Thursday, 25 September 2014

Wine Filtration Fun-damentals

As vintage starts to heat up, we have been trying to clear as much storage room as possible in the winery tanks, ready to accept grape juice for ferment. The week just gone has been a flurry of filtering and bottling activity, primarily for contract parties.

It has been an excellent learning experience for me to participate and assist in this process, as I have not had much to do with filtration of wine previously. In saying that, I have had a lot to do with filtering mineral slurries in my past work in the mining industry, so many of the technical concepts are very similar. 

Basically, when a wine is being filtered prior to bottling, the primary goal is to remove particulate matter that may still be suspended in the liquid, so that it does not end up in the bottle. In some applications, this can serve to assist in sterilising the wine by removing any bacterial particles and in most cases it can improve the aesthetic value by ensuring customers are not put off by any 'floaties' in their wine.

Filtration is a mechanical process whereby the wine is pumped through a medium in which solid particles can be trapped for removal. There are two major considerations which must be taken into account to maintain the quality of the wine during filtration.

Oxygen Contact
Firstly, it is essential to minimise the amount of oxygen contact, or aeration. To cut a long story short, chemical oxidation of wine can result in the formation of acetaldehyde, which causes browning, loss of fruity aromas, and formation of aldehydic odor (similar to nailpolish or vinegar). Oxidation can be minimised by injecting nitrogen air into the flowing wine to displace oxygen from the system, a process called sparging.

Filtering Speed
The speed at which the wine is pumped also needs to be moderated to ensure that all the wine has the opportunity to contact adequately with the filter medium and also so that the filter medium does get 'clogged'.
Setting up the Pad Filter

Filters come in different shapes and sizes. I have been using two different types of filter this vintage. The first is a plate and frame style Pad Filter where the pads are made from Diatomaceous Earth (they can also be made from cellulose). This is a form of 'rough filtration' and the passing size we were using was 100 micron, which is adequate for removing larger particle sizes from the wine. 
Finer filtration can be achieved using a Membrane Filter (also known as a Cartridge Filter).  The ones I have been using have sieve-like discs which can remove particles down to as small as 1 micron, so very fine filtration. For this reason, membrane filtration can be used to finish or sterilise the wine.
Membrane Filter
The hot topic when it comes to filtration is whether or not this process can detract from the quality of the finished wine. It is suggested that some of the aroma and flavour components from the wine can be stripped during the process, leaving the wine as a less complex version of its former self.
According to UC Davis Enology, the equilibria between aroma compounds re-establishes itself afterwards and the intensity returns. Their studies show that 'expert tasters are not able to recognize filtered versus unfiltered control wine'. However, they do concede that 'unfiltered wine may allow continued microbial activity, which may change the character of the wine if it is aged significantly post-fermentation'.
Whether you believe UC Davis or think the jury is still out, the degree of filtration, if any, is always carefully considered by the winemaker.

I can now set up and run the filtration process myself and have been tasked with this on a number of occasions, even with reserve wines. It has been an unexpected insight that I did not count on having the opportunity to undertake during vintage.

If you are interested to read more about the details of wine filtration, an excellent summary is available from UC Davis.