June 3, 2010

The smell of baking and toasting bread: Entity of the Month

Filed under: ChEBI — Duncan Hull @ 7:47 am
Tags: , , , ,

ToastRelease 69 of Chemical Entities of Biological Interest (ChEBI) is now available, with 584,456 total entities, of which 21,369 are fully annotated to three star level. This months Entity of the Month is the smell of bread (baked and toasted), or more precisely 6-acetyl-2,3,4,5-tetrahydropyridine. The text below is reproduced from the ChEBI website where data is available under a Creative Commons license.

Chemistry, like most other fields of human endeavour, has a tremendous capacity for both good and evil. However, arguably one of the best and most delightful reactions in chemistry is the Maillard reaction.

It occurs when amino acids are heated together with sugar and is therefore a prominent reaction when baking bread or brewing beer: many of the reaction products provide the characteristic flavours of these foods, which we all enjoy so much.While the chemical structures and identities of most of the products of this form of “non-enzymatic browning” are only poorly characterised or unknown, our Entity of the Month 6-acetyl-2,3,4,5-tetrahydropyridine (CHEBI:59533) is an exception. It is a well known aromatic compound, which is responsible for the flavour of white bread, popcorn and tortillas and has an extremely low odour threshold, between 0.02 and 0.06 ng l–1[1]. It exists in a tautomeric equilibrium with 6-acetyl-1,2,3,4-tetrahydropyridine, the two forms usually occurring in foods in a 1:2 ratio.

The compound can be synthesized in a simple three-step procedure. In a first step, BOC-protected 2-piperidone is treated with 1-ethoxy-1-lithioethene in a bid to build up the acetyl side-chain. This results in ring opening and the formation of a linear ketone which, after treatment with toluene-p-sulfonic acid, reforms the heterocycle in the form of an ene-carbamate. Treatment of the latter with potassium hydroxide yields the final product [1].

The Maillard Reaction is named after Louis Camille Maillard, a precocious French physiologist, who first described it in the 1910s. Maillard is also known for his contributions towards the diagnosis of kidney disorders.

The image top right shows freshly toasted bread – and the brown colour (the Maillard reaction is a method for non-enzymatic browning) is indicative of the reaction having taken place and is taken from the Wellcome Trust Image Collection


  1. Harrison, T., & Dake, G. (2005). An Expeditious, High-Yielding Construction of the Food Aroma Compounds 6-Acetyl-1,2,3,4-tetrahydropyridine and 2-Acetyl-1-pyrroline The Journal of Organic Chemistry, 70 (26), 10872-10874 DOI: 10.1021/jo051940a

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