“Oxygen can make or break a wine.” – Louis Pasteur. Oxygen is an integral part of life and plays an important role in different biological and chemical reactions.
Dissolved oxygen is the free molecular oxygen in solution and is expressed as milligram per litre (mg/L), parts per million (ppm) or percentage saturation (%sat). The extent of oxygen solution into wine during air contact is influenced by temperature, atmospheric pressure and the pH of the wine. Lower temperature and an increased atmospheric pressure favour the solubility of oxygen and higher pH decreases the percentage molecular sulphur dioxide, which limits the influence of oxygen as an anti-oxidant. At standard temperature and atmospheric pressure wine is saturated with oxygen at a dissolved oxygen concentration of 6 mg/L (www.hannainst.com).
The general perception is that oxygen is detrimental for wines, although certain stages of winemaking exist where it can be beneficial, if managed properly. This includes the hyperoxidation of juice, the role of oxygen during the initiation of alcoholic fermentation and the micro-oxygenation of red wines.
The aim of hyperoxidation prior to alcoholic fermentation is to protect the resulting wine against browning or oxidation during the further winemaking processes. It is also stated that it can improve the shelf life of such wines. It comprises the enzymatic oxidation where oxygen reacts with certain phenol groups in the absence of sulphur dioxide to form yellow quinones. The latter compounds react further with oxygen to form brown coloured products, which precipitate and can be removed by racking. Wines made from such juice are consequently protected against further browning. The uncertain potential advantages of such procedure depend on various factors of which the vineyard and cultivar can play a role. Different opinions also exist regarding the influence of such practice on the sensory quality of wine.
Oxygen is essential for the multiplication of yeasts and formation of flavour profiles in the beginning of alcoholic fermentation. Most of the dissolved oxygen results from the crushing of the grapes, pressing of skins and rackings. Depending on the temperature, equipment and the executed processes it can even lead to the saturation level of oxygen at 6 to 9 mg/L oxygen. If insufficient oxygen is present in the juice it can lead to sluggish or stuck fermentations. This is as result of insufficient sterole formation in the yeasts. Sterole formation is essential for yeast propagation and after various generations of yeast propagation a shortage of oxygen may develop for it. Dissolved oxygen levels of 4 to 6 mg/L are required at the beginning of alcoholic fermentation to overcome this problem …