Crystals: colours and flavours

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Crystals provide flavour and colour to our daily world. An important part of the colours that we see around us is due to the optical properties of certain crystals, whether through absorption in the case of pigments, diffraction in the case of structural colours, or other more complex phenomena such as in gems. An important part of what we eat is solid, and solids tend to crystallize. Among our daily food there are many crystals and many more for which crystallography brings methods that facilitate their production or improve their organoleptic or nutritional properties.

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The salt that we consume everyday is sodium chloride crystal. In some cases it is extracted from rocks that were formed by the drying of geologically distant seas, and in other cases by crystallization in salt flats from saltwater rivers or essentially from seawater. The crystallization of salt was the first engineering operation performed by man. Its simple cubic structure was the first to be studied using X-ray diffraction.

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The sugar we consume is soluble crystalline carbohydrate. Its composition depends on the source from which it is obtained, which is normally sucrose, a disaccharide that once ingested divides into fructose and glucose. Sweetening capacity depends on the size of the crystals, because larger crystals dissolve more slowly. Crystallization is a process of purification. In the sugar refineries, they successively crystallize the first sugar, called white for being purer. With the remaining solution, richer in other compounds of the sugar cane, a second, darker type of sugar is crystallized, and finally the third crystallization is the so-called brown or dark sugar.

image009  Cubito de hielo, azul

When water freezes it forms a hexagonal crystalline structure. The ice cream we like so much is made up of approximately 60% ice crystals together with sugar, micelles of fat and milk protein, all emulsified with air bubbles. The ice cream texture is fundamentally determined by the size and morphology of the ice crystals, which must be controlled during the manufacturing process. During transportation and storage, temperature fluctuations can cause the ice to recrystallize, altering the ice cream texture. The same thing happens with all frozen products. The crystallization of water is one the most important lines of research in crystallography.

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A good chocolate, aside from the flavour, should have the appropriate shine, consistency and texture, and should melt in the mouth, not in the hand. These properties depend on the crystallization of the fatty acids of the cacao butter, the principal ingredient of chocolate.

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Pigments are coloured for the selective absorption of light, which depends on the molecular bonds, the crystalline structure and the crystal size. The control of these properties by various industries – cosmetic, paint and coatings, and plastic manufacturing – requires crystallographic research and development. Controlling the forms of crystals is also critical in the cosmetic industry.

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Structural colours, such as those of this peacock feather, some butterfly wings and Moctezuma’s headdress (in the main photo), do not come from pigments, but from the physical interaction of the light with periodically ordered biological nanostructures. The processes of interference that give rise to these “structural colours” are being imitated in nanotechnology laboratories to produce materials with particular optical properties, called photonic crystals.

image019 The Logan Sapphire, from Sri Lanka, weighing 423 carats and with a beautiful blue colour, is a perfect example

The colour of gems can be due both to absorption, as in the case of pigments, and structural colours (for example, opal), but the slight variations in colour that decisively influence the economic value of the jewels are usually linked to more subtle effects such as “colour centres” or electronic transitions in crystalline materials. This is the case with the blue sapphire, which owes its colour to the transmission of electrons between the iron and titanium atoms in its crystalline structure.

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