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Lettuce, Lactuca sativa / Compositae
Postharvest Atmosphere Management
Lettuce
In the case of lettuces, pre-cooling is an indispensable operation if we search for an optimal quality of the produce, since the time of preservation decreases as the number of hours between the harvesting and the temperature drop at 2ºC increases.

The temperature must be lowered to 1ºC; the most suitable methods to do so are vacuum cooling, forced humid air and water cooling. In the most suitable one, the vacuum method, the lettuce is already packaged. Water cooling is recommended for Roman lettuces and other cabbage lettuces unless Iceberg. The packages must bear ventilation orifices so as to enable a more uniform cooling. Forced humid air cooling is slower than the previous, although it is also appropriate.

The storage of lettuces must be made at temperatures between 0 and 1ºC, and relative humidity over 95%. The produce must be introduced in the pre-cooled store rooms. If they are not packaged individually, it is necessary to cover the packages with perforated plastics so as to reduce dehydration. In this way they are perfectly stored between 5 and 30 days.

Lettuces are sensitive to ethylene; thus, the store rooms must be well-ventilated. The lettuces must not be stored with species producing this gas, like melons, tomatoes or apples.

The use of modified atmosphere has been tested; this method extends storage up to 1.5 months and improves the quality in short storage periods. The suitable conditions are temperatures at 0ºC, high levels of humidity, oxygen levels between 1 and 5% and carbon dioxide levels of 1%. Actually, this system is only used for pre-cut produce.
Distribution
During transport, temperatures must be low, around 0ºC and not exceeding 6ºC. For sea transport of sprouted lettuces modified atmospheres are also used. The room must be suitably ventilated and mixed loads with species producing ethylene must be avoided.

The distribution must also be made at low temperatures, between 0 and 10ºC. The moisture levels must be high, so as to avoid dehydration; the distribution of lettuces next to products like tomatoes must be prevented, since they give off ethylene.
Postharvest Problems
The lettuce may undergo various alterations and suffer the attack of parasitic diseases during the storage. Among the first disorders are blight, yellowness or the "brown stain’. Among the diseases stand out Botrytis cinerea, Sclerotinia sclerotiorum and Bremia lactucae.

The lettuce is a species with a high respiratory rate, greater among those types that do not sprout.

Blight: lettuces may loose a great quantity of water if the appropriate measures to avoid it are not used; otherwise, they loose the desirable fresh appearance. They must be stored at low temperatures and covered with plastic.

Yellowness: it is a normal process caused by the ageing of the lettuce, but it may accelerate with the proximity to ethylene.

"Russet spotting": it is a physiological alteration that affects the Iceberg lettuce mainly. It is characterized by the manifestation of small spots, 1mm wide and 2-4mm long. They are yellow at the beginning turning afterwards into a reddish colour. They usually appear in the inferior part of the central nerve of the leaves, and are caused by small ethylene concentrations.

"Tip burn": it is a physiological disorder occurring in the field, but it may worsen during storage. It is characterized by a burn on the tip of the youngest leaves. Later on, the affected areas are invaded by pathogens like Erwinia carotovora.

"Pink rib": this disorder gives rise to a pink coloration in the main nerve. It is more common during post-harvest.

"Rib discoloration": this alteration causes the browning of the main nerves. It mainly affects the external leaves and it occurs in the field.

"Browm stain": it is a symptom of damages caused by carbon dioxide. It is characterized by superficial spots of oval to irregular shape, 0.5-1.5cm of diameter. The external part of the heart’s leaves are usually affected by this disorder.

Among the diseases affecting this species during storage we find the following ones:

Botrytis cinerea is a fungus that causes grey rot. It gives rise to some areas of grey greenish or brown colour. The tissues soften and later on a grey down covers them.

Sclerotinia sclerotiorum is a fungus that gives rise to areas with watery appearance, of pale or pink-brown colour. A kind of white cotton covers them.

Mildew: this disease is caused by the fungus Bremia lactucae, that attacks in the field. During storage, the fungus reaches the nearest inner leaves. At the beginning, there are some yellow areas that later on end up dying. The affected area acquires a white characteristic dust.

Soft bacterial rots: they are characterized by soft areas that become watery. Some of the bacteria causing this disorder are those belonging to the sort Erwinia and Pseudomonas.
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