Here we go.
Have you ever wondered why coffee tastes bitter? Have you, like me, gone looking for an answer, finding that caffeine is a naturally bitter substance? Has a barista ever told you that overextraction pulls out too much caffeine, which makes it more bitter?
I told my customers this for coming up to four years, believing every word. Until I read an article of Sweet Maria's (I'll post the link at the end). The article was on coffee chemistry and freshness, and contained so much actual chemistry that I needed to spend two days teaching myself bits and pieces just to understand it. Caffeine, it turns out, is in too low a concentration to affect the taste of coffee all that much. Bitterness is, for good or bad, contributed by the presence of Chlorogenic Acid Lactones (CALs from here on in), derived from chologenic acids, and Multiply Hydroxylated Phenylindanes. CALs are the predominant polyphenol in green coffee. So what's a polyphenol? It's a structural class of natural, synthetic or semi-synthetic organic material, characterised by the presence of large multiples of phenol units. Great. So what's a phenol? It's a class of chemical compound directly linked to an aromatic hydrocarbon group. The term aromatic refers to their generally sweet aroma.
When the green beans are roasted, two things happen.
- The phenolic acids (such as CAL) break down into di- and trihydroxybenzenes. These are aromatic compounds which are in turn classed as phenols.
- The acids epimerise (they become epimers...I couldn't find a simple description of what an epimer was, so please guys, let me know) and dehydrate to give various lactones that provide a pleasent, 'coffeelike' bitter quality.
If the roasting continuesthe lactones break down and form 4-vinylcatehol as a highly reactive intermediate. This is a highly energetic molecule which breaks down very quickly into a more stable molecule, in the case Multiply Hydroxylated Phenylindanes. These yield a lingering, harsh bitterness, almost always associated with over-roasted coffee.
That, unfortunately, is all I know. I hope it's been of some help, and if anyone has any further details, feedback, etc, let me know. It's what the e-mail and Twitter accounts are for :) Quiz me, make me go and search for things. Keep me busy. All the contact details are to the right hand side of the window, at the top. Hope to speak soon. Oh, and if you thought this helped, tell your friends.