The process of racking involves siphoning the wine away from the sediment. Sediment is primarily comprised of inactive or dead yeast cells. While yeast is beneficial to the must in the beginning, over time it becomes multiplied many times over. As each new generation of yeast is produced, older generations die off. Most of what you will see at the bottom of your fermentation container is dead yeast cells.
As the fermentation nears the end of the cycle, there should be a significant amount of active yeast cells which are beginning to wind down because they are running out of fuel; or sugar. After the sugars have become consumed the active yeast will begin to starve and feed on itself. An enzyme will be produced that will break down the inactive yeast cells. The purpose of this is to release the nutrients in the dead yeast cells that can be used by the yeast cells that are still active. This process is called autolysis. Over a period of time, an otherwise perfectly good batch of wine may be ruined as a result of this process.
Racking is also important because it can assist in the clarification of the wine. If wine is not racked, the end result will be wine that may be clear on top; however, the bottom will be somewhat thick and hazy.
The racking process is not terribly difficult. In fact, it is quite simple. A siphon is necessary for racking. The key; however, is to make sure that you siphon off the wine without stirring up any of the sediment. It should be noted that while food grade hosing can be used many winemakers do prefer to use what is known as a racking tube in order to rack their wine. This is because, if you are not careful; using a hose to rack the wine can cause the sediment to become stirred up and it may also move around. A racking tube fits on the end of the hose and serves as a type of wand that gives you control over where in the container you draw the wine.
In the old days, wine was racked by manually sucking on the end of the hose; however, today there have been numerous advancements made in winemaking equipment. One of those advancements is an automatic siphon that works with a pump in order to start the siphoning action.
Keep in mind that during the first two to three rackings, it is not essential to remove all of the sediment. You should try to siphon as much of the wine liquid as possible; however, even if you do bring up a small amount of the sediment this will be fine. It is during the final racking that it becomes imperative that you make sure that all of the sediment is left behind with none following through into the wine.
After the fermentation process has been completed, it you still need to rack the wine (such as when storing wine long-term in bulk) it may be necessary to add Campden Tablets to the wine. The purpose of this is to ensure that any air that may have entered the wine during the racking process is driven out. Campden Tablets will also help to minimize oxidation. Keep in mind that this is not a problem when racking wine while the fermentation process is ongoing as the CO2 that is produced during fermentation will naturally eliminate any air that may have been introduced.