Fermenting the Must

Once the fruit has been crushed it is now a stew of grape juice, grape skins, grape seeds, and grape pulp and is called must. The must is placed in a vessel to begin fermentation. Fermentation refers to the chemical process of yeast converting the sugar in the grape juice into alcohol, heat, and carbon dioxide (CO₂).

At no other point in the winemaking process can the winemaker have a bigger effect on the finished product (for good or ill) than during fermentation. A host of factors will interact during the fermentation process to bring about the finished wine, so the winemaker is going to set up the fermentation environment based on their experience and what kind of wine they are trying to make.

Maceration

Maceration is a term that refers to the amount of time the juice is in contact with the skins. This is very important to the wine because the skins are where the wine will get all its color and most its flavor. In fact, most people are surprised the first time the see the juice of grapes such as Cabernet Sauvignon immediately after crush — without skin contact this juice is indistinguishable from Chardonnay or Sauvignon Blanc! They are all a pale yellow color until the dark purple pigments in the Cabernet Sauvignon skins begin to release their colors into the must, rapidly darkening the juice to the dark color that we associate with Cabernet-based wine.

Since all the good stuff comes from maceration it would seem that 'more is better', but this is not always so. Too long on the skins and the wine can be overly tannic and out of balance. Not enough time on the skins and the wine can be simple and light. The trick (or skill) is knowing how long to let the wine sit in contact with the skins before pressing the must. This is accomplished by experience combined with regular tasting of the must (and possibly chemical analysis).

The Effect of Temperature

As a rule of thumb, the hotter the fermentation, the more extraction from the skins is achieved. (Extraction refers to the amount of chemical components that are absorbed by the wine from the skins and other solid materials that will later be removed from the wine through pressing and racking.) Here again, it would seem that 'more is better', but this is not necessarily the case. There is danger in very hot fermentations though (above 90º Fahrenheit); if the must gets above around 94º it can kill the yeast (depending on the strain). Also, there tends to be a higher concentration of volatile acidity in wines that ferment very hot — and this should be avoided. If the fermentation is too cool however, the wine may not absorb enough of the chemical components of the grape skins and other solids, resulting in a wine that is not as deep and complex as it could be.

Issues with Large Volumes

In large wineries where fermentation vessels tend to be quite large there is the potential of too much heat being created by the fermentation process, which could kill the yeast and cause what's know as a stuck fermentation. Often times these stuck fermentation can be re-started, but if they can't the entire batch may need to be thrown out. Large fermentation vessels are often wrapped in a water jacket that pumps coolant around the tank to control the temperature.

Issues with Small Volumes

On the other hand making small batches of wine (like I have done) presents the opposite problem; how to conserve the heat of fermentation to encourage the desired extraction. In my case I have had to wrap my fermentation vessels in fiberglass building insulation so that the heat generated by the fermentation can be kept in the vessel and not dissipated into the cool air around it.

In either case, temperature is something to be watched very closely to ensure that fermentation is conducted within the desired range.

Selecting a Yeast Strain

In the old days the winemaker would put the grapes into a shallow tank, jump on them to crush them, and within several days the wine would start to ferment on its own. This is because the grapes are already coated with wild yeasts from the vineyard that, if left to their own devices, will ferment the wine. Though some winemakers swear by them and say they produce better, more complex flavors it is more reliable to use cultured yeast to bring on fermentation.

There are numerous strains of commercially available yeast, each one imparting a different flavor to the finished wine, so the choice of which strain of yeast to use is one of the choices the winemaker must make based on what kind of wine they want to produce. I have been using a yeast called Premiere Cuveé (Prix de Mousse). In my experience it produces wines with pronounced dried fruits, and a complex character that I like.

Cap Management

When the wine is actively fermenting the CO₂ that is released pushes the grape skins upward and — if allowed to — actually pushing about half of the floating cake of skins out of contact with the juice below. With no skin contact the juice is not extracting any of the chemical compounds it needs from the skins, so the winemaker must somehow bring the skins back into contact with the juice. This is generally accomplished by one of three approaches:

1. Submerged Cap — holding down the skins so they don't float up and stay in contact with the wine
2. Pumping Over — wine from the bottom of the fermentation vessel is pumped over the top of the skins to wet them down and drive them back into the must
3. Punching Down — physically pushing the cap into the wine to re-submerge the skins

I have always used the third method, going out to the garage 2 or 3 times a day and pushing the floating cap of skins down into the wine by hand. It is messy, but it ensures that I get a clear picture of the temperature and relative status of each batch of wine on a constant basis, as well as knowing that the skins are fully mixed back into the wine.