Distillation - main principles

This page presents distillation :

Origins of distillation

Essential oil distillationDistillation appeared well before rhum agricole and seems to be known from the 3rd or the 4th millennium B.C and could have originated in Mesopotamia or in Pakistan. Distillation was at first intended to produce essential oils (rose, ...) rather than alcohol. Distillation was conducted using apparatus similar to the on illustrated here.

An aqueous solution was heated and the small droplets condensing on the lid were conveyed towards the peripheral gutter. The solution in the main tank was hence more and more concentrated.

Distillation was later perfected by alchemists searching for the philosopher's stone. They made the first "aqua vitae" (water of life), supposed to bring longevity. From 1000 AD, Abulcasis (Abu al-Qasim al-Zahrawi) an Arab surgeon in Cordoba, distillates wine and later at the 12th century, European alchemists perfected the process. Alcohol is produced in very small quantities and is only use as a medication.
Alcohol kept its medication status until the 15th century. In 1411 tax papers attest the sale of aygardent (burning water) in Gascogne (South west fo France) and the alcohol business did not really develop itself until the 17th century with whisky, vodka and cognac.

Distillation principles

Distillation is a method of separating the constituents of mixture (either a liquid or q gaseous one). It is a physical process and not a chemical reaction using the different boiling temperatures of the constituents to separate them from the others. In the rhum agricole context, we are dealing with alcoholic distillation, a way of separation alcohol from water. Alcohol can be segregated from water thanks to the different boiling temperatures (78,4°C/173°F for ethanol and 100°C/212°F for water). Alcoholic vapours are first to get out of the heated solution. Cooling those vapours creates a solution more concentrated in alcohol than the first one.
Since the solution to be distilled contains other constituents than ethanol and water, the distilled liquid contains some of these too. Some constituents are undesirable (i.e. methanol) others are highly sought as aromatic components which gives rhum its distinctive perfume. Those aromatic components are specific to the way the rhum was produced (fermentation, distillation, ...).




Illustration of an alembic, adapted from a Wikipedia illustration

(a) Curcubit, containing the wine to be heated;
(b) Lid, covering the curcubit;
(c) Tube conveying vapors towrads the condenser ;
(d) Condenser where vapors, cooled by a water flow, condense.

First modern stills were created by Albucasis. Those apparatuses quickly cooled the alcoholic vapours, enabling the operator to retrieve liquid alcohol.

These first units could not separate esters from alcohol. Esters being responsibles for bad tastes, alcoholic beverages used to be highly flavored (anise, juniper, ...). The process was perfected with the use of pot stills where the liquid obtained from the first distillation was re heated and introduced  once again for a second distillation. These two distillations allow to separate the heart (alcohol) from heads (esters, methanol) and tails (water). A well known example of this kind of still is the charantais alembic, used in the French Charente region to produce Cognac :

alambic charentais
Charentais alembic illustration,
Rhum, eaux de vie de canne D Kervegant - 1946

During the 19th century, distillation technics were perfected with the introduction of reflux stills. Those alembics were imported from English speaking islands since those islands' rums were highly regarded with a quality clearly outperforming the quality attained ion the French islands. Those apparels are adapted and called Père Labat alembic even if the priest actually died more than a century ago, without even designing anything close.

alambic pere labat
Père Labat alembic,
Rhum, eaux de vie de canne D Kervegant - 1946

Distillation columns

From the second half of the 19th century, distillation went continuous with the introduction of column still distillation.