The Science of Tomato Colour – Yellow vs Red

Fruits and vegetables are probably the most colourful foods on our table. Meats and fish are all pretty similar (white/reddish), carbs are often yellow/brown/white, but fruits and vegetables can be so many different colours. These colours aren’t just pretty, they are also fascinating for scientists to study. Colour molecules are often highly complex and often seem to have nutritional benefits.

Whereas we were used to red tomatoes, nowadays there’s also yellow tomatoes in our supermarkets and market stores (as I also saw when visiting a greenhouse!). It got me wondering what the differences between the two are. Is it just colour molecules, or is there more to it?

From green to yellow or red

During ripening of tomatoes various processes occur simultaneously. The sugar concentration in the tomatoes will increase, but the most apparent one is the change of colour. Tomatoes are green when they start growing and only when they stary ripening is when they start turning red.

The green colour in tomatoes is caused by chlorophyll. The concentration of chlorophyll decreases a lot during ripening. The concentrations of carotenoids (the molecules that will give the tomatoes their colour) on the other hand will increase significantly. In the case of red tomatoes (see more below) the concentration of lycopene increases a lot.

This ripening process is not unique for tomatoes, a lot of other fruits and vegetables have similar ripening processes.

What makes a tomato red?

The colour red of a tomato is caused by mostly one molecule: lycopene. Lycopene belongs to the group of carotenoids. It is a quite strong colour red. It has this red colour thanks to all the double bonds. These bonds can influence light in such a way that only specific wavelengths are reflected.

Apart from lycopene, rd tomatoes also contain quite some β-carotene. This molecule also gives oranges and carrots its orange colour, it has an orange/red hue.

What makes a tomato yellow?

Yellow tomatoes on the other hand have a very different concentration of these colour molecules. They contain far less lycopene and β-carotene than the red variety. In some cases the concentration goes down to 0%, but in most cases there is a reduction of more than 90%! This explains why they’ve lost their red/orange hue.

Nevertheless, yellow tomatoes still contain a lot of carotenoids. Carotenoids are a very large group of molecules, to which also carotene and lycopene belong. However, the yellow tomatoes contain slightly different types of cerotenoids. They seem to contain more lutein, this is a colour molecule which makes fruits and vegetables, yes indeed yellow!

The fact that these colour molecules are all closely related makes it easier for growers to obtain different colours of tomatoes. They don’t have to aim for species that make completely new molecules, instead, they only have to tweak the existing processes.

Red vs. yellow tomatoes

The main difference between red and yellow tomatoes simply is their colour. This also causes their compositions to be slightly different. They contain different molecules, simply because it’s these molecules that change the colour. There are some other differences, for instance in vitamin C content and other minor components. However, the exact differences depend a lot on the exact variety tested.

The same goes for taste. Some claim yellow tomatoes are sweeter, but it will strongly depend which yellow tomato you’re comparing to which red tomato. The same goes for tartness and the related acidity.

Sources

I’ve used at least two scientific articles that have analyzed yellow and red tomatoes (1 and 2). The New York Times also wrote about the differences between the two colours of tomtoes.

The ripening of tomatoes and the concentrations of components during ripening has been investigated by Dutch researchers.