FACTORS AFFECTING STORABILITY OF ROOTS AND TUBERS

Deterioration following the harvesting of fresh roots and tubers and the consequent losses are caused by:

1. mechanical damage,
2. physiological changes within the plant,
3. infection by decay organisms and pest infestation.

The losses caused by these processes may occur throughout all stages of the food system, from crop maturity through harvesting, transport and storage. (Table 2.1)

Table 2.1 Causes of loss in roots and tubers and their effects (Adapted from: FAO, 1981)

Factor	Mechanism	Stage Affected	Resulting Loss
Mechanical	Rupture	Harvest	Moisture loss
	Bruising	Harvest, Transport, Storage	Access to pests and diseases
	Crushing	Transport, Storage	Total loss
Physiological	Transpiration	All stages before processing	Water loss
	Respiration		Dry matter loss
	Sun scorch	In field after lifting	Tissue degradation
	Greening		Toxins (potatoes)
	Chilling	Cold storage	Loss of palatability
	Inversion of starch	End of dormancy	Increased transpiration and respiration
	Sprouting	Storage
Pathogenic bacteria & fungi	Necrosis and tissue degradation	Pre-harvest	Partial to complete loss
		Storage	Downgrading
Insect infestation	Boring & Chewing	Pre-harvest	Partial loss
		Storage (fresh or processed products)	Access for decay organisms
Rodent & bird damage	Chewing	Pre-harvest	Partial loss
	Pecking	Storage	Access for decay organisms

Pre-harvest factors are largely responsible for significant post-harvest losses. The factors include; field pests, infection by disease organisms, infestation by pests, environmental and cultural practices and also genetic factors. A further complication is the interrelationship and interaction between the different components of production and harvesting. Their effects are greatly influenced by the condition of the product itself and, during storage, the temperature and ambient relative humidity. For these reasons the total production and marketing system (local as well as urban) needs to be addressed as a whole. The main causes of loss are discussed below.

2.1 Mechanical Damage

The skin of a mature root or tuber is normally an effective barrier against most potentially invading bacteria and fungi causing rotting of the tissues. Any rupture of this barrier caused by damage or injury to the skin will provide an entry point for infection and will also stimulate physiological deterioration and dehydration.

There are different degrees of mechanical damage, from small bruises to deep cuts and they may be sustained at any stage, from pre-harvest operations, through harvesting and subsequent handling operations when the product is graded, packed and transported for market or, simply, even carried to the farmer's house. Serious mechanical injury, which may result in the product being rejected during grading, is a direct loss. Damage to the tuber skin that is not immediately obvious can lead to physiological deterioration and allow the entry of pathogens.

2.2 Physiological Factors

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2.2.1 Respiration

2.2.2 Transpiration or evaporation of water from tubers

2.2.3 Dormancy and sprouting

2.2.4 Pathological factors

2.2.5 Attacks by pests

2.2.6 Damage by extremes of temperatures

2.2.1 Respiration

Roots and tubers are living organisms and as such, they respire. The respiration process results in the oxidation of the starch (a polymer of glucose) contained in the cells of the tuber, which converts it into water, carbon dioxide and heat energy. During this transformation of the starch the dry matter of the tuber is reduced. The respiration process can be approximately represented by the oxidation of glucose.

For respiration to occur freely a supply of oxygen is needed and the resulting carbon dioxide and heat have to be removed from the products' environment. A limited supply of oxygen and inadequate removal of carbon dioxide may lead to effective asphyxiation and the death of product tissue. Ideally the complete combustion of 1g of glucose yields 1.47g CO₂ + 16 kJ of energy. In practice about 5.1 kJ (32%) of this energy is used as metabolic energy and 10.9 kJ (68%) is released as heat. The rate of respiration is assessed either by measuring the uptake of oxygen or the quantity of carbon dioxide released and is expressed in milligrams of CO₂ per kilogram of tuber per hour.

2.2.1.1 Factors affecting the respiration rate

During its physiological development through growth, harvesting, storage and subsequent planting as seed, the living tuber passes through different stages during which the respiration rate varies with;

· the physiological age of the tuber,
· whether it is sprouting or in its dormant phase,
· whether or not it has been damaged and is healing its wounds,
· the storage conditions, mainly temperature.

Generally, the rate of respiration is relatively high at harvest, followed shortly by a decrease, especially during storage, then followed by an increase once sprouting begins. For example, the dry matter losses for potatoes stored at 10°C are approximately 1% to 2% during the first month after harvest, 0.8% per month during storage but rising to 1% to 5% per month when sprouting is well advanced (Burton, 1966; Rastovski et al., 1981).

