Previous Page Table of Contents Next Page

Chapter 3 - Principles of Storage for Roots and Tubers

3.1 Control of Mechanical Damage
3.2 Control of Temperature
3.3 Control of Sprouting
3.4 Control of the Spread of Diseases
3.5 Control of Damage Caused by Insects
3.6 Control of Nematodes

Root and tuber crops are still living organisms after they have been harvested and losses that occur during storage arise mainly from their physical and physiological condition. The main causes of loss were discussed in Chapter 2 which indicated these were associated with mechanical damage, physiological condition (maturity, respiration, water loss, sprouting), diseases and pests. To ensure effective storage of root and tuber crops, these major causative factors need to be properly understood and, where appropriate, be properly controlled, taking into account the socio-economic factors which prevail in the areas of production and marketing.

3.1 Control of Mechanical Damage

3.1.1 Curing of root and tuber crops
3.1.2 Proper packaging and handling

Root and tuber crops need to be handled gently to minimize bruising and breaking of the skin because of its relatively soft texture compared, for example, to cereal grains. The effect of mechanical injury resulting in external and/or internal bruising and tissue discolouration is often underestimated. Severely damaged tubers should not be stored for three reasons;

· because of lower quality
· because of the increased risk of subsequent pathogenic losses.
· because of the risk of introducing disease organisms into sound produce.

Most mechanical damage occurs as a result of careless handling at harvest and during transport to and within a store since, generally in the tropics, food handling procedures are poorly developed and fresh produce is all too frequently treated as an inert object.

Careful harvesting and proper handling of roots and tubers is, therefore, an important step towards successful storage. Crops are most likely to be injured at harvest by the digging tools, which may be wooden sticks, machetes, hoes or forks. Therefore, immediately after harvest, the crop must undergo the operation of curing. Curing was previously discussed at section The need for curing as a method of reducing the onset of disease is well recognized and the technique is becoming more widely understood and practiced.

3.1.1 Curing of root and tuber crops

Roots and tubers have the ability to heal their skin wounds when held at relatively high temperatures and humidities for few days after harvest (Table 3.1) whilst at the same time there is a general strengthening of the skin. The term "curing" refers to the operation of self-healing of wounds, cuts and bruises. There are two steps involved in the curing process:

· Cells suberization. The production of suberin and its deposition in cell walls;

· The formation of cork cambium. The production of cork tissue in the bruised area. The new cork tissue seals the cut or bruised areas and helps prevent the entrance of decaying organism while also reducing water loss.

Although wound healing is important in preventing the invasion by pathogens, it also important in limiting the rate of respiration and water loss. Since water loss in badly suberised and damaged tubers may be very high, it is important that the curing operation is carried out as soon as possible after harvest. This will encourage as complete healing as possible although the extent of healing will depend on the on the type of damage; the older the tuber, the less well a wound will heal. Four factors affect the healing of wounds;

· temperature of the commodity
· oxygen and carbon dioxide concentration within the commodity
· humidity within the commodity
· the use of sprout inhibitors

It is perhaps unfortunate that the curing operation when carried out properly involve conditions of high temperature and high humidity within the commodity, which also favour the rapid growth of microorganisms and commodity deterioration. Therefore it needs to be carried out very carefully and quickly and it follows that after curing further handling of the produce should be reduced to a minimum so as to avoid further injury that may result in further losses.

The length of time for proper curing cannot be precisely defined as it depends on many factors, such as condition of the crop at harvest, type of wound, season, storage temperature and relative humidity.

Table 3.1 Conditions required for curing root and tuber crops (adapted from Booth, 1974)


Temperature °C

Relative humidity (%)

Duration (a) (days)


15 - 20

85 - 90

5 - 10

Sweet Potato

30 - 32

85 - 90

4 - 7


32 - 40

85 - 95

4 - 7


30 - 40

90 - 100

4 - 4

(a) The length of time for proper curing cannot be definitely stated as it depends on many factors, such as condition of the crop at harvest, type of wound, season, storage temperature and relative humidity. In practice the curing period generally ranges from 5 to 20 days.

