Banana (Musa paradisiaca) is one of the most commercially important tropical fruit in the world. Bananas are harvested at various stages of its maturity depending upon the purpose for which it is cultivated, such as culinary, table purpose etc., and distance to the market. Bananas are not usually allowed to ripen on the tree as it takes long time. Moreover, the fruit-peel splits, fruit ripens unevenly and fails to develop good colour and aroma, hence the marketable quality deteriorates. Therefore, banana needs to be ripened artificially. Factors affecting plantain and banana ripening can be physiological, physical, or biotic. Physiological factors relate to fruit maturity or environmental factors, which affect the metabolism of plantain and banana. Physical factors include mechanical damage, or relate to dimensions of the fruit. Biotic factors include attack from pests and diseases.
The more mature plantain is at harvest, the shorter the ripening period. armers have to match the date of harvest with the transportation time to the market. However, an early harvest reduces yield. As fruits mature, the cross-sectional diameter increases. Fruit angularity also changes during growth and maturation. As fruits approach full maturity, fruit angles become less acute. . Fruit angularity can be used to predict the optimum harvest date.
Physiological studies on bananas show that storage life decreases as external temperature increases over the range 15-3°C. A 1°C reduction increases storage period by 1-2 days. However, at temperatures below 11°C, fruits suffer chilling injury. Therefore, optimum storage temperature for plantain and banana fruits is 13-14°C. This temperature will maintain fully mature, ripe and unripe fruits for 1-2 weeks. Storage period can extend to 4 weeks when plantain and banana are harvested up to 4 weeks before full maturity.
The relationship between ripening period and temperature is due to fruit respiration. Fruit respiration depends on many enzymatic reactions, and the rate of these reactions increases exponentially with increase in temperature. Studies show that ripe fruits respire at approximately 4 times the rate of unripe fruits. Consequently, ripe fruits lose sugar resources at a higher rate than unripe fruits. This explains why ripe fruits deteriorate quickly.
Water loss and humidity:
Where fruit is sold on a weight basis, loss of water means economic loss. Additionally, water loss reduces visual quality. Water loss causes plantain to lose its firmness, the peel becomes soft and shriveled, and ripening period reduces. Detailed studies on plantain show a curvilinear or power relationship between fruit weight loss and ripening period. For a 2% change from 2% to 4% weight loss per day, ripening period reduced by 9 days or 50%. However, for the same 2% change from 8% to 10% per day, only a 1 day or 5% reduction in the ripening period occurred. Therefore, at a low rate of weight loss, a small increase in weight loss has a critical effect on ripening.
The rate of water loss depends on the ambient relative humidity (RH). RH is the amount of water vapor present in the air, relative to the maximum amount of water vapor that can be held in the air, at a given temperature, saturated air being 100% RH. When a water-containing material such as fruit is placed in an enclosed space, for example, a sealed container, the water content of the air within the container increases or decreases until it is in equilibrium with the fruit. The water equilibrium principle applies when fruit is stored. The rate of water loss depends on the ambient RH. At an ambient RH of 95-100%, fruit loses little or no moisture, and ripening period is unaffected. However, as humidity decreases, the rate of water loss increases, and ripening period reduces. Excessive wetting can also be a problem. When plantain/banana is stored in wet conditions, such as in moist coir (coconut fiber), the uptake of water from the coir to the plantain/banana leads to peel splitting.
Exposure to direct sunlight reduces the ripening period of plantain and banana. Sunlight increases fruit temperature above ambient temperature, which increases respiration, and possibly the rate of water loss. Therefore, in traditional African markets, some traders shade their produce.
Ethylene (C2H4) is a gaseous plant hormone which determines the time between harvest and senescence. The time from harvest to the climacteric respiratory response is called the 'green life' or pre-climacteric period. Ethylene shortens the pre-climacteric period; at high concentrations, ethylene causes rapid initiation of the climacteric respiratory response and accelerates ripening.
All fruits produce small amounts of ethylene during development and when damaged or stressed. During ripening, climacteric fruits produce larger amounts of ethylene than non climacteric fruits. When ethylene is applied to climacteric fruits, at a concentration as low as 0.1-1.0 ml/l, for 1 day, ripening starts. Once ripening starts, climacteric fruits ripen within 1-2 days.
When non climacteric fruits are exposed to ethylene, fruits show an increased rate of respiration. However, respiration rate falls when ethylene is removed. A rise in respiration rate may occur more than once in non climacteric fruits. However, for climacteric fruits, the climacteric is autocatalytic, that is, once started, the process cannot be stopped until the fruit is ripe.
Poor storage methods allow a increase of ethylene, stimulate the climacteric response, and reduce the ripening period. For example, plastic sheets placed over stacks of fruit for shade increase the level of ethylene within the plantain stack and increase the rate of ripening. Therefore, store plantain in thatched or ventilated areas to prevent the increase of ethylene. Also, do not store unripe fruits with ripe fruits. During the pre-climacteric period, fruits are less susceptible to physical damage and pathological attack. This is the best time for handling, transportation, and marketing.
Mechanical damage is a physical factor affecting ripening. Fruit damage during handling generates ethylene. If ethylene production is sufficient to start the climacteric respiratory response, fruit immediately starts to ripen. Damage can also reduce ripening period by causing moisture loss. The effect of damage can easily be measured by recording fruit weight loss over time. Cuts and abrasions on the surface membrane cause the most weight loss. After harvest, fruits lose the ability to repair ruptured peel.
Surface to volume ratio:
The ratio between surface area and volume determines the rate of water loss. Greater the surface to volume ratio, there is a shorter the postharvest life. A leaf which has two large surfaces with little volume loses moisture faster than a fruit. Large fruits lose less water than small fruits.
Fruits with thin peel lose more water, a higher peel permeability leads to a higher rate of water loss and a faster ripening rate. Also, fruits with thick peel, for example melons, withstand damage better than fruits with thin peel, such as tomatoes.
A higher density of stomata may cause a higher rate of water loss, which accelerates ripening. French plantain cultivar Obino l'Ewai has a more dense arrangement of stomata than other cultivars.
Fungi, bacteria, viruses, and insects also account for a great share of total postharvest loss. Pests and diseases reduce both ripening period and overall quality. However, attack by pests and diseases are often secondary because a pest exploits a damaged area of the fruit. Careful fruit handling often prevents such attacks.
About Author / Additional Info:
Scientist at NBPGR, New Delhi