An old saw among chefs and food marketers proclaims that we eat first with our eyes. Whether we're admiring the meticulous plate composition of a Michelin-starred bistro or basking in the light green of pistachio ice cream, sight whets our appetites as much as smell. This gastronomic quirk presents particular challenges for the purveyors of frozen foods, whose cardboard-wrapped wares lie in stacks behind frosty glass.
If you want to stand out in this frigid carton wilderness, you're going to need great brand recognition ... or a pretty good gimmick. It's said that the human mind is ruled by habit and novelty, so if you want to break customers' death grip on the former, you'd best ramp up the latter, whether that means offering real fruit juice, gluten freedom or a color-changing confection.
Food already changes color without our help, of course. Think of a banana ripening in your countertop fruit bowl or a steak browning as it cooks. Novelty foods that change colors as you mix or eat them might raise these natural processes to a fanciful art, but they tap into the same basic comestible chemistry and food physics. There's cereal that reveals its true hue after it's submerged in milk, as well as toothpastes and cocktails that become transparent at given temperatures or shift hues in acidic or alkaline environments [source: USPTO]. Some foods entertain in other ways, like the ice cream that glows using lick-activated jellyfish proteins [source: Harris].
The topic of color-changing ice cream heated up in July 2014 when Spanish physicist Manuel Linares and colleagues announced Xamaleon, a tutti-frutti-flavored ice cream that changes colors three times when licked. According to Linares, the trick to the treat involves temperature change and the acids in the human mouth. A quick spray of a mysterious substance he calls a "love elixir" speeds up the switch from periwinkle blue to pink and finally purple [source: Yirka].
Creating such a treat requires a keen understanding of what causes color and color changes in food, and a knack for molecular chemistry doesn't hurt, either.
Nature's Food Coloring
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.
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.
All About That Base (and Acid)
To anyone who has used litmus paper or owned a swimming pool, the fact that pH differences can prompt color changes should come as no surprise. But what do acidity and alkalinity have to do with color? The answer, once again, has to do with the molecular structure of pigments.
The term pH stands for "potential of hydrogen" or "power of hydrogen." You can think of pH as a logarithmic scale that describes the abundance or lack of hydrogen ions. Acidic solutions have an excess of hydrogen ions and a pH lower than 7, whereas alkaline solutions, aka bases, have an excess of hydroxide ions and a pH greater than 7.
Because of this, bases tend to yank hydrogen ions off pigments, forcing the molecules into a structural arrangement that alters their absorption patterns and, consequently, their colors. Acidic solutions, with their abundance of hydrogen ions, need no purloined electrons and weakly interact with pigments. Acid-bathed colors, unlike acid-washed jeans, tend to remain unchanged.
Our old friends the anthocyanins are prime examples of pH-controlled pigments. Most anthocyanins appear red in acidic sap but turn blue in alkaline solutions. In a neutral environment, they are violet. Thus, the same pigment that accounts for the red of roses and dahlias can provide the blue of cornflowers [source: Encyclopedia Britannica]. That's much more impressive than those color-changing T-shirts sold in the '90s.
Several patent filings for color-changing foods take advantage of pH's prodigious chromatic powers. One patent describes a "frozen dessert novelty which changes color" via pH alterations. The treat consists of two zones: One contains a low-pH substance colored with a pH-sensitive pigment, and the other contains a high-pH substance, which may or may not contain a pH-sensitive colorant. When the two parts mix through stirring, licking or swirling, the pH shift causes the color to change.
This approach provides one possible (and completely speculative) explanation for Xamaleon ice cream. It's an appealing one, because the color changes involved cover the same spectrum as anthocyanins, which scholars have nicknamed the "vegetable chameleon." Coincidence?
Linares, Xameleon's inventor, admitted to the press that the change takes place due to acids in the human mouth and temperature, which has an effect on the richness of the color of some anthocyanins. It's also possible to prepare colorless solutions containing anthocyanins and activate their color by adding the right chemicals, which could explain the necessary "love elixir" spritz [sources: Heines; Yirka].
Or not. If there's one lesson from all this, it's that chemistry provides too many color-related tricks for us to assume we've gotten the scoop on Linares' secret. But a little armchair chemistry makes for good conversation between licks of tutti-frutti.
Author's Note: How Color-changing Ice Cream Works
Researching this article rekindled my interest in color perception even more than the now infamous "is it blue or is it white?" dress on the Internet. It's a topic that everyone thinks they understand until they start researching it. But it also reawakened an interest in the rich history of pigments, a history dominated as much by happy accident as by careful chemistry, in which monopolies on particular colors could drive fortunes.
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