Table 3.2: Percentage weight loss of cured and uncured root crops (Booth 1972)


Duration of Storage (days)

Percent Weight Loss







Sweet Potatoes








3.1.2 Proper packaging and handling

The ideal in packaging is to protect the produce from damage during handling, transport and storage and to provide containers of uniform size that are conveniently stacked and handled, easily accounted for in quantity and, where appropriate, in weight.

In many developing countries traditional baskets, and various types of trays or buckets are used for transporting produce to the house or to village markets. These are usually of low cost, made from readily available material and serve the purpose for transport over short distances. But, they have many disadvantages in large loads carried over long distances:

· they are difficult to clean when contaminated with decay organisms;

· they often lack rigidity and distort when stacked thus applying severe local pressure to their contents

· they are frequently very variable in shape and therefore are difficult to load, especially for long journeys.

· being of local manufacture they are often rather crude and may have sharp edges or splinters causing cuts and punctures to the commodity.

Many authorities have observed that produce being transported and marketed in commercial quantities needs better packaging in appropriately sized units if losses are to be minimised and to achieve economical use of transport. The shape of packages is significant because of need to load for maximum capacity and stability. Round baskets, whether cylindrical or tapered, hold considerably less produce than boxes occupying the same cubic space; a cylindrical basket contains only 78.5% by volume compared with a rectangular box occupying the same space on a vehicle.

However, packaging can be a major item of expense in produce marketing, especially in developing countries where packaging industries are not well developed. The selection of suitable containers for commercial scale marketing requires very careful consideration. Among the various types of packaging material that are available1, natural and synthetic fibre sacks and bags as well as moulded plastic boxes seem to be more suitable and have greater promise for packaging roots and tubers and for their transport to distant markets (Figure 3.1).

[1 Including sawn wooden boxes, cardboard boxes, moulded plastic boxes, paper or plastic film sacks, natural or synthetic fibre bags.]

Figure 3.1: Poor synthetic fibre sacks and bags Natural and synthetic fibre sacks and bags

Bags up to 5 kg capacity and sacks from 25 to over 50 kg can be made from natural fibres, jute or sisal, or from synthetic fibres of polypropylene or polyethylene. These sacks are ubiquitous and have been in continuous use for packaging and transporting root and tuber crops to markets for a considerable time. Their main problem in many developing countries is that often they are too large and when filled (and rarely are they not filled absolutely full) they are too heavy for careful handling and are often dropped or thrown down resulting in severe bruising and damage to the contents. Finely woven sacks, particularly of synthetic material impair ventilation when stacked. Moulded plastic boxes

Moulded high-density polythene boxes, allegedly reusable, are widely used for transporting perishable produce in many countries. They can be made to almost any specifications, are strong, rigid, smooth, easily cleaned and can be made to stack when full of produce and to nest when empty in order to conserve space. Their disadvantages are:

· they can be produced economically only in large numbers but are still costly;

· they have to be imported into most developing countries, adding to the cost and pressures on foreign exchange;

· there has to be a good organisation and sound reliable cooperation between the market traders, transport services and producers if there is to be a effective container return service.

· the containers, particularly the cheaper varieties, deteriorate rapidly when exposed to sunlight, unless treated with an ultraviolet inhibitor.

Before deciding on what packaging to use, the producer or trader has to ensure that the cost of packaging does not exceed its benefits. A decision should involve consultation between the transport contractors and market traders. The factors to be considered include;

· the type of produce;
· the level of losses occurring during marketing;
· the comparative cost of the present and improved packaging;
· the regularity of supply of the packaging material;
· the acceptance of the packaging method to the market.

3.2 Control of Temperature

Temperature has a great influence on many factors that cause loss during storage. It is the single most important factor affecting the rate of respiration, it also influences the rate of sprout growth, the development of rotting micro-organisms and insect infestation. Figure 4.8 illustrates the effect of different temperature regimes on the storage of potatoes. At 10°C, the rates of sprout development, rotting and respiration are shown to be moderate but at 4°C, sprouting is stopped, while rotting and respiration continue but at very low levels.

