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Apple, Malus domestica / Rosaceae
Postharvest Atmosphere Management
Apples have a relatively high capacity of conservation and there are techniques that extend it. The storage in controlled atmosphere allows storage practically until the following year’s harvest. This fruit has also a high capacity to produce ethylene, which makes it necessary to control the concentration of this gas during the storage.

The cold storage rooms keep the apples at temperatures slightly superior to 0ºC; according to the variety, the optimal temperature varies between -1ºC and 3º. Relative humidity in these premises is kept at 90-95%.

In order to benefit apples as soon as possible from cold - prolonging their storage – the temperature must be lowered before they are introduced in the storage rooms. This is done by means of hydro pre-cooling or in rooms for air pre-cooling.

In spite of their good capacity of conservation, as it happens with any other product, they tend to loose quality after a period of time. The " farinose’ sensation in the apples is a varietal characteristic but it also increases with storage. This characteristic is caused by the pectin, a substance found within the cells that cements them. This substance is degraded and the cells become " loose".

Apples have been widely researched on for their commercial importance, and nowadays we know the optimal conditions of storage for all the varieties of commercial interest.

Ethylene may accelerate the rate of senescence and loss of firmness in the " Fuji’, " Gala’ and " Granny Smith" types. The diminishing in the concentration of this gaseous hydrocarbon in these varieties may reduce their susceptibility to scald. The apples "Gala’ must be cooled very fast since they soften at great speed.

Ethylene stimulates maturation in " Golden Delicious". The fruit of this cultivar that must be stored for more than a month shall benefit from the use of controlled atmosphere in terms of pulp firmness, acidity and colour of the skin. Under conditions of 1 to 3% of O2 and 1.5 to 3% of CO2 the maximum period of storage is 10 months.

For Fuji apples, the recommended composition of controlled atmospheres must be less than 0.5% of carbon dioxide and 1.5 to 2.0% of oxygen. If these conditions are kept, storage may extend up to 8 months. It must be taken into account that Fuji apples of late harvesting (beyond 180 days from flowering) must not be subject to controlled atmosphere. Even with 0.5% of CO2 internal browning may occur.

Controlled atmospheres in apples Gala have been used successfully at intervals of 1 and 2% of carbon dioxide and between 1.5 and 2.0% of oxygen. The storage may last up to 4 or 5 months.

With regard to Granny Smith apples, the following composition of controlled atmospheres is recommended: 1.5% of oxygen and 1.0% of carbon dioxide.

Controlled atmospheres maintain the firmness and acidity in the three above mentioned varieties, reducing the susceptibility to bitter pit and scald.

For the " Red" type apples, the recommended composition of controlled atmosphere is 1 to 2% of O2 and 2 to 4% of CO2. Those fruits that shall be stored for more than a month will benefit from controlled atmosphere in terms of firmness and acidity retention and reduction of the incidence and severity of scald. The potential period of storage in this atmosphere extends up to 10 months, whereas in conventional storage the duration is of 6 months.

The Web site of the postharvest section of the University of Davis, ( http://postharvest.ucdavis.edu/Produce/ProduceFacts / ) includes recommendations to maintain the optimal postharvest quality of the varieties Fuji, Gala, Granny Smith and Golden Delicious. This Web page contains, for each apple, the index of maturation, index of quality, optimal storage temperature and the relative humidity, respiration rates, rates of ethylene production, response to ethylene, response to controlled atmospheres, disorders or physiological alterations, microbial disorders, and specific recommendations for warehouse handling.
Distribution
During transport, apples must be transported with the maximum care so as to reach the consumer in the best possible condition. They must be protected from the atmospheric agents and kept in the appropriate conditions of temperature, atmospheric composition and relative humidity.

The transport vehicles for apples must be freezer trucks and they must maintain apples at temperatures slightly superior to 0ºC; according to the variety, the optimal temperature varies from -1ºC to 3º. These premises are kept at 90 to 95% of relative humidity.
Postharvest Problems
Apples may undergo, after their storage, different physiological alterations or diseases.

Bitter pit - It consists of small spots - like freckles- that deepen in the skin; they are formed by corky tissue. This physiological alteration is caused by calcium deficiencies; it already occurs in recently harvested apples but it worsens if they are stored.

Scald - the skin shows a more or less extensive brown area on the surface of the fruit. It occurs in the warehouse; the reason why this occurs is not very clear, although it is thought that it is due to the ethylene that the apples emit, reaching high concentrations.

Brown core - the apples centre turns into a brown colour, caused by too low oxygen concentration in the storage room.

Blue rot - It is the commonest and more destructive disease in post-harvested apples. It is caused by the fungus Penicillium (the same sort of the cheese) giving rise to a bluish short hair.

Black rot - it is caused by the fungus Alternaria, that brings along some injuries covered by black short hairs, not very deep, without defined limits. Alternaria affects many other species.

Brown rot – cause by the fungus Monilia. On the surface of the wound appears a short hairy area of the colour that gives name to this disease.

Grey rot - it is caused by Botrytis, a fungus that produces a cottony down of grey colour that attacks many other products.

Core rotting – some varieties of apple keep the communication between the heart and the outside during development, through the chalice tube. In this type of apples occur the growth of fungi in the heart, next to the pips.
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