Pigments and Tomato Color

I’m going to tell you why a tomato can be green and ripe at the same time.

Plants use pigments for various advantages.  Green is of course the most prevalent color that we see when we look at plants.  The green color we observe is a reflection of light cast at our eyes by the plant pigment chlorophyll.  Chlorophyll is the most prominent pigment in plants and dominates plant tissue so that nearly all non-woody tissue appears green.  Think of a tomato plant.  Except for the flowers which are yellow the entire above-ground plant is green, with leaves and stems and unripened fruit full of chlorophyll.  Chlorophyll appears green to the human eye because the rest of the light spectrum is absorbed as an energy source (in particular the red and blue spectra).  Plants use the energy they capture from red and blue light to split water which begins a complex carbohydrate-forming process called photosynthesis.  Photosynthesis is the most important chemical reaction to life on earth and has the magical power of turning gaseous carbon dioxide into solid plant matter.  Think of a wooden chair.  Nearly all the material in the wooden chair was constructed from gaseous carbon dioxide, literally being formed from thin air.

Chlorophyll is the most dominant plant pigment but it is not the only one.  Plants present other colors to  us.  Two examples would be the red or pink in a red onion and a red, ripe tomato.  Although both of these pigments appear red to us they are actually quite different and serve different functions for the plant.  The red color of the onion is from a pigment called anthocyanin.  Anthocyanin is water-soluble and is used by many, many different plants to protect against solar damage (injury from the sun).  Anthocyanin has certain health benefits for our diets and imparts different flavors.  Another example of a similar, but unrelated, type of pigment is found in beet roots.  Anyone who boils beets knows how quickly the color of the beet will mix with the boiling water.  That is because the betacyanin pigment in the beet is highly water soluble.

The tomato is different.  Inside of most tomato fruit are little packets of oil called chromoplasts.  When the tomato is young and green the chromoplast, or plastid, is immature and lacks color (in fact it might actually be a photosynthetic chloroplast to start).  But when the fruit begins to mature the plastids’ pigments mature and become colorful, often red.  These plastids are found in the fruit packed in oil and are oil soluble.  Think of making a tomato sauce and how well olive oil mixes with the sauce.  Now think of pouring oil into a pot of boiling beets.  The oil will sit on top of the purple water and not mix with the pigment of the boiling beets.

Chromoplast pigments are functionally different from anthocyanin pigments.  Instead of protecting the plant from harm (from the sun, for example) the pigment inside a ripening tomato fruit is used to attract animals in order to spread the plants’ seeds around.  The oil-based nature of a plastid is a result of their complexity compared to simpler structures like anthocyanin pigments.  Not all fruit have oil-based pigments (many actually use anthocyanin), but tomatoes do.

Color is also a strong attractant to human beings and we get excited about new and unusual colors.  People are often surprised when they encounter heirloom tomatoes for the first time because they are not used to seeing tomatoes that are not red in color.  Its fun to know that ripe tomatoes can come in a spectrum of color: white, pink, yellow, brown, purple, even green.  Tomato color is determined by the blending of pigments in the skin and in the flesh.  Think of a red tomato.  It’s skin is actually yellow, not clear.  And it’s flesh is pink, not red.  Laying the yellow skin over the pink flesh gives you a red color.  Pink tomatoes on the other hand have a clear skin over a pink flesh.  This becomes more complicated as you get into varieties like Striped German which has multiple colors in it’s flesh and variegated skin (clear and pigmented).

Now, to wrap up.  A ripening tomatoes chlorophyll breaks down as it ripens and chromoplasts take over (giving the plant color).  Some chromoplasts are themselves green in color, or yellowish/green.  So when a tomato that has these green-chromoplast genes begins to ripen it’s flesh softens (like any other ripe tomato) and it’s green color lightens as the dark chloroplasts give way to lighter chromoplasts.  Although the tomato is not brightly colored we can identify that it’s ripe by it’s softness and by light shades of yellow or pink mixed with the green color.


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