How vermouth is made

The preparation of the wines on which the vermouth will be based uses processes common to all wine and wine-based products. High molecular weight compounds and trace metals that could compromise clarity are removed from the wine by coagulation and colloid-forming agents such as gelatine, bentonite, and charcoal. Small quantities of potassium ferrocyanide may also be necessary to help remove trace metals.

As is commonplace in all vinification, sulphur dioxide (SO2) may also be added to the wine to inhibit or kill unwanted yeasts and bacteria, and to protect wine from oxidation. After up to several days contact time the blend is decanted and filtered.

Although alcohol is generally added at a later stage, the base wines of some vermouths, particularly in France, may be fortified at this part of the process. The base wine may be aged in wood, sometimes for up to a year, and may also undergo a form of biological aging similar to the flor process the Spanish use in sherry making before botanical flavours are added.

Botanical flavours (herbs, spices, roots and barks) are incorporated into the vermouth using various methods, the most usual being by concentrated extract (a tincture) produced by macerating the botanicals in neutral alcohol, or a distillate (alcoholate) made by distilling the botanicals in alcohol, often after a period of steeping.

During maceration, the most common method, the botanicals are placed in a tank, covered with aqueous alcohol and agitated periodically. This is often performed in rotating tanks and may last for several weeks before the flavoured extract can be drawn off and the botanicals pressed.

Distillation is used to extract the flavour of more volatile substances contained in botanicals such as fruits and St John's Wort, and to exclude compounds with a higher molecular weight which could have detrimental physical or organoleptic properties. The botanicals are distilled in a mixture of neutral alcohol and water, often after a period of maceration.

Some vermouths, most notably Noilly Prat, are flavoured by macerating the botanicals directly in the wine already fortified with alcohol.

Vermouths are usually sweetened, some with added sugar, while others take advantage of the natural sweetness of grapes by checking fermentation with the addition of alcohol, so killing the yeast before it is able to convert all the natural sugars to alcohol. This low-alcohol and high-sugar wine is known as 'mistelle'.

The sugar or mistelle is generally blended into the base wine, which is then mixed with alcohol, water, botanical extracts and caramel (if required) in blending tanks. After homogenisation, the vermouth is allowed to marry for a period, usually several weeks.

It is common to stabilise and clarify the vermouth by refrigerating it for several days at temperatures close to its freezing point (-8°C). This precipitates substances that, if left, may form a deposit if the bottled vermouth is subjected to low temperatures during storage, transport, or contact with ice when served. Finally, the vermouth is ready for bottling.