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.
Researchers have identified that yellow-skinned apple cultivars produce mainly acetic acid-derived esters, while red-skinned cultivars produce mainly butyric acid-derived esters. However, most apple cultivars produce a range of versions of these compounds. The infographic shows four key esters that make significant contributions to apple aroma and flavour. But it’s the varying balance of concentrations of these esters and others in different apple cultivars that determines their exact aroma character.
That, then, covers the basics of why different apples might smell or taste different. But what about the varying sweetness of the cultivars? Some apples taste sweet as honey, while others have a puckering tartness. This property has little to do with the aroma and flavour compounds the apples give off and more to do with the delicate balance of sugars and acids.
Like alcohol, sugar is another word which, in everyday use, means a specific chemical compound. When we say “sugar”, we’re usually talking about sucrose. But as with alcohol, there are various sugar compounds — in fact, sucrose itself is what we call a disaccharide, made up of two sugars bonded together: glucose and fructose. Fructose is the most abundant sugar in most fruits and the dominant sugar in apples. Sucrose and glucose are also found, but in much smaller quantities.
While fructose might be the go-to fruit sugar, the profile of acids in fruits is more diverse. In apples, we’ve already mentioned two of the acids involved in ester formation, but neither is dominant. That place is taken by malic acid, a fact that’s less surprising if you know your Latin: “malic” derives from the Latin word for apple, malum. The origins of the name date to the first isolation of malic acid from apples by Carl Wilhem Scheele in 1785, though the name itself came from fellow chemist Antoine Lavoisier in 1787.
The balance between sugar and acid levels defines whether an apple will taste sweet or tart. We can express this as the ratio of sugar and acid concentrations in a particular apple. The higher the value of the sugar/acid ratio — that is, the more sugar there is in relation to acid — the sweeter the apple’s taste will be. Apple varieties such as Fuji, Gala and Red Delicious can have up to 7 grams of fructose per 100 grams of fruit. More tart varieties, such as Granny Smith, might only contain around 5 grams of fructose per 100 grams of fruit. Apples with higher acid levels are often considered better for cooking.
Absolute values for the sugar/acid ratio for different apple varieties are hard to come by because, even for a single apple, it isn’t constant. The sugar/acid ratio will increase as an apple ripens and the quantity of sugar it contains increases. When stored, apples lose acidity, so over time, their sweetness will increase further.
A final group of compounds that can affect apple flavour are tannins. These are large compounds that add bitterness and astringency to apple flavour, so unsurprisingly, most varieties of apples grown for eating have low tannin levels. However, tannins are important in apple varieties intended for making cider, where they add flavour and depth. There’s more on cider chemistry in this previous infographic.
To conclude, if you also happen to find yourself at an apple festival in the coming weeks, you’ll hopefully be less bamboozled by the different varieties. Or, at least, you now have a rudimentary understanding of the chemical differences underlying their flavour!
References and further reading
- Biochemistry of apple aroma: a review – Miguel Espino-Díaz 1, David Roberto Sepúlveda 1, Gustavo González-Aguilar 2, Guadalupe I Olivas (2016)
- Characterization of ester odorants of apple juice by gas chromatography-olfactometry, quantitative measurements, odour threshold, aroma intensity and electronic nose – Yunwei Niu, Ruolin Wang, Zuobing Xiao, Jiancai Zhu, Xiaoxin Sun, Pinpin Wang (2019)
- Chemometric classification of apple cultivars based on physicochemical properties: raw material selection for processing applications – Maiqi Zhang, Yihao Yin, Yantong Li, Yongli Jiang, Xiaosong Hu and Junjie Yi (2023)
- Factors affecting apple aroma/flavour volatile concentration: a review – Jonathan Dixon & Errol W. Hewett (2000)
