Compound Interest: The Chemistry of Bell Peppers

Bell peppers come in a range of hues, from fresh greens to vibrant reds. Chemical pigments are behind these, but what changes to cause peppers to travel through this spectrum of colours? Here we take a look at the compounds behind the colours (as well as some pepper aroma chemistry) – and find that peppers have some extraordinary chemistry to thank for some of their hues.

Peppers start off green, which unsurprisingly is due to the presence of chlorophyll pigments. These are vital for photosynthesis in plants, and actually come in two subtly different forms, chlorophyll a and chlorophyll b. As the pepper ripens, these chlorophyll pigments start to decompose, and other types of pigments start to take their place.

All of the different colours of peppers that follow green are due to the presence of carotenoid pigments. Small amounts of these pigments are present even in green peppers, but their synthesis is increased as the pepper ripens. Yellow peppers owe their colouration primarily to violaxanthin, though a number of other yellow-orange pigments, including lutein and beta-carotene, also play their part. Lutein also contributes to the yellow colour of egg yolks, whereas beta-carotene is well known as the compound behind the bright orange colour of carrots.

The chemistry really starts getting interesting when we get to red peppers. The red colouration is due to the production of the carotenoids capsanthin and capsorubin, both of which are found near-exclusively in red peppers as most other plants are not able to synthesis them. Taking a closer look at what’s going on to create them at a chemical level makes it even more amazing that peppers are able to.

Synthesis of capsanthin and capsorubin, also known as paprika ketones, requires a type of chemical reaction known as a pinacol rearrangement. This reaction usually requires a concentrated acid catalyst, but peppers can carry it out in near-neutral conditions and at room temperature with the help of hitherto unidentified enzymes. That the humble bell pepper can do with these enzymes what chemists require specific lab conditions to achieve is pretty impressive.

Changes in colour aren’t the only effects of pepper ripening. It also affects the compounds that give peppers their aroma. In green peppers, a significant contributor to aroma is 2-methoxy-3-isobutylpyrazine, more commonly referred to as ‘bell pepper pyrazine’. This compound, as the name suggests, has an smell that is very characteristic of green peppers. It has an incredibly low odour threshold, meaning that its smell can be detected below the part per trillion level.

Other compounds are present too, and make minor contributions to the pepper aroma. These include ‘green’-smelling compounds such as cucumber-like and grass-like aldehydes. The concentration of most of these volatile compounds actually falls as the pepper ripens, with a few notable exceptions. The concentrations of both (E)-2-hexenal and (E)-2-hexanol, associated with sweet and fruity aromas, increase with ripening. This helps to explain the subtle difference between the aromas of peppers of different colours.

There’s even more to pepper chemistry once you venture outside of the Capsicum annuum species, of course. Chilli peppers belong to the same genus, and their colour chemistry is due to the same chemicals described here. They have an added spicy kick, however – which you can learn more about here!

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