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Courgette, Cucurbita pepo / Cucurbitaceae
Postharvest Atmosphere Management
Courgette
Pre-cooling is a hardly used technique. Hydro-cooling and air cooling are the simplest methods to lower the temperature in this species and it is applied to increase the self life of the fruit.

Cold storage is basically used than to regulate the market, since it enables the produce to be kept in a satisfactory state of development and to keep it for some days if the prices are low. The susceptibility to cold limits the use of low temperatures on this species. At temperatures between 0 and 4ºC and 90% of relative moisture, they can be stored for 4 to 5 days without any problem. Longer periods may bring about some problems caused by cold; whenever the produce is stored over one week the temperature must be higher, between 7 and 10ºC, thus extending storage for two weeks.

The suitability of each product for cold storage depends on the generic material. The new yellow cultivars are less appropriate to be stored.

Modified atmospheres with a low concentration of oxygen between 1-4% and high levels of carbon dioxide (5% and over) are suitable to reduce the sensitivity of courgettes to low temperatures and the agressivity of some pathogenic micro-organisms. However, percentages of oxygen around 1%, as well as 10% of carbon dioxide percentage, favour the development of strange tastes.

The best results are obtained at 5ºC together with an atmosphere containing a 5% of carbon dioxide. Greenhouse courgettes show less susceptibility to chilling injuries occurring at 4ºC when storage extends for more than 4 or 5 days. Closed packages create a modified atmosphere that surrounds the produce and protects it.
Distribution
Relative moisture during transport must be high; the average temperature recommended ranges from 10 to 20ºC.

The produce ventilation for an effective distribution must be average. Courgettes are sensitive to bruises.
Postharvest Problems
Courgettes are unripe fruits harvested when they are still in continuous growth. They have a high metabolic rate and scarce protection on the surface, thus having a short post-harvest life.

The alterations affecting the loss of quality in courgettes are: chilling injuries, blight and loss of weight, yellowness, some diseases and changes of composition.

Chilling injuries

Temperatures below 4-5ºC more than 4-5 days may cause chilling injuries, showing small depressed areas that may penetrate into the pulp. The symptoms are already visible in the store room if storage extends for 6 days, increasing its intensity when the produce is put at environmental temperature.

If courgettes are harvested when they are still very tender, with high contents of water, they are more sensitive to low temperatures. Produce affected by chilling injuries, when removed from the store room, shows a greater loss of weight and higher rates of ethylene production and carbon dioxide than those of healthy courgettes.

Blight and loss of weight

The loss of water causes the loss of turgescence and firmness, along with a loss of weight.

The rate of loss depends on factors such as the development of the produce, the temperature and the type of preparation. There are differences within the varieties, which are less important.

The most tender-harvested fruits undergo an easier dehydration, since their skin is less formed. In one week, courgettes weighing around 100 g may loose up to 7% of their weight.

The higher the storage temperature, the greater the loss of water, although it actually happens the other way around, if the cold storage room has a good system to control the relative moisture.

The loss of water is reduced if the produce is pre-packaged in trays covered with a plastic film.

Yellowness

As it happens with other green vegetal organs, after the harvesting the chlorophyll undergoes some degrading processes that give rise to the development of yellow tones. It is an undesirable phenomenon representing the loss of freshness of the produce.

Diseases

The main diseases affecting the harvested produce are wet rot, rot caused by Sclerotinia, grey rot, and rot caused by Alternaria.

- Wet rot: Caused by the fungus Rhizopus stolonifer and characterized by the presence of watery, soft and sunk areas spreading over a wide surface. Even though the rot penetrates inside the fruit, it does not cause decay. The surface shows the mycellium and reproductive structures of the fungus, of a dark colour. Produce affected by bruises and damages during their transport are very susceptible to this fungus. Low temperatures, between 5 and 8ºC, stop the development.

- Rot caused by Sclerotinia: The development of the fungus Sclerotinia sclerotium is favoured by temperatures between 22 and 25ºC. This fungus attacks a wide rank of hosts and gives rise to depressed and humid alterations, that spread all over the surface towards the heart of the vegetable. The most affected area is covered with a white cottony mycellium.

- Grey rot: Cinerea Botrytis is another fungus affecting courgettes, causing the lateral and style rot. The produce is covered with a cottony mycellium and the tissues soften.

- Podredumbre by Alternaria: Caused by the fungus Alternaria tenuis. Even though it may also attack in the field, it usually occurs after the harvesting, in vegetables that have been damaged during handling or by storage conditions. The vegetables grown in plants debilitated by diseases are also prone to suffer this attack. It is characterized by brown spots that may cover a wide area. The external tissues dry and become spongy, the dark efflorescence of the fungus covers the affected area. The effect of this disease is reduced when the harvest takes place under conditions of low relative moisture. Closed plastic preparations allow the creation of an atmosphere with high relative moisture. If we add high temperatures to these conditions, we will create an ideal environment for the pathogen’s development. The low-oxygenated atmospheres or high in carbon dioxide reduce the effect of some diseases, since this type of concentrations have an influence on the pathogens, diminishing their metabolism.

Changes of composition

Once harvested, the fruit’s vegetal components continue their evolution at a rate that is determined by the temperature. At 10 and 15ºC the main changes are an increase of the pectins soluble in water and cellulose, along with a decrease of starch and protopectins. Sugar and hemicellulose do not show great variations. It has been proved that the yellow cultivar "Multipik" undergoes a reduction of 10-15% of vitamin C content after the harvesting.
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