. apples, avocado, bananas, pears, peaches, plums, tomatoes
- Ethylene sensitive produce: e.g. lettuce, cucumbers, carrots, potatoes, sweet potatoes
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