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Sweet potato, Ipomoea batatas / Convolvulaceae
Postharvest Atmosphere Management
The optimal storage conditions for sweet potatoes are fixed between 11-15ºC and 85-90% of relative moisture for several months. Storage at temperatures below 12ºC may cause wrinkling of the roots, a brown colour of the flesh, superficial hollows of small size and secondary cryptogamous attacks.

At home, they must be kept in a fresh, dry, dark place with good ventilation, where they can be stored for 7-10. The environmental temperature must not exceed 15ºC since they could germinate or ferment. They must not be kept in the fridge, since their taste is altered by cold temperatures. Once cooked they can be kept in the fridge for one week or even more if they are frozen.

Pre-cooling is not a usual procedure for this species since it would inhibit the process of healing of the wounds. On the contrary, right after the harvest, they are subject to temperatures higher than the suitable for long storage, which is known as " curing". The curing is applied before marketing or storage and enables the healing of the tissues which have been damaged during the harvesting. Otherwise they would be an authentic way to loss water and to let micro-organisms in. The healing tissue is generated in two or three days. The optimal conditions for curing are ventilated enclosures where temperatures are kept at 30ºC and 85-90% of relative moisture for 4-7 days. Once the process has finished, sweet potatoes must be cooled down to 13 or 15ºC, the ideal temperature for their storage.

Sprouting during the storage can be prevented or reduced by means of growth inhibitors like hidracida maleica that is it also used for onions. However, in Spain there are no specific inhibitors for sweet potatoes.

In order to stop the development of fungi in already harvested sweet potatoes, it is quite useful to apply some fungicides (in Spain there are no fungicides registered to apply in post-harvested sweet potatoes).

This species is well adapted to long periods of storage, provided they have been subject to suitable curing, they have no excessive humidity in the ground during the days previous to harvesting, they have not been subject to chilling injuries, and the curing has been made two days after the harvesting. The optimal conditions for storage of sweet potatoes are fixed between 11-15ºC and 85-90% of relative moisture. Temperatures at 10ºC do not cause any damage if they are subject for a short period of time. But if they are stored for several days at this temperature there appear some symptoms of chilling injury. Temperatures over 15ºC are detrimental since they stimulate sprouting, the hollowing of the pith and the development of a disease produced by a virus known as ‘internal cork’.

Controlled atmosphere is not a recommended technique in the conservation of this species.
Distribution
The same environmental conditions recommended for storage are suitable to maintain the quality during transport and distribution of sweet potatoes.

The transport from transoceanic markets is made by sea and the suitable conditions are the same ones as for storage under controlled conditions. Handling must be careful for the wounds, that would hardly heal in the temperature conditions for transport.
Postharvest Problems
Sweet potatoes show a low rate of respiration, similar to that of mature onions and potatoes. This species is quite sensitive to cold, so the appropriate temperatures for storage are over 5ºC.

The main causes of post-harvest losses are: mechanical damages, diseases, sprouting and secondary roots, dehydration, chilling injuries, freezing injuries and other physiological alterations (hollow fruit and internal collapse), insect attacks, carbohydrates evolution, ethylene response, and curing.

Mechanical damages

This species is very prone to suffer from damages during harvesting and handling. Bruises, even though it does not affect conservation, must be avoided since they will be evident when marketing the product.

Diseases

During storage, the diseases that affect sweet potatoes are multiple. These are: soft rot (Rhyzopus), black rot (Ceratocystis fimbriata), scab or Mummification (Monilochaetes infuscans), blue mould rot (Penicillium), charcoal rot (Macrophomina phaseoli), dry rot (Diaporthe batatitis), stem-end rot (Diaporthe fusarium), stem rot (Fusarium oxysporum), foot root (Plenodorum destruens), grey mould (Botrytis), internal cork (virosis), Java black rot (Diplodia tubericola), mottle necrosis or bacterial ring rot (Pythium ultimum and P. scleroteichum) and mucor rot.

Soft rot (Rhizopus) is the main disease occurring during storage, transit and marketing of sweet potatoes. All of them are subject to develop this disease. The parasite penetrates the wounds produced during handling, here lays the importance of a good development. The first symptoms are the softening and wateriness of the affected tissues, without any colour change. If we make a cut and we press it we can see the exudation of a yellowish brown liquid with fermentation odour. With time, these affected tissues turn into a cinnamon or brown colour, the loss of water cause them to dry up and they become firm. In extremely dry conditions the sweet potato may even mummify. The infected items affect all the tissue, although in some cases the infection stops and a part of the sweet potato remains healthy. The disease is identified by the development of a superficial grey mycelium provided with black sporangia (an organ that carries the spores, resembling the rounded small head of a needle).

Black rot (Ceratocystis fimbriata) is one of the most serious and widespread diseases of sweet potatoes. The first symptoms are superficial spots, slightly depressed, of brown colour and circular shape. They usually have half centimetre of diameter during the first days, and grow until they reach 1 to 5 centimetres, of a black to greenish black colour with black small spots.

Scab or Mummification is caused by Monilochaetes infuscans. It causes drying and mummification by the roots during storage. This process is quite slow, except in warm and dry environments. Under this type of conditions there occur some cracks that in 4 or 6 weeks devalue the whole of the produce.

Blue mould rot (Penicillium sp.) usually occurs in sweet potatoes that have suffered freezing or chilling injuries. The spores of the fungus penetrate through the wounds or other types of damages in the tissues (other pathogens’ attacks). The symptoms affect large areas of the roots, even the whole of the product in case of chilling injuries. The characteristic mycelium of this fungus is of a white and bluish-green colour.

