Winemaking Processes: Fermentation, Separation, and Finishing

Posted on Jul 27, 2025 in Medicine & Health

Fermentation Management: Stirring and Pumping Over

During the fermentation process, two key techniques are employed to manage the must and extract desired compounds:

Bâtonnage (Stirring)

  • Purpose: To break the cap (the layer of skins and solids that forms on top of the fermenting must) and plunge it back into the liquid mass.
  • Methods: Can be done manually using poles or metal/wooden pistons, or automatically with a hydraulic piston designed to enter and remove the skins according to a set schedule.

Pumping Over (Remontage)

  • Process: Fermenting must is drawn from the bottom of the tank and re-entered at the top, allowing the wine to fall over the cap in a rain-like fashion to spread evenly.
  • Volume: Typically, 50-100% of the tank volume is circulated each day until a density of 1.000 is reached.
  • Aeration: This technique allows for minimal or no aeration (closed circuit) or maximum aeration (open circuit), depending on winemaking goals.

These techniques can be used jointly or separately. Their primary effect is to facilitate the extraction of substances from the grape skins and their dissemination throughout the liquid mass. They also provide oxygen (O2) to activate the yeast and keep the cap wet, which helps prevent the growth of undesirable acetic acid bacteria.

Wine Separation: Devatting and Pressing

After fermentation, the wine is separated from the grape solids through two main processes:

Devatting (Draining and Racking)

This process is crucial for determining the degree of maceration achieved during fermentation. Several types of devatting exist, depending on the desired wine style:

  • Short Devatting: 4-5 days, performed before the end of fermentation, typically for young red wines.
  • Average Devatting: 6-10 days, for young, balanced red wines.
  • Long Devatting: 2-3 weeks, completed after alcoholic fermentation, for wines intended for aging.

Devatting is generally done in two ways:

  1. Homogenizing and Draining: The wine is homogenized (e.g., by pumping over), and then the skins are separated by a system of draining and pressing.
  2. Free-Run Wine Draining: Wine is drained directly from the side valves or bottom of the tanks. This “bleeding” or free-run wine typically accounts for 40-60% of the total wine volume.

Once the free-run wine is drained, the remaining skins (pomace) are removed. This can be done manually, requiring several people to drag the pomace to the tank door, or automatically, often with carbon dioxide control to prevent oxidation.

Pressing

The highest quality wine, often called “free-run wine” or “flower wine” (vin de goutte or vin de fleur), is obtained directly from the initial draining. The remaining wine is extracted from the pomace after devatting, which then goes to the press. This pressed wine is generally of lower quality.

Pressing is performed gradually:

  1. Initially, wine flows freely from the press before pressure is applied.
  2. A low-pressure pressing yields good quality wine.
  3. As more pressure is applied, the quality of the extracted wine gradually decreases.

The remaining moist pomace (husks) is typically sent to distilleries for further processing.

Finishing Stage: Malolactic Fermentation

Malolactic fermentation (MLF) is a secondary fermentation that often takes place spontaneously if conditions are suitable. It is carried out by lactic acid bacteria (primarily Leuconostoc, Lactobacillus, and Pediococcus), which transform malic acid into lactic acid and carbon dioxide.

MLF is important for three main reasons:

  • Wine Deacidification: Reduces the wine’s acidity, as malic acid is stronger than lactic acid.
  • Flavor Improvement: Contributes to a softer, rounder mouthfeel and can introduce buttery or nutty notes.
  • Microbial Stability: Prevents later spoilage by malolactic bacteria in the bottle.

However, negative effects can include a slight increase in volatile acidity if other compounds, such as citric acid, are degraded by bacteria, forming acetic acid.

Conditions for Malolactic Fermentation

Several factors influence the success and progression of MLF:

  • pH: The optimum pH range is typically between 4.2 and 4.5. Activity increases rapidly with higher pH; below pH 2.9, MLF usually does not occur.
  • Temperature: The ideal temperature is around 18°C (64°F). This temperature helps prevent the attack of lactic acid bacteria on other components besides malic acid, which could increase volatile acidity. Accepted margins are generally between 20-28°C (68-82°F).
  • Alcohol Content: Above 13% v/v, the activity of malolactic bacteria can be limited. At normal alcohol concentrations, bacteria are capable of degrading all malic acid, but lactic acid formation is greater at lower alcohol concentrations.
  • Sulfur Dioxide (SO2): Lactic acid bacteria are more sensitive to SO2 than yeasts. High levels inhibit them, while small doses can sometimes promote their proliferation. Sulfite addition is typically avoided after alcoholic fermentation if MLF is desired.
  • Aeration: Both extreme anaerobiosis (lack of oxygen) and excessive aeration can be negative for MLF.
  • Nutrients: Lactic acid bacteria have greater nutritional demands than yeasts, primarily for amino acids. If these are absent, malolactic fermentation may not occur.

Based on these factors, MLF typically lasts from 5 days to 4 weeks. It can sometimes be stopped and restarted several months later if necessary.