Wine production yields non-pertinent solids which make their way to the bottom of the fermentation/aging vessels. The figure below illustrates the scope of the problem and the solution -- racking -- that has been devised to separate the wine from this potentially harmful residue.
According to terroirsdumondeeducation.com, "... the act of racking stands out as a pivotal stage in crafting a wine's unique character and ensuring its quality ... Despite its operational simplicity, racking plays a crucial role in determining the final wine's quality and characteristics."
The racking methods available to the winemaker, factors influencing racking, and common issues and solutions are summarized in the following chart.
Chateau Margaux, the famed Bordeaux producer, avers that the success of barrel aging "depends to a large degree on the quality of the racking" operation. During the two years or so of aging that the estate's wines undergo, an average of seven or eight rackings are conducted (at 3-month intervals in Year 1 and 4-month intervals in Year 2).The principle, according to the estate, "consists of separating, with the greatest possible precision, the clear wine from the sediment deposited during the preceding months." Towards that end, the estate adheres to the following practices:
- Do not disturb the dregs
- No pumps
- Use suction to push the wine delicately from one barrel to another
- Real decanting begins when the greater part of the wine has been taken out
- Requires coordination of two cellar workers
- One worker gently tilts the barrel
- Drawer checks the consistency of the flowing wine with the help of a candle and as soon as the wine becomes hazy, the operation is halted.
Macroaeration
For red wines, racking produces benefits beyond the physical realm. Great care is generally taken to protect white wines from oxygen exposure -- except in the case of hyperoxidation and some skin contact wines -- but the higher phenolic content of red wines affords them greater protection from oxygen's deleterious effects. Macroaeration -- aerating fermenting wines by vigorous racking -- takes advantage of these facts to aid in producing softer, less astringent wines which manifest more fruit character and improved color stability.
Joy Ting of Winemakers Research Exchange fleshes out these concepts:
Phenolics such as anthocyanins and tannins are responsible for the color and mouthfeel (astringency) of red wine... Tannins themselves are polymers, large molecules composed of smaller subunits bonded together in a specific way. After tannin polymers are extracted from the skins and seeds of grapes, acid hydrolysis in the grape juice and wine breaks these large polymers into smaller polymers and monomers ... when oxygen is introduced to the wine, it reacts with metals to start a chain reaction that leads to the activation of monomers, making them more likely to react with other monomers to reassemble tannin monomers into polymers. Each time two monomers (or a growing chain and a monomer) bond, they become even more likely to bond again, leading to elongation of the tannin polymer. Put simply, activation by oxygen can begin a cascade that results in faster formations of phenolic polymers. These chains continue to grow, adding more phenolic subunits, until they bond with an anthocyanin. When an anthocyanin is added to the chain, it is not likely to bond with anything else, effectively capping the chain.
These chain reaction of tannin molecules have important effects for the color and astringency of the wine. The perceived mouthfeel of tannins is, in part, related to the length of the phenolic chains. Longer chains have more reactive sites for interaction with salivary proteins, thus the higher the degree of polymerization, the more astringent tannins seem. Micro-oxygenation ... is thought to bind anthocyanins to growing chains before they are lost to precipitation, enzymatic attack, or binding to lees, resulting in better color retention and shorter tannin polymers ... Polymeric pigments (tannin polymers capped with anthocyanins) are less susceptible to oxidation and browning than monomeric anthocyanins so micro-oxidation also aids in color stability by preserving anthocyanins in solution in their colored form.
The racking procedure that is used aerate the red wine is called délestage, or rack and return, and is illustrated graphically in the (a) portion of the chart below. The figure illustrates the protective cloud of carbon dioxide over the solution, preventing oxygen from getting through to the wine. In the first step the wine is racked to a secondary container -- loudly. The figure shows the solution passing through some type of screen which removes the skins and seeds. This is purely optional and is not an official component of the rack-and-return process. If a winemaker seeks to limit tannin extraction, he/she may opt to implement this step.
Step 2 of the (a) component shows the wine being racked back to the original vessel. The solids at the bottom of the tank are rehydrated during this part of the process.
There is a school of thought which posits that the wine still does not get enough oxygen exposure with this process and part (b) of the chart has been implemented to meet this market. An oxygen ingress port (a venturi) is added to the tube prior to its entry into the vessel and the flow of wine draws in air, increasing oxygen levels in the wine.
When grapes that have been over-treated with sulfur-based vineyard mildew and fungus inhibitors are allowed to macerate with the grape skins, some solids can dissolve into the juice. If this hydrogen sulfide is not removed quickly, it can result in mercaptan formation. Mercaptans, according to wineanorak.com, is a large group of very smelly sulfur compounds with smells such as "cabbagey, rubbery, struck flint, or burnt rubber." Vigorously racking the wine can reduce the level of hydrogen sulfide therein and reduce the potential for mercaptans later in the wine's life.
