Once the nut is dried it is kept in silos for 5-6 months at a temperature of 7 to 10ºC. The optimal temperature is of 0 to 10ºC; the lower it is, the longer the postharvest life. They can be stored frozen at -18ºC for more than a year, maintained in satisfactory conditions.

According to the moisture content of the nuts, that may vary from 2 to 20%, the optimal relative humidity in order to increase the storage must be from 55 to 70%. It is recommended that the packages protect the product from moisture.

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

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

There are physiological alterations or diseases caused by bacteria and fungi.

There may occur some problems like: the nut physical damage or crack, which reduces the life in storage; the transference of smell due to the high lipid content of the nuts; for that reason they easily absorb the smell and therefore they must not be stored with other produce with strong smell. The damage due to ammonium causes the blackening of the external tissues; they are very sensitive to the ammonium.

Moreover, there are some insects like the grain weevils that may damage the stored nuts. In order to diminish the attack of insects there must be good hygiene and the produce may be fumigated with authorized chemicals. However, they are less and less used, since they may bring about some problems with residues. Other forms to control them are by means of irradiation, heat treatments and controlled atmospheres.

Concerning the diseases that affect the nuts, they may be mainly caused by bacteria and fungi.

1 Bacteriosis
The bacteria that cause bacteriosis are of the genus Xanthomonas and they appear with heavy rain and high temperature. They affect the leaves, the buds and the fruit, and they may reduce the harvest to half. The fruit affected shows dark spots on the surface and a cracked centre.

2 Aspergillus flavus
The nut 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.