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Potato, Solanum tuberosum / Solanaceae
Postharvest Atmosphere Management
The main procedures after harvesting will be the curing, during which potatoes are put under temperatures around 20ºC to heal the injuries in the skin. They can also be put under cooling treatment, although it is not very usual. Potatoes intended for human consumption and stored for long periods of time will be put under chemical treatment in order to avoid sprouting and to prevent the attack of insects.

The behaviour of potatoes after harvesting will depend on the type of potato, whether it is early or late.

In the case of late varieties, potatoes must be regarded as an organ which has stopped growing, thus the activity of its metabolism will be low. Its skin has matured and therefore it will be less sensitive to the microorganisms attack, dehydration and mechanical damages. All the opposite in the case of early potatoes.

The postharvest actions that must be carried out are curing, precooling and treatments to avoid germination.

- the curing consists of submitting the recently harvested potatoes under high temperatures for a period so as to heal the harvesting damages. This process helps to the regeneration of the damaged skin. During the curing there is an important loss of water, although the beneficial effects are greater.

If the damages are healed, we prevent other types of diseases caused by microorganisms that penetrate through the open wounds.

The curing will take place at 15-20ºC, 85-90% relative humidity for 5 to 10 days. This can be also done at lower temperatures, 7-15ºC; it is a slower process, but there are less rots.

Relative humidity must be high, but one must avoid the appearance of free water films on the tuber, since that would stop the skin’s cellular regeneration and will favour the development of bacteria.

-precooling is not often applied to potatoes, since there are better results if temperatures are slowly reduced. Nevertheless, in exceptional warm temperatures periods, they use hydrocooling, that is to say, water cooling.

- treatments to prevent germination are applied on potatoes intended for fresh consumption after long time storages. These chemical treatments prevent sprouting and they can be done in the field, during the weeks previous to harvesting, or during the postharvest.

There is another irreversible way to inhibit germination, applying gamma rays. With this irradiation insects can also be controlled. However, the cost of the facilities and the consumer’s reluctance for ‘irradiated’ produce limits its use.
Postharvest Problems
They are many alterations in potatoes, from mechanical damages produced after the harvesting, sprouting, blight, the attack of insects and bacteria that cause rots, chilling injury, the presence of sweet flavours and the greening caused by light expositions.

- Mechanical damages
This phenomenon is more appreciable in early potatoes, since they are less protected. Peeling, bruises, cuts and breaks are the main damages, being sensitivity at its high degree when the potato’s temperature is low. The bruised tubers are removed from the rest, since they have a short conservation and they are easily attacked by microorganisms.

These damages may appear in any of the handling stages, since there is a high degree of mechanization.

- Black spots
They are the evidence of the tuber’s bad handling. This dark zone appears in the surface and deepens in the pulp. Sensitivity to this injury increases when the tubers are cold, reason why it is recommended not to harvest or handle in these conditions.

- Sprouting
The tubers sprouting proves that the period of maturation has finished. The buds reduce the produce commercial value and accelerates deterioration.

This phenomenon takes place at temperatures over 10ºC. Late varieties develop it 2 or 3 months after the harvesting even at 10ºC.
The good temperatures handling and the application of chemical produce that prevent sprouting are the tools to control this phenomenon.

- Blight
Also called tubercle wrinkling, it is caused by the loss of water, which is greater in immature tubers and increases with the presence of buds. High relative humidity reduces the effect of this alteration, whereas the application of wax coverings is ineffective. However, since humidity in these cases is next to saturation, one must avoid the formation of a water film that would make the attack of microorganisms easier.

Parasitical diseases:

1 Fungi diseases
- Dry rot or Fusarium wilt
It is caused by Fusarium solani. The symptoms are areas with firm, brown rots, spreaded along the tuber. The part affected is wrinkled, often with round folds, and on this area the white-pink fungus develops.

The damages make the fungus penetration much easier. In order to avoid these damages it is advisable to disinfect the premises and the machinery, and try to avoid mechanical damages to tubers.

