Setting the wine to fermentation.
Fermentation vessel, is it a balloon, or a barrel, it must be closed as soon as it is filled with juice; only then can fermentation be prevented from outside influences. The liquid in the vessel must not come into contact with the ambient air. As I mentioned before, the cork itself is not enough for this. Once fermentation had started, it would have been blown up under the pressure of carbon dioxide. I also wouldn't recommend the method, which can still be observed today in backward winemakers, relying on this, that the cork or plug is put on the opening of the vessel, without pressing it. The gas may then leak, but at the same time harmful bacteria can get inside the fermentation vessel.
Rags or cotton wool are also not suitable for this; admittedly they pass gas, but also bacteria, undesirable in fermenting must.
The only permissible, because an effective closure of the fermentation vessel is a hydraulic closure. There are various designs of it, but each can be fixed in a cork or in a rubber spike. The cork or plug closes the opening of the balloon or barrel (when purchasing, you must specify the circumference or diameter) and the gas generated during fermentation can only escape through the fermentation tube.
The fermentation tube is constructed like this, gas or air may leak to the outside, but cannot get in. For this purpose, the tube is filled with water. The overpressure of the fermentation gas pushes a small column of water upwards and the gas can escape.
The function of the hydraulic plastic closure is different. A tube stuck in a stopper or pin widens upward in the shape of a double-walled cylinder open at the top. Water is poured between the walls. Second cylinder, open at the bottom, fits exactly into the space between the two sides and when applied, the edges of its walls are immersed in the water. The gas produced during fermentation rises up through the tube, pushes out the covering cylinder and escapes through the water.
• Only a fermentation vessel with a closure equipped with a fermentation tube allows for a clean and free of harmful bacteria fermentation of the juice.
• Different sizes of glass and plastic hydraulic closures are available.