Semi-perishable crops

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In contrast to grain, crops such as potatoes, yams, carrots and onions are more perishable and require carefully managed storage conditions to maintain top quality. While market value is seldom great enough to justify the expense of ideal levels of temperature and humidity control, the desired conditions will be discussed and then various methods of achieving levels as close as economically feasible will be described.

Properties

The properties of the many horticultural crops are far more varied than grains and pulses. This in turn results in highly varied storage characteristics. For example, yams and potatoes can be stored adequately for several months, while cassava can be kept for only a few days without deterioration if not processed.

The initial moisture content following harvest is much higher in these mature crops than in grain. With grain, a loss of moisture is desirable for storage and does not affect the use of the crop. This is not the case with fruits and vegetables. Loss of moisture may cause the crop to become unmarketable. Yet with a high moisture content, storage of these crops is more difficult since there is a greater likelihood of insect and fungal problems. Whereas lowering the moisture content of grain will inhibit sprouting, though not affect viability, the high moisture vegetable crops, which cannot be allowed to dry out, are more prone to sprouting. However, there is generally a period of dormancy following harvest which can be used to good advantage.

The perishable and semi-perishable crops are living organisms and as such, continue to respire. Consequently any storage will need ventilation to remove the heat and moisture of respiration and to prevent condensation on cool surfaces.

Fruits and vegetables are nearly always susceptible to physical damage such as bruising, cutting and cracking. Much of this results from dropping the fruits or tubers on to hard surfaces or on to other fruits and tubers as they are loaded into containers or bins. In many cases, 200 to 300mm is a maximum safe drop.

Further losses can occur if the heat of respiration is allowed to cause a temperature rise. "Black heart" in potatoes, for example, is a serious problem resulting from high temperatures under storage conditions.

In contrast, low temperatures approaching freezing produce a characteristic sweetening in potatoes.

Losses can also be caused by disease. This tends to be worse if the crop has been damaged, thus allowing the disease organisms to enter through cuts and cracks in the surfaces. Removal of earth from the crop and careful loading before storage can help to reduce this problem.

Storage Requirements for Potatoes and Other Horticultural Crops

Potatoes are the most commonly stored root crop and the greatest amount of research has been conducted relative to ideal storage requirements. In general, however, very similar facilities and operating conditions are suitable for several other crops of varying perishability. The following sections will deal primarily with potatoes, but much of the information, including the storage facilities described, will be suitable for other semi-perishable crops.

As mentioned, some bruising and cutting of the tubers is likely to occur during harvest. These fresh wounds are an ideal entry place for disease and rot organisms. The infection can be minimized by storing the potatoes for the first one to two weeks at a temperature of 13 to 20°C and a relative humidity of 90 to 95%. During this curing period the skin toughens, making the tubers much less subject to further injury or disease problems.

Potatoes are naturally dormant for about two months. However, it is often necessary to store them for longer periods of time by extending the dormancy period and by keeping shrinkage to a minimum. Temperature and humidity are important factors in this respect. Suitable temperatures for long term storage are related to the eventual use of the potatoes.

For seed stock, temperatures of 3 to 5°C will delay sprouting for up to 8 months. For ware potatoes, 4 to 8°C will allow several months (4 - 8) of storage without serious sprouting, while lower temperatures increase the risk of sweetening, that is, the conversion of starch to sugar. Finally, for processing potatoes, a minimum temperature of 7 to 10° C is required in order to prevent discolouration and to keep sweetening to an absolute minimum. In stores with higher temperatures it is possible to control sprouting in ware and processing potatoes for up to 6-8 months by using a sprout supressant chemical.

The relative humidity (RH) of the air in the store is of great importance. Low RH will lead to shrinkage and weight loss while too high RH will cause condensation on the surfaces. This is objectionable since free water on the potatoes greatly increases the possibility of rot and the spread of diseases. A potato tuber is about 80% water and strictly speaking, air is in equilibrium with the tuber at a relative humidity of 98%. However, in practice, to avoid condensation the relative humidity is kept between 90 and 96%.

Potatoes exposed to direct or indirect sunlight will turn green and develop a bitter taste which is poisonous and make the tubers unsuitable for human consumption. Stores should therefore be without windows and ventilation openings should have light-traps.

Potatoes that have been held at low temperatures tend to be brittle and subject to considerable damage if handled. If the store has been maintained at low temperatures throughout the storage period, it is best to warm the store to about 10°C for a few days before the potatoes are removed.

Storage Without Buildings

Delayed Harvest

The simplest form of storage for some crops is to leave them in the ground and harvest them only as required. There is risk of pest and rodent damage, but the deterioration which may take place after harvest may exceed the field losses; hence delayed harvest is a reasonable choice. This is particularly useful for cassava where field deterioration is normally substantially less than post harvest losses due to even short term storage. On the other hand, some crops deteriorate substantially in quality if left in the ground beyond a certain stage. Carrots, for example, tend to become tough and woody.