- Gangrene
It is caused by several fungi belonging to the Phoma genus. It is the most important disease affecting the potatoes stored in many Spanish warehouses. The first symptoms are small areas, irregularly circular, which are dented and dark. In short time the areas enlarge, joining one to the others, and at the same time the crust wrinkles and dents a bit more; sometimes even some cracks appear. There are no circular folds nor fungi, as it happens with the fusarium wilt. The pulp has necrosis that can be removed.

- Pythium
It is known as " wet rot of the injured tubercles" and is caused by Pythium debaryanum and Pythium ultimum. The symptoms appear after the harvesting, in the surface of a damage. The first symptom is a watery spot with that grows quickly turning into a dark and humid spot. When it is pressed it seeps liquid. The tissues affected by this rotting stretch, giving place to splits from where the dark yellow liquid seeps.

- Silver scab
It originates in tubers that come from the field and it is caused by Helminthosporium solani. The symptoms are irregular zones of the bark with a slight colour alteration, becoming ash-coloured. With time, the spots are more evident and turn into pale gray.

- Stem rot and tubercles
It is caused by Sclerotium rolfsii. It does not develop until the tuber is in the warehouse. The symptoms are mainly placed in the insertion of the runner; it is a circular and irregular rot, the bark is slightly dented and eventually it tends to darken. The rotten tissues are easily removed, and this makes it easy to distinguish these potatoes from those affected by Fusarium.

- Stain in the skin
It is a very important alteration in places like Scotland, Ireland and Canada and it is caused by Oospora pustulans. They appear after a long period of time. First of all, a small protuberance develops to a dented zone with irregular edge and 2-3 millimeters of diameter; the central zone is lifted up. It gives rise to an irregular tuber with dented areas, wrinkled skin and cracks.

2 Bacterial
- Soft rots
They are produced by diverse bacteria. One of most important is Erwinia carotovora, which causes the loss of tissue consistency; sometimes it turns into pink and afterwards it darkens. In many cases these rots go along with characteristic but not unpleasant smells.

- Annular rot
It is caused by Clavibacter michiganense. This bacterium attacks in the field, and the symptoms are seen when the potato is cut cross-sectionally. In the beginning it is characterized by a slight transparency in the tissues that surround the vascular system; little by little the vascular ring underneath the skin turns into yellow, darkening and even separating from the adjacent tissues. Unless the tuber is cut, the disease is not detected in later stages. The disease is visible when there are external deformations and protuberances that split a viscous liquid when they are pressed.

3 Insects
- Potato moth
The larvae develop inside the potato, digging some galleries.

4 Sweet flavours
The main reserve substance in potatoes is the starch, and at environmental temperature the balance between starch and sugar favours the starches. The sugar accumulation comes along with low temperatures and the aging of the product. The minimum temperature for these symptoms to appear varies, but it takes place at temperatures below 10ºC, reaching its maximum at 2 to 4ºC.

This phenomenon takes place because at low temperatures the respiration rate is very low; thus, the sugar from the starch is not consumed; it is accumulated in the tuber conferring a sweet flavour. In order to avoid this phenomenon, storage temperatures are usually over 4ºC.

5 Chilling injury
Some varieties suffer from browning when they are stored at 0-2ºC.

6 Pressure lesion
Found in the potatoes placed in the inferior layers of the storage piles, and it is characterized by black spots developed where they touch with other tubers.

7- Greening
It is one of the main problems in the marketing stage and it is caused by the exposure of the tubers to light.The green colour is due to the formation of chlorophyll and to the solanine, another compound with bitter taste. It is toxic when it appears in great quantities.

The light of the fluorescent tubes used in the stores shelves cause a greater degree of greening than any other type of light. On the other side, the use of packages that protect products from the light has a drawback: the consumer cannot see the product’s qualities.

8- Others
Black core
It is characterized by the darkening inside the tubers, specially in their central area. It develops under conditions of scarce aeration and/or high temperatures.
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