With the exception of highland areas, low temperature storage in the tropics within the range 10° to 15°C can only be envisaged by using a refrigeration process of some kind. At subsistence or small farmer level this is generally not practical because of cost implications and the technical support needed to sustain conventional refrigeration technology. Only in very dry areas is simple evaporative cooling at all successful but even this simple technology needs a prime mover to be operating almost continuously. Therefore, successful storage of roots and tubers in any sort of structure depends very much on natural ventilation to remove respiration heat, to remove carbon dioxide, which in concentration can lead to the breakdown of dormancy, and to keep the temperature of the crop as low as possible. Ventilation should be with the coolest possible air, night time ventilation (see section 4.3.2) is not only the coolest but has the highest relative humidity, so that water loss through transpiration is also held to a minimum.

3.3 Control of Sprouting

3.3.1 Storage at low temperatures
3.3.2 Gamma irradiation (Table 3.3)
3.3.3 Chemical sprout inhibitors

How long roots and tubers, with the exception of cassava, can be made to remain dormant with limited endogenous metabolic activity is, generally, the determining factor in how long the commodity can be stored. The end of dormancy leads to the initiation of sprouting which, in turn, means increased respiration and dry matter loss. Therefore, if the duration of storage is to be longer than the natural dormancy period an alternative method to prevent or delay sprouting is needed. One or more of three methods can be used:

· storage at low temperatures,
· the use gamma irradiation,
· the use of chemical sprout inhibitors.

3.3.1 Storage at low temperatures

The effects of temperature on dormancy have been discussed in section For potatoes stored at 4°C sprout growth is negligible; for yams storage at 15° to 16°C prolonged the dormancy period for six months. But, as discussed in Chapter 2, socio-economic constraints in most developing countries will limit the use of refrigeration in the storage of roots and tubers at farmer level.

3.3.2 Gamma irradiation (Table 3.3)

Gamma irradiation is known to induce lesions on nucleic acids and cellular proteins thus preventing them from multiplying. When applied at doses of about 7.5 head, gamma irradiation has been reported to inhibit sprouting of yams, potatoes and sweet potatoes However, this technique has not yet been applied on a commercial scale in the tropics and is unlikely to be of practical value to farmers because of the cost of the high technology involved. Furthermore it is a treatment that is not widely acceptable to consumers or a permitted treatment for food commodities in some developed countries.

3.3.3 Chemical sprout inhibitors

In stores using natural air ventilation, with relatively high ambient temperatures (20°C to 30°C such as are normally experienced in tropical and subtropical lowlands) and for any period of storage beyond the normal or natural end of the dormancy period, the use of sprout inhibiting chemicals is the only practical means of controlling sprouting. This treatment has proved effective on potatoes and sweet potatoes.

CIPC (isopropyl-N-chlorophenylcarbamate) is the chemical most commonly used as a sprout suppressant. It is mixed with an inert filler to give a concentration of about 2%, and applied as a dust on potatoes at a rate of 1-1.5kg per ton of produce and it has been very effective in delaying and reducing sprout growth. As sprout suppressants also inhibit the development of wound cork, they must be applied only after the curing operation has been completed or a few weeks after harvest when wounds have healed and any lesions would have naturally sealed off (see section 3.1.1).

Sprout suppressants have almost all proved to be quite ineffective on yams. This is probably because yam tubers are unique amongst plants propagating vegetatively in not having pre-formed buds. Most sprout suppressants, such as CIPC affect the meristematic cells of the sprouting loci. On potato tuber these loci are well formed at the time of harvest and are located at the tuber surface. In yam sprout initials are formed only just before the end of dormancy and then rise from beneath the periderm. When a sprout inhibitor is applied on the yam tuber just after harvest there are no sprout loci on which the chemical can act. Once sprouting has been started, the application of sprout suppressant may then inhibit further growth of the sprout initials.

Table 3.3 Monthly cumulative percentage loss in weight of irradiated yam tubers, the radiation (expressed in head) (Adapted from Adesuyi, 1982)

Storage Duration Months

12.5 (krad)

10.0 (krad)

7.5 (krad)

5.0 (krad)

2.5 (krad)


























































3.4 Control of the Spread of Diseases

Efforts to control the spread of diseases should aim to be preventive rather than curative. Simple and low cost preventive measures which to help control the incidence of post-harvest diseases include:

· gentle handling to minimise the risk of injury to tubers during harvesting, transport and storage;

· adequate cultural practices and especially using disease free planting material;

· good phytosanitary practices, including regular inspection of fields and premises, proper disposal of diseased tubers and plant debris, the cleaning and sterilising of implements, boxes, buildings, etc.

