Bananas, the distinctive yellow fruit you can find in any supermarket, are tasty and great as a healthy snack. Dense in carbohydrate, they can provide a burst of energy to help you through your busy day, but there’s a lot more science involved than you might think – from radioactivity to antioxidants and even the ripening process which allows them to appear on our shelves in UK supermarkets, despite the fact they are grown thousands of miles away!
1. In the picture above, you can see some Musa acuminata ‘Dwarf Cavendish’, the most commonly available type of banana, along with a glass beaker used in many different chemical reactions in the lab, mainly to hold solids, liquids and mixture. The molecule on the left is isoamyl acetate, the main component of banana oil, while on the right is ethylene, a gas given off by bananas as they ripen.
Although native to Indonesia, Malaysia and Australia, the two main species of banana we eat today, Musa acuminata and Musa balbisiana, are now grown all over the world, particularly in Central America, India and China, and are one of the most widely consumed foods around. They are highly nutritious, rich in vitamin B6 and fibre as well as being good sources of potassium and manganese. In fact, it is the potassium which makes bananas slightly radioactive! A small proportion of potassium molecules found in nature are a different isotope, which is unstable. Molecules of this isotope decay, giving off radiation in the process. Although there is not enough radiation in a banana to do you any harm, this has given rise to the “banana equivalent dose”, a unit of radioactivity used to give an idea of different levels of radiation in a way that is easy to understand – it is simply the amount of radiation that will be absorbed if a person eats a single banana.
2. A large component of the taste and smell of bananas comes from a single molecule called isoamyl acetate (also known as banana oil), also known as 3-methylbut-1-yl ethanoate. You might be familiar with this already as it is the compound which gives sweets, such as foam bananas (and even pear drops), their distinctive flavour. Because of its arrangement of oxygen atoms (Fig. 1), isoamyl acetate is classified as a type of molecule called an ester. Many different fruit get their characteristic smells from similar esters and, considering their simple structures, it is remarkable that each different ester has an immediately recognisable scent. The strong smells are due in large part to their high volatility – they tend to evaporate and therefore bind easily to our olfactory receptors – special sensors in the nose which are responsible for our sense of smell.
Bananas are usually picked and exported while unripe and green, and then ripened in ethylene gas-filled rooms on arrival at their destination, increasing the shelf-life of the fruit. Ethylene, H2C=CH2, is a “plant hormone” which promotes ripening and is also produced by the plant itself it is damaged. A peak in the production of ethylene stimulates the ripening process by initiating a number of genetic and chemical processes in the fruit, using messenger molecules known as “ethylene response factors” or ERF’s (Journal of Experimental Botany, 2013, 64, 2499-2510). Unripe bananas contain high amounts of starch, degradation of which is one of the key ripening processes. Enzymes are large protein molecules which speed up, or “catalyse” specific biological processes. They often have distinctive 3-dimensional shapes which allow only very specific molecules to bind to them. The enzyme amylase is able to break down the starch strands by hydrolysing (adding water across) the links between the sugar units, which consist of two glucose molecules joined together (Postharvest Biology and Technology, 2006, 40, 41-47). The result? As ripening occurs, starch is converted to sugars accounting for the much sweeter, less “mealy” taste of ripe banana. Another important ripening process involves pectin, the “glue” which holds plant cells together. Enzymatic degradation of pectin in the cell walls causes the softening of the fruit (another property associated with ripening) as the cells can slide past each other more easily (Pakistan Journal of Botany, 2011, 43, 1501-1506).
Clearly, the most obvious sign of ripening is the colour change from green to yellow. This is due to the degradation of chlorophyll, the green pigment in the peel, and is a consequence of the lower-than-tropical temperatures used in the artificial ripening process. A curious side-effect of this colour change causes ripe bananas to glow blue under UV light, as the intermediates in the metabolism of the chlorophyll which accumulate in the skin fluoresce (emit light) at a wavelength in the ultra-violet. This could be a mechanism to alert animals who see mainly in the UV range to the presence of the ripe fruit, facilitating seed dispersion (Angew. Chem. Int. Ed., 2008, 47, 8954-8957).
3. Isoamyl acetate (banana oil) can be easily synthesised from acetic acid and isoamyl alcohol (3-methyl butan-1-ol) in a process known as a Fischer esterification, shown in Figure 2. Protonation of the acetic acid and consequent attack by the alcohol on the activated carbonyl results in a tetrahedral intermediate. This intermediate then undergoes proton transfer, creating a good leaving group in the form of a water molecule and facilitating the collapse of the transition state. The carbonyl reforms, and ejects the water molecule, giving the isoamyl acetate product after the proton on the carbonyl oxygen is captured by a water molecule, regenerating the H3O+ – the sulphuric acid acts as a catalyst.
Although it is called banana flavouring, a solution of isoamyl acetate in ethanol is often added to foods to confer a pear flavour (and is known as pear oil), highlighting the sensitivity of the nose to small changes in the composition of scented molecules. Interestingly, the same molecule has been identified as a major component of the alarm pheromone of honey bees (Apis mellifera) – a mix of volatile substances released when a bee stings, which provokes a hostile reaction in other bees. This was only discovered because researchers investigating bee stings noticed a sweet smell “reminiscent of banana oil” (Annual Review of Entomology, 1969, 14, 57-80).
As isoamyl acetate is widely used not only in food flavouring, but also in the cosmetic and pharmaceutical industries, much research has been done to discover and develop enzymatic synthesis routes, which permit the resulting products to be labelled as containing “natural” additives (something which is increasingly in demand in the marketplace). Various lipases known to catalyse esterification reactions in organic solvents have been utilised to this end, and good results have been obtained in the laboratory with lipases such as the commercially available Candida antarctica lipase B (Postharvest Biology and Technology, 2006, 40, 41-47). Although most of these methods are currently only practical on a laboratory scale, the many advantages of using enzyme catalysis (environmentally friendliness, high specificity/selectivity and the fact they rarely call for extreme reaction conditions) make them the subject of ongoing research (see for example: P. Znidarsic-Plazl, A. Pohar and I. Plazl, in Icheap-9: 9th International Conference on Chemical and Process Engineering, Pts 1-3, ed. S. Pierucci, Aidic Servizi Srl, Milano, Editon edn., 2009, vol. 17, pp. 1077-1082; S. Torres, A. Pandey and G. R. Castro, in Bananas: Nutrition, Diseases and Trade Issues, Nova Publishers, Editon edn., 2010, pp. 225-244.
Contributors: Anna Lawrence (text, images), Stephanie Harris (editing), Natalie Fey (editing)