Blog 135

Distillation Part 2 - Pot & Column Stills


Ben, is a director of Bar135 in Bristol.

Pot Still

This is sometimes referred to as an alembic or alembic still. It looks like a large copper kettle and is heated by direct heat. The vapours collect in the head and are led off through a narrow tube at the top, called the swan’s neck or lyne arm, from where they go to the condenser. Here they are liquefied. Such a still is not very heat-efficient, but it produces spirits with character. Pot still distillation is a small batch process, as the still must be refilled each occasion, and because of the time and labour intensity involved, this method can be much more expensive than using the continuous still.

Redistilling, often several times, is necessary to achieve the appropriate alcohol level. Most spirits made with a pot still are double distilled, but sometimes it is done in three or even four stages (e.g. Irish whiskey, distilled three times). Several spirits are produced using the pot still including Cognac, brandy, Scotch malt whisky, Irish whiskey, American Bourbon whiskey and some rums (usually the darker ones). The pot still, with its painstaking thoroughness, produces distillates that retain the character and personality of their source ingredients.

The largest pot still in the world is in the Old Midleton Distillery, Co. Cork Ireland. It has a capacity of 31,618 gallons (approximately 140,000 litres). It is no longer in use.

Copper Pot Still

Separating the poisonous heads and tails

The first distillation, which is called ‘broullis’ in Cognac and ‘low wines’ in Scotland, and which has an alcoholic strength of about 35% ABV, is then redistilled. The first vapours coming off contain a high proportion of volatile poisons, particularly acetaldehyde, which has a boiling-point of 28°C (82°F) and methanol. Passing from the domed head at the top of the boiler, they condense in the worm and are collected in a special receiver. Some of the less volatile substances fall back into the boiler, to be revaporised: the molecules of the different substances shake loose from each other. The volatile poisons are lighter than alcohol, having a lower specific gravity, and can be detected by using a hydrometer. When the hydrometer readings, and the still man’s nose, indicate that purer alcohol is coming over, the still man will switch the stream of water-white liquid coming from the worm into another receiver. This ‘heart’ or potable fraction mainly contains Ethanol (C2 H5 OH) and some flavour components. As the distillation progresses, a rank smell and a rising hydrometer reading will indicate that the poisonous fusel-oils are starting to come over in greater concentration, so the still man again switches the stream back to the first receiver. The tails, which are milky in colour, contains undesirable higher alcoholic compounds like Propandl and Butanol fusel oils, plus an increasing quantity of water. These parts are separated from the fourth part (milky tails) which contains poisons which are heavy. The still man, during this second distillation, separated the poisonous ‘heads’ and ‘tails’, called respectively ‘foreshots’ and ‘feints’ in Scotland, from the good ‘heart’.

The poisons can be extracted from the hearts by further re-distillation, as in Irish whiskey, or by long maturation in wood as with Cognac.

Column Still

Also referred to as the Patent, Column or Coffey Still, the continuous still was invented by Robert Stein in 1820, a relative of the Scotch whisky Haig. It first went into commercial production in Cameronbridge distillery in Fife, Scotland. An Irish Customs Officer based in Dublin named Aeneas Coffey developed the patent still further and his version eventually caught on worldwide. Ironically Aeneas Coffey never received a single order for his still from distillers in Ireland who regarded his still as seriously comprising the craft of whiskey distilling. Basically this still consists of two tall columns, each about sixty feet in height, called the analyser and the rectifier. The alcoholic wash is broken down into its constituent vapours, or analysed, in the analyser, and the vapours are selectively condensed, or rectified, in the rectifier.

Column Still

The analyser and rectifier

The still consists of two vertical columns, the analyser and the rectifier. Steam enters the bottom of the analyser, rises and meets the alcoholic wash, which has heated in the rectifier, descending the column. The alcohol in the wash is vapourised as it encounters the hot steam. It rises and is passed to the bottom of the rectifier. As the hot vapour rises it is cooled by the pipe carrying the cold wash and condenses. This distillation is a continuous process, with the various vapours being condensed and drawn off the still at different alcoholic strengths, according to where in the still the spirit plate is placed. The spirit can have a high degree of alcohol and purity, so only one distillation is needed. The result of using the continuous still is a more neutral spirit than that obtained from a pot still. If the spirit plate is placed to remove a lower-strength spirit, it will have more flavour and character, but will be much harsher. Up to 100 plates may be used in this still during production, but the fewer plates, the more flavour you obtain. Several spirits are made using the continuous still include vodka, grain scotches, light rums and gin.

Now we have a basic understanding of the distillation process we can use this to distinguish between spirits and identify where the different flavours come from.  The flavours aspect is covered by our next post - maturation

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