· pre-harvest crop application of chemicals to control the diseases in the growing crop;

· curing of the crop before storage;

· only storing produce that has been dried before putting into store, avoiding produce in store getting wet and storing at the optimum temperature.

Some diseases can be prevented or controlled by the direct application of chemicals to the produce by dipping the produce, applying sprays or dusts. To be effective chemical treatment requires the application of the appropriate compound, at the recommended dosage, by the most appropriate method and at the most suitable time. It is important to have a thorough knowledge of the pathogens being treated in order that the correct treatment can be selected and properly applied.

Thiabendazole and Benomyl are presently the most commonly used fungicides for post-harvest treatment of pathogenic diseases of roots and tubers. The best results are obtained when the chemicals are applied not later than three days after harvest and not after the pathogen is well established. One drawback to these particular chemicals is that they are not always readily available to farmers or they are too expensive. There is a very real risk, which is frequently borne out in practice, that farmers who, for any reason, cannot obtain these chemicals will use what ever is at hand without regard to the danger of applying chemicals which are suitable for growing plants but which are dangerously toxic when applied directly to stored food produce, whether or not the produce will be treated, as a minimum being washed in potable water, before being consumed.

3.5 Control of Damage Caused by Insects


Insect pests can be the cause of serious losses in stored roots and tubers, yams and sweet potatoes in particular. Surveys carried out in 1981, 1983 and 1984 in Côte d'Ivoire showed increasing levels of infestation of stored yams over a period of four months storage, with 63% of stored tubers being infested by moths and weight losses of 25% attributed to insects. (Sauphanor and Ratnadass, 1985).

Good hygiene is of paramount importance in insect control including, particularly, the destruction by burning of infested tubers and rubbish that can act as host to a variety of insect pests and cleaning and disinfection of the store structure. In many areas it may still be necessary to use some form of chemical control especially if storage is extended over several months. Various methods of control of the potato tuber moth in potato stores have been tested in many countries. In stores in Bangladesh dried and crushed Lantana camara, as well as the insecticide Decamethrin (Decis), has been reported to be effective. In Kenya moth damage was reduced significantly by the repellent weeds Lantana camara, Minthostachys sp. and Eucalyptus sp. (Centro International de Papa (CIP) Annual Report 1988). Deltamethrin used as a spray of 2.5g active ingredient per 100 litres of water, has been reported to be effective in controlling moths (Tineidae sp) on stored yams (D. alata et D. cayenensis) (Sauphanor and Ratnadass 1985).

Insecticides may be applied as dusts on the planting material, on the soil during the tuber-forming period, or as sprays applied to the growing crop. The same observations on the misuse and application of unsuitable chemicals apply equally to use insecticides as to fungicides. Allegedly safer and more practical alternative control methods are continually being developed which are particularly suitable for subsistence agriculture. Some level of control can be achieved through the use of established insect repellents, such as "lantana" and through crop rotation and forms of shifting cultivation which reduce the likelihood of a serious build-up of soilborne pests. Another promising control is the use of cultivars that have a marked genetic resistance to insects and new cultivars are being bred with resistance characteristics.

3.6 Control of Nematodes

Nematodes are well known to cause serious losses of roots and tubers, of both quantity and quality. There are three possible methods of control:

· Application of chemicals to soil and plants. This treatment has not proved to be economical;

· Treatment of propagative material prior to planting by immersion in nematicides or hot water (50°C for 15-60 minutes is reported to give good results (Bridge, 1980));

· Cultural practices. The most common and reliable method. It involves growing alternative crops for several years which are not suitable hosts for the specific nematode pest. It is important that the alternative crops really are unsuitable hosts for the nematode and that there are no ground keepers and weeds left growing that will permit a colony to survive. (see section

Of course, the ideal solution to the problem of nematode infestation is to grow nematode-free seed material in soils from which nematodes have been eliminated.

Previous Page Top of Page Next Page