The vast majority of distillers in the cognac region are bouilleurs de cru, distillers who grow their own grapes, rather than the bouilleurs de profession, distillers whose sole purpose is to distil the wines for growers and merchants. There are probably nearly a thousand of the former group who sell their brandies under their own name, and it is these whose reputations have developed over the centuries from their family skills. Of course many of these bouillers de cru also sell their cognacs to a blender, who may keep the cognacs for a few years in casks before selling them in the style of the house or negoçiant who eventually sell the products in the market.
Distillers all operate under the same strict rules. The wines have to be distilled twice to a maximum strength of not greater than 72 percent, in copper stills with heads that are shaped rather like onions. The distillation must be in two boilings, with the second one not exceeding 25 hectolitres of liquid. Distillation starts in late November and must be completed by 31st March the following year.
The basic design of the alembic charentais was perfected by the Dutch in the 17th century and it has not changed significantly since then. The basis of the operation is to gently heat the wine to a very high temperature, which makes it evaporate. The very essence of the wine is collected in the head of the still, escapes through a long curved pipe in the top where it runs through a condenser and returns to a liquid. Control of the process is of vital importance - it must not be allowed to get too hot. In the early days distillation was difficult to control, since wood from the local forests, even coal was used for heating.
Today most stills are heated by natural gas. Oil fire boilers are not allowed as there is risk of contamination from the oil and its residue. Modern distillation has not changed significantly over the years, but control of the process is now much better understood and the various stages at which chemical changes occur can be controlled accurately, thus minimising waste. Whilst there are many standardised practices, the distiller does have a considerable number of choices available that can influence the style and flavour of the final product.
Most distillation in Cognac evolves around the ugni blanc grape and assuming this is a standard factor (it is not always), the first choice is the size and shape of the still and the still head. On the second distillation, the maximum quantity brouillis allowed in the boiler is 25 hectolitres, however 12 and 15 hectolitre stills are indeed quite common, especially amongst the smaller distillers. Many small firms believe, with some justification, that the flavour can be improved when smaller quantities are heated. A crucial factor though, is the design of the head or chapiteau. This collects the vapours from the boiled wine before escaping through the “swans neck”, the col de cygnet. It is then be condensed in cooled coils, the serpentin, where the vapours condense into liquid.
In order to extract the fuller flavours from the wines it is desirable to minimise the levels of rectification. Rectification is when the vapours that have condensed in the still head drip back to be re-boiled, thus neutralising the spirit to a greater extent. A straighter sided head will allow more vapours to pass uninterrupted. A further factor which can influence rectification is the height of the swans neck - the shorter the distance the vapour travels before it is condensed and collected in the barrel, the more flavour will be present in the final eau de vie. It is this process, with lower rectification levels, that the big houses who buy from hundreds of producers for blending, are trying to stop, thus encouraging spirit neutrality to make it easier to blend.
There are many variations allowed in the distillation process which can affect the final quality and taste of the cognacs. One of the areas of modern methods is the argument relating to the use of the chauffe-vin, a simple pot sitting between the boiler and the condenser. Most modern distilleries do use this form of pre-heating the wine before it enters the boiler. The heating is provided by the hot spirit as it passes through pipes which pass through the pre-heater. There is a danger of the wines becoming oxidised if they get too hot and it is sometimes argued that purity is greater if the chauffe-vin is not used.
Perhaps the greatest influence on taste is created by the use of the lees, the mushy solid content of the grapes, which provide the fruitiness from the yeast found in the lees. The yeasts form a number of esters which tend to enrich the final product, and many distillers use this process when trying to develop greater individuality in their cognacs. The practice of distilling on the lees is forbidden by some of the big negoçiants, who are seeking greater neutrality to simplify blending with hundreds of other brandies.
Further variations can occur in the cooking time of the wines, which can be as great as 6 to 12 hours, depending on the size of the still and the heating process. The slower the cooking the more thoroughly the essential qualities of the fruit are extracted. This occurs most effectively in the first distillation, which concentrates the wine approximately 3 to 4 times to produce a brouillis at a strength of around 27-30%. Whilst this is a small variation, it can create a huge difference in the taste! It is in this first distillation that most of the important chemical reactions take place.
The final distillation is required to be between 67 and 72 degrees. The first vapours are too strong and they are set aside from the main part of the distillation, the final vapours are too weak and are added back into the still for re-boiling.