Learn about a variety of biochemical processes with these quick, simple experiments using one of the world’s favourite fruits.
Bananas are one of the most popular fruits across the world – over 100 billion of them are eaten every year. They are not only a staple food for millions of people, but also an essential source of income and employment, especially in Latin America.
Along with plantains, bananas are part of the Musa genus, which includes around 70 species. Today, however, most sweet bananas come from a single variety called Cavendish. These bananas are cultivated using asexual reproduction so they are genetically identical. This loss of biodiversity, however, leaves them particularly vulnerable to parasitic attacks – as is currently happening in a global infection by the fungus Fusariumw1.
Perhaps surprisingly, bananas also provide a good way to understand biochemical processes. In this series of quick activities involving bananas, students can learn about sugar metabolism, starch storage, melanin production and the molecules responsible for the banana flavour. The activities are suitable for secondary school students and involve chemistry at several different levels.
Before starting, present your students with six bananas of differing ripeness (see bananas 1–6 in table 1, which are numbered in terms of increasing ripeness). Which banana do your students guess is the sweetest? Most students vote for the black banana, but is their assumption correct? Students can conduct a blind tasting test to judge which sample tastes sweetest. Do their taste buds identify the correct answer? To find out, students then measure the sugar content of various bananas using a refractometer. The activity will take around 20 minutes.
Here, students use Lugol’s iodine solution to test for the presence of starch in different bananas, showing that the amount of starch declines during the ripening process. This activity takes approximately ten minutes.
During the ripening process, the enzyme amylase catalyses the breakdown of starch (a polysaccharide) to simple sugars – first to maltose (a disaccharide) and then to glucose (a monosaccharide). In the activity, the banana with the highest sugar content (in the form of monosaccharides and disaccharides) is banana 3 in table 1. After this stage of ripening, the sugar content starts to decline because the majority of the starch has been converted into simple sugars and the banana continues to use glucose for cellular respiration.
This phenomenon is also demonstrated by the reaction with Lugol’s iodine staining: unripe bananas have a higher starch content than ripe bananas, as shown by a darker staining with Lugol’s iodine solution. With further ripening, the staining is considerably reduced due to the lower amount of starch.
In a blind tasting test, students might not choose the banana with the highest sugar content as the sweetest. Other factors that can affect the taste include the softness of the banana and its water content. Moreover, since amylase is also present in our saliva, foods that contain lots of starch (but little sugar) start to taste sweet after chewing, as the amylase breaks down some of the starch into sugar.
Discuss some of the following questions with your students:
In this activity, students prepare microscopic specimens using banana pulp from the most and least ripe bananas. Under the microscope, students can view the cellular organelles that store starch (amyloplasts) and understand why these organelles disappear during the ripening process. This activity takes approximately 15 minutes.
banana pulp for viewing
under the microscope
Samuel Ginsburg
Amyloplasts are responsible for the synthesis and storage of starch granules through the polymerisation of glucose. In unripe bananas, staining with Lugol’s iodine solution turns the starch and amyloplasts a deep purple colour (figure 3). In riper bananas, there is no reaction with the solution, because the amount of starch is reduced and the amyloplasts are no longer present in these samples (figure 4).
Discuss some of the following questions with your students:
a banana in boiling water, the
skin turns black due to the
production of melanin.
Sacha Glardon
Students learn about the effect of melanin production and cell damage in bananas in this simple activity, in which a banana is submerged in hot water, causing the skin to turn black. The activity takes approximately five minutes.
When the banana is submerged in hot water, its cells are exposed to heat, causing cell damage. In response, the dark pigment melanin is produced, which results in the banana skin turning black. This pigment protects external cells (in both plants and animals) from ultraviolet damage and also counteracts the presence of free radicals due to its antioxidant properties. Melanin is produced from the oxidation of the amino acid tyrosine, which is catalysed by a copper-containing enzyme called tyrosinase. When the cells of the banana are damaged, tyrosinase is released. Melanin production does not affect the ripening process, but since riper bananas are older and have therefore suffered more cell damage, they are browner.
Discuss some of the following questions with your students:
This activity is suitable for students aged 16–19 and involves synthesising artificial flavours. Using 2-methyl-1-butanol, students can produce the organic compound isoamyl acetate (3-methylbutyl ethanoate), which has a scent similar to bananas and pears. Or, using 1-pentanol, students can produce the compound amyl acetate (pentyl ethanoate), which has a scent similar to bananas and apples. The activity will take approximately 15 minutes. You may wish to divide your class so that one half produces isoamyl acetate, and the other half produces amyl acetate. Groups can then compare scents.
This experiment involves the use of sulfuric acid. Students should wear safety glasses and gloves, and follow the usual safety rules for chemistry classes. As with any chemistry experiment, students should not taste the flavouring. See also the general safety note.
Isoamyl acetate and amyl acetate are the main components of banana flavouring, although flavour manufacturers usually keep their exact recipes a secret. Isoamyl acetate is produced by a condensation of 2-methyl-1-butanol with acetic acid, whereas amyl acetate is produced by a condensation of 1-pentanol with acetic acid. Both esterification reactions are catalysed by sulfuric acid (figure 6). As with all fruits, the natural flavour of banana is a mixture of several componentsw2, so it is difficult for artificial flavours to copy the flavour and smell exactly.
Discuss some of the following questions with your students:
There are a number of additional experiments you can carry out to learn more about the biology and chemistry of bananas, as well as to link to other interdisciplinary topics. For example, you could: