Why a Lemon is Yellow

No, we can’t answer the philosophical question, of why a lemon has a yellow color. But, we can explain what it is that makes a lemon yellow. Hint, it’s molecules, a specific group of molecules called carotenoids.

Colour of lemons is given by carotenoids

Lemons are a type of citrus fruit and like most of their related species, such as oranges and grapefruits, they have a bright colour. Lemons of course, are yellow. Despite all being a different color, the color of most citrus fruits is due to the same group of molecules: carotenoids. These molecules have a unique structure that allows them to reflect and absorb light in such a way that we perceive the products they’re a part of as yellow, orange, or even reddish.

Did you know that tomatoes, which can be yellow as well, are also colored by carotenoids?

It’s a unique mix of carotenoids

Despite a lot of fruits being colored by carotenoids, there still is a wide range of colors out there. This is because there are hundreds of different carotenoids out there. They each have a slightly different structure and thus interact with light a little differently, giving the fruit a different color.

Yellow is a slightly less bright and strong color than say a dark orange. The reason lemons have this relatively moderate color is that they contain a lot of colorless carotenoids. About a quarter of the carotenoids in lemons are colorless. Those that do have a color, β-cryptoxanthin and carotene for instance, give it the light yellow color.

Not all yellows are identical

You may have noticed that not all lemons have the same yellow color. There’s still quite some variation. This is again because of slight nuances in composition of those carotenoids. A little more of one carotenoid compared to another will slightly change the color again. There is such a wide range of colors that can be made by carotenoids, and perceived by our eyes!

Chlorophyll hides color in unripe lemons

Lemons only turn yellow during the ripening process. However, during ripening, no additional yellow carotenoids are formed. Instead, the green color, which overpowers the yellow, is broken down during ripening. This is actually a very common process to occur in many fruits. The green color stems from chlorophyll and is very common in unripe fruits. Only when it breaks down do lemons turn yellow, apples turn red and oranges orange.

The colors of fruits & vegetables are very diverse and all rooted in (somewhat) complex chemistry. Aside from carotenoids, there are a select few groups of molecules that significantly contribute to color in fruits and vegetables.

References

Y. H. Hui, József Barta, M. Pilar Cano, Todd W. Gusek, Jiwan S. Sidhu, Nirmal K. Sinha, Handbook of Fruits and Fruit Processing, John Wiley & Sons, 28 Feb 2008, p. 300, link