Clamp

In areas that have low mean soil temperatures, a simple ground clamp (Figure 9.31) may be suitable, especially for potatoes. They are piled on the ground in a long row and covered with 150 to 200mm of straw or coarse grass. Chicken wire is put all around the base to resist rodents and then soil is dug out around the pile and placed on the straw. This store is not likely to be satisfactory for more than a month or two unless the soil temperature is near 10° C and night air temperatures are 10° C or less. To control soil pests the ground can be treated with an insecticide before the clamp is made.

Figure 9.31 Simple roof crop clamp.

Covered Clamp

Another simple store for short time storage is the covered clamp (Figure 9.32) consisting of a raised platform on which the potatoes are heaped and then covered with 10cm of grass or straw. Air is free to circulate through both produce and straw. A thatch roof above provides shade to help reduce daytime temperatures. Protection from rodents will be needed.

Figure 9.32 Covered clamp raised from the ground.

Storage in Multi-purpose Buildings

Slatted Boxes or Bins

Square boxes of slatted construction, each holding about 1 m³ of potatoes offers a good possibility for small as well as large scale stores. The boxes can be larger but not deeper than 1m. If located in a well isolated building, the fluctuation of daytime temperatures will be reduced. The boxes should be raised about 250 to 300mm above the floor so that air can circulate freely. With little insulation and only natural ventilation this method is best suited to cooler areas and then for relatively short storage periods of 3 to 4 months.

Smaller boxes can be handled manually, while the larger boxes of 1m³ and more can not be moved manually when filled. See Figure 9.33.

Figure 9.33 Box store for root crops.

Clamp on Floor

Using a building similar to that shown in Figure 9.33 a clamp offers an alternative to boxes. To allow adequate ventilation with cool night air, a duct under the crop is included as shown in Figure 9.34.

Figure 9.34 Duct under produce heap.

Naturally ventilated stores

Figure 9.35 shows an example on how to build a potato store suitable for small scale production. The store, which holds about 1500 kg., is naturally ventilated and measures 150 x 160cm square. The walls are 1 50cm high and a slatted floor is placed 90cm off the ground to keep rodents away. The store shown in the figure is made of offcuts but other materials may be just as good. For insulation the walls have a 20cm thick layer of straw which will be compressed to about 10cm when the store is loaded. The floor should be covered with about 5cm of straw before loading and on the top 20cm of straw should be spread evenly to protect the potatoes from sunlight and drying.

Figure 9.35 A Naturally Ventilated Store.

The method of operating the store is depending on the average temperature on the place. If the average temperature is above 20°C it is necessary to extend the walls on three sides down to the ground like an apron. The forth side will have a flap that is kept open only at night in order to take advantage of the cooler air for the ventilation.

For higher altitudes with mean temperatures below 20° C it is possible to operate this potato store with continuous ventilation and the apron and the flap can be left out. In this case the store legs should be fitted with rat guards.

The ventilation should be just enough to remove heat caused by respiration without causing any excessive loss of moisture.

Larger Stores

Buildings to store large quantities of potatoes or other root crops in bulk must be of substantial construction to resist the force of the crop against the walls. Further, the walls and ceiling must be well insulated whether outside air or refrigeration is used for cooling.

The wall sills must be securely anchored and the studs firmly fastened to the sill in order to withstand the high lateral force of the potatoes. It is desirable that the concrete floor be tied to the foundation with reinforcing bars.

Tie beams should connect the top of the side walls on opposite sides of the building to resist the load and braces at frequent intervals are needed to withstand uneven loading.

Insulation and Vapour Barriers

Regardless of the climatic area in which they are built, large air cooled or refrigerated stores should be well insulated. In uplands e.g. in Southern Africa some insulation will prevent freezing of the potatoes in midwinter. In contrast, in hot regions where mechanical refrigeration may be necessary, substantial insulation will help to reduce the cost of operation. An R value of 4 to 5 in the ceiling and 3 to 3.75 in the walls should be adequate to prevent condensation in a cold climate and to allow economical operation in warm areas. These large storage are expensive buildings and it is important to install high quality commercial insulation.

As discussed in Chapter 7, vapour barriers are essential to prevent the accumulation of moisture in the insulation. It was pointed out that moisture travels from the warm side to the cold side of a wall or ceiling. Thus the vapour barrier is installed on the warm side. A refrigerated store in a warm area poses no problem in choosing the outside of the wall and ceiling as the warmest and therefore the proper place to install a polythene vapour barrier. However, air cooled stores are much more difficult to design as the outside temperature may be warmer at the start of the storage season and the inside warmer later on. A very careful assessment must be made to choose whether vapour barrier should be used or not, and if so which side to install the vapour barrier. Alternatively a nonpermeable rigid type insulation can be installed to resist moisture penetration from either side.

