Seeing Green

Have you ever wondered why coffee is green?  Most people will assume it’s due to chlorophyll, which is also an assumption made in popular literature (Rao, 2014), but does it make sense?  Chlorophyll is the energy centre of the plant cell converting CO2 into sugar facilitated by exposure to sunlight, but why would a bean buried under several layers of exocarp, mesocarp . . . etc. need this?  It wouldn’t see the light of day for quite some time, and coffee beans prefer darkness to sprout, which made me quite sceptical that this assertion was correct.  Surely someone had measured chlorophyll in green coffee beans?

A literature search would reveal that few studies have investigated (or perhaps reported, as there was nothing to see?) the chlorophyll content of green beans (Clarke, 1985).  Those that I have encountered suggest that the silver skin around the bean contains some chlorophyll[1], but that the bean itself does not, so why then are green coffee beans green?  In truth, raw coffee beans are not always described as green, sometimes they are thought to be yellow, while other times blue.  Chlorophyll a and b absorb light below 500 nm (blue-violet) and above 600 nm (orange-red), resulting in their perceived colour falling between 500-600 nm (green-yellow), and consequently their chemistry only partially explains the colours reported to be found in “green” coffee. 

Although these pigments were observed by Leeuwenhoek[3]  in the 1600’s (Leeuwenhoek, 1951), they would be officially discovered during the initial characterization of chlorogenic and caffeic acid in the early twentieth century, by the scientists that would bestow these molecules with their generic names (Groter, 1908, Groter, 1909, Groter, 1911).  As a scientist it is always quite satisfying to find the point of conception of industry expressions, in this case caffeic acid and chlorogenic acid, which unsurprisingly, but satisfyingly, originate from studying coffee, making their findings directly applicable to the product.  

Groter (REF) demonstrated that these pigments could only be formed from either chlorogenic acid or one of its hydrolysis products, caffeic acid (structure I in the figure above).  Suggesting that the caffeic acid moiety (part of the molecule) was involved in the generation of the pigment.  While pure chlorogenic acid is white, or colourless, pure caffeic acid crystals are a pale yellow, signifying that it is neither the chlorogenic acid nor caffeic acid in their native form, but rather a modified version that contributes to the generation of the vivid green pigmentation described by these authors.

Sharing an etymological origin with chlorine, chlorogenic acid was named after the Greek word khloros (χλωρός) and the suffix ghenos (-γένος) meaning “giving rise to [a] pale yellow-green [colour]”.  Coincidently, chlorine is also strong oxidizing agent that was used at the time, capable of oxidizing chlorogenic acid (step 1 within the figure above).  Due to the toxicity of chlorine gas its use within labs and industry has been systematically tapered off, having been replaced with safer alternatives, including the use of ozone or hydrogen peroxide.  We also no longer smoke in labs or taste our experiments (unless executed with food grade chemicals, instruments and environment).

In nature oxidation can occur through the gradual absorption of oxygen, generally by exposure to air, as commonly occurs in lipid oxidation.  This chemical process can be accelerated by the application of heat, but more often than not oxidative enzymes, e.g. polyphenol oxidase (PPO) or peroxidase, are present in vivo cellular systems, like coffee cells.  These enzymes are capable of efficiently catalysing the conversion of phenols (structure 1, within the figure above) into quinones (structure 2, within the figure above).  Frequently, associated with the defence response of the cell these enzymes are triggered when the integrity of the plant’s cell wall is compromised exposing the cell to air, i.e. PPO in fruit browning or bruising of green coffee during milling.  

Quinones (structure 2, within the figure above), due to their di-carbonyl structure, are very reactive species.  They can cross-link with one another and with surrounding molecules commonly creating brown pigments and on occasion contributing to product astringency, as is seen in tea manufacturing or intermittently found in coffee.  Additionally, the carbonyl structures permit quinones to participate in the Maillard reaction (Step 2, within the figure above).  Carbonyls are the participating functional group within reducing sugars leading to their participation in the Maillard reaction, allowing for quinones to replace sugars within the pathway (step 2 within the figure above)(the Maillard reaction will be covered separately).  An important distinguishing feature of this Maillard reaction pathway, as opposed to others, is that it only proceeds with primary amines.  

Participation of the quinone structure in the Maillard reaction, is accompanied by the cyclization and incorporation of an additional molecule of chlorogenic acid in the generation of a yellow pigment (structure 3 within the figure above).  Contrary to what is commonly stated about the Maillard reaction, it does not require heat.  If conditions are favourable, the Maillard reaction proceeds, resulting in green coffee’s pigments being Maillard reaction products.  Pretty cool, eh?

There has been some debate regarding the sequence of reaction steps;  Does oxidation occur prior to the Maillard reaction? The reaction sequence presented above is that believed by this author, as the structural configuration supports cyclization in the manner that encourages the formation of the polycyclic structure of the pigments, within the figure above.    

The following two steps (step 3 and 4 within the figure above) are oxidation-reduction (REDOX) reactions that modify the structure and consequently the wavelengths the structures absorb; transitioning from yellow-to-green-to-blue in the case of oxidation and back upon introduction of reducing conditions, e.g. introduction of a reducing reagent like ascorbic acid.  Furthermore, step 4 (within the figure above) has been found to be initially (step 4 within the figure above) reversible, with several authors remarking that this pathway becomes permanent over time (REF), likely due to the precipitation of the blue pigment out of solution.