Gases protect and preserve flavour compounds in winemaking
Wine growers and wine connoisseurs know that the quality of a wine is created “on the vine”. It is only the grapes and what has accumulated inside them during the ripening process that can subsequently tickle the taste buds of wine lovers. This includes around a thousand volatile compounds that have so far been identified by oenology – the science and study of wine. The foremost task of vintners is to bottle as many of these flavour-giving substances as possible. In doing so, they have to contend with some strong adversaries though: bacteria, fungi, heat and oxygen all pose a threat to the sensitive flavour compounds. In modern winemaking, gases are one of the methods of choice for keeping these adversaries in check.

Every wine grower is glad of dry and sunny weather at harvest time. The grapes get one last ripening boost and the harvest stays healthy – only in this way can truly great vintages be created. However, a warm autumn also encourages premature fermentation. As soon as the grapes are pulled from the panicles and get slightly squashed in the process, the ubiquitous microorganisms start to attack the must. They multiply particularly quickly in warm temperatures. If the wine grower does not intervene, they will immediately start to convert the fructose into alcohol.

Golden October spells danger
While this conversion – the fermentation process – is what turns the juice of the grapes into wine, it would be premature at this stage, because the must needs some time to allow important constituents to be released from the grape skin. This includes some of the flavour compounds that help give the wine its fruity taste. The tannins, which are essential to the “body” of the wine, and the colour compounds, which give it its shimmering lustre, are also predominantly derived from the grape skin and extracted prior to fermentation.

Colouring bath of skins
To ensure that this subtle process is not interrupted by premature activity on the part of the alcohol-forming yeast cells, many vintners rely on so-called cold maceration. The process was originally developed in Burgundy to help give the not so intensely coloured Pinot Noir a darker density. In most cases, a temperature of approximately five degrees Celsius is aimed for. This is achieved by adding dry ice with a temperature of minus 78 degrees Celsius. The cryogenic carbon dioxide cools the must to the desired temperature and also keeps the ambient air away from it: It changes from the solid state to the gaseous state (sublimation) and rises above the must, remaining just above it as a blanketing CO₂ cloud since it is heavier than air. After a while, fermentation can finally begin. With good red wine, it takes place “on the skins” – the must and grape skins stay together. In the case of white wine, the two are first separated. The must is pressed, and this juice is then clarified, id est turbidities such as leftover particles of grape skin is removed.

Nowadays, with larger batch sizes, the flotation process is predominantly used for this step. Many wineries use nitrogen for this as it does not react with the sensitive contents. Nitrogen is fed into the grape juice inside a pressure tank. At a pressure of five to six bar, it remains absorbed in the liquid, but is then released again as soon as the pressure is reduced. Gas bubbles rise to the surface, taking with them the particles to which they are still adhering. This leaves clear must at the bottom, which can simply be drawn off. The process also works with compressed air, but the oxygen it contains can cause undesirable reactions, oxidation and a loss of flavour.

Tank flooded with gas
During fermentation, the microorganisms themselves produce plenty of carbon dioxide, thus preventing air from getting into the fermentation tanks during this phase. However, before these are filled, as well as a few weeks later when transferring the fermented young wine into the storage tank, the question of how to avoid contact with oxygen arises again. The best way to prevent changes in the flavour and colour of the wine is to use inert gases – nitrogen or argon for red wine and carbon dioxide for white wine. These gases displace the air during the storage, transfer or bottling of wine and thus prevent oxidation by atmospheric oxygen. They are stored in gas tanks or gas cylinders and dosed via pipes and hoses as required. There is another very simple method of tank inerting. This involves manually adding a measured quantity of dry ice to the empty tank. Inside the tank, it sublimates to gaseous carbon dioxide and pushes the air out through the opening at the top of the tank.

The same inert gas processes are used when different wines are blended to make a cuvée, as is the case with the majority of the fine Bordeaux wines for example. These expensive wines are made from the wines of different grape varieties, which are made separately before being mixed at a later stage. Here too, of course, it is desirable to avoid contact with oxygen, and this can be achieved by using inert gases. When the vintner bottles the wine later on by drawing it from stainless steel wine tanks, it is often done in several stages. This leaves a number of half-full tanks, quite often over a longer period of time. Inside these tanks, a layer of the relatively heavy noble gas argon offers the most efficient protection against flavour-destroying oxidation. It is generally used in a nitrogen-argon mixture and can also preserve a wine’s freshness and flavours in bottles that have been opened.