The word crystal comes from the Greek kryos (cold) and means super-cooled water. Nowadays, knowing in detail how water crystallises is crucial in food conservation technology, in meteorology and in climate change. As well as ice – that is, crystallised water – many food products are crystalline, such as salt and sugar, and many others contain crystals, like ice cream, butter and chocolate.
Can you imagine why the quality of ice cream depends on the shape and size of the ice crystals? Do you know why the exquisiteness of chocolate depends on how it crystallises? Do you know what the difference is between brown sugar and white sugar?
The poster you have just been reading could not be more crystallographic. Snowflakes are actually crystallised water, and ice cream is, to a high percentage, just that – crystallised water – but its quality depends on the size and the form of those ice crystals.
Moreover, in this International Year of Crystallography this is the perfect topic, because precisely one of the milestones that we are celebrating is that four hundred years ago the scientist Johannes Kepler not only discovered the hexagonal symmetry of snowflakes but also came up with an explanation not far from the reality. He wrote it in a booklet only a few pages long as a Christmas present to his benefactor Johann Matthäus Wacker von Wackenfels in 1611. It had the short title: “De nive sexangula”.
Representation of the compact stacking of spheres from Kepler’s “Mathematici Strena seu de Nive Sexangula”.
Today wonderful photographs of ice crystals are attained with optical microscopes. One of the best photographers of ice flakes is Kenneth G. Libbrecht, who took the three photos you can enjoy here, and many more on his webpage.
|Symmetry of order 6 or hexagonal symmetry: if we rotate this ice crystal 60 degrees without you knowing, when you look back at it you would not be able to distinguish that we have done so. The same would happen if we turn it 120º, 180º, 240º, 300º or, obviously, 360º (a complete revolution, leaving it as it was). We say that the crystal has “rotational symmetry of order 6” or “hexagonal symmetry”.|
The snowflakes that fall on our shoulders all have hexagonal shapes, that is, with symmetry 6, as Kepler recognised, because they belong to the most stable polymorph in environmental conditions, the hexagonal structure. Up to twelve different structures of crystallised water, of ice, are known. But the snowflakes that fall on our shoulders and our head are of the hexagonal variety. They adopt such a variety of shapes that it is said that no two snowflakes are alike. But they all have something in common: the hexagonal plates or sheets, the hollow hexagonal prisms, and the so-called dendritic crystals, which take the shape of branches or stars, all crystallise following a hexagonal pattern.
Snowflakes are formed from water vapour in the clouds when the temperature is below 0ºC. The morphology type of the crystal does not depend on the nature of the nucleus nor on its size, but on the humidity and, especially, the temperature of the air in which they form. Among the most common classifications is the “International Commission on Snow and Ice” which groups together a total of 35 categories in 7 basic types of ice crystals that are shown in the table below:
|Hollow hexagonal prism type ice crystals that form in cirri.|
The ice cream we like so much contains approximately 60% ice crystals along with sugar, micelles of fat and milk proteins, all emulsified with air bubbles.
|The structure of an ice cream at -5ºC. From Dairy Technology, P. Walstra et al. published by
Marcel Dekker, 1999
The texture of ice cream is fundamentally determined by the size and morphology of the ice crystals, which must be controlled during the manufacturing process. During transport and storage, fluctuations in temperature can produce recrystallization of the ice, altering the texture of the ice cream – in other words, the small ice crystals dissolve, increasing the size of the remaining crystals, converting a smooth texture into a hard and unpleasant product.
When an ice cream is not conserved adequately, the initially rounded and tiny-sized ice crystals that do not need chewing convert into larger sized crystals that create an unpleasant texture.
The same happens with all frozen products. The crystallization and study of the properties of crystallized water is one of the most important lines of research in crystallography.