Understanding the magic behind color-changing ice cream

A well-worn saying among chefs and food marketers is that we first eat with our eyes. Whether we're admiring a beautifully plated dish at a Michelin-star restaurant or enjoying the soft green shade of pistachio ice cream, our visual senses play as significant a role in our appetite as our sense of smell. This quirk of our senses creates challenges for those who sell frozen foods, with their products often hidden behind frosty glass in unappealing cardboard packaging.

To stand out in this icy market, you'll need a strong brand presence... or a clever gimmick. It's said that the human mind thrives on both routine and novelty, so to break the cycle of the former, you need to elevate the latter. Whether that means offering fresh fruit juice, gluten-free options, or a color-shifting treat, creativity is key.

Color-changing foods are not a new phenomenon. From a banana ripening on your kitchen counter to a steak turning brown as it cooks, nature itself provides these color transformations. Novelty foods that shift hues as you mix or consume them elevate these natural processes into something whimsical, but they rely on the same basic food chemistry and physics. Think of cereals that reveal their true color once submerged in milk, or toothpastes and cocktails that become clear when exposed to certain temperatures or change colors in acidic or alkaline conditions [source: USPTO]. Some foods even dazzle in other ways, like ice cream that glows thanks to jellyfish proteins activated by licking [source: Harris].

The concept of color-changing ice cream gained attention in July 2014 when Spanish physicist Manuel Linares and his team introduced Xamaleon, a tutti-frutti-flavored ice cream that changes color three times when you lick it. According to Linares, the magic happens through temperature changes and the acids in your mouth. A special spray he calls a "love elixir" accelerates the shift from periwinkle blue to pink, and finally to purple [source: Yirka].

Creating this type of treat demands a deep understanding of what triggers color changes in food, along with a strong grasp of molecular chemistry.

Nature's Food Coloring

To grasp why foods change color, it's helpful to first understand why they have color to begin with.

Color appears when visible light interacts with the cones in our eyes, triggering nerve signals that are interpreted by the brain's vision centers. We can only see light within a certain range of wavelengths (from 400 to 700 nanometers, or violet through red), and only if it's reflected or refracted. Light that is absorbed never reaches our eyes, but it alters the colors we perceive by subtracting specific wavelengths from the light that does reach us.

Plants display a wide array of colors thanks to the natural pigments present in their cells. Chlorophyll a, a pigment found in photosynthetic organisms, primarily absorbs violet-blue and red-orange wavelengths, giving it a green appearance unless other pigments mask it. To capture as much energy as possible, plants also contain accessory pigments that absorb wavelengths not captured by chlorophyll a. For instance, chlorophyll b absorbs red-orange and green light. Other food pigments include:

These pigments also contribute to one of the most celebrated color changes in nature: the arrival of autumn. Anthocyanins are present in the sap of red maples year-round, but it's only after the more dominant chlorophyll pigment breaks down that the vibrant reds and purples are revealed.

What determines the colors absorbed by pigments? It all comes down to their molecular structure and composition. For instance, lycopene is an isomer of carotene, meaning it shares the same chemical formula but has a different structure. This structural variation explains its unique absorption pattern.

Conjugating Colors

Let's dive deeper into some of the molecular characteristics that influence color absorption, focusing on the arrangement of molecular bonds and chains.

Atoms connect to form molecules in different ways, but color absorption is particularly tied to covalent bonds, where atoms share electrons. In a single covalent bond, two atoms share one pair of electrons, while a double bond involves two shared pairs. (Can you guess how many pairs are involved in a triple bond?)

Conjugated molecules feature chains of alternating single and multiple bonds. While they’re not the only determining factor, these conjugations significantly influence the colors that plant pigments absorb. Longer chains tend to absorb longer wavelengths like red and orange light [source: NBC].

This connection explains why processes that break these chains or rearrange molecules, like transforming carotene into its isomer lycopene, can change a plant’s color. One such process is a change in the acidity or alkalinity of the environment, measured by pH. Consider sliced apples: they turn brown when phenols and enzymes, usually separated in the cells, mix with oxygen. However, when lemon juice is applied, its acidity alters the enzymes, preventing them from reacting with phenols, keeping the apple fresh [source: Wolke].

Acidity can also indirectly influence plant color. For example, hydrangeas can be blue or pink based on the amount of aluminum in their flowers: a higher aluminum content results in blue petals, while none leads to pink ones. The soil’s acidity plays a role, too, as plants absorb nutrients and substances like aluminum better when the soil pH is between 6 and 6.5. In alkaline soils, hydrangeas bloom pink — a perfect example of how pH affects color [source: Williams].

These processes provide insights into how color changes might occur in novelty foods, but they’re only scratching the surface; let’s dive deeper.

All About That Base (and Acid)

For anyone who has used litmus paper or cared for a swimming pool, the fact that pH variations can trigger color changes shouldn’t come as a surprise. But how do acidity and alkalinity influence color? The answer, once again, lies in the molecular structure of pigments.

The term pH stands for "potential of hydrogen" or "power of hydrogen." You can imagine pH as a logarithmic scale that measures the concentration of hydrogen ions. Acidic solutions have an abundance of hydrogen ions and a pH below 7, while alkaline solutions, or bases, have more hydroxide ions and a pH above 7.

Because of this, bases tend to remove hydrogen ions from pigments, forcing the molecules into new structural arrangements that alter their absorption and, as a result, their color. Acidic solutions, which are already rich in hydrogen ions, don’t need to steal electrons and interact with pigments weakly. Acidic colors, unlike acid-washed jeans, generally stay the same.

Our familiar anthocyanins are perfect examples of pH-sensitive pigments. Most anthocyanins appear red in acidic environments but shift to blue in alkaline ones. In a neutral setting, they are violet. So, the very same pigment that gives roses and dahlias their red hue also provides cornflowers with their blue [source: Encyclopedia Britannica]. This is far more remarkable than those color-changing T-shirts from the '90s.

Several patent applications explore the vibrant potential of color-changing foods, leveraging the remarkable chromatic effects of pH. One such patent outlines a "frozen dessert novelty that shifts color" as a result of pH changes. This treat is designed with two distinct sections: one features a low-pH substance, tinted with a pH-sensitive dye, while the other holds a high-pH substance, possibly with its own colorant. When these parts combine through stirring, licking, or swirling, the resulting pH shift triggers a color transformation.

This method offers one speculative (and entirely theoretical) explanation for the color-changing phenomenon in Xamaleon ice cream. It's an intriguing possibility, especially since the color variations span a range similar to that of anthocyanins, often referred to as the "vegetable chameleon." Could this be more than just a coincidence?

Linares, the creator of Xamaleon, confessed to the media that the color change occurs due to the acids present in the human mouth and the temperature, which influence the depth of color in some anthocyanins. There's also a method to prepare colorless anthocyanin solutions and activate their hues by introducing the right chemicals. This could explain the necessity for the "love elixir" spray [sources: Heines; Yirka].

Or perhaps not. If there's one takeaway from this, it's that chemistry offers so many color-related tricks that we can't be certain we've uncovered Linares' true secret. However, a little speculative chemistry makes for an enjoyable discussion while savoring a tutti-frutti treat.