2.2.1.2 Physiological age and sprouting

During dormancy, the endogenous metabolic rate of tubers is at its minimum and the dry matter losses are correspondingly reduced. For example, it has been shown that immediately after harvest yam tubers (D. rotundata) respire at a rate of 15mL CO₂/kg fresh weight/hour at 25°C. The respiration rate will later drop as low as 3 ml CO₂/kg/hr and remain at that level until sprouting starts. During sprouting the respiration rate increases dramatically to over 30mL CO₂/kg/hr (Cooke et al. 1988a).

2.2.1.3 Skin permeability

The permeability of the skin of the tuber is a function of its maturity and is a very significant factor in the rate of respiration. The periderm of freshly harvested immature tubers is most permeable and thus permits greater levels of respiration than similarly harvested mature tubers. Immature potatoes are reported to respire at a rate of about 17mL O₂/kg/h immediately after harvest, compared to a rate of 5ml O₂/kg/h when physiological mature. (Burton, 1966; Rastovski et al. 1981) (see Table 2.2).

Table 2.2 Loss of weight in Yam attributable to respiration (Passam, 1982)

Age of Yam tubers	Total weight loss (% per day)		Percent of total weight loss due to Respiration
	25°C	35°C	25°C	35°C
After harvesting	0.22	0.36	27	30
During dormancy	0.15	0.28	7	10
During sprouting	0.21	0.34	35	20

Figure 2.1: Respiration rates of potatoes at various storage temperatures (Burton, 1978)

2.2.1.4 Skin damage

Damage and wound healing greatly influence respiration. It has been found that simply cutting through a potato will double its respiration rate and dropping the potato from a height of about 1 metre will so damage a potato skin as to increase respiration by 30% to 50%. For sweet potatoes the rate of respiration of a damaged tuber doubles in response to a wound after a delay of about 20 hours. This effect is known as wound respiration response.

2.2.1.5 Effect of storage temperature on respiration

Temperature is the single most important factor affecting the rate of respiration. For biological material this can be expressed as; Q₁₀ = 2, i.e. the rate of respiration is doubled for every 10°C increase in temperature over the range 5°C to 25°C. Significant variations to this general rule will occur through interactions with other factors affecting the rate of respiration, as discussed in the previous paragraphs.

For potatoes, the rate of respiration is at a minimum around 5°C. Below this temperature the respiration rate tends to increase (Figure 2.2). For yams, it is known that low temperatures reduce the rate of metabolic activity of the tubers (Table 2.2), but temperatures in the range 10° to 12°C cause damage through chilling which, because of a breakdown of internal tissues, increases water loss and increases susceptibility to decay. Very low temperatures, below 5°C, similarly affect sweet potato and cassava. The symptoms of chilling injury are not always obvious when the tubers are still in cold storage, they become noticeable as soon as the tubers are restored to ambient temperatures.

2.2.2 Transpiration or evaporation of water from tubers

Transpiration is water loss through the skin pores of the tuber and is, effectively, evaporation. Because roots and tubers are characterised by having high moisture content, even in the ambient conditions prevailing in the humid tropics, they will continually lose water to the surrounding air. This loss of water can be significant in several ways. Whilst the original food value may not be affected a large water loss will adversely affect the quality of the produce, for example a loss greater than 10% will result in a bigger peeling loss because of the shrivelled texture of the skin and the culinary quality may also be affected. A weight loss becomes an economic loss when the produce is marketed on a weight basis as well as being less attractive to potential customers.

It is very difficult to give precise data on transpiration weight loss, even under precisely defined conditions because of the wide range of factors by which it is influenced. These include:

· temperature,
· relative humidity,
· the rate of air movement surrounding the tuber,
· most significantly, the permeability of the skin and how this may have been affected.

2.2.2.1 Effect of temperature and relative humidity of the air

There are interstitial spaces within all plant tissues that allow water vapour and air to move throughout the plant. The water vapour within these spaces exerts a water vapour pressure, which pressure is a function of the amount of free water contained within the tissues and its temperature. The rate at which water is lost from fresh tubers depends on the difference between the water vapour pressure within the tubers and the water vapour pressure of the surrounding air, with moisture passing from the higher pressure to the lower. (For simplicity, although it is not really accurate, the phenomenon can be considered as a movement of moisture caused by the difference in relative humidity of the air within the plant tissues and the air surrounding the plant.) If there is a considerable difference between the temperature of the produce and the surrounding air, temperature becomes the dominating influence on water vapour pressures, whereas when both are at similar temperatures it is the amount of water vapour that has the most significant effect. It therefore follows that to minimise water loss from high moisture content produce, the produce should be kept in atmospheres that have a comparable water vapour pressures. In practical terms this means in a cool moist atmosphere.

