The Colour of Whisky

The colour of Whisky ranges from the water-clear Glen Kella to the pitch-black Loch Dhu. The appearance can be an interesting criterion to describe a Whisky.

Often Whisky has a wonderful colour. Learn about the chemical background of how the natural colour of Whisky is created. We also go into the matter of artificial colouring Whisky with caramel as a colouring agent, which is common practice at some distilleries.

The new make distillate is a water-clear liquid. The colour only appears in the spirit during maturation in oak casks. During this time, various factors affect the distillate and thus influence the colour of the Whisky, for example:

• The length of maturation: the longer the spirit is in contact with the wood of the cask, meaning the longer the maturation period, the more intense the colour.
• The type of cask: Usually, the casks are freshly charred oak casks (USA) or used Bourbon or Sherry casks (Scotland). Each type of cask gives a distinctive colour to the Whisky.
• The number of previous maturations: Casks are usually used more than once for maturation. With each maturation, the distillate extracts colour from the wood. After each maturation, there is consequently less colouring available. This means that after each maturation the next Whisky is less intensively coloured.
  
• The size of the cask: The smaller the cask, the more intense the contact of the distillate with the wood. It matures faster and absorbs colour more quickly.
  
• The temperature during maturation: The solubility of the dyes increases with the temperature, which means that the distillate absorbs colour more quickly at higher temperatures.

In some cases, colour is also added to Whisky after maturation, for example with artificial colouring: Some distilleries add tiny amounts of caramel to their products, either to maintain a consistent colour tone or to give their Whisky a more valuable appearance.

The intensity of the colour depends on the concentration of the colourant dissolved in the Whisky. Or in chemical terms: the more colour molecules are dissolved, the more intensive the colour of a Whisky. The colour is created when light hits these colour molecules.

How does the yellow hue come about? Sunlight normally appears white to us. However, it can be broken down into the entire visible colour spectrum from red to blue using suitable equipment (for example a prism - or even in the case of a rainbow). Depending on the energy of the light, a different colour is produced. This energy can be described by the wavelength of the light.

The visible colours are in the range of 400 to 800 nm (nanometres). Light with wavelengths in the range of

• 400 to 490 nm is blue
• 490 to 570 nm is green
• 570 to 600 nm is yellow
• 600 to 610 nm is orange
• 610 to 800 nm is red

If light of all wavelengths is present, we perceive it as white light. If only light in the range around 590 nm is present, we see a yellow light. We have the same effect if part of the blue light is missing, in that case, too, we see yellow light.

If light hits the colour molecules of our Whisky, they consume part of the energy-rich blue light and we see a yellowish liquid. The more colour molecules the light hits, the more blue light is lost and the more intensively the Whisky is coloured. The probability of a colour molecule being hit by the light therefore depends on the quantity of colour molecules. The probability also increases with the distance the light has to travel through the Whisky. The longer this distance, the more intensively the Whisky appears to be coloured.

If we now want to judge the colour of a Whisky, we should consider the following factors, after knowing these basics:

• Light source: the light spectrum of sunlight differs considerably from that of a neon tube or a fireplace. Sunlight can also have different spectra depending on the time of day and the weather.
• Light path: The path of the light through the Whisky depends largely on the vessel in which it is stored. For example, the colour is completely different when viewed in a glass, a 0.7 litre bottle or a miniature bottle.
• Light receptor: Our perception of colour depends largely on our eyes. Depending on age and condition, it varies - sometimes considerably. Of course, all these circumstances do not make it impossible to judge the colour. When using the same vessels, rough judgements are easily possible. Nevertheless, all these influences always lead to an individual judgement of the colour tone.

A photometer is used to measure the colour of liquids. The instrument can measure the intensity of light of different wavelengths so that a complete light spectrum is obtained. The following examples show that the spectra of Whiskies are almost the same. Only in the intensity of the light reduction, there are clear differences. Even rum has a spectrum comparable to that of Whisky. This is hardly surprising, however, as all these spirits derive their colour from wood.

Since all spectra are the same, it is sufficient to know the light reduction at one wavelength to be able to adequately determine the intensity of the colour. For the following overview, this was the wavelength 405 nm. The light reduction is given in /m (absorption coefficient). The measurement result shows the light reduction for a light path of one metre. Based on the absorption coefficient, the colour intensity of the different Whiskies can be read very finely. However, the example of Highland Park also shows that the value for Whisky can vary from bottling to bottling.