Filtering Your Wine
You can find any number of how-to articles on filtering wine—a Googling yields over forty million hits—and at least that number of opinions on it.
What you don’t see is the 'why-to'. You'll read endless opinions about how filtering strips wine. Those opinions all share one common thing: they are 100% wrong in every way it is possible to be wrong on a single topic. As it goes with these things, the strongest opinions come from people who have a little knowledge and are willing to share it at top volume, but even some famous winemakers weigh in as opposed to filtering.
They're wrong too. Filtering is great--not necessary, but in no way harmful to wine when done correctly and in every way beneficial. I filter almost all of my wines, and 100% of the wines I intend to serve to other people or give as gifts.
Do I have to filter my wine?
Not at all: wine kits have very low levels of suspended solid material. Fining agents like bentonite, Chitosan, isinglass and Kieselsol, deal with haze-causing proteins and 99% of the yeast give you clear wine that will taste great and age well.
There's an analogy my friend Jeff came up with twenty years ago that works well: the difference between a fined wine and a filtered wine is the difference between a freshly washed car and a freshly waxed car. They both look good, but the waxed car looks spectacular and is much more appealing.
Long-term you can look forward to greater stability, as the wine will not change in flavor or appearance during aging. Sediment will almost always form in unfiltered wines after enough time. These consist of dead or dormant yeast, tannins, and colloids (proteins, mostly).
The issues around filtering are really the same as any handling or processing in winemaking, like racking or stirring. There's a possibility of introducing oxygen or stray contaminants when you expose the wine to the filter.
Both of these are easily avoided by good winemaking practice. Clean and sanitize your filter by taking it apart, soaking and scrubbing, rinsing and sanitizing, and you won't need to worry about bugs.
Oxidation is even less of a worry. While filters agitate the wine as it passes through them, they don't actually add oxygen to it. Most home filter set-ups are pressurized, meaning they use a pump to force wine down a hose and through the system. If there is a leak somewhere in them, wine will flow out under pressure and oxygen won't get in.
Does filtering strip colour, flavour or aroma from a wine?
The tightest filters available to home winemakers stop around the 2µ mark (two microns). A regular yeast cell is around 3-4µ, and a freshly budded daughter cell is around 2µ. It’s more common to see filters that allow the passage of material as large as four microns in size.
Colour molecules, the aforementioned anthocyanins, are not on this scale. They are much (much!) smaller they can’t be seen with a microscope. So tiny in fact, you can’t filter them out.
If you take a look at used filter pads they are often stained the same color as the wine. That stuff isn't the actual color molecules. It's color compounds bound to other kinds of goo in the wine. When color molecules bind to tannin they fall out later as a deposit or as sediment. That is simplifying things, but the bottom line is that you can't filter out color—not any color that wouldn’t fall out on its own anyway.
What goes for colour compounds goes for flavour and aroma: they’re just too small to stick to filter pads. Sure, wine can go a little bit numb after filtering or handling. That's a phenomenon called bottle shock, and it can come from racking and bottling as well. Leave the wine for a week or two to recover and it snaps right back.
Summing up, you don't have to filter, just like you don't have to wax your car. But if you do, you'll be rewarded with a beautiful visual treat the delights the eyes just as much as it delights the palate.