2.2.2.2 Air movement and water loss

The greater the velocity of air moving over fresh produce the faster is water lost though transpiration. Air movement (or ventilation) through produce is essential to remove the heat and CO₂ produced by the respiration of the produce but the rate of air movement needs to be kept as low as practical to prevent excessive loss of moisture.

2.2.2.3 Influence of the permeability of tuber skin

The skin of tuber crops will allow water vapour to diffuse through it during respiration and transpiration. The factors discussed earlier (variety, degree of maturity, damage, extent of suberization) which affect the rate of respiration also affects the rate of transpiration through the skin. At 10°C, undamaged mature potato tubers are reported to lose water at a rate of 62 to 109 ug/cm²/hr/kPa, while immature tubers harvested 24 hours previously lost water at a rate of 1-6mg/cm²/hr/kPa, a factor 15-100 times greater (Rastovski et al., 1981).

2.2.3 Dormancy and sprouting

Yam, cocoyam, potato and sweet potato tubers propagate vegetatively. To counter what is often an unfavourable climate at the end of their growth period, they go into a dormant phase. The beginning of this period is considered to be the point of the physiological maturity of the tubers, also called "wilting point". The dormancy period can be defined as the period of reduced endogenous metabolic activity during which the tuber shows no intrinsic or bud growth, although it retains the potential for future growth. Dormancy is both a species and a varietal characteristic. It is also affected by other factors, temperature is the most important but others, including moisture, oxygen and CO₂ content of the storage atmosphere, the extent of wounding and any disease of the tuber, real or putative, although normally of lesser importance may, occasionally, have an over-riding effect.

The cassava root, as opposed to other roots and tubers, is a plant of perennation and not propagation. It has no dormancy and it senesces soon after harvesting. The post-harvest deterioration of cassava is discussed in section 4.1.

2.2.3.1 Effect of variety

Passam (1982) suggested that differences in the dormancy of yam species are the result of the ecological environments in which the different species have evolved. For example, varieties of yam native to regions with marked arid seasons have a longer period of dormancy than those that are native to regions with shorter dry seasons. D. cayenensis, which originates from the West African forest zone where the dry season is very short, shows almost continuous vegetative growth. In contrast, D. alata and D. rotundata, originating respectively in Asia and Africa, appear to be adapted to climates where there is a longer dry season during which the plant survives as a resting tuber. These inherent differences in dormancy are responsible to a large extent for variations in the ability of different species to store well. (Table 2.4)

2.2.3.2 Temperature effect on dormancy

Lower storage temperatures are widely practiced as a technique for reducing the metabolic activity of roots and tubers and prolonging their dormancy. Temperatures of 16° to 17°C have been used to prolong the storage period for D. alata tubers for up to four months, provided the tubers were properly cured prior to storage in order to control infection by wound pathogens.

For potatoes, sprout growth is practically negligible at 4°C and below and increases with increasing temperature. However, avoidance of sprouting by low temperatures leads to sweetening of the tuber, which is considered to lower the value of the stored crop. (Table 2.4)

Table 2.3 Dormancy periods of the major edible yams (Passam, 1982)

Yam species	Locality	Length of dormancy (weeks)
D. alata	Caribbean	14 -16
	West Africa	14 -18
D. rotundata	West Africa	12 - 14
D. cayenensis	West Africa	4 - 8
D. esculenta	West Africa	12 - 18
	Caribbean	4 - 8
D. trifida	Caribbean	4

Table 2.4 Cumulative percentage of sprouting of yams tuber stored at different temperatures (Adesuyi, 1982)

Storage period (Months)	15°C	20°C	25°C	Yam Storage Barn (ambient)
0	0	0	0	0
1	0	14	0	0
2	0	40	28	30
3	0	54	94	88
4	0	54	100	100
5	0	56	100	100

2.2.3.3 The end of dormancy or sprouting

While roots and tubers remain dormant they can be stored satisfactorily, (provided they are undamaged and free from disease). As soon as dormancy is broken and sprouting begins, the rate of dry matter loss increases dramatically since the formation of sprouts requires energy, which is drawn from the tubers' carbohydrate reserves. The rate of water loss also increases and if this becomes excessive the tubers dry out allowing pathogens to penetrate the tuber, potentially causing severe damage if not total loss, making continued storage quite impracticable (Table 2.5).