Ventilation System

There are many different types of air distribution systems incorporated into large stores, not only for potatoes but for several fruits and vegetables as well. They range from simple natural ventilation to manually controlled fans and inlets and finally to sophisticated automatically controlled dampers and variable speed fans. The choice of system will be determined not only by environmental needs but also by economic factors.

A ventilation system of medium complexity is shown in Figure 9.36 and can be installed in a store similar to that shown in Figure 9.37. The ventilation system allows complete exchange of air, complete recirculation or various mixtures in between. Although automatic controls will provide more accurate regulation of the system, manual control is possible because conditions change slowly in a large store. To control the relative humidity in the store, a humidifier can be installed in the ventilation system. This also reduces the temperature of the incoming air.

Air Distribution

Air from the proportioning system is forced into a main distribution duct and from there into lateral ducts cast into the concrete floor. The laterals may be covered with removable 50 by 100mm wood slats, thus allowing an elevator to be set up in the duct for unloading the bin.

The spacing of the lateral ducts are limited to 80% of the height of the heap i.e. 0.8H between centres and designed in size to limit air velocity to no more than 5m/s. They should be tapered or stepped in order to maintain a fairly uniform velocity as air is fed off to one bin after another. Because the potatoes cover about 75% of the open area, the wood slats should be spaced to give 4 times the area needed for the correct velocity.

Evaporator Size

As described in Chapter 7, evaporator size influences the temperature at which it can operate, and the difference in evaporator temperature and store temperature greatly affects the relative humidity of the store.

It is satisfactory to choose an evaporator size that will require about 6°C temperature difference during the loading period. Then when field heat is gone and the heat load is much smaller, the difference can be dropped to less than 2°C and an adequate humidity will be maintained. Unit blower evaporators are most commonly chosen for produce storage.

Any cool store should have an adjacent room for grading, packing and shipping the produce. It should be well lighted and adequate in size to store empty containers and packed produce to be shipped immediately.

As mentioned earlier, potatoes need to be warmed to at least 10°C prior to handling after a period of cold storage. If they have been stored in bulk in the store, they must be warmed in place. If they have been stored in pallet boxes, they may be warmed in the packing room which can be maintained at a temperature comfortable to the workers.

Figure 9.36. Ventilation system.

Figure 9.37 Large bulk store.

If the cool store is used for other produce it may be desirable to have some refrigeration in the packing room so that grading and packaging of perishable produce can be completed under cool conditions.

Later in this chapter the storage requirements for a number of fruits and vegetables is discussed. In many cases the temperature and humidity requirements are similar to those for potatoes and what has been covered in relation to potatoes holds equally true for the other produce with few reservations. If produce is held in storage for a short time, the air distribution system is probably not necessary and unit-blower evaporators will be adequate. Further, the non-compatibility of several fruits and vegetables to simultaneous storage, even when they require similar conditions, should be noted.

Grading and Handling Facilities

Grading crops for sale is more likely to be required where large volumes are handled. The principle requirements of a structure for this purpose are to protect the crop while being handled and to allow grading to be carried out without being affected by the weather.

Both the stored produce and the workers require protection from sun, rain, wind and dust. In some cases a pole building without walls will be adequate. In other situations, an enclosed room with lighting, ventilation and perhaps either heating or cooling will be required.

Seed Potatoes Stores

Seed potatoes must be kept from one season to the next. It is clearly important to maintain the tubers in good disease free condition and to keep the viability as high as possible. Seed potatoes may be satisfactorily held in a refrigerated store at 4 to 5°C for up to 8 months, but that is not always possible. Alternatively, and at lower cost, potatoes can be held in naturally ventilated stores at ambient temperatures where sprouting is allowed under the influence of diffuse sunlight. This technique is well proven and seed held for long term has been nearly as viable as that held in refrigerated stores for a similar length of time. This method of using the ambient temperature together with diffuse sunlight which allows chits (short sturdy sprouts) to form, can be used for seed potatoes as soon as the dormancy period has come to an end. Once the chits have developed, however, it is important to control aphids by routine application of a systemic insecticide, otherwise, virus diseases are likely to be introduced.

Potato Chitting Trays

Regardless of how seed potatoes are stored, it is desirable for the tubers to chit (sprout) before planting and this is done by deliberately exposing them to either artificial or diffused natural light. The light must reach all of the potatoes and consequently shallow trays with slatted bottoms are required both for good light distribution and adequate air circulation. A good design is shown in Figure 9.38. To give good light penetration, the alleyways between stacks of trays should be at least a meter wide and lines of trays should be placed in the store to give the best lighting from the sides and top (if lighting panels are fitted in the roof). Space under the bottom trays is essential for air circulation.

