Most people probably know that citric acid is the source of a lemon’s sourness and acidity. However, it’s not the only acid found in fruits, or even in lemons. In fact, there are a whole range of different acids, with the particular ones present varying from fruit to fruit. This graphic takes a look at some of the main players and the fruits they’re found in.
Since we’ve already mentioned citric acid, it makes sense to start there with our discussion. It’s well known for being present in lemons and limes, which contain it in the highest amounts, and is also present in other citrus fruits. Perhaps less well known is that it’s also the principle acid in many berries, including strawberries, raspberries, and gooseberries (to name but a few).
Citric acid’s lends a relatively sharp acidity, and is often used independently as a flavouring in foods and drinks. Additionally, it can be used in some limescale removal and cleaning products, as well as in pharmaceuticals and cosmetics to help adjust their acidity. More broadly, it’s also found in most living organisms, including you, as salts of citric acid are part of the citric acid cycle (also called the Krebs cycle) which occurs in all cells.
The other organic acid found in a large number of fruits is malic acid. Apples are probably the fruit in which its presence is best known; in fact, its names comes from the Latin for apple, mālum. It also crops up in fruits with stones, such as cherries, apricots, and peaches. Watermelon’s acidity is relatively low compared to other fruits, but its principle acid is also malic acid. Away from fruits, it’s the main contributor to rhubarb’s tart flavour.
If you’re a fan of the ridiculously sour sweets which come with a warning not to eat too many at once, then you’re a fan of malic acid, as this is the compound used to give the sweets their excessive sourness. It’s also sometimes used to give the vinegar flavour in salt and vinegar crips. Wines, too, contain a fair amount of the compound. Like citric acid, salts of malic acid are found in the cells of most living things as well, as they are involved in some cellular reactions.
The final acid of the three most common organic acids in fruits is tartaric acid. In fact, this is found in comparatively fewer fruits than both citric and malic acid, primarily occurring in grapes along with malic acid. It’s also found in avocados, and in the tamarind fruit, as a principle acid.
Like malic acid, tartaric acid is sometimes used in sour sweets. You’re probably most familiar with it, however, from its presence in wine. The tartaric acid still in the wine contributes to the tartness of the wine, along with malic acid. ‘Wine diamonds’, the particles left lurking at the bottom of a bottle of wine, are mostly composed of potassium bitartrate, a salt of tartaric acid.
Potassium bitartrate is also a significant chemical compound in chemistry history. In 1815, a French chemist, Jean-Baptiste Biot, noted that crystals of the compound could cause rotation of plane-polarised light. Another famous French chemist, Louis Pasteur, continued this work further, and found that the salt had two possible structures which were mirror images of each other – what chemists now refer to as enantiomers, or optical isomers.
Tartaric acid’s role in this discovery is still, in a way, immortalised in one of the terms chemists use in this area. A ‘racemic mixture’ is a mixture of the two mirror images which doesn’t have any effect on plane-polarised light. The word ‘racemic’ comes from the name initially given to a mixture of the two enantiomers of tartaric acid, ‘racemic acid’, which itself comes from the Latin racemus – meaning ‘a bunch of grapes’.
Though the acids we’ve discussed here are the three main acids found in fruits, there are plenty of other acids found in much lower amounts. These include isocitric acid, found in blackberries, oxalic acid, found in low amounts in a number of berries, and quinic acid, found in plums and other stone fruits.
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References & Further Reading
- Natural acids of fruits & vegetables – Hawkins Watts
- On wine, chirality and crystallography – Z Derewenda
- Pasteur & the discovery of molecular chirality – G Totem
- Food – the chemistry of its components – T Coultate