As I have mentioned, we have our first bottling of the year scheduled for Monday and Tuesday where amongst other wines, we are bottling our Pinot Gris and Verdejo. The last major step before bottling is to make sure that they are sterile filtered.
What exactly is filtration? Simply it is the passing of wine though a filter medium, capturing particles that are larger than the hole size of the particular medium. Case in point
pre and post filtered Verdejo
These are two glasses of the exact same wine. The glass on the left is un-filtered compared with the same wine after filtration.
As is true of most processes in winemaking, there are many types of filtration, but you can either distinguish between  Depth filtration and  Surface Filtration.
Depth filtration is a type of filtration where wine is pushed through a series of pads made of cellulose fibers while surface filtration means running the wine along a thin-film of polymer material filled with holes tinier than the particles filtered out.
At Keswick we use a plate and frame filter, where a series of filter pads are inserted into the filter machine and the wine is subsequently passed through it, hence depth filtration.
plate and frame filter with sheets inserted
In the case of these white wines, filtration is used not only as a means of improving the wines visually, it is of critical importance in ensuring these wines are stable and remain stable long after they have been bottled. Both of these wines are regarded as dry [there is no residual sugar], however they do contain a fair amount of malic acid. Lactic Acid Bacteria [Oenococcus oeni] can consume malic acid to liberate energy [Carbon Dioxide] and as such these wines needed to be sterile filtered to remove any active yeast cells and bacteria. Sterile filtration, means running the wines though sheets that have a porosity of 0.45 micron [the smaller the number the tighter the porosity].
So today was basically spent running the wines though the plate and frame, each time inserting tighter sheets until I was able to push 1ooo Gallons of wines through the 0.45 sheets. 5 Filtration runs and mission accomplished. All that is left to do is adjust sulfur levels and get it into the bottle, thank goodness someone else does that for me though.
filtering the pinot gris from right tank into left