We are in the midst of the fermentations. As mentioned in the last post, white wines are made by fermentations of the juice, whereas red wines in fermented “on the skins” . There are many considerations made prior to getting to this point, some are empirical and some are more practical.
One example is the configuration of the fermenter, or “tank size”. As an aside this includes the choosing of wood vessels too (barrels vs puncheons vs “ovals” etc ). One consideration is the volume vs surface area of the fermenter. Early in my wine career there was the talk of the “1 x 1 ratio” whereas the “fill point” of a red fermenter was the height of the red fruit in a fermenter was equal to its diameter. That was considered to be optimal for the extraction process. White wine fermenters, especially in champagne production were tall and skinny, allowing very efficient settling of the yeasts at the end of the ferment. Open top fermenters, as the name implies a tank with no top, were utilized primarily in fermenting “red burgundies” (pinot noir) as the open top allowed “punchdowns” versus “pumpovers”. Today in addition there is the popular “egg” which is primarily a white wine fermenter made of concrete and stainless steel that is the shape of an egg.
But as is usually the case, there are considerations that need to be made when purchasing these vessels, most driven by the practicalities of logistics and expense. A larger diameter tank can hold far more grapes for a given height and if the winery is producing larger volumes is probably a good call, but note that these larger diameters take up more floor space which means a bigger building, and the cooling of a VERY large diameter tank is not as efficient as smaller diameter. A fermenting “must” produces a LOT of heat, so the “cooling jackets” (an outer skin welded to the tank allowing cooling glycol to circulate) on a large diameter tank are quite some distance from the “core” of the tank. The heat transfer from the core to the skin of the tank is slow so it’s possible for a large diameter tank to really heat up in the middle while the fermentation near the skin of the tank is cool. These considerations also must include the design and capability of the chilling system for the tanks. Additionally, there is quite a large amount of CO2 produced and this must be evacuated efficiently.
Red winemaking is pretty much like making your morning coffee in that since “the good stuff” is in the skins, you must in some method get the skins in contact with the liquid. A fermentation produces a lot of CO2, so much that it will push up the skins from the liquid, a “cap” that is buoyed above the liquid. Obviously, this is preventing good skin contact with the wine, so the two primary methods of getting extraction are again, like making your coffee. One method (pumpover) as the name implies is too use a pump to suck the wine off the bottom of the tank and spray the wine over the skins at the top of the tank. This will release some heat and as the wine sinks back down through the skins it does its extraction. The other method is the “punchdown” whereas (with an open top fermenter) one physically pushes the “cap” ( little bits at a time ) down into the fermenting must, over and over physically immersing the skins in the liquid (think of this as your “French press”) This will also release some heat trapped under and within the cap. In either case the winemaker will have a system by which the timing, frequency and duration of these methods is practiced, depending on the how they wish the extraction to show in the finished wine.
The punchdown method is considered quite gentle relative to the pumpover in addition cannot pull seeds from the bottom of the fermenter where they may be abraded via pumping thereby releasing bitter “seed tannins”. Automated system on tanks for those wineries doing large volumes can have a ring of air injectors that can be programmed to inject forced air into tanks (still primarily used on red burgundies) at pressures that “blow apart” the cap, which then sinks temporarily getting extraction, before being buoyed again. Further there is a technique termed “ rack and return “ whereas the wine in a tank is pumped to another tank, the cap sinks to the bottom and is allowed to “ compress “ under its own weight, thereby extracting itself, and after a period he wine is pumped back over the cap. Usually the cap will be slow to rise so the wine, now over the cap gets a good extraction as the CO2 pushes it back up.
Time and temperature and extraction are the consideration for the reds. Yeasts and bacteria all have a temperature “sweet spot”, and it is generally known that “phenolic extraction” happens early in the process. That is to say it has been shown that “the maximum extraction” of phenolics (the colors and flavors) in red wines occur in the first days of the process, and in fact the concentration of some phenolics decreases over time in the fermenter. A hot solution will generally be capable of “holding” more extraction than a cold solution, yeasts have optimal temperature ranges as do bacteria. The warmer a ferment, the faster it will proceed and as the alcohol levels increase, the more it will extract, but remember seed extraction, stem material, and so forth is bad. Is the varietal a “ big tannic “ varietal or a delicate red ?
Of course it gets more complicated when the fruit is NOT at its optimal condition, which is pretty rare anyway. If the fruit is under-ripe or over-ripe or has some molds or damage ( bird damage is usual ) whereas the fruit already has some spoilage growing before processing, “ things “ get a lot more interesting. In whites the action of some molds can produce compounds ( glucans ) that don’t necessarily affect the appearance or flavors, but the glucan molecules are “ sticky “ and can make filtration before bottling almost impossible. Molds all have a distinctive aroma or flavor and keeping a wine on the skins in the presence of molds can impart undesireable characteristics so it might be a good idea to “ get the wine off the skins “ quickly during a ferment. Spoilage organisms as a result of fruit damage in the vineyard can and will affect the ferments in a negative manner and may change the method the winemaker is trying to utilize “ just to get the ferment to finish “.
White wine ferments are performed cooler with the main considerations to “ protect the varietal character “ and / or promote the unique characteristics brought out by the yeasts. Many wine aromatic compounds are “ volatile “ and overly vigorous ferments can “ blow off “ much of what one is trying to capture. While white wine ferments tend to be friendlier than the red ones, the characteristics one is attempting to achieve are somewhat “ more delicate “. Whether or not to allow or promote the bacterial fermentation during the yeast ferment is one of the main considerations in a “ white ferment “ as the result of a secondary ( bacterial or malo-lactic ) ferment has dramatic influence on the presentation of the finished wine.