Winemaking - cleaning up and bottling
At the end of the maturation period the wine needs to be cleaned up in preparation for bottling. There are a number of techniques that can be used to stabilise and clarify wine:
Fining - this is done by adding a fining agent to the wine which causes the lees to fall to the bottom from where they can be racked off. The main fining agent for white wine is a clay called Bentonite. Red wines are traditionally fined with beaten egg white, or other protein rich substances like gelatin.
Centrifugation - by spinning a wine at high speed yeast and bacteria can be removed from a wine. Cold treatment (or 'tartrate stabilisation'). Many white wines are held at -3 degrees for a week just before bottling. This cold treatment precipitates out any excess tartaric acid in the form of tartrate crystals, hopefully preventing a crystal deposit forming in the bottle later.Filtration - this is the process used to remove any remaining yeast or bacteria from a wine. There are three main types of filter:
1. Kieselguhr (or 'diatomaceous earth') - to a layman it looks as if the wine is being filtered through mud. Used as the first stage in cleaning up a wine containing lots of yeast.
2. Cellulose (or 'plate and frame') - these filter pads look like thick pieces of white cardboard. Effective in removing most remaining yeast. 3. Membrane (or 'sterile') - made of synthetic polymers, a membrane filter will remove all remaining yeast or bacteria.
Pasteurisation - heat treating a wine just before or during bottling is still occasionally used to be sure that there are no active microbes present. However, the damage caused to wine by flash pasteurisation, coupled with the availability of membrane filters, has seen a decline in the use of pasteurisation in wine production. Hence most wines nowadays are 'cold sterile bottled'.
If all or most of the above techniques are used on a wine it will almost definitely be clear and stable when bottled. However, each of these manipulations inevitably strips some flavour and character from a wine. There is an ongoing debate in the wine industry as to how much a winemaker should or shouldn't do to his wine to reconcile the need for stability on the one hand, with flavour and character on the other.
Modern wine bottling lines are clean, highly automated facilities. Hygiene and sterility are priorities to try and ensure that the risks of contamination and oxidation are minimised. Various techniques exist to ensure that the bottles themselves are clean, and that the wine does not suffer too much air contact during the filling itself. The big debate at the moment concerns the use of natural cork to close the bottle. Natural cork is a tried and tested material, but it is increasingly expensive, and can spoil a wine through cork taint or by allowing air into the bottle. The use of synthetic cork is on the rise, whilst others advocate the use of screw tops or beer bottle tops for day-to-day wines.