segunda-feira, 4 de julho de 2016


THE CHEMISTRY OF THE PINEAPPLE by Kiera Watson, Tim Harrison and Natalie Fey - School of Chemistry, University of Bristol, UK
The pineapple (Ananas comosus) is a tropical plant that is a member of the Bromeliaceae family. The name pineapple was originally used in English to describe an organ of the Conifer tree which is now known as a pine cone. The name was also used for the fruit of the pineapple plant due to their resemblance to the original namesake. Pineapples are known by a variety of different names, such as piña, abacaxi or ananas in Spanish, Portuguese and French respectively.

The pineapple plant is native to southern Brazil and Paraguay (see the following page for much more information on Pineapples: J, Morton, Fruits of warm climates, J. F. Morton, Miami, 1987, pg. 18-28), but is now grown all over the world. Costa Rica are top (2013) when it comes to annual production, growing ~2 700 000 tonnes per year, followed by The Philippines and Brazil reported to each produce around 2 500 000 tonnes per year.

The pineapple plant was also transported from its original territory to the Caribbean by native Indians who called it “anana”, which translates to “excellent fruit”. The European explorer Christopher Columbus first encountered this exotic fruit in 1493, while exploring the Caribbean island of Guadeloupe. Columbus brought the sweet fruit back to Europe where it became a symbol of wealth and privilege.

Pineapple fruit and juice

Pineapples make a great and healthy snack (often ingested in juice form, with thanks to the piña colada!) due to their high vitamin C and manganese content. Pineapples also contain a high concentration of bromelain, a mixture of enzymes that are responsible for the tenderisation of the tongue often experienced when pineapples are consumed (more on this later), but also for the medicinal uses of this plant. It is most often extracted from the stem of the pineapple.

The characteristic smell of the pineapple is the ester ethyl butanoate. Esters are molecules which contain the ester functional group (R-COO-R’). Esters often have distinct aromas; those with a small number of carbons tend to smell of typical organic solvents (as often found in glues) whereas those with longer carbon chains have more interesting smells that are often described as similar to those of fruits. The esters shown in figure 1 are displayed in their skeleton format where only the covalent bonds between carbon atoms are shown together with any non-carbon atoms. Hydrogen atoms are not shown as all chemists know that any bonds a carbon forms that are not shown will be to hydrogens. The carbon at the end of the chain has only one bond so it would be attached to three hydrogen e.g. –CH3, also known as a methyl group.

Figure 1: The structures of some esters and their aromas.

It is worth noting that scents can be very subjective such that descriptions for the scent of butyl methanoate range from raspberry to ‘fruity’ and rum.

Esters are synthesised by reaction between a carboxylic acid (RCOOH) and alcohol (ROH). The reaction requires an acid catalyst, such as concentrated sulfuric acid (H2SO4), to produce the ester and water as a by-product. The pineapple ester ethyl butanoate can be synthesised in the lab by reaction of butanoic acid with ethanol (Scheme 1).

Scheme 1: The synthesis of ethyl butanoate (CH3CH2CH2COOCH2CH3) from the acid catalysed reaction of a butanoic acid (CH3CH2CH2COOH) and ethanol (CH3CH2OH).

Many people experience tenderisation of the tongue when they consume fresh pineapple, which is due to a substance known as bromelain. The term bromelain refers to the cysteine proteolytic enzymes found in the pineapple fruit and stem (proteolysis refers to the digestion of proteins). The bromelains extracted from stem and fruit in pineapples are in fact mixtures of different enzymes. Stem bromelain is most commonly used as it is cheap to produce from the stem which is a waste product in commercial pineapple production; it also has a much higher bromelain concentration than other parts of the plant. Bromelain has been used for medicinal purposes. Tinned pineapple does not have the same effect as the enzyme is denatured when the sealed tins of pineapple are heated as part of the production. Denaturing changes the shape of the enzyme and so its active sites are no longer the correct shape for activity.

Bromelain has also found a use in the culinary world, as a meat tenderiser. The enzymes in bromelain are responsible for this effect through the breakdown of peptide bonds in protein structures, such as the collagen in steaks.

The Authors:

Kiera Watson, at the time of writing, is a final year undergraduate student in the School of Chemistry, University of Bristol, UK.

Tim Harrison is the Bristol ChemLabS Director of Outreach, School of Chemistry, University of Bristol, UK.

Natalie Fey is a Lecturer based in the Centre for Computational Chemistry, School of Chemistry, University of Bristol, UK.

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