- The Absinthe Buyers Guide would like to thank the ICBD for producing this article on the distillation of spirits and Absinthe.
For more information about the ICBD, visit their website at:
- Dr. Annie Hill
The International Centre for Brewing and Distilling (ICBD)
Department of Biological Sciences
Heriot Watt University
- Production of Spirits
- Distillation is an essential step in the production of spirits such as whisky, bourbon, brandy, rum, gin, vodka, akvavit, tequila and eau de vie.
The first step in the process is the preparation of an alcohol solution with a strength of 6 12% alcohol by volume (abv), that is to say a liquor in the range covered by strong beers or wines. The key raw material is a source of starch, which can be broken down by enzyme activity to give sugars. These sugars are fermented by yeast to produce alcohol and carbon dioxide.
One source of starch is the grape, which produces fermentable sugars as it ripens and it is the air-borne yeasts on the grape skin, which carry out the fermentation when the grapes are pressed. Such wine can be distilled to make brandy. A second source of starch for alcohol production is cereals, which were traditionally used in northern climes. In the whisky process barley malt provides the enzyme activity, mainly through amylases, which breaks down the starch reserves in the endosperm of the grain. Cheaper sources of unmalted cereals (wheat, maize, barley or rye) can be used together with barley malt to produce a liquor, at about 8% abv, which can be distilled to produce a spirit such as whisky, bourbon and akvavit.
Cane sugar products, specifically molasses and sugar cane juice, are another source of starch, which can be fermented and distilled to produce spirits such as rum. Gin and vodka are made from either cereals or molasses. Tequila is unusual in that the starch source is the sap of the plant Agave tequilana, which is cultivated in Mexico. Eaux de vie use fruit as the raw material, for example calvados is made from apples, kirsch from cherries and slivovitz from plums.
Distillation can be defined as the separation of the constituents of a liquid mixture by partial vaporisation and separate recovery of the vapour and residue. It utilises the fact that alcohol has a higher vapour pressure and lower boiling point than water. If a mixture of alcohol and water is heated to its boiling point the vapour will be richer in alcohol than the liquid. Therefore when the vapour is condensed the liquid collected will have a higher alcohol content than the original mixture. It should be realised that the products of fermentation are not simply alcohol and carbon dioxide but include many other compounds, such as aldehydes, esters and higher alcohols albeit at low levels and these feature in the distillation process.
Distillation can be carried out either as a batch or a continuous process.
Malt whisky production uses batch distillation usually with two copper pot stills (these are onion shaped vessels although each distillery has its own design). The vapour from the first still is condensed to give a mixture with about 23% abv and this is then distilled in the second pot still to give a spirit with an alcohol content of about 70% abv.
A continuous still consists of a number of cylindrical columns fitted with perforated trays and enrichment of vapour in alcohol is obtained by a counter-current flow with liquid descending and vapour rising. The driving force is the injection of steam at the bottom of the column. Continuous stills are more energy efficient than pot stills and can be built to have much higher throughputs. They can be operated for several weeks without a break producing a continuous stream of spirit with an alcohol content as high as 96% abv.
The fundamental difference between the two approaches is that continuous distillation can produce a spirit of higher strength and purity. The flavour compounds in spirit, known as congeners, are more pronounced in spirit distilled at lower levels of alcoholic strength as less separation of the constituents has taken place. Consequently batch distillation is usually used for spirits where there are flavours derived from fermentation, whereas continuous distillation is always used for the production of a more neutral spirit in flavour terms.
A feature of some spirits, such as whisky, bourbon, brandy and rum, is that after distillation they are matured in oak casks for several years, which makes a major contribution to both colour and flavour. Others are prepared from neutral spirit to produce gin, where it is flavoured with botanicals, chiefly juniper, coriander and angelica, and vodka where it undergoes charcoal filtration to give a flavourless spirit.
Absinthe can be made from neutral spirit using either cereals or grapes as the starch source.
The spirit obtains its flavour from herbs, rather in the same way as gin is flavoured with botanicals. After the herbs have soaked in the spirit, a further distillation is carried out. The key herb is wormwood, but secondary flavouring comes from hyssop, fennel, angelica, aniseed and liquorice.
Wormwood, Artemisia absinthium, is a perennial plant with bitter tasting and deeply incised leaves. It is wormwood which gives the spirit its green colour and bitter taste but it is also the ingredient that has caused concern. The reason is that one of the compounds extracted from the herb is a mono-terpene called thujone (Figure 1), which is toxic.
- Figure 1.
- Absinthe was traditionally produced at a high strength, about 75% abv, and was very popular in France in the period 1880 1914. It was however thought to cause addiction and hallucinations and was banned in 1915 with wormwood bearing most of the blame.
However wormwood has been used historically as a medicinal herb for gastric pains and as a cardiac stimulant. It is unlikely that the levels of thujone in absinthe were the cause of serious problems. Thujone is also present in sage and tarragon, and vermouth is flavoured with wormwood. The level of thujone in absinthe during its growth in popularity was probably in the range of 60 90 ppm. The EU currently allows production of absinthe with up to 10 ppm thujone.
Traditionally absinthe was poured in a precise ritual. A tall glass with a shot of green thick absinthe was diluted slowly with water. During this process the absinthe changed to a milky green colour caused by the precipitation of the essential oils of the herbs. The diluting water, perhaps three or four times the volume of the spirit, was dribbled through a lump of sugar held over the glass in a perforated spoon. The drink was referred to as La Fee Verte or the Green Fairy.
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