How Color-changing Ice Cream Works

By: Nicholas Gerbis

Conjugating Colors

The color of hydrangeas varies depending on the pH of the soil they’re planted in.
The color of hydrangeas varies depending on the pH of the soil they’re planted in.
© Umon Fukushima/Corbis

Let's take a closer look at some of the structural qualities of molecules that influence color absorption, specifically the arrangement of molecular bonds and chains.

Atoms "stick" to one another to form molecules in various ways, but color absorption is closely linked to covalent bonds, in which atoms share electrons. Single covalent bonds occur when two atoms share one pair of electrons; double bonds involve two shared pairs. (Can you guess how many pairs a triple bond entails?)


Conjugated molecules contain chains of alternating single and multiple bonds. Although they're not the only deciding factor, these conjugations help determine the colors that plant pigments absorb. Longer chains absorb longer wavelengths, such as red and orange light [source: NBC].

Given this relationship, it makes sense that a process that can break these chains, or rearrange molecules like carotene into isomers like lycopene, can affect a plant's color. One way this can occur is through a change in the acidity or alkalinity of the pigment's environment, measured by pH. Take, for example, sliced apples. The apple portions turn brown because two chemicals normally kept apart in their cells, phenols and enzymes, are free to mingle with oxygen. But when you squeeze lemon juice on the apples, its acidity deforms enzymes so they cannot react with phenols, and the fruit stays fresh [source: Wolke].

Acidity can also affect plant color indirectly. Hydrangeas can have a blue or pink hue depending on the amount of aluminum in their flowers: A lot of aluminum produces blue petals, while none causes pink ones. How does soil acidity fit in? Plants can better absorb nutrients and other substances, including aluminum, when soil pH is around 6 to 6.5. Thus, in alkaline soils, hydrangeas blush pink — another example of the power of pH to affect color [source: Williams].

Processes like this offer clues into how color changes might occur in novelty foods, but they're really just the tip of the iceberg; lettuce delve deeper.