Braeburn, Bramley, Clivia, Dabinett, Gala, Jonagold and Reinette. No, not an excerpt from the class list at your local private school, but a selection of the names of the over 7,500 different cultivars of apples. Each of these cultivars has its distinct flavour and character — and a bit of biochemistry can help us understand why.
Apple festivals are a regular autumn occurrence across England, and it was at one last weekend that I got thinking about the chemical variety in different apple cultivars. I confess to not being a huge apple fan unless they’re juiced, baked in a pie, or in the form of cider. Consequently, I’m aware that the delights of all of these varying cultivars are mostly lost on me. However, I was still curious as to what molecular differences underlie the contrast between sweet and tart apples, and whether there are straightforward chemical explanations for the varied flavours of differently named apples.
Apple flavour develops as apples ripen, and the compounds they contain at different stages of ripeness vary. Before apples are ripe, aldehydes are the dominant compounds. Some of these, such as hexenal, are also found in the smell of fresh-cut grass, so you might find their grassy aromas familiar. While some aldehyde compounds come from the breakdown of fatty acids in the fruit, others form from particular amino acids.
Aldehyde levels drop as apples ripen and as they convert into other compounds. Chief amongst these are alcohols. We’re all familiar with ethanol, which we generally refer to as alcohol, but alcohols are a whole family of compounds. In apples, enzymes convert aldehydes to alcohols through a reaction known by chemists as reduction. Many of these alcohols hang around once apples are ripe, accounting for up to 16% of the compounds given off by the fruit. Their aromas range from sweet to fruity.
The main players when it comes to the smell of fresh, ripe apples are the esters. Like aldehydes, apples form these compounds in a couple of ways. Many esters form from the esterification of alcohols: the alcohols and acids such as acetic and butyric acid link together to form these pleasant-smelling molecules. The breakdown of the branch-chained amino acid, isoleucine, can form branch-chained esters.
This article was originally published by Compound Interest. For read the full version of this article, please visit Compound Interest/Apples.
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