If you’re anything like me, it begins as a sweet, shapeless sense of anticipation at the margins of the mind. By the time I am home, it has become more than a gratifying thought, enshrined in ritual and expectation. And yet, how often do we consider the marvel of alchemy that made it possible? The remarkable process that turned simple grape juice into this toothsome wonder? It’s easy to give recognition to the quality of the fruit or the skill of the winemaker, but without the love-hate relationship between yeast and oxygen, the party simply cannot start.
As both catalysts and custodians of fermentation, yeast takes its job seriously. These little powerhouses can start and finish juice fermentation on their own but oxygen helps them do a better job of converting sugars to alcohol and carbon dioxide. It also makes them more resilient, helping them to ferment all the sugars in the must to absolute dryness. There is always some oxygen present in the freshly pressed juice, and the yeast happily mops it up.
Oxygen is essential at all stages of the winemaking process, but how much depends on the stage and the variety and style of wine. Too much oxygen in the finished wine can reduce its shelf life and quality. Whereas lower levels of oxygen can furnish the wine with a much sought-after, earthy, flinty, and mineral expression, as it sits in the cellar. It is a true creator/destroyer.
The dynamic tension that arises from yeast and oxygen’s co-dependence during fermentation can be manipulated by the winemaker depending on what he or she is trying to achieve.
In aromatically intense wine styles, like Sauvignon Blanc, the winemaker works to minimize oxygen in the juice, as he or she wants to protect flavor precursors that lead to fruit intense wines. This allows for the production of much more pungent aromatic wine styles. Even in this instance, the yeast still needs a certain amount of oxygen in the first third of fermentation. Without it all sorts of fault problems can arise due to yeast lacking the necessary population or strength to finish its job: Residual sweetness; vinegar-like volatile acidity notes, and even garlic, cabbage and/or onion notes, to name just a few.
In less aromatic wine styles, like chardonnay, where overt fruit expression is undesired, some winemakers actively expose the juice to oxygen pre-fermentation, to oxidize phenolics. In many instances this allows the wine to age better once in bottle.
After fermentation, it is a different story. In all but the more esoteric styles, oxygen behaves like a jilted lover – facilitating interactions to further the process of decay by oxidation. This is one of the reasons why aromatic wine styles are bottled and put to the market so quickly: Once in bottle flavor and aroma recede with every day that passes. If you like your wines super aromatic, buy your year’s stash as soon as it hits the market, and cellar/store it at cooler temperatures (where possible) to slow down the oxidation process.
In less aromatic styles, winemakers who rely more on winemaking philosophy rather than the varietal expression imparted by the yeast, often like to leave the wine in contact with the gross lees (the dead yeast cells and other particles remaining in a wine after fermentation) to improve both complexity and texture. These yeast lees have the ability to absorb significant amounts of oxygen acting as a natural preservative and protecting the wine as it matures in barrel, or tank. Good old yeast battles away till the bitter end! Less oxygen can mean less need for anti-oxidant type additives like sulphur dioxide and thus (assuming the wine is otherwise well made) a fresher more natural and vital flavor in the finished wine.
Oxygen is a vital component of healthy wine fermentation, but like any symbiotic relationship it is a fine balance. Wines designed to age are best enjoyed at the apex, where the vital characters of youth intersect with the more complex and subtle characters of age. Beyond this ethereal point, the wine – with the aid of its pall-bearer, oxygen – faces a steady decline towards its ultimate resting place: the kitchen sink.