The Chemistry of Plums & Prunes

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Dried plums, more commonly referred to as prunes, have a reputation for being a good remedy for constipation. This is, in part, due to their high fibre content – but is there actually any proof for their efficacy? And if so, are there additional chemical reasons? This graphic takes a look at the facts, and also finds an unusual connection between prunes and chewing gum.

Plums are in season in the UK at the moment, which is what prompts the topic for today’s graphic. There’s plenty of interesting chemistry pertaining to them before we even move to discuss prunes. As with many fruits, a huge range of volatile compounds are contained in plums, and a number of these have been shown to contribute to their aroma. Benzaldehyde, gamma-decalactone, linalool, and methyl cinnamate are just a small number of the compounds that contribute. Individually, these compounds can have a variety of aromas – it’s the particular combination of them that produces the fruity plum scent.

If you’ve been out picking plums, or even just buying them in the shops, you’ll have notice their skin is often covered by a light, dusty white coating. Around 20% of this coating is made up of long chain alkanes, whilst approximately 48% is made up of long chain alcohols. In both cases, the compounds in question are mainly those containing 29 carbons. This coating serves two purposes for the fruit: it helps to protect it from fungal infection, and also helps prevent it from losing water. Additionally, it can help trap compounds, such as nonanal, which contribute to the flavour of plums.

Other compounds of interest in plums are those contained within their pits. Many stone fruits are cyanogenic – that is, their seeds contain compounds called cyanogenic glycosides, such as amygdalin, that themselves contain cyanide, and can release it when ingested. Apples are another common fruit with amygdalin-containing seeds. Though we don’t commonly eat the seeds of these fruits, it might initially seem concerning to think that they contain poison-releasing compounds. However, it’s another classic case of the ‘dose makes the poison’ mantra that we often bring up. The amounts of cyanide released by a single seed in your stomach are many times below the lethal dose, and don’t cause any harm.

Moving back to the topic at hand, and prunes are very well-known for being a cure for constipation – so much so, in fact, that apparently they’re now often referred to as ‘dried plums’ on packaging instead, by manufacturers who don’t want their products becoming the butt of jokes. A number of studies have been carried out to determine whether prunes are actually an effective cure for constipation; a 2011 study found that they were more effective than psyllium (a seed-based form of fibre), whilst another study in 2008 found that prune intake in subjects suffering from severe constipation was associated with an increase in frequency of bowel movements. So, their reputation as a remedy for constipation certainly seems to hold up – but why do they help?

Part of the reason is down to the high fibre content of prunes: they contain around 6 grams of fibre per 100 grams. However, there’s also a particular compound present in prunes that lends a significant hand to their laxative effect. That compound is sorbitol, and prunes contain almost 15 grams per 100 grams. Sorbitol has a known laxative effect, and you may also have heard of it in its capacity as a sweetener. In fact, it’s often used in sugar-free chewing gum. If you’ve examined a pack of chewing gum closely, you’ll have noticed the warning that ‘excessive consumption can cause a laxative effect. This is often due to sorbitol; chewing gum formulations contain around 30g per 100 grams, or around a gram per chewing gum stick. Other sugar alcohols sometimes used as sweeteners, such as xylitol, can also induce this effect.

There are other compounds found in prunes that are also thought to contribute to the laxative effect of prunes. These include neochlorogenic acids and chlorogenic acids. If you’ve ever noticed a similar effect on your bowel movements after your morning cup of coffee, there could be a connection there too! Both of these classes of compounds are also found in coffee, as Reactions discussed in their recent video on the subject.

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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