Chapter 6

Wine Fermentation

Cap Management for Red Wines

During red wine fermentation, yeast converts sugar into alcohol and in doing so, it creates extra carbon dioxide. Carbon dioxide produced during fermentation causes the grape solids (skins, seeds, stems) to rise, forming a cap that floats at the top of the tank. There are a number of reasons to manage this cap, including to: (1) keep the cap moist enough for the yeast to remain active; (2) maintain a constant temperature throughout the tank; (3) break up the cap so it doesn't become stratified thus allowing the yeast to use all the sugar present; (4) keep solids in the cap in anaerobic conditions, preventing volatile acidity (VA), etc.; and (5) allows for a steady, continuous extraction of polyphenols, which contribute to color, flavor, and tannic structure in the finished wine.

Punch-Down

Punch-down, also known as pigeage, is the process of breaking up the cap and submerging it in to the must. Punch-downs work best with open fermentors or small fermentors with large lids. Under normal circumstances, punch-down is also a fairly oxidative cap management regime.

Manual Punch-Downs

Punch-downs are carried out using a metallic or wooden stick with a flat plate end to increase contact surface (Figure 6.3). This is a tedious process and is only suitable for small wineries and for small volume fermentations of short duration.

Mechanical Punch-Downs

Manual punch-downs are somewhat limited by the size of the fermentor. Large fermentors tend to generate fermentation caps that are too thick for manual punchdowns. Larger fermentors which are intended for punchdowns, should have a semi-automatic or fully-automated punch-down systems (Figure 6.4). There are several mechanical devices (e.g., screw, helix, jack, and pistonbased) that have been developed to perform punch-down.

Pump-Over

Pump-over, also known as remontage, is the process of drawing off the must from the bottom of the fermentation vessel and pumping it over the cap, soaking it, extracting flavor, tannin and color and oxygenating the must. An effective extraction takes place as the liquid percolates through the skins. Pump-overs are one of the more oxidative cap management techniques.

Manual Pump-Over

If done manually, it requires emptying fermenting must from a racking valve at the bottom of a tank and then pumping it to the top of the tank through a 2 1/2-in - or 3-inch hose and sprayed over the top of the must in the same tank (Figure 6.5). The pressure of the falling must produce an emulsion which facilitates oxygen dissolution. An effective pump-over should thoroughly leach the pomace cap.

Automatic Pump-Over

If the decision is made to have dedicated, fixed equipment, liquid will be hard piped from a racking arm to a pump situated next to the tank. The pump will draw the liquid from above the lees layer containing mostly yeast and grape solids and then send the fermenting juice up a riser (vertical pipe) and into the top of a tank to a spray device.

Number of Pump-Overs

If the frequency of pump-overs is not excessive it contributes to the tannic structure of wine and favors the extraction of the highest quality tannins, making wine rich and supple, but an excessive pump-overs can increase the tannin concentration leading to hard, aggressive, disagreeable wines.

Pumps

Typically, each tank has its own pump, which is controlled by a PLC for programming the frequency and duration of the pump-over based on temperature, time, or other factors (Chapter 23). Many wineries utilize centrifugal pumps for this purpose; however, experience has shown that centrifugal pumps tend to grind and break up any grape seeds that are sucked in by the pump and become part of the flow.

Submerged Cap

Submerged cap, also known as chapeau submerge, is a technique which was widely used in Europe during the 19th century (Figure 6.7). Opposed to, although not mutually exclusive, punching down or pumping over, the submerged method holds the cap under the surface of the must for a fixed period of time meaning it remains permanently in contact, but not disrupted, with the must.

Rack-and-Return

A more oxidative technique widely undertaken is rack-andreturn, also known as delestage. Shortly after fermentation has become sufficiently vigorous to produce a cap, the juice is drained into an intermediate container letting the juice flow freely across a screen to capture and remove the harsh tannins from seeds (Figure 6.8). The seed-catching screen is emptied as necessary to keep a good flow and avoid overflowing the container. The wine is sprayed into the holding tank which becomes aerated. The cap of grape skins loosens and falls to the bottom of the tank producing an increase in pressure, which generates a very intense but gentle extraction.

Pulsair System

A new technology used for cap management is Pulsair's pneumatage system, which works by sequentially pulsing large bubbles of compressed filtered air or another gas into the juice below the cap, using a wand, or through fixed injection port probes near the bottom of fermentation tank. The fast, rising air pulses completely separate the grape cap into individual berries to enhanced infusion of phenolics, flavor, aromatics, tannins, and color.

Rotary Fermentors

Rotary fermentors are horizontal, automated tanks that rotate on an axis to mix the pomace and the juice, thus allowing for maximum of extraction in the minimum amount of time (Section 22.1). They are ubiquitous in Australia where they have helped create the Australianstyle, fruit-driven, juicy red wines. Producers and marketers of rotary fermentors cite many benefits. Perhaps the most important is the style of wine produced, mainly a soft, accessible, early-drinking red wine needing little aging.

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