Charcoal rot is caused by the fungus Macrophomina phaseoli. The wounds occurring during the harvesting represent a way for the fungus to peneate. The inial external symptoms are spots of a pale brown colour on the surface and the inner tissues. The size and shape varies, but it is quite common to see a clean line separating the ill tissues from the healthy. While the disease grows, the skin wrinkles and the affected tissue turns into a dark brown colour, while the texture remains firm.

Eventually, the root mummifies, showing a hard, black and dry appearance.

In dry rots (Diaporthe batatitis) the symptoms are evident from the beginning of the infection: a firm dark brown rot. As the pathogen develops, the damaged tissues loose water very fast and they dry, becoming black and hard. The surfaces and tissues that are close to these areas are quickly covered with black small mottles.

The stem-end rot is caused by several organisms, among which we find Diaporthe and several species of Fusarium. These fungi penetrate through wounded or weaken tissues, causing several types of symptoms, among which stand out a firm and dry rot characterized by the drying of the sweet potato’s stem end.

Stem rot is caused by Fusarium oxysporum. The first symptoms are small, superficial, circular mottles, of a pale brown colour. They progressively enlarge and the surface becomes somewhat depressed, hardly changing its coloration. The advanced disease shows some mottles of pale brown colour, circular perimeter, half centimetre deep and free of moulds and fruitful matters on its surface.

Foot rot is caused by the fungus Plenodorum destruens. The symptoms are fully visible in post-harvesting. The damaged product shows a rot with firm to spongy texture of brown-dark colour; the affected tissues are wrinkled.

Grey mould rot is caused by Botrytis sp. The infection takes place through the wounds, so it usually occurs in the tips. The damaged tissue has a greyish brown colour and is moderately soft, emitting an amylaceous scent.

Internal cork is virus-like infection. The infected sweet potatoes show a set of spots inside the pulp, of irregular size, brown to black colour and hard and corky consistency. The contiguous tissues remain healthy. The sweet potato’s surface shows no clear symptoms, except for a slight depression of the skin revealing the cork spot underneath.

The Java black rot is caused by Diplodia tubericola. In initial states the rot has a brown colour and it is moderately firm. As the rest of tissues are invaded, the central part of the area affected turns into a paler brown colour and it wrinkles on the surface.

Mottle necrosis or bacterial ring rot results from the wound of harvesting caused by Pythium ultimum and P. scleroteichum. The first symptoms consist of small, depressed spots of greyish brown colour, around the joints of the secondary roots. Low temperatures favour the development of a soft rot of grey colour. Under conditions of relatively high temperatures, the manifestations are veins or spots of greyish-brown to reddish-brown colour, along with small concavities in the flesh. The third type of symptom is a greyish-brown superficial rot that grows forming an edge around the sweet potato.

Mucor rot is generated by the fungus Mucor racemosus. Sweet potatoes are infected through unhealed wounds or dead small roots. The affected tissues are wet, whitish, and at the beginning of the infection they have a characteristic amylaceous smell. Later on they become spongy and when they are split open they have a fibrous consistency. The symptoms at the beginning are similar to those caused by Rhizopus, reason why it is necessary to use the microscope.

Sprouting and rooting

An excessively humid atmosphere induces the sprouting of sweet potatoes.

Dehydration

Water loss is the main cause of the loss of weight; as a consequence, the product softens and wrinkles, loosing its fresh appearance. The main cause of loss of weight is transpiration. It is during the process of curing when the greater loss of weight takes place; during storage the loss is slower.

Chilling injuries

Sweet potatoes suffer from chilling injuries if they remain for 10 or more days at temperatures below 12-13ºC. The external manifestation of these damages is an area covered with pitting that, in severe attacks, gather and form some injuries that develop into fungic infections. The main symptom inside the vegetable is a change of colour, although the texture and the taste also change. Scattered spots of brown to pale black colour surround the vascular ring. Sweet potatoes damaged by cold are subject to be affected by pathogens; among the most frequent are Mucor, blue mould and grey mould.

Frost injuries

The freezing temperature for sweet potatoes is relatively low (-1,6ºC) if compared with that of other vegetables. Frost injuries are manifested by a change of colour of the vascular ring, from brown into a yellowish colour. The rest of tissues have a watery appearance and a green-yellowish colour.

If sweet potatoes have been subject to freezing temperatures for too long, the tissues collapse and the vegetable becomes soft and flabby, and at the same time it releases water.

Other physiological alterations

Among them are the hollow fruit and internal decay. The hollow fruit consists of the formation of hollow cavities inside the root. It takes place when the loss of weight is greater than the loss of volume. The internal decay is an alteration that occurs occasionally during storage. The cut of the tissues has a dry and spongy appearance; some cavities grow in the central part of the root. The spongy areas have a white or pale yellow colour, with a cottony texture.

Insect attacks

During storage sweet potatoes may suffer the attack of insects. Cylas formicarius, the sweet potato’s grain weevil, is found in Africa, Asia, the Pacific Islands and North and South America.

Evolution of carbohydrates

Carbohydrates (starch and sugars) are the main components of sweet potatoes and their evolution during storage depends on the temperature and the characteristics of the variety.

The temperature makes the balance between starch and sugars conditional. At low temperatures, the sugars gather, specially the sucrose. This phenomenon does not represent a negative effect on this species, since it must be sweet. However, low temperatures may cause chilling injuries, which cancel the advantages of sweetness.

Response to ethylene

Sweet potatoes have usually a low rate of ethylene production and low sensitivity to this gas, so its concentration in the atmosphere does not represent any problem. However, this compound may cause some changes during post-harvest. High concentrations in the storage environment would cause unpleasant smells and tastes, affecting also the coloration. Nevertheless, some positive effects have been shown with the application of ethylene, reducing chilling injuries and improving the resistance to the attack of the fungus Ceratocystis fimbriata.
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