Oxygen makes up about 20% of the air we breath and is found everywhere in the winery. In general, we as winemakers are mostly aware of the detrimental effects exposure to oxygen can possibly have on our wines: at best, a dulling of the fruit with a loss of once-present vitality, with VA and sherry-like, aldehydic flaws developing in a worst-case scenario. In fact, it is because of these potentially negative reactions that most winery decisions (ex: processing fruit, racking, bottling, etc.) usually seek to carefully limit or even eliminate a wines’ exposure to oxygen in the first place. However, there is one time when oxygen exposure on the macro level is actually quite beneficial to any wines’ development, and that is during the fermentation itself. When correctly applied, oxygen interacts with both the yeast and the wine/must in such a way that yeast health is improved, fermentations encounter less problems, and the resultant wine quality is often more approachable with fresher aromas and tastes than it had been previously. While the reasons for this are complex, they are indeed accessible. Essentially, two separate elements: the yeast and the reduction-oxidation potential of the wine itself interact to form a symbiotic relationship. This combined system then has specific reactions when exposed to oxygen. By better understanding the quality and timing of these reactions, we as winemakers can hope to take advantage of the positive effects while avoiding the negative ones.
In order to best do this, we will need to be sure that we have a solid working foundation for each of the two elements in the system. So, let’s begin by taking a closer look at the first of these two, the yeast itself.
The yeast cell is like a small balloon and it survives by selectively letting nutrients in and passing waste and by-products out through its skin, or membrane. The more healthy the membrane, the more efficient this transport mechanism will be and the yeast will be better prepared to handle whatever adverse conditions it may find itself in; and at various stages of a fermentation these can potentially be quite varied and challenging.
At the very beginning when the fruit has just been crushed there a great deal of sugar in the must. The presence of this sugar makes the juice thicker/heavier than water and this density actually creates stressful pressures that need to be regulated against if the yeast is to effectively survive. A helpful example would be to imagine how little effort it takes for us humans to take a couple of deep breaths while lounging by the side of a pool. Now try to take the same deep breaths while you are standing in the pool with the water up your neck and you can now see that the same action requires much more effort than before. This is because water is thicker than air and even though what you are doing is usually an easy action you are now having to fight against the pressure of the water and this requires more effort. Well, it is the same for the yeast in the must and the greater the sugar level, the greater the density will be and that translates to added physical stresses that the yeast will have to work against in order to survive. In fact, a must with a high enough °Brix level (≥25°Brix) should actually be thought of as being potentially toxic to the yeast.
Towards the end of fermentation, however, the yeast finds itself in quite a different set of conditions than it started out. At this point we see very little stress from the density of the sugar in the must since most of it has already been consumed. But now, along with a scarcity of nutrients, there is a fair amount of alcohol present. As was the case with sugar, at high enough concentrations alcohol is also toxic to the yeast and can therefore become an antagonizing factor, as well.
So, we can see that at various points in a fermentation there may be times when environmental conditions can come together and make it difficult for the yeast to do its job. It should be noted that the ability of yeast to gracefully work through these adverse conditions will be directly related to its state of health. But, just why is this important? Well, the answer is simple: If the yeast becomes unhealthy and stressed it will lose efficiency and become sluggish, possibly running the risk of stopping altogether. In addition, it can start producing excess Hydrogen Sulfide (H2 S) and often Volatile Acidity (VA), as well. This H2 S and VA will have a negative impact on the must and will begin to mask the fruit flavours and aromas while causing the wine to appear harsh and aggressive on both the palette and the nose. Furthermore, this excess H2 S and VA, if present in large enough quantities can now begin to inhibit and stress the yeast itself (along with the subsequent ML culture, if desired), causing them to make even more of these undesirable compounds and the ugly cycle continues. Finally, if left unchecked, these flaws and their consequences can effectively lower a wine’s final quality.
Therefore, it becomes obvious that it makes sense for winemakers to try to do everything we can in order to keep the yeast as stress-free as possible. So, in addition to the common practices of balancing the must when needed, controlling the fermentation temperatures, and using a comprehensive nutrient schedule, we can now look at how the effects of oxygen can be used to lend a helping hand. When oxygen is present, yeast use it to synthesize fatty acids and sterols. These specific types of chemicals are then used for cell wall maintenance and population growth. This, in turn helps the yeast stay healthy and plentiful throughout the course of fermentation. It should be noted, however, that the yeast need the presence of oxygen to facilitate these processes and that in the absence of it production will stop. In the beginning, this is not a problem as oxygen is indeed present for the yeast both in the saturated form from being dissolved into must during the mechanical processing of the fruit, as well as from surface area exposure at the top of the fermentation vessel. However, once the fermentation starts in earnest, the yeast will have consumed the dissolved oxygen in the must and the top of the vat will be blanketed with CO2, effectively cutting it off from the oxygen in the surrounding air. In short, if the yeast is to receive any more oxygen to help it stay healthy and limit the production of undesired sulfur compounds during the remainder of the fermentation, the winemaker must take measures to add it them selves.
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