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Walnut, Juglans regia / Juglandaceae
Postharvest Atmosphere Management
The walnut is dried in order to reduce its moisture content from 30-45% to 12-15%. They may be sundried or dried with hot air in a kiln.

Once dried, the whole walnut can be kept in a warehouse with satisfactory sanitary and ventilation conditions, for 5-6 months at a temperature below 15ºC and relative moisture inferior to 70%. At maximum temperatures of 10ºC in cold storage rooms and relative humidity of 60-70%, the walnuts are kept up to 12 months. They may be stored frozen at -18ºC for more than a year, keeping them in good conditions.

According to their moisture content, that may range from 2 to 20%, the optimal relative humidity so as to increase the time of storage must be of 55 to 70%. It is recommended that the packages protect the produce from the moisture, in order to avoid the fungi. If the atmosphere is too dry, there may occur some losses of weight and bring about the rancidness.

The effect of controlled atmospheres vary according to the levels of their components. Thus, the levels of oxygen below 1% are very effective to delay the rancidness and other symptoms of deterioration; the oxygen concentrations below 0.5%, balanced with nitrogen, and/or carbon dioxide levels in the air over 80% may be effective to control the insects attacking the stored products and may be an alternative to fumigation.

For all this, it is recommended the vacuum packaging or the nitrogen flows, in order to remove the oxygen and maintain the quality of the walnuts.

The kernels must be kept without light and with limited conditions of moisture, temperature and oxygen. It is recommended to keep them in opaque packages in cold storage rooms at temperatures of 4-6ºC and relative humidity of 60-79%.
Postharvest Problems
The walnuts may suffer from physiological alterations that are prevented with suitable storage conditions (physical damage, absorption of foreign smell...) and by diseases caused by bacteria and fungi.

There exist some problems like the physical damage, that is to say, that the nut breaks and for that reason, the storage life shortens; their giving off smell due to the high content of lipids in the walnuts, they absorb very easily the smell. For that reason, they must not be stored together with other products that give off strong smell. They are also very sensitive to the damage by ammonium, that causes the blackening of the external tissues.

Moreover, there are several insects that may damage the walnuts, such as Carpocapsa or worm infestation. The fruit attacked may be distinguished by the presence of excrement in the entry point. To control this pest, the larvae are destroyed with different chemical procedures or biological control.

Concerning the diseases that affect the walnut, they are mainly produced by bacteria and fungi.

1) Bacteriosis
The bacteria that cause the bacteriosis belong to the genus Xanthomonas and they appear in conditions of heavy rains and high temperature. It affects the leaves, the buds and the fruit, and they may even reduce the harvest to the half. The fruit affected shows dark spots on the surface and a cracked centre.

2) Aspergillus flavus
The walnut infection may begin before the harvest and its development is favoured by rainy and damp weather, and when the walnuts are damaged by insects. The best way to avoid the growth of fungi in products already harvested is to keep them in optimal conditions of temperature and relative humidity during all the handling process.
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