To understand why foods change color, it's useful to know why they have color at all.
Color appears when visible light interacts with the cones in our eyes, sparking nerve signals that the brain's vision centers interpret. We only see light that falls within our perceptual range (wavelengths between 400 and 700 nanometers, or violet through red) and only if it's refracted or reflected. Absorbed light never reaches our eyes, but it affects the colors we perceive by subtracting particular wavelengths from the light that does.
Plants assume a variety of colors because of the natural pigments in their cells. Chlorophyll a, a pigment common in photosynthetic organisms, absorbs mainly violet-blue and red-orange wavelengths and appears green unless masked by other pigments. To drink in as much energy as possible, plants also contain accessory pigments that absorb the spectral ranges that chlorophyll a does not. Chlorophyll b, for example, absorbs red-orange and green light. Other examples of pigments in food include:
- Carotene, part of a group of accessory pigments called carotenoids, gives carrots and sweet potatoes their orange hue and lends dandelions and marigolds their bright yellows.
- Lycopene helps tomatoes, watermelons and rose hips pop with their characteristic reds.
- Anthocyanins partly account for the deep purples of grapes and blueberries.
These pigments also provide one of the most celebrated color-changes in nature: the arrival of autumn. Anthocyanins lurk in the leaf sap of red maples year-round, but it's only after the more dominant chlorophyll pigment decomposes that the purples and reds can shine through.
But what determines which colors these pigments absorb? The answer has to do with their molecular structure and their composition. For example, lycopene is an isomer of carotene, which means it has the same chemical formula but a different structure. This structural difference accounts for its absorption pattern.