Due to the presence of skins during fermentation, red grapes have several unique processes that winemakers need to make sure to consider to ensure successful fermentation.
Also referred to as cold maceration, the practice of cold soaking grapes has become more prevalent in the modern wine industry. After processing into the winery, the must is not inoculated. Depending on the winery's program, this may be a few days to upwards of two weeks. The aim of cold soaking is to allow the extraction of water-soluble color and flavor compounds without harsher phenolic compounds like tannins, which are significantly more soluble in the presence of alcohol.
Cold soaking proponents contend the resulting finished wines increase color, fruitiness, flavor intensity, and mouthfeel with lower astringency and bitterness. Another major benefit of cold soaking must is more accurate chemical analysis prior to fermentation onset as the juice 'soaks up' for sugar and acid from the skins.
The main requirements for successful cold soaking are pretty simple.
- Keep must temperature cold, ideally below 10° C (50° F).
- Minimize oxygen in must. This will limit microbial activity and oxidation and can be accomplished by using inert gas coverage, usually CO2.
- Add sufficient SO2. Again, this will help limit microbial activity and oxidation.
- Conduct cap management on a daily basis, but don't over do it. 1-2 times daily is sufficient.
A 'fermentation cap' is created when CO2 produced by the yeast pushes the grape skins to the surface of the tank. The cap can pose some problems if it is not managed properly.
- Skin contact is the primary reason red grapes are fermented on skins, and this will minimize it.
- The cap will dry out relatively quickly, promoting the proliferation of undesired microbial activity.
- CO2 and heat will be trapped in the fermentation vessel.
- Oxygen will not be able to enter the fermenting.
Different cap management regimes have a different effect on extraction of skin and seed components. Choosing the right method depends on wine style and time constraints. Cap management programs will often combine method.
- Punchdown - Depending on the fermentation vessel size, a handheld or piston-like device is used to push the cap down into the center of the tank. This is repeated until the entire cap has been saturated. Punchdowns are commonly completed 2-4 times daily during active fermentation. Though the action can be gentle or more rigorous, punchdowns are usually associated with higher extraction.
- Pumpover - Juice from the bottom of a fermenting vessel is pumped over the top, wetting the cap but not fully submerging it. These are commonly completed 2-3 times daily during active fermentation. Pumpovers can also be completed in gentle or rigorous fashion depending on the desired rate of extraction.
- Rack and Return - Juice is drained from the bottom of the fermentation vessel into a separate vessel, leaving behind only the grape solids (primarily skins). The juice is then pumped back over the top of the tank, either immediately or several hours later. Many believed this is the gentlest way for mixing an entire tank, and is also used for seed removal. Seeds, particularly less-than-ripe seeds, are believed to have more astringent/bitter tannins that many wineries try to remove once fermentation is underway. Rack and returns are not usually completed everyday during active fermentation, but used in conjunction with a punchdown or pumpover regime.
- Rotary Tank - Rotary fermentation tanks can be rotated on their axis, mixing the tank thoroughly and promoting faster extraction and more uniform temperature throughout the must. The extraction is so high that many winemakers often ferment must to 1/2 or 2/3 brix depletion in a rotary fermentor before draining and pressing to a different tank to complete fermentation.
Winemaking additives are in no way exclusive to red varieties or considered necessary during winemaking activities. All but one (non-ellagic tannins) of the additives discussed below are naturally occurring in grapes and/or wine at some levels. Separate articles discuss fermentation nutrients and wine additives in depth.
- Sulfur dioxide (SO2) - SO2 is usually added to red grape varieties at the destemmer/crusher stage when fermentation will not be immediately induced. For example, when cold soaking will be completed. SO2 helps protect the must from microbial activity and oxidation. Common additions prior to cold soaking are 30-50 ppm.
- Enzymes - Enzymes can help stabilize color and increase extraction of stable phenolic compounds. Similar to yeast strains, numerous proprietary enzymes are available and designed for creating different wine styles. Enzymes should not be added to must with high SO2 levels, which will inhibit their activity. They should always be added prior to fermentation, ideally immediately upon arriving in the fermentation vessel.
- Tannins - There are several different types of tannins naturally occurring in grape stems, skins, and seeds. These are classified as polyphenolic compounds and attribute to a wine's astringency, mouthfeel, aroma, and color stability. A broad range of proprietary tannin products are available, sourced not only from grapes but oak wood, nuts, and other fruits. Tannin additives may be made for a variety of reasons: increasing color stability, increasing phenolic structure, inhibiting specific microbials, and decreasing oxidation. Most tannins are designed for addition prior to or during fermentation.