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Issue 95 - Fluid Dynamics

Whisky Magazine Issue 95
April 2011


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Fluid Dynamics

Ian Wisniewski asks what is reflux, and what influence does this have?

?Reflux is a complex process during distillation, which significantly influences the character of the resulting spirit. This is because reflux determines the proportion of lighter and richer flavour compounds within the spirit, and the greater the level of reflux the higher the proportion of lighter flavour compounds. The level of reflux is determined by the size and shape of the stills, and the rate of distillation, whether faster or slower, which is set by each distillery. The level of reflux that occurs, and the influence this has on spirit character, is individual to each distillery.

Reflux begins once the still is heated and flavour compounds within the charge (i.e. liquid in the still) begin turning from a liquid into a vapour form. Different flavour compounds have their own volatility (boiling point). Lighter flavour compounds are generally the most volatile, due to a simpler molecular structure known as ‘short chain,' as they typically comprise up to a couple of linked units. This means that lighter flavour compounds, including certain esters (fruity notes), evaporate at a lower range of temperatures than richer flavour compounds, such as some cereal notes. These have a heavier molecular weight, being either ‘medium chain' (several linked units), or ‘long chain' compounds (10 or more linked units), and so require a higher range of temperatures to evaporate.

Consequently, lighter flavour compounds evaporate first, rising from the boil pot (base of the still) i...

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