The Coffey (or Column) Still is a bit more complicated. Composed of two main parts – the Analyzer (A) and Rectifier (B).
The Wash (1) enters the Rectifier where it is heated. It then is fed into the top of the Analyzer where it is mixed with steam (2) which causes the alcohol in the wash to boil off into vapour. The wash trickles down through plates in the Analyzer and by the time it gets to the bottom, all the alcohol is extracted and only ‘spent wash’ (3) remains which is drained continuously.
As the alcohol boils off the Wash, the alcohol vapour is fed continuously into the Rectifier where it is cooled (by the incoming cold Wash) and thus condenses (7) and is collected (6). At the same time, the less volatile substances are collected (5) and re-fed into the Analyzer with the Wash (to ensure all the alcohol is collected).
So what does the Coffey still offer the Pot still cannot – well purity for one. The collected raw spirit from a pot still usually runs around 60-80% pure. The other 20% is made up of water and flavour compounds (esters and oils). The Coffey Still produces raw spirit at 90%. This normally means that the more robust spirits such as whisky and bourbon are produced in the pot stills specifically for these flavour compounds. The raw spirit is then aged in wood (oak) to eliminate the unpleasant flavours and impart new flavours to the whisky.
Due to the Coffey Stills much purer spirit – it is usually considered lacking in the flavour compounds so important to these spirits. The distillers at Nikka, however, have experimented over time to extract as much flavour as possible from the wash using the Coffey Still. And the proof of their success is there for the tasting. I believe the Nikka Coffey Malt is the biggest seller of the Nikka Range – and is set to become even more popular as they ramp up production for an eagerly awaiting world.