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Chemical element (O) whose Latin name oxygenium is derived from the Greek "oxys" for "sharp, sour". Actually, the name is wrong because oxygen used to be held responsible for the formation of acids. The fact that hydrogen is the cause of acidity was only recognised later. With 21%, the colourless, odourless and tasteless gas forms the atmosphere with nitrogen (78%) and carbon dioxide (0.04%). It is the main component in water and is also present in numerous compounds in the earth's crust. At around 50%, it is the most abundant element on Earth. It is released during photosynthesis in plants, which involves sunlight, water and carbon dioxide. Almost all animals and most plants need oxygen to live. In high concentrations, however, it is toxic to most living things. Oxygen is extremely reactive and forms compounds with most substances very quickly.

In winemaking, it plays a decisive role either through aerobicity (presence) or anaerobicity (absence). Oxygen can have both positive (see aerate) and negative (see oxidation) effects on must and wine. At the beginning of fermentation, oxygen ensures that the yeasts multiply. The smallest amounts dissolved from the must are sufficient for this. Larger quantities are usually harmful. In various stages of winemaking, sulphur compounds in particular and also other substances are used as oxidation-preventing, preserving agents. The carbon dioxide produced during fermentation protects against oxygen, but in the end oxidation must be prevented, otherwise vinegar or a spoiled, oxidised wine will inevitably result. This is best achieved by using closed fermentation tanks made of stainless steel, the empty space of which is filled with inert gas (protective gas) if necessary. Alternatively, in reductive ageing, a low and controlled contact with oxygen is deliberately allowed.

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