Among the activities taking place in the winery this time of year is racking. There many reasons to rack. The most common reasons are: natural clarification of the wine, getting the wine off the lees and gently getting more oxygen into the wine. The first racking follows the settling of the wine following pressing to get it off the "gross lees." The gross lees are the solid matter; tiny bits of grape skin and pulp that is suspended in the wine after pressing. This matter has no good effect on the wine, so it's removed right after an overnight (or longer) settling period.
Later rackings, if all goes smoothly, are to get the wine off the "fine lees,'" or spent yeast hulls, following fermentation. The fine lees settle out of the cloudy wine and they are usually kept in contact with the wine and even stirred on a regular basis for an extended period because they impart complexity and better mouthfeel into the wine. They even have an awesome ability to absorb small aroma and taste defects from the wine. Racking at this point is done to begin clarification of the wine and given enough time and a few rackings, even difficult-to-clear wine can become sparkling clear. Patience is truly needed to get the wine clear and production oriented wineries shortcut this step by fining and filtering the wine.
Racking can also be needed on an ad hoc basis if the winemaker detects any off aroma in the wine during aging. Frequently, this kind of racking follows the detection of "reduction," or oxygen need in the wine when there is a note of egg or sulfur. The winemaker will lose no time in racking to get the wine off the lees, as the lees have probably absorbed all the defect they can. Even more important, the racking introduces the wine to oxygen in a controlled manner and if all goes well, allows the wine to blow off the unwanted aroma and get at the root of the defect by getting the wine the oxygen it needs to develop correctly.
The photo shows a typical racking where we're using gravity, which is more gentle than pumping to move wine from tank to barrel. The tank valve is a couple of inches above the bottom of the tank which allows us to leave the lees behind.