Harvesting and handling
A harvested banana fruit passes through three physiological development stages, namely the pre-climacteric or “green life” stage, the climacteric and ripening stage and finally the eat-ripe and senescence stage.
The green life stage
The “green life stage” of a harvested banana fruit represents the period from harvest until the initiation of the respiratory climacteric. It is a period of low metabolic activity and the commercial objective is to prolong this period as long as possible. This is achieved in three ways, namely, by inducing in the harvested bunch an inherently long green life with good pre-harvest management, secondly, by harvesting at an early stage of fruit maturity and, finally, by low temperature control (13°C) during transport of packed fruit. The use of sealed polyethylene bags has the effect of reducing water loss, reducing O2 concentration and increasing CO2 concentration, all of which extend the pre-climacteric, and these bags are widely used.
The reduction of O2 and increase in CO2 reduce the respiration rate, and increased CO2 also inhibits the synthesis of ethylene. High humidity delays the internal synthesis of ethylene by preventing water loss. Pre-harvest factors which induce a short green life are a depleted leaf area, leaf disease, fruit mechanical injury (stimulates water loss and ethylene production), anthracnose lesions on the fruit and delayed harvesting. Postharvest factors are high temperatures, low humidity and traces of ethylene from individual fingers which have started to ripen.
The climacteric stage
All fruits produce ethylene but in climacteric fruits like banana there is a rapid and massive increase in ethylene production which precedes the respiratory climacteric. Once initiated, the climacteric is irreversible.
The climacteric group of products is characterised by a sharp increase in respiration after harvest. This increase in respiration is associated with fast ripening of the product. In practice this means that climacteric fruits ripen after harvest and can therefore be picked in an ‘unripe’, but mature condition to ensure maximum storage, transport and shelf-life with minimum quality loss.
Several noticeable changes take place simultaneously during the ripening process. Tissue softening commences, during which starch is degraded to sugars in both the pulp and peel, and rupture strength of cell walls slowly deteriorates. The peel of the fruit turns to light green and then to yellow as chlorophyll is broken down.
During colour change the pulp becomes softer and sweeter as the ratio of sugars to starch increase, and a characteristic aroma is produced. Various enzyme systems are involved in all the changes. Eventually the peel becomes spotted brown and then completely brown and the pulp loses its firm white texture to become brown and gelatinous. Bananas may be divided into seven different degrees of ripeness in accordance with their external colour. Likewise, a visual check of the angularity of the fingers and colour of the flesh will reveal ripeness; as the fruit ripens the fingers become rounder and the flesh changes colour from white to yellowish.
Minimum acceptable size (length and diameter) and grade standards for export markets vary by banana variety and market specifications.
Mature bananas left to ripen naturally will eventually soften but the change in colour will not be uniform and the peel will be dull, pale yellow and unattractive. In order for the fruit to attain a bright yellow peel colour, a firm pulp texture, and good flavour, bananas are ripened by releasing ethylene into a sealed chamber or room and a controlled temperature and RH. Immediately after harvest bananas do not respond to ethylene treatment or, in the best scenario will initiate ripening but will never attain the characteristic bright yellow coloration. One main reason for controlled ripening is to provide retailers and wholesalers with fruit at a stage of ripeness desired by consumers.
The colour of the peel is used as an indicator of ripening. A scale of 1 to 7 is convenient where 1 is dark green, 2 is light green, 3 is more green than yellow, 4 is more yellow than green, 5 is yellow with green tips, 6 is full yellow, and 7 is flecking. Bananas are usually ripened to colour stage 3 to 4 before delivery to distribution centres, retailers or wholesalers.
A premium quality banana is very clean (free from defects such as scars, physical damage, insect injury, and latex staining), free from decay, has an adequate finger length and diameter, does not have excess curvature, and upon ripening has the desired uniform bright yellow colour and sensory attributes in flavour (sweetness, acidity) and aroma.
Cooling and storage
During long distance transport, bananas must be kept in the pre-climacteric state, so that ripening can later be induced artificially in the ‘ripening rooms’.
If premature ripening is to be avoided, then besides cutting the fruit at the appropriate maturity, three other requirements must be fulfilled. First, there must be no delay between cutting and loading the (sound) fruit into the refrigerated space. Secondly, the cooling process itself must be as rapid as possible (preferably within 24 hrs. after harvesting/cutting). The third requirement is the prevention of ethylene accumulation in the atmosphere surrounding the bananas.
For storage purposes (Cavendish) bananas can be kept at +13,2°C up to approx. 28 days in regular packs and up to 40 days in ‘Banavac’ packaging. This consists of polyethylene bags 0.4 mm thick, in which the carbon dioxide content is raised to 5% and the oxygen content is reduced to 2% (“modified atmosphere”). The ethylene which arises is absorbed by adding potassium permanganate. This makes the fruit dormant, i.e. its respiration processes are interrupted, so extending storage life. Bananas are packed in corrugated fibreboard boxes as whole hands, clusters or individual fingers holding an average weight, for premium fruit, between 13 and 18 kg., depending on market preferences.
When subjected to a very high reefer mode, the flesh of the banana goes mushy, they bruise readily, spoil more rapidly, develop an insipid taste and split open at the skin ends.
Chilling injury is an important disorder of bananas. Both green and ripe fruit are susceptible, with green fruit being slightly more sensitive than ripe fruit. Chilling injury results from exposing fruit to temperatures below about 13°C for a few hours to a few days, depending on cultivar, maturity, condition of the fruit, temperature and duration of exposure. Chilling injury is mainly a peel disorder. Symptoms include sub-epidermal discoloration visible as brown to black streaks in a longitudinal cut, a dull or greyish (smoky) cast on ripe fruit, failure to ripen, and in severe cases the peel turns dark brown or black, and even the flesh can turn brown and develop an off taste. Chilled fruit are more sensitive to mechanical injury. Ripe fruit, if chilled, turn dull brown when later exposed to higher temperatures and are very susceptible to handling marks; the slightest pressure causes discoloration. Inflicted chill in green or ripe fruit may not become apparent until 18 to 24 hours after actual damage has occurred.
Skin abrasions result from skin scuffing against other fruits or surfaces of handling equipment or shipping boxes. When exposed to low (<90%) relative humidity conditions, water loss from scuffed areas is accelerated and their colour turns brown to black. Dropping of bananas may induce browning of the flesh without damage to the skin.
Bananas are not to be shipped with ethylene-producing commodities.
O2 below 2% may cause dull yellow or brown skin discoloration, failure to ripen properly, and off-flavours. CO2 greater than 5% may cause the fruit to soften while still green and confer undesirable texture and flavour. Presence of ethylene can cause premature and rapid ripening during transit.
Main benefits of controlled atmospheres include delaying of ripening, reduction of crown rot incidence, and a much fresher condition (latex flowing through the vascular tissues) upon arrival at destination. The beneficial and detrimental effects of reduced O2 and/or elevated CO2 are temperature and time-of-exposure dependent. Removal of ethylene gas can have an additional benefit on extending green-life of bananas, under both ambient and modified atmosphere conditions. Especially during overseas transportation, consideration should be given to ventilation procedures.
An under-peel discoloration resembling chilling injury has been observed on green bananas when transported long distance under CA conditions at below 14°C; this disorder has also been associated with exposure to very high temperatures, by desiccation and by pressure.
Anthracnose, Black heart, Black sigatoka, Brown spot, Chilling injury, Crown rot, Finger drop, Heat injury, Phytophthora, Pitting, Premature ripening, Rhizopus rot, Sooty mould.