[Information based on DipWSET D1 material]
I have to say that studying oxidative versus reductive winemaking methods has really put into perspective some of the aromas I pick up when tasting. I distinctly remember sitting around a table at work, tasting through wines with my colleagues, and it was quite obvious one of them was off. I knew it smelled, I knew it tasted funny. But I couldn’t tell you why. And then one (industry veteran) piped up to say that this wine had undergone reductive winemaking and the winemaker took it just a tad too far.
Well, studying this bit, I now understand how he came to that conclusion.
An interesting bit of information my book supplies is this: oxygen itself is not very reactive with many compounds in must or wine, but the reactions it takes part in create products that then go on to react with must and wine compounds. Those reactions are oxidation reactions.
For example, when it comes to our fresh and fruity Sauvignon Blancs and the like, oxygen is not exactly our friend. Sauvignon Blanc’s aroma precursors, in the form of thiols, will break down in the presence of oxygen, eliminating those fresh, fruity aromas typical of the variety. Similarly, oxygen may even create unwanted aromas, like acetaldehyde (created from ethanol’s exposure to oxygen), which gives off nutty, apple aromas. This latter point may not be a negative depending on the wine style being produced, but if we’re thinking of our fresh, fruity Sauvignon Blanc, those tertiary notes are not typically desirable.
Furthermore, white wine, because of its lack of phenolics, is more susceptible to color change—the wine will turn gold and eventually brown if exposed to too much oxygen.
In white wine, some winemakers may choose to hyperoxidize the grape must before fermentation in order to create more stability against oxidation further down the line. Hyperoxidation is also thought to increase the aging potential of white wine as well.
Red wine, on the other hand, includes phenolic compounds that act as antioxidants and, thus, protect from oxidation. In fact, in red winemaking oxygen is essential for the polymerization of tannins (which will soften with the help of oxygen) anthocyanins (color compounds which will become more stable with the help of oxygen).
Oxygen is essential to all winemaking: it helps kick start fermentation and promote healthy yeast populations. When it comes to aging and storage, if wine is kept away from oxygen for too long, this can lead to reductive flavors, which in large amounts can be off-putting and even considered a fault. (This can happen if a bottle closure has low levels of oxygen ingress [for example, some cork conglomerates and screw caps]). Exposure to oxygen over time (for example, with time in barrel) can lead to favorable aromas and flavors, adding tertiary notes and thus depth and complexity to the wine.
So, as we can see, how much oxygen a producer decides to expose his/her/their wine to and when plays a pivotal role in the style and quality of the wine.
Oxygen can be limited by…
- Avoiding headspace (ullage) in storage vessels by either filling that vessel to the top, regularly topping off, or flushing that headspace with inert gas (below)
- Use of inert gases, such as nitrogen, carbon dioxide, and argon to flush oxygen out from machines, pipes, and storage vessels
- SO2 additions
- Use of impermeable containers (stainless steel, concrete)
- Maintain a cool, constant environmental temperature
Oxygen can be increased by…
- Cap management techniques that spray/splash the must or wine (pumpovers, rack and return)
- Use of small wooden barrels (this will increase the ratio of wine to oxygen)
- Increase number of rackings and/or lees stirring during aging process
- Allow ullage without protection of inert gas
- Hyperoxidation or micro-oxygenation
A NOTE ABOUT SULPHUR DIOXIDE (SO2)
Sulphur Dioxide acts as both an anti-oxidant and an anti-microbial. It can be applied in gas, liquid, or solid form, and a small amount is naturally created during the fermentation process.
Timing of SO2 is critical. It’s typically added immediately after harvest and/or reception to the winery (to act as both an anti-oxidant and anti-microbial), but can be added at different stages of the winemaking process.
When SO2 is added to must or wine, it dissolves and some will react with compounds in the liquid—this is called bound SO2 and it is ineffective as either an anti-oxidant or anti-microbial. The stuff that doesn’t react with those compounds stays free, so is called free SO2 and is mostly inactive—but a small portion will exist as molecular SO2 which IS effective as both an antioxidant and antimicrobial.
Greater proportion of free SO2 is in the molecular form at lower pH levels. This means that a greater amount of SO2 needs to be added to musts or wines with high pH in order to enjoy those antioxidant and antimicrobial properties and benefits.
Adding a larger amount of SO2 when grapes are crushed, at the end of MLF, and at bottling is more effective than adding small amounts of SO2 throughout the winemaking process.
How’d I do? Did that all make sense? Anything I forgot or you want to add?
Thanks again for studying with me!
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