The banana colour scheme distinguishes seven stages from ‘All green’ to ‘All yellow with brown flecks’. The green, unripe banana peel contains leucocyanidin, a flavonoid that induces cell proliferation, accelerating the healing of skin wounds. But once it is yellowish and ready to eat, the chlorophyll breaks down, leaving the recognisable yellow colour of carotenoids.
Unripe (green) and ready-to-eat (yellow) bananas.
The fruits are cut from the plant whilst green and on average, 10-30 % of the bananas do not meet quality standards at harvest. Then they are packaged and kept in cold temperatures to reduce enzymatic processes, such as respiration and ethylene production.
However, below 14°C bananas experience ‘chilling injury’ which changes fruit ripening physiology and can lead to the brown speckles on the skin. Above 24°C, bananas also stop developing fully yellow colour as they retain high levels of chlorophyll.
Once the green bananas arrive at the ripening facility, the fruits are kept in ripening rooms where the temperature and humidity are kept constant while the amount of oxygen, carbon dioxide and ethene are controlled.
The gas itself triggers the ripening process, leads to cell walls breakdown and the conversion of starches to sugars. Certain fruits around bananas can ripen quicker because of their ethene production.
By day five, bananas should be in stage 2½ (’Green with trace of yellow’ to ‘More green than yellow’) according to the colour scale and are shipped to the shops. From stage 5 (’All yellow with green tip’), the fruits are ready to be eaten and have a three-day shelf-life.
A fruit market. Image: Gidon Pico
The very short shelf-life of the fruit makes it a very wasteful system. By day five, the sugar content and pH value are ideal for yeasts and moulds. Bananas not only start turning brown and mouldy, but they also go through a 1.5-4 mm ‘weight loss’ as the water is lost from the peel.
While scientists have been trying out different chemical and natural lipid ‘dips’ for bananas to extend their shelf-life, such methods remain one of the greatest challenges to the industry.
In fruit salads, to stop the banana slices go brown, the cut fruits are sprayed with a mixture of citric acid and amino acid to keep them yellow and firm without affecting the taste.
Bananas are a good source of potassium and vitamins.
The high starch concentration – over 70% of dry weight – banana processing into flour and starch is now also getting the attention of the industry. There are a great many pharmaceutical properties of bananas as well, such as high dopamine levels in the peel and high amounts of beta-carotene, a precursor of vitamin A.
Whilst the ‘seven shades of yellow’ underpin the marketability of bananas, these plants are also now threatened by the fungal Panama disease. This vascular wilt disease led to the collapse of the banana industry in the 1950’s which was overcome by a new variety of bananas.
However, the uncontrollable disease has evolved to infect Cavendish bananas and has been rapidly spreading from Australia, China to India, the Middle East and Africa.
The future of the banana industry relies on strict quarantine procedures to limit further spread of the disease to Latin America, integrated crop management and continuous development of banana ‘dips’ for extending shelf-life.