As with anything to do with plant biology, the ripening of chillies is a complex process.
In fruit cells, chlorophyll molecules absorb blue and red light which means that green light is reflected (which is why the fruit are green) Also, molecules of anthocyanins and carotin are present in smaller quantities in the fruit and reflect yellow and red / purple respectively . In most plants, chlorophyll is present in massively larger quantities and completely masks other pigments during the active growing stage.
Chlorophyll is unstable and has to be replaced regularly throughout the life of the plant. As the summer turns to autumn, shorter days and cooler temperatures trigger the formation of a water-proof, cork-like substance ( suberin ) in a layer between the cells at the stem of the fruit. This layer progressively reduces the flow of nutrients into the leaf and chlorophyll production ceases.
Shorter days and cooler temperatures also trigger the plant to produce ethylene (a simple gas) which in turn triggers the production of several different enzymes. These enzymes perform a number of functions in the fruit. One of these is the break down of starches into sugars, which is why ripe chillies are sweeter than the green equivalent. Other processes are the break down of pectin (the glue between the fruit cells) making the chillies softer (that ripe feel) and the break down of acids eliminating the sour taste that green fruit can have.
With the reduction in nutrients, the chlorophyll breaks down and this enables the previously controlled carotene and / or anthocyanin pigments to show through. In some chillies, the anthocyanin levels are very high as the result of a secondary action of simple sugars in the presence of sunlight (which is why fruits ripen red, orange or purple on the sunny side). Chillies with increased levels of anthocyanins in their leaves turn red or red-dish purple when the chlorophyll has broken down.
Rather like an artist mixing primary colors on his palette, the presence of the two main groups of
pigments – the flavonoids including anthocyanins give yellow through red to blue and the carotenoids – carotin which give yellow through to orange. These are then split into numerous color pigments such as cyanin (red), pelargonidin (brick red), delphinidin (blue) etc.
Anthocyanin is a fascinating because it yields different colors according to pH of the liquid within the pigment bearing cells. The same pigment molecule gives us blue in Purple Tiger chillies when cell contents are alkaline and red in Jalapenos when the cell contents are acid.
The color can remain almost stable through the life of the chilli or may change markedly. This serves as an indicator to birds whether the chilli is ready to eat or not as birds are not affected by the capsaicin heat
I hope that has helped make clear the process that chillies go through as they ripen and why you see the broad spectrum of colors that are possible in chillies.
Source by Nigel Laubsch