2.2.4 Pathological factors

All living organisms are subject to invasion by microorganisms, fungi, bacteria and viruses, which constitute the most serious cause of direct post-harvest loss in tropical root crops. These disease organisms are widely distributed in the air and soil and on dead and decaying plant material. The extent of the occurrence and the magnitude of losses due to pathogenic microorganisms are very variable. The time of infection varies with the crop and with different diseases, it can occur in the field before harvest or at any time afterwards. Since many post-harvest pathogens are introduced through wounds, one of the major factors governing the incidence and magnitude of such losses is the physical condition of the produce. The cork layer surrounding the roots and tubers is intended to serve as a barrier against bacterial and fungal attack. As discussed in Section 2.1, when this protective barrier is damaged the plant is predisposed to pathogenic infection.

2.2.4.1 Sources of infection

The infection may start:

· Before harvest, through natural pores in the above and below ground parts of plants, which allow the movement of air, carbon dioxide and water vapour into and out of the plant.

· Due to injuries caused after harvest by careless handling, by insect or other animal damage.

· By direct penetration of the intact skin of the plant; the time of infection varies with the kind of crop, its maturity and the type of disease organism; it can occur in the field before harvest or at any time afterwards.

Pre-harvest field infection does not necessarily become apparent until after harvest but can occur at any time between the field and the final consumer. The infecting organism may be distributed by infected seed or other planting material, from crop residues or rejected produce left decaying in the field or field boxes or packhouses, from contaminated water used to wash produce and by contaminating healthy produce from diseased produce in the same package. Many diseases can survive by using weed plants or other crops as alternative hosts.

2.2.4.2 Types of pathogenic loss

Losses reducing the quantity of sound produce are, generally, the more serious losses but are often underestimated because they are not easily recognised or evaluated. They are often caused by infection of the produce in the field before harvest either by a primary infection or a secondary infection following an initial infection by one of a few specific pathogens, normally through a wound. The initial infections cause a breakdown of the host tissues and once these primary pathogens are established, they are followed by an invasion by a broad spectrum of secondary pathogens.

Losses affecting the quality of the produce and which occur when the disease affects only the surface of the produce do not necessarily affect the intrinsic quality or quantity of the commodity but makes the crop less attractive to the consumer or buyer in the market. In crops grown for domestic consumption, the result is not necessarily serious since the affected skin can often be removed and the undamaged interior can be used. For crops intended for a commercial market, qualitative losses usually result in financial loss. In yams, sweet potatoes and potatoes, diseases causing internal blemishes also reduce the final quality of the crop.

2.2.5 Attacks by pests

Post-harvest and storage losses are caused by pests, which include: insects, nematodes and animals.

2.2.5.1 Insects

Insects damage roots and tubers in two ways:

· by boring holes in the tubers, reducing the quantity and quality of the produce and sometimes the germination capacity;

· by damage to the epidermis providing entry for moulds and bacteria to penetrate the tuber.

Estimates of storage losses of roots and tubers due to insects are very scarce. Some investigations in Côte d'Ivoire on damage to stored yams suggest that, under certain circumstances, insect damage losses may reach 25% in just four months of storage (Sauphanor and Ratnadass 1985).

- Insect pests of yams

The following are the most prevalent insect pests; there are others that are of smaller economic significance.

· Pyralid moth (Euzopherodes vapidella). First reported during the 1970s causing extensive damage to stored yams in Nigeria and Côte d'Ivoire. The insect preferentially attacks D. alata, generally during the first few days following harvest. Infestation may also start in the field on those parts of the tuber emerging from the mound. The insect lays its eggs generally on wounds or holes dug by its larvae from a previous generation. The damage is visible from the "dust-like" excrete on the surface of the tuber.

· Tineidae sp. These moths seems to prefer tubers with a lower moisture content which may explain why its attacks generally follow that of E. vapidella on D. alata. The young larvae of Tineidae penetrate the yam tuber from holes previous bored by E. vapidella, remaining unobserved whilst the inside of the tuber is extensively damaged.

· Two species of mealy bugs (Aspidiella hartii) and (Planococcus dioscorea). These are common on D. cayenensis and D. alata. The attack by these pests results in white looking colonies, which can cover the whole tuber, causing mainly a loss in quality and also in reduction in germination.

