Termochromatic labels in beer bottles

It’s impressive how modern technology can influence our lifestyles in ways we could have never imagined. Day by day activities are often enhanced in ways that are beyond our imagination. Here is an example of that:

It’s quite normal to hang out with some friends and have a beer while talking through the events of the week. Sometimes, however, one does not enjoy his or her beverage because it’s not as cold as beer should be. Most of the times, if you tell the waiter or waitress that your beer is not as cold as it should be, they’ll change it. But every once in a while you come along one that simply touches your beer and says: “Seems quite cold to me”. A beer brand from the Canary Islands, in Spain, has solved that problem, by fitting their labels with what they call a “perfect cold indicator”.

The idea is quite easy: they’ve issued new labels, which allow you to visualize the perfect consuming temperature of the beer, thanks to thermocromic painted labels which change colors. In particular, the brand’s logo, a dog’s head, turns blue when the temperature is perfect.

The brand in question, Tropical, has always bet on innovation as a key factor to making the consumer experience better while improving their product. They often use special offers and campaigns throughout the year to attract customers, and during one of them in the early summer, called “Tropical Cold” they attached one of these thermocromic labels to the beers. Initially it included a little thermometer, that would turn blue when the beer reached the ideal consuming temperature, which is around 6ºC or 42.8 Fahrenheit.

The huge acceptation and good results have led the brand to design a new label which would include the cold indicator. In July 2013 they launched the new format, which replaced the thermometer for a dog (which is their logo) that would turn blue when the temperature is perfect to drink it. This format lasts up until now, and it's really appreciated, since it gives you a sense on when your beer is well served.

New format was introduced in several packaging options: bottles of 75, 33 and 25 centiliters, or 25.4, 11.16 and 8.45 fluid ounces as well as cans of 33 and 50 centiliters or 11.16 and 16.9 fluid ounces.

How does thermocromic paint work?

Thermocromism is the ability of substances to change color when the temperature gets hotter or colder. The word thermocromism comes from the greek words thermos, which means heat, and chroma, which means color. These materials are present in a very big amount of things we use or see daily. These things may vary between mood rings, forehead strip thermometers, coffee mugs that show you when your coffee is hot, or boilers that change color when the water is already heaten up. And now more recently, it has been applied to beer bottles and cans, to show when they’re perfectly cold for consumption. Notice all of the first items I named show when the temperature rises above a certain level, and the beer labels actually turn another color when they get cold enough. This is just a response to the different materials that are in thermocromic paint or paper.

There’s not only one type of thermocromic materials. We can use the incandescense of any metal as simple example to explain what this is all about. When we hear “red hot” referred to iron, we know it’s very hot, but what does it actually mean. If we throw iron into a furnace, we can gradually see how it changes color in various stages: it will go from its original color to grey-black at 600ºC to red hot at 950ºC to yellow hot at 1100ºC and after that it will turn to white hot. This change of color is because of the energy it receives while getting hotter. The atoms in the iron start vibrating as they get excited and they let the energy out again in form of light.

Thermocromic materials though don’t have to do with incandescence. They change color at lower temperature and because of different chemical reactions. There are a lot of natural thermocromic materials but the most widely used are liquid crystals, which constitute most of the computers or cellphone displays, and organic dyes known as leuco dyes.

Liquid Crystals

Liquid crystals are more used when a precise temperature is required, because their behaviour can be controlled, but their colors are limited to a scale. This type of thermocromic material is used for example in strip thermometers. They are solid in some respects and liquid in others. The ones we’re interested in are in a nematic form, where molecules are arranged to point in more or less the same direction. When light shines into nematic liquid crystals, some will reflect back in a type of reflection called iridescence. The incoming light reflects off nearby crystals and add together in a process called interference and this produces the reflection. The crystals in the material can move closer or more apart, depending on the temperature that is acted upon them. This phenomenon will cause the color of the reflected light to be different, always depending on how close the crystals are. This means that liquid crystals can adapt themselves to multiple temperatures showing different colors, all the way through the red-violet spectrum.

Leuco Dyes

Leuco dyes are present in those objects that not need a big precision. They can show simple information such as “hot” or “cold” as opposed to the various information that can be found out from Thermocromic Liquid Crystals (TLCs). For example coffee mugs, that change color when filled with hot content or our beer labels and cans.

The molecules in a leuco dye can acquire two forms (leuco and non-leuco) one of which is colorless and the other one has a designated color. In the labels we are currently discussing, at a normal temperature (25ºC or 298K) the leuco dye is colorless, and when cooled under a certain barrier (which in this case is around 6ºC or 42ºF) in changes color to blue.