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French bean, Phaseolus vulgaris / Leguminosae (Fabaceae)
Postharvest Atmosphere Management
French bean
Pre-cooling is a necessary method in order to keep a high quality. French beans must be cooled down immediately after their harvesting, although the temperature must not descend below 4-5ºC so as to avoid chilling injuries. A fast decrease of the temperature is specially important when they are placed in large packages, since the mass of product accumulated complicates the spontaneous loss of heat. The temperature must be risen to 7-10ºC by means of pre-cooling.

Irradiation is not a common practice in French beans, however, good results are obtained mainly with ship consignments to South Africa.

Storage is made at temperatures between 5 and 10ºC, with high levels of relative moisture (90-95%). Storage lasts for some days, depending on the variety, the rapidity at which the field heat was eliminated and the temperature of storage. At 7ºC they can be stored for 7 days or so. Temperatures at 4.4ºC enable the produce to be kept in good conditions for approximately 10 days, but it must be consumed immediately after the storage, reason why these conditions are only recommended in beans intended for industry. If they are subject to temperatures below 4.4ºC for more than 2 days they will suffer from chilling injuries during trade.

The application of controlled atmosphere with an oxygen concentration of 2-3% and carbon dioxide between 5-10% has a moderate effect: the efficacy of this technique is a potential interest for the industry.

For further information on the basic indicators of post-harvest handling for French beans consult the University of Davis Internet address ( http://postharvest.ucdavis.edu )
Distribution
The environmental conditions for storage are suitable for the transport and distribution stages. The increase of temperature must be avoided since they cause water condensation on the produce, thus favouring the development of micro-organisms.
Postharvest Problems
French beans, like other produce which has been harvested physiologically unripe, have a high metabolic activity and, consequently, the rates of transpiration and respiration are also high.

After the harvesting, some problems of blight and loss of weight may arise, scald, chilling injuries, physiological ageing or "Rusting", browning of the wounds, yellowness, increase of fibrousness, seed’s growth, damages caused by freezing, diseases, compositional changes and sensitivity to ethylene.

Blight and loss of weight
The loss of water has as a consequence the loss of turgescence of the pods, that are withered and of a dull colour. This species has a relatively high tolerance to the loss of water since it is considered that the reduction of weight allowed is up to 37% before they become unfit to be traded. At 27ºC and relative moisture of 60% the loss of weight after 24 hours is around 4%. The rate of water loss below 90% of relative moisture is very fast, even at low temperatures.

The pre-packaging in plastic film with orifices protects the vegetable from dehydration: we must take into account that the humid atmosphere favours the development of micro-organisms, reason why the temperature control must be accurate.

Scald
Under this term are included a group of alterations like yellowness, increase of fibrousness, blight, burns evidenced by browning, development of mould..., when they are a result of the exposure of the harvested pods to the sun, as well as to conditions of scarce ventilation when the produce has not yet been pre-cooled, in which case an insufficient air circulation is still more harmful than low temperatures.

Chilling injuries
Temperatures below 4.4ºC cause chilling injuries whenever storage lasts for more than two days. These damages are manifested by red stains and small depressed areas. These damages usually appear in the first or second day after returning the produce to environmental temperature. Some damages may also occur at temperatures below 7-10ºC. The produce damaged by cold shows rots (Botrytis, Rhizopus, etc.).

Physiologic ageing or "rusting"
It is manifested by a superficial browning in the pods. It generally takes place after some days of preservation and the most affected beans are those coming from "old’ plantations.

Browning of the injuries
The damages resulting from the harvesting or the preparation develop and acquire a brown tone, which makes them more evident.

Yellowness
Some time after the harvesting, the pods tend to lose chlorophyll, as well as other tissues. The process is faster at temperatures of 8-10ºC than at lower temperatures. An environment with low levels of oxygen and high levels of carbon dioxide slow down the process.

Increase of fibrousness
As in the case if yellowness, the pod’s fibrousness increases in the course of post-harvest life and the phenomenon is also faster at high temperatures.

Growth of the seeds
The pods continue their evolution during storage and the seeds continue their developing; they increase their firmness and enlarge their size.

Damages caused by freezing
They occur in produce subject to temperatures below freezing (-0.7ºC) evidenced by oedemas in the surface.

Diseases
The most outstanding are: grey rot, Sclerotinia, Antracnosis, Rhizoctonia, Pythium, Rhizopus, Sclerotium, halo blight and bacterial blight.
- Grey rot: It is a polyphagous fungus that may affect both in the field as the already harvested fruit, manifested by a greyish mycellium of the pathogen.
- Sclerotinia: The fungus Sclerotinia sclerotiorum brings along during cultivation the disease known as "blight ’, that may cause the death of the plants affected. The attacks are usually stronger during the post-harvesting. The infection extends to healthy pods and the symptoms are humid spots of brown colour and indefinite edge.
- Antracnosis:: Caused by the fungus Colletotrichum lindemuthianum. It is usually developed during the storage in humid packaged lots, from unnoticed injuries during classification. This disease has a severe effect in rainy and humid weather. The spots may grow affecting wide areas of the pods.
- Rhizoctonia: The affected pods infect the healthy contiguous pods, producing "nests’ of a brown mould.
- Pythium: The fungi of this genus infect the pods in the field when there is humid weather, and they are evident once harvested.
- Rhizopus: It is visible right after the produce is removed from the fridge, since temperatures of 4-5ºC prevent its development. The most susceptible pods are those damaged by chilling.
- Slerotium rolfsii: It propagates from infected pods to the healthy ones forming "nests’ of rot; its development is prevented at temperatures of 8ºC.
- Halo blight: The affected pods may show the symptoms during storage their high temperatures and relative moisture promote its development.
- Bacterial blight: The pods affected by the bacterium Xanthomonas campestris p.v. phaseoli may develop the symptoms during storage at high temperatures.

Compositional changes
The vitamin content undergoes some modifications in the stages following the harvesting. The most significant change is that of ascorbic acid (vitamin C) that diminishes. The behaviour of thiamine and riboflavin, two vitamins that belong to the B complex, differ, so the first one increases with time and the second one diminishes.

Sensitivity to ethylene
French beans have a low rate of production to ethylene but they are quite sensitive to this gas, that accelerates the process of senescence.
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