The Chemistry of Watermelons

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Watermelons are a popular, refreshing summer fruit. There’s also a lot of intriguing chemistry behind them, from the colour of their flesh and the complexity of their aroma, to the tales of exploding watermelons in China, and even the claims that they can have a viagra-like effect. This post takes a look at each of these and the molecules behind them.

Let’s start with the colour. Everyone’s familiar with the vivid pink colouration of watermelon flesh. Of course, there are some varieties of watermelon that don’t actually exhibit this colouration, but it’s the pink-fleshed watermelon that’s by far the most popular. The cause of this colouration is a chemical called lycopene.

Lycopene is a carotenoid pigment, like beta-carotene (the compound that gives carrots their orange colour). It’s also found in tomatoes, and responsible for their red colour. Watermelon actually contains even higher levels of lycopene than tomatoes; it can contain up to an estimated 72 micrograms per gram, whilst the raw tomato maximum is estimated at 42 micrograms per gram. Lycopene has been linked with a range of health benefits, though the scientific evidence for the majority of these is currently conflicted or limited.

Watermelon aroma, on the other hand, has been extensively researched. This is perhaps partly a result of watermelon flavour being notoriously hard to produce artificially, and a number of studies have tried to pin down the exact molecules that contribute to its aroma and taste. The aroma molecules are produced as a consequence of the watermelon being cut into – this releases enzymes from cells in the melon, which in turn aid the oxidation of fatty acids into aroma compounds.

The view on the main components of watermelon aroma has differed over the years. Initial studies suggested that the primary odourants were C6 and C9 alcohols. (Z,Z)-3,6-nonadien-1-ol, in particular, was suggested as the predominant compound contributing to the aroma, and has a smell individually described as ‘watermelon rind-like’. The alcohols were suspected due to their relatively high concentrations; however, they also have higher odour threshold values (the level at which they can be detected by the human nose) than some of the other compounds in the aroma mixture.

It’s now thought that it’s the C6 and C9 aldehydes that make the main contribution to watermelon aroma. It was suggested that previous researchers had failed to realise this, as they collected the compounds over long periods of time, during which enzymatic action converted the aldehydes into the alcohols. Scientists now think that (Z,Z)-3,6-nonadienal is the predominant watermelon odourant, and that several other aldehydes also contribute. One of these is (Z)-3-hexenal, the same compound largely responsible for the smell of fresh-cut grass – helping to explain why many people may find the two aromas similar.

Whilst (Z,Z)-3,6-nonadienal may well be the compound that gives watermelon much of its distinctive aroma and flavour, it’s unfortunately easily broken down, which limits its potential for use as an artificial flavouring. Attempts to produce structurally similar esters that might provide a similar aroma and flavour have also been unsuccessful to date, so it doesn’t look like the problem of substandard watermelon flavourings is one that’s going to go away any time soon.

Watermelon chemistry has made the news for a number of reasons. One of these is its supposed viagra-like effect, with some websites even referring to it as ‘nature’s viagra’. The chemical reasoning was that watermelons contain decent levels of citrulline, which is metabolised into arginine in the body. Arginine is an amino acid involved in the synthesis of nitric oxide in the body, which in turn is associated with the widening of blood vessels. It does this by increasing levels of cyclic guanosine monophosphate (cGMP).

Now, viagra works by breaking down an enzyme (phosphodiesterase type 5) which is responsible for breaking down cGMP. Watermelon doesn’t have any effect on this enzyme, so proclaiming it as a natural alternative to viagra is inaccurate. Additionally, even if watermelon did have an organ specific effect, you’d have to eat a lot of watermelon to significantly boost your levels of arginine, and there’s no evidence that arginine by itself would boost anyone’s ability to gain an erection anyway.

Another more amusing watermelon chemistry story hit the news back in 2011. Farmers in Eastern China were confronted by the comical sounding sight of acres of watermelons spontaneously exploding over a period of a few days. As it turned out, it was most likely due to the misuse of a growth promoting chemical, forchlorfenuron.

Forchlorfenuron is approved for use on kiwi fruits, grapes and raisins in the US, and whilst its use is completely safe, it’s not considered appropriate for use on watermelons. The reason for this is that it leads to somewhat misshapen fruit, and also causes the seeds to turn white. It’s thought the Chinese farmers used too much of the growth chemical on their watermelons, and at the wrong time of the year, during a particularly wet period.  They also used it on a particularly thin-rinded melon, so all of these factors made the ultimate fate of their crop grimly predictable.

Learn more about the chemistry of food and drink with the upcoming Compound Interest book, “Why does asparagus make your wee smell?”, coming in October 2015 and available to pre-order now.

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