Destillation

Field of application

Distillation enables the separation and purification of volatile food products from aqueous blends. Distillation can be used to separate flavours or essential oils, but is mainly used either for the production of potable alcohol or spirits, or for the industrial production of alcohol from agricultural raw materials (e.g. fruit, grain), which can then be used in alcoholic beverages (liquors). Spirituous beverages are regulated by EC Council Regulation (EEC) No 1576/89. They are prepared from the distillate of the alcoholic products of yeast fermentation of preparations of agricultural origin. Distillation normally follows alcoholic fermentation.

Description of techniques, methods and equipment

The process takes place in two basic types of equipment; the pot still and the column still. Stills may be operated singly or in groups. The addition of heat enables the separation of alcohol/aqueous compounds from the initial liquid feed in the still. Condensed aqueous alcohol is removed as a liquid spirit from the head of the still, whilst a residual stream is discharged from the base.

a) Column still

In column distillation, the alcoholic liquid, or beer, enters a distillation tower heated with vapour. In each contacting device (generally trays) an equilibrium is created between the vapour enriched with volatile components and the condensed liquid. Crude alcohol is drawn off from the top of the tower and then rectified through another tower where 95% of the alcohol is separated from higher alcohols. At the bottom of the first tower, an aqueous blend or stillage is drawn off. A condensed water or stillage water contaminated slightly with organics is drawn off at the bottom of the second tower once the alcohol has been dehydrated. 95% alcohol can be turned into anhydrous alcohol by a number of different technologies: azeotropic distillation using a third component, adsorption by molecular sieve or dehydration by a membrane technique. For most alcoholic beverages distilled in a column still, the alcoholic strength of the distillate is only between 60 and 84%.

b) Pot still

The pot still can be operated in a batch or continuous manner. In the former case, a batch of material is charged to the still pot, boiling is initiated, and the vapours are then continuously removed, condensed and collected until their average composition has reached a desired value. When operated in a continuous manner, feed is continuously passed to the still pot with the vapour and liquid portions being continuously removed. The use of distillation in the Food, drink and milk industry can be illustrated by the following two examples; the distillations of whisky and cognac:

c) Scotch Whisky

Distillation units in the production of Scotch Whisky range from simple pot stills to continuous multi-column stills. Energy is introduced in the form of steam to the bottom of the still and selectively volatilises the alcohol and other components from the fermented liquids and predistilled alcohol water mixtures. The volatile components are recirculated within the still to achieve the correct separation, selection and concentration of alcoholic/aqueous compounds for the many varied and desirable congener profiles required by the various types and brands of products. Stills can be operated singly or in series. The volatile components are condensed by heat exchange with water in condensers and removed as a liquid spirit. The residual material known as pot ale, spent lees, stillage or spent wash, is discharged from the bottom of the still. (In column-stills other fractions are also removed such as fusel oils and high feints. The high feints are fed back into the stills and the fusel oils (largely amyl alcohol) are sold as a coproduct.)

d) Cognac

Cognac is obtained by the distillation of white wines harvested in the Controlled Appellation area. The distillation of Cognac is a two-stage process:
• stage one; a first distillate, known as "brouillis" is obtained. This has an alcoholic strength of between 28 to 32% volume.
• stage two; the "brouillis" is returned to the boiler for a second heating, known as "la bonne chauffe". The distillation "heads", “secondes” and "tails" are separated, leaving only the "heart" of the spirit. There is a restriction on the maximum strength of distillation, which must not exceed 72 vol% alcohol.

The time of each stage is about 12 hours.

Distillation is carried out in two 'chauffes' (two separate heatings) in a special Charentais copper pot still comprising a characteristically shaped boiler, heated with a naked flame and topped with a cowl shaped like a turban, an olive or an onion. A Charentais-still often has an energysaving wine reheater. This optional device, in which the heat is provided by the alcohol vapours passing through it, preheats the wine, which is to be distilled in the next cycle. The final day for distillation is March the 31^th of the year following the harvesting. Maturation must be carried out in Limousin or Tronçais oak casks. As with other brandies and matured wine spirits, maturation was aided by the use of oak extracts, but this practice is becoming less common in Cognac. The French regulations prohibit the use of additives in the final product, other than water and sugar or caramel and restrict the minimum strength to 40 vol% alcohol.