Compound Interest: Aroma Chemistry – The Smell of Freshly-Baked Bread

Aroma Chemistry – Fresh-Baked Bread — Click to enlarge

As we took a look at the complex chemistry of bread-making last week, this week it seemed to make perfect sense to look at some of the chemistry that results from putting the end result of that process into the oven! There are a host of compounds that contribute towards baked bread’s aroma; here we take a look at a selection of them, how they are formed, and what they contribute.

Several factors can influence the aroma of your bread, and a couple of them have an effect even before your loaf is put in the oven. The ingredients used are an obvious source of some of the compounds that go on to help form the aroma; however, the volatile compounds found in the flour are really only minor contributors to the end result.

More significant are the compounds generated by the fermentation process. Enzymatic activity in the dough can help produce fermentable sugars that yeast can use to produce a whole range of compounds. Precursors to some of the most important aroma compounds in the bread’s crumb are created as byproducts of the fermentation process. In sour dough breads, the bacteria present can also generate flavour and aroma compounds, such as lactic acid.

It’s the reactions during baking that also make a big contribution to the smell when you remove the bread from the oven. There are essentially two different classes of reaction occurring: Maillard reactions, which occur between sugars and amino acids in the bread, and sugar caramelisation reactions. Both types of reaction help to develop the brown colouration of the bread’s crust; both also help form aroma and flavour compounds, though the Maillard reactions are more significant in this regard. The amino acids present in the dough influence the types of products formed.

Anyway, enough about the reactions that form them; what compounds are we actually talking about here? There’s unsurprisingly a bit of variance in the compounds found in the crust and the crumb. The crust is largely the domain of compounds that give cracker-like, malty aromas. These include the aptly-named compounds maltol and isomaltol, both created as a result of the caramelisation of sugars in the bread. Both compounds are also found in roasted malt, and it is from this that their names are derived. They impart a sweetness to the aroma of bread.

The most significant aroma compound in the crust of wheat bread is 2-acetyl-1-pyrroline (2AP). This compound is formed during Maillard reactions, and imparts a roasted, cracker-like aroma. A similar-looking compound, 2-acetyltetrahydropyridine, is also found in the crust, and formed in a similar manner. Both of these compounds have low odour thresholds (0.6ng/L for 2-acetyltetrahydropyridine, and 0.2ng/L for 2AP) meaning that it doesn’t take a lot of them for their scent to be detectable. A higher yeast content in the bread has been shown to lead to higher levels of 2AP.

The levels of these compounds are much lower in the bread’s crumb than in the crust. Instead, a range of aldehydes resulting from Maillard reactions during baking can be found. Interestingly, amongst the most significant of these are (E)-2-nonenal and (E,Z)-2,6-nonadienal, which are also important odour contributors in cucumbers. More oddly, (E)-2-nonenal is also a compound that’s been linked to the changes in human body odour as we age.

Other compounds found in the crumb include 2,3-butanedione (more commonly known as diacetyl) which lends a buttery note, and methional, which adds a potato-like scent. Methional is found in higher levels in rye breads (in both the crust and crumb) than in wheat breads, as is 3-methylbutanal, a compound found mainly in the crust which offers a malty aroma.

Although we’ve highlighted a selection of the most significant contributors here, there are of course a large number of other compounds found in bread, many of which can also contribute to the aroma to some degree. In addition, every bread is different, and will contain differing amounts of the compounds highlighted here, leading to its own unique aroma. That said, scientists have actually managed to ‘simulate’ the crust aroma of baguettes with just 14 different molecules – though the real thing probably still has an advantage over the simulation, in that you can eat it afterwards!

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