All hail the humble banana

About half of the bananas in Limpopo rot after harvesting, yet unripe bananas have the potential to become a staple food, like potatoes or maize, research indicates.

In 2009-10, bananas contributed R1.2-billion to the R2.1-billion turnover from subtropical fruits produced in South Africa, according to the department of agriculture, forestry and fisheries. This makes bananas the most important subtropical fruit grown in the country, but research conducted in Limpopo in 2012 found that about 50% of harvested bananas are wasted, mainly because the fresh fruit isn’t actually being used or converted into other forms of food.

One of the reasons for these significant losses, confirmed by the department of food science and technology at the University of Venda, is the nature of bananas: they ripen very quickly. This is mainly because they produce a lot of ethylene. Ethylene is a plant hormone which drives the ripening process.

An unripe banana secretes a lot of ethylene to make itself ripen. This means that the fruit has a short shelf life after harvest because it will quickly transition from being ripe to being overripe.

Although people prefer to eat bananas when they’re ripe (because they have a high sugar content and make a tasty snack), they can also be eaten when they are unripe. Unripe bananas are mostly starch – they are estimated to comprise about 70-80% starch when unripe – much like the seed germinating parts of maize and the white potato.

If this perishable fruit is, instead, converted into shelf-stable flour and other value-added products, we can reduce wastage and increase food security in the country. At the department of food science and technology at the University of Venda, our research group is investigating the nutritional content of value-added banana-based foods, such as flour made from unripe local banana varieties which can be used for baking bread, cakes, pastries and other foods.

We are looking at mature unripe local banana varieties, namely Luvhele, Mabonde and Muomva-red as well as the popular Williams banana. These indigenous varieties are smaller in length and size than the typical large yellow Williams banana sold in grocery stores. The Luvhele variety has a very sugary taste when ripe, unlike the Mabonde which is less sugary and more starchy. Muomva-red has red skin, hence the name, which means “red banana” in Venda. These three local bananas are seldom eaten and are usually set alight by farmers to prepare their farms for planting of “more valuable” crops.

In conducting this research, we removed the unripe bananas’ peels, and sliced the hard pulp into minute pieces. We soaked them in organic chemicals such as ascorbic, citric and lactic acid to reduce the browning of the fruit when it reacts to air. By drying the soaked pulp at 70°C and then milling it, we managed to produce unripe banana flour.

Although these bananas are considered a nuisance by farmers, when used unripe for flour, they had great nutritional value, in terms of their water-holding capacity, bulk density, ability to increase in volume and their colour. The bulk density and colour affect packaging and consumer acceptability, while their water-holding and oil-holding capacity and swelling power determine how easy they are to bake with.

Our research team also determined the amount of antioxidants that can be obtained from the flour through monitoring its antioxidant activity and polyphenol content. Polyphenols are antioxidants that contain phenols, which preserve foods by reducing their reaction with oxygen. Numerous studies have shown that polyphenols are vital for human health and when present in fruits, polyphenols help reduce the incidence of certain cancers and degenerative diseases.

We can determine the antioxidant levels in a banana flour sample by using a simple assay, DPPH. DPPH stands for 1,1-diphenyl-2-picrylhydrazyl, a dark crystalline powder that is able to measure the antioxidants in the banana flour and, through this, the ability of the unripe banana flour to transfer hydrogen to free radicals in the body.

Free radicals are highly reactive molecules, which cause cell damage when they combine with our DNA and proteins. Antioxidants combine with free radicals to neutralise them.

We found that that the total polyphenol content in Muomva-red bananas was higher than that of fresh berries (such as strawberries, blackberries and raspberries), fresh kiwifruit, fresh oranges and fresh plums.

Flour from this red indigenous banana had the highest polyphenol content of the three local varieties. According to a study of polyphenol content in fruits published in the International Journal of Food Science and Technology in 2012, only fresh grapes and freeze-dried mulberries had higher total polyphenol values than the Muomva-red banana flour.

Unripe banana fruits also contain a number of essential minerals, such as calcium, magnesium, phosphorus, potassium and sulphur, which promote body growth and function.

Unripe bananas, especially the local ones, could be used for producing value-added foods.

Our research team is now exploring how to commercialise this flour, conducting consumer trials, testing its application in baking, and also informing rural South Africans about the livelihood that these previously unwanted and discarded bananas could offer them.

Anyasi Tonna Ashim attends the University of Venda.

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