· The yam beetle (Heteroligus spp.). The beetle attacks the tuber during the growing period damaging the epidermis and permitting the penetration by microorganisms during storage.

- Insect pests of sweet potato

Cylas formicarius (F.) (commonly called sweet potato weevil), widespread through the tropics, and Cylas puncticollis which exists in only a few African countries, are the two most damaging of the many insect pests that attack sweet potato in tropical and subtropical regions (Talekar, 1982). These pests damage the sweet potato roots while the crop is growing as well as during storage. It has been reported that weevil infestation is usually more prevalent in roots harvested during the dry season. In the field the insect lays eggs on the stems and on exposed roots. The larvae, which are responsible for most of the damage, bore holes through the stems and the roots allowing the introduction of other disease organisms.

- Insect pests of stored potato

The potato tuber moth (Phthorimaea operculella) is considered the principal insect pest of stored potatoes. In India it is a serious pest in areas where potatoes are stored through "country methods", i.e. not using cool storage (Pushkarnath, 1976). Pest surveys in East Africa carried out by CIP in 1987 showed that the potato tuber moth caused the majority of damage problem. The moth was also observed to cause extensive damage in stored potatoes on seed farms in Burundi and Rwanda (Centro International de Papa (CIP) Annual Report 1988). It is the larval stage that causes the most severe damage. It is about 1 cm in length, has a dark brown (or black) head and a body which may be white, yellow, pink or green. They tunnel extensively into the tuber flesh causing the infected tubers to rot, mainly because of a secondary infection of pathogens

2.2.5.2 Nematodes

Nematodes mostly infect growing plants and remain on the tubers after harvest. They damage not only the tubers themselves, but also create points of entry for other pests and pathogens. While nematodes are not normally considered of significant importance in stored produce, they have been shown to cause necrotic areas on stored yam tubers. Many species of nematodes affect root and tuber crops. Among these, Scutellonema bradys, Pratylenchus coffeae and Meloidogyne spp are alleged to be the most significant, particularly on yams (Bridge, 1980).

· Scutellonema bradys has been recorded on yams all over the tropics and is recognized as an economically major nematode pest in West Africa. An infestation results in high quantitative losses, as well as loss in the marketable value of the affected tubers. The eggs are most often laid in plant tissues but they are almost always to be found in the soil. Very large populations can build up in the tuber. A tuber disease commonly associated with S. bradys is referred to as "dry rot", where the nematodes feed within the tuber causing tissue breakdown and producing cavities. Initially small yellowish lesions develop beneath the periderm and as the infection spreads these turn into areas of dark-brown or black coloured rot. External symptoms are slight to deep cracks on the tuber skin and malformation of the tuber.

· Pratylenchus coffeae, commonly called "lesion nematode" is found in many countries throughout the world and is known to cause serious damage to a wide range of crops. It causes a marked reduction in yield and dry rot on stored yams. The symptoms on the affected tubers are similar to that of S. bradys (Bridge, 1980).

· Meloidogyne spp, also known as "root-knot nematode" is very common as root pest of tropical crops often causing serious yield reduction.

The main source of inoculum is reported to be infested seed material. Host populations of S. bradys know a wide range of crops and some weeds. including: cowpea, melon, pigeon peas, okra, tomato, and sorghum.

2.2.5.3 Rodents, birds and other animals

Attacks by vertebrate pests, even including monkeys, are not very frequent and are not well documented in the literature. When they do happen, the resulting losses may be extensive because of subsequent rotting of the damaged tubers.

2.2.6 Damage by extremes of temperatures

Roots and tubers are susceptible to extremely low or high storage temperatures. Yams, cassava and sweet potatoes are known to suffer from chilling damage at 12°C or below, while for potatoes, cocoyams and sweet potatoes this type of damage occurs at 2°C or below. The extent of chilling damage usually depends on a time/temperature interaction. The most common symptoms are internal tissue breakdown, increased water loss, susceptibility to decay, failure to sprout and changes in culinary qualities, cooking and taste. Potatoes respire strongly at temperatures of 30°C and above. At these high temperature levels the tubers require a great deal of oxygen and respire a considerable volume of carbon dioxide. At a certain point, the rate of respiration is so great that the cells can no longer obtain sufficient oxygen to sustain the rate induced by the temperature and the carbon dioxide formed cannot be disseminated. This ultimately leads to the death of the cells, giving rise to what is commonly known as "black heart" (Rastovski et al, 1981).

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