Figure 9.38 Potato chitting tray (approx. 350 x 500mm).

Small Seed Potato Chitting Stores

For the small landholder who requires a limited quantity of chitted potatoes, a rack similar to that shown in Figure 9.39 built under the eaves of the family home is a simple and inexpensive solution.

Figure 9.39 Small-scale chitting racks.

Buildings for chitting stores can be very simple. They may be built of poles, blocks, bamboo, reinforcing wire and netting and are constructed so that the sides let in light and ventilation. The interior is always at ambient temperature and lit by indirect daylight. As a result, once potato dormancy finishes, the tuber sprouts grow but only slowly, remaining short, green and strong.

Figure 9.40 Medium-sized chitting store with shelves.

Direct sunlight must be avoided and if roof-lights are installed, a shading device should be fitted below the roof to diffuse the light. Whitewashed strip-bamboo curtains suspended about a metre below the roof-lights serves this purpose well.

These naturally ventilated ambient temperature stores are best suited for areas or altitudes with maximum temperatures in the 18 to 24°C range. Results have shown losses somewhat higher than in expensive refrigerated stores, but satisfactory seed quality remains after 5 to 6 months providing the insecticide application has been continued on a regular basis.

Larger stores having similar characteristics can be built to suit the amount of seed to be stored see Figure 9.41. It is also quite possible to use the maize crib shown in Figure 9.5 for chitting seed potatoes if it is not needed for maize storage at the time.

Figure 9.41 Larger-scale potato chitting store.

Perishable crops

Fruit and Vegetables

The majority of fruits and vegetables are highly perishable commodities with a short storage life. The exceptions, including apples and potatoes can, if well stored, last for several months. Table 9.8 describes the primary differences between the non-perishable and perishable crops.

Table 9.8 Comparison of Cereals vs Horticultural Crops

Storage Requirements

The major requirements for the storage of perishables are the need to lower temperature substantially and to retain moisture in the produce. Table 9.9 illustrates the storage conditions and storage life for a number of fruits and vegetables.

Mixing Commodities

Some crops produce odours in storage while others emit volatile gases such as ethylene. Ethylene stimulates the ripening of many fruits and vegetables. This is negligible at low temperatures but may be a nuisance at higher temperatures.

Consequently, even when two or three crops require the same storage conditions, it is not advisable to store them together.

Products that emit ethylene include bananas, avocados, melons, tomatoes, apples, pears and all fleshy fruits. Lettuce, carrots and greens are damaged with stored with fruits or vegetables which produce ethylene. Even very small amounts can be harmful. It is recommended that onions, nuts, citrus fruits and potatoes each be stored separately.

Onions

The following technique for the harvesting, drying and storage of onions has been developed:

• 1 Onions are harvested when at least one third of the tops have fallen over
• 2 If weather is dry, the onions are left in the field to dry. The neck must become tight and the outer scales should rustle when dry. This is most important and successful storage depends on full drying or curing.

If weather is unsuitable for outside curing, the onions may be placed on slatted shelves in a well ventilated open shed. Layers should not be more than 10 to 15cm deep. (The seed potato store can be used for this purpose).

Onions will keep at higher temperatures than shown in Table 9.9 and this seems practical, particularly in dry areas. This involves placing cured onions in a slatted-floor store which is freely ventilated except during damp conditions.

Storage Structures for Perishables

A Low-cost Cool Store

A simple low-cost structure in which vegetables can be stored for the few hours between harvesting and transporting to market should be useful to growers of all sizes. The basic construction is similar to that shown in Figure 9.32. A simple frame is constructed with poles or other low cost materials. Covered with grass or other thatching material, protection is provided for the produce from excess temperature and moisture loss until it can be transported to market.

Table 9.9 Ideal Storage Temperatures, Relative Humidities and Expectecl Storage Life of Fruits and Vegetables

The wall should be extended to ground level on three sides but left open on the fourth (prevailing wind) side for ventilation. This allows for free air movement most of the time, but canvas flaps should be provided for closing the ventilation openings if desirable.

The grass roof and walls can be kept wet with a sprinkler pipe-line, or if that is not available, the thatching can be hand sprinkled as required. The interior will be kept cool and moist with temperatures as much as 5 to 8°C lower than outside. More important, produce harvested late in the afternoon can be cooled during the night with resulting temperatures the following noon as much as 10°C below ambient.

Commercial Cool Store

As shown in Table 9.9, only a few crops, including potatoes, onions, carrots and apples can be stored for periods longer than a few days or weeks. However, the wholesale merchant will require short term refrigerated storage for his produce and as indicated, separate rooms will be needed for crops that are not compatible with each other in storage. As with refrigerated potato stores, attention must be given to adequate insulation, good vapour sealing and large size evaporators which help to maintain high humidity.

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