We all know that we need air and oxygen to survive (well perhaps some, also known as oxygen thieves, less than others). Take a deep breath… Now hold your breath and read further to see what role oxygen plays in grape juice and certain white wines…
When wine is made the grapes are normally crushed and the juice is then exposed to air and obviously oxygen. This exposure of oxygen leads to a process called oxidation. You probably heard of different styles of wines that are classified as either more reductive or more oxidative. Now, reductive is just the opposite of oxidative, in other words when wine is made reductively oxygen exposure or oxidation is limited, while when a wine is made in a more oxidative style more oxygen was introduced in the process.
When grape juice oxidizes certain compounds in the juice reacts with the oxygen. These are mostly what we call phenolic compounds of which tannins are one group. When this happens the oxygen concentration in the juice, as well as the phenolics levels in the juice, decreases. Phenolics are then oxidised and forms brown compounds called quinones. This process can happen quite quickly in grape juice to which oxygen has been added. (The same reaction takes place when you take a bite of an apple and it becomes brown in a few minutes). We have measured the oxygen levels in fresh grape juice and found that it can decrease from around 5 mg/L to almost zero within a few minutes in certain cases. The reason for this that the oxidation is driven in the juice by so-called oxidation enzymes. You have all heard of enzymes, these are little proteinaceous compounds that enhances a chemical reaction, without them life on earth would be quite boring and extremely slow… Now winemakers do not want too much juice oxidation to take place when they make certain styles of wine (not really the case in red wines, but definitely when it gets to certain whites). We have found that especially levels of the volatile thiols, causing the guava, passion fruit aromas in Sauvignon blanc, which I discussed on this blog a while ago, can decrease with too much juice oxidation. Interesting enough we also found that the compounds causing the green pepper, grassy notes in Sauvignon blanc, called methoxypyrazines, are much less sensitive towards oxidation.
So how do winemakers prevent juice oxidation from ruining their Sauvignon blanc wines? They either exclude oxygen from entering the juice by replacing it in the press or tanks with an inert gas such as carbon dioxide (yes, the stuff we exhale…) or they can add an anti-oxidant, such as sulphur dioxide or sulphur as most people refer to it, to the juice. The sulphur dioxide actually inhibits the oxidation enzymes and changes the brown coloured quinones I mentioned earlier into a colourless form, thereby both preventing and fixing oxidation to a certain extend. In the following picture taken of the same Sauvignon blanc juice the effect of juice oxidation and sulphur dioxide is shown. The first one received no oxygen and sulphur dioxide, number 2 received oxygen and no sulphur dioxide and number 3 received both oxygen and sulphur dioxide. It is clear that where oxygen entered the juice in number 2 it became completely brown, but in number 3 where sulphur dioxide was present the oxygen could not turn the juice brown.
In some other white wines, such as certain styles of Chenin blanc or even Chardonnay winemakers do not care too much about juice oxidation, as different compounds contribute to these wine’s aroma than volatile thiols. However, too much oxidation or reductiveness in wine, like all things in life, is not good and we will look at that next week in part 2.
You can breathe now again…. (-;