Inert substances (inert = inactive, uninvolved) are substances that are inert and do not participate in certain chemical processes because they do not contain oxygen. These are almost all gases (noble gases), except the very reactive oxygen. In viticulture, for example, these are nitrogen and carbon dioxide, which are often used as protective gases against undesirable oxidation in wine containers. This prevents, for example, the formation of mould (wine faults). So-called inert gas mixtures are produced by burning combustion gases and then contain nitrogen and carbon dioxide instead of oxygen. These mixtures are cheaper than, for example, pure nitrogen, which is, however, the preferred choice.
There are bottling plants which fill the empty space (head space) with inert gas before the bottle is closed in order to exclude oxidation. This is particularly important when using screw caps, because the amount of oxygen in the empty space is much greater here. In the case of the so-called internal sealing closures, such as the cork (whether natural or plastic), sealing under vacuum has also been used for a long time, because this prevents too high a pressure in the head space due to the cork being pushed in (too high a pressure could lead to it being pushed out). However, sealing under vacuum is now also possible with screw caps. In Australia, snow-dropping is also used, in which dry ice (solid carbon dioxide) is added. When the dry ice comes into contact with the wine, it is immediately evaporated, thereby pushing the air out of the bottle neck. Afterwards the bottle is immediately sealed. On this subject, see also oxidation, oxygen management and closures.