This is specific to white wine making. Between the press-stage and alcoholic fermentation, the juice of the white wine grapes can be clarified. The goal: reduce the amount of suspended solids, produced from grape skins, seeds, and even stems.
This is an optional winemaking technique, as some winemakers may choose to retain all (or a higher percentage of) solids to increase the wine’s texture, astringency, and even add some flavors/aromas. The higher the level of solids, however, the less of the more fruity aromas will be available in the resulting wine. As such, the technique of lees aging is more commonly used on non-aromatic grape varieties, like Chardonnay. Although, some aromatic grape varieties may see some lees aging, just with a smaller percentage of said lees.
Typically, however, when we think of our more aromatic varieties, like Sauvignon Blanc, or the more subtle-fruited varieties, like Pinot Grigio, it’s a more common winemaking technique to clarify the must. NOTE: Some level of solids are needed, as those skins, seeds, stems provide needed nutrients required by yeast for the fermentation process. (See Nutrients Needed.) Do not over-clarify.
Here are the key ways in which to clarify grape must…
1. Sedimentation: (Pictured above.) This is considered the simplest form of clarification, as it occurs naturally over time. Sometimes referred to as “settling,” the suspended solids in the must are simply left to fall via gravity over time. If relying on sedimentation, it’s important the wine is chilled (to around 39°F) to prevent oxidation, spoilage, as well as spontaneous fermentation. Sedimentation will take longer in larger and taller vessels due to their depth. Time for sedimentation can range anywhere between 12 and 24 hours. The juice is then racked off the lees and into a separate fermentation vessel, leaving all solid particles behind.
NOTE: Sedimentation is also used to clarify final wines as well.
2. Flotation: This technique uses gas to create bubbling up through the must. As the bubbles rise within the liquid, they bring the solid pieces up with them which can then be skimmed off the top. This is a quicker technique than sedimentation, though notably more expensive given the equipment needed. The benefit is that the must doesn’t need to be chilled since the process is so quick.
NOTE: If oxygen is the said gas, this technique can double as hyperoxidation—a process that deliberately and speedily introduces oxygen into white wine to oxidize those elements most susceptible to oxygen (turning them brown—but don’t worry, they precipitate during fermentation, so your white wine will turn white again). This is intended to stabilize the wine from post-fermentation oxidation as well as remove bitter compounds from some of those solids. BUT, if not wanting to introduce oxygen into the wine at this early stage, an inert gas can be used for flotation.
ANOTHER NOTE: Fining agents (compounds added that help bind particles together) need to be used for flotation to be successful. (See Filtration Methods for a breakdown on fining agents.)
LAST NOTE: Flotation only works with must NOT wine.
3. Centrifugation: This machine, as the name implies, uses centrifugal force to separate solids and liquids via a rapidly rotating container. It’s a continuous machine (as in you don’t have to do this in batches), so is best suited to high-volume production. The process does expose the must to some oxygen, however since it’s an inclosed container, it can be flushed with inert gas to prevent that.
NOTE: Centrifugation can be used on both must and wine.
4. Clarifying agents: There are several different compounds that can help speed up the rate of sedimentation. Some are more appropriate for must as opposed to wine. For example, pecotlytic enzymes break down pectins (found in plant cell walls). This breakdown speeds up the separation of liquid and solids.
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