Wine Filtration 101

You want to start a heated and emotionally charged discussion on a controversial topic? Ask a group of amateur winemakers for their thoughts on the impact of filtration on wine. Chances are you will get diametrically opposed views on the pros and cons of filtration, and, quite often, those views and opinions are not necessarily based on scientific data, but rather on traditional beliefs and myths.

The reality is that the vast majority of commercial wines are filtered, except for some ultra-premium wines. And wine marketers have muddied the issue by labeling their high-end wines as unfiltered — with a price to match — suggesting that unfiltered wines are far superior to filtered ones. How so? Even Émile Peynaud, one of the greatest enologists of the twentieth century, commented on “the sensory consequences of filtering” in his book “Knowing and Making Wine.” According to Peynaud, “the mechanical action of filtering has never had a negative influence on quality.” And Ribéreau-Gayon, et. al., state in their classic work “Handbook of Enology: Volume 2 – The Chemistry of Wine, Stabilization and Treatments” that “[winemakers’ reservations about filtration], alleged to make wine taste thinner, are probably excessive.” They go on to assert that filtration “is intended to eliminate turbidity, foreign bodies and impurities that would, in time, form the lees. It would be ridiculous to suggest that these substances make a positive contribution to flavor.”

So why is filtration such a shunned practice, particularly among makers of “premium” homemade wine? There is a deep belief that filtration strips wine of its color and, more importantly, of integral flavors. Filtration does not strip wine of its color, albeit, it lightens the wine, but that’s because particles or colloidal matter that would otherwise precipitate in the bottle or stain the glass are removed. Wine connoisseurs expect some sediment in their ultra-premium wines (hence the main reason for decanting), but the vast majority of consumers do not understand or accept sediment in wine. And flavor compounds are much smaller than filter pore size and are therefore unaffected by filtration. But a wine can surely be over-filtered when not done properly: An excessively filtered wine can show signs of “fatigue,” oxidation or poor color. So let’s examine the principles and chemistry of filtration to help you understand how filtration affects wine, clarify the pros and cons of wine filtration, and help you make informed decisions on whether you should filter or not.

Understanding filtration

Filtration is the process of filtering or passing wine through a filter medium for the purposes of clarification by mechanically separating particles (or colloidal matter) still in suspension to produce a crystal-clear wine in preparation for bottling. Filtration also promotes microbial stabilization by removing yeasts, bacteria and other unwanted microorganisms that can spoil wine once bottled, a process often referred to as sterile filtration. (I feel that “stabilizing filtration” is a better term because we cannot achieve an truly sterile environment via filtration.)

In filtration technology, particle size defines the type of filtration and the required retention rating of the filter medium. The unit of particle size is the micron, short for micrometer or µm, defined as 10–6 meter. (As a reference, the average size of a human hair is 70 µm.) But beware, a filter medium rated for 3 µm does not mean that it will filter out all particles bigger than 3 µm; it can actually filter out smaller particles too. This is because of filter manufacturing technology and economics. These ratings are referred to as nominal retention ratings, and that’s why these are often provided as a range. In critical applications, such as sterile filtration, filter media with absolute retention ratings are used to ensure a higher degree of microbial stability.

Clarification often involves multiple filtration passes depending on the wine’s physical aspect, which depends on how the wine was treated. For example, if treated with enzymes to break down colloidal matter, or with fining agents to treat pectin or to clarify the wine, fewer filtration passes may be required. In all cases though, the wine is always racked off its sediment before any filtration.

Basic steps in filtration:

Prior to bulk aging the wine, or in preparation for bottling, a rough polishing filtration pass is performed to remove coarse particles larger than 5–10 µm. Immediately preceding bottling (i.e., on the same day), a polishing filtration pass is performed to remove particles larger than 1–4 µm. If the wine was fined and then racked multiple times over the course of the bulk-aging period, a polishing filtration may be all that is required. Some will even skip this step if the wine is very clear, particularly in the case of whites; for reds, it is always recommended owing to the much higher concentration of polyphenols, namely, anthocyanins, the red pigment compounds. A fine filtration pass, also known as pre-sterile filtration, is performed to remove particulates larger than 0.5–0.8 µm. In home winemaking, this is usually the finest filtration possible with the equipment available. The final step before the wine is bottled and corked is sterile filtration — also known as membrane filtration — which is performed to remove residual or latent yeast cells, which are greater than 1.2 µm, and spoilage bacteria, most of which are greater than 0.65 µm.

The table on this page summarizes filtration terminology and targeted particle sizes. Manufacturers of filtration technology, suppliers of filtration equipment and winemakers often use different terminology for the type of filtration, and so it is best to always understand particle size.

Except for sterile filtration, which requires specialized equipment, the clarification filtration steps prior to bottling can be run sequentially by cascading filter systems, or the wine can be filtered in multiple steps when only a single filter unit is available, as is usually the case in home winemaking. The last clarification filtration feeds into sterile filtration, which then feeds the bottling line. Rough, polishing and fine filtration use depth filtration technology, while sterile filtration uses membrane technology.

Depth filtration technology uses filter pads, manufactured from layers of cellulose fibers, set in a plate-and-frame system, or pleated or wrapped sheets, manufactured from polypropylene spun microfibers, set in a cartridge then mounted in a special housing apparatus in a filter system. Wine is forced through the filter medium by the mechanical action of a pump and then filtered throughout the thickness of the filter medium. Depth filtration, therefore, has high particle retention or filtering capacity. Filtering efficiency can, however, vary significantly from the specified retention rating.

Membrane filtration technology uses a membrane manufactured from specialized material, such as cellulose esters or other polymers, with minuscule holes or pores. Wine is filtered at or near the surface (much like the action of a sieve) of a disposable or reusable filter medium. Because this type of filtration requires a large surface area for efficacy, the membrane is pleated. Wine is forced through the tiny pores in the membrane by mechanical action of a pump, and particles larger than the membrane’s rating are filtered out. Being a surface filtration technology, membrane filtration has low particle retention or filtering capacity; however membrane filtration meets the stricter requirements of specific applications, such as preventing microbial contamination.

Sterile filtration is the domain of commercial wineries, and so, here, we will focus on depth filtration using filter pads, as used in home winemaking. Those interested in implementing sterile filtration in their home winemaking can consult “Should I Membrane Filter My Wine?” in the August-September 2007 issue of WineMaker.

Depth filtration

Depth filtration systems consist of three components: a filter pad housing assembly, a pump, and filter pads. Some systems incorporate the housing assembly and pump into a single piece of equipment. Let’s examine each component in a little more detail.

Filter pad housing assembly

One of the most popular filter pad housing assemblies is the clear or red-colored round casing with top and bottom plates plus an insert to channel wine into the system, all fastened with nuts and bolts. The unit uses two round filter pads for small (5-gallon/19 L) batches. A separate pump is required for this type of system. Another popular type is the plate-and-frame assembly, and it invariably integrates a pump matched to the capacity of the system. Capacity of a filtration system is defined by the maximum number of pads it can hold. Popular systems operate using three pads for small and medium batches, or 10, 20 or more pads for larger batches. However tempting, pads of different ratings should never be mixed in the same assembly in the hope of achieving different passes in a single filtration step; it would result in reduced filtration efficiency and compromised quality of the wine.


The pump is used to displace wine from a carboy or holding tank through the filter medium and into a receiving carboy or tank. A good pump matched to the capacity of the filter system is essential for efficient filtration. The most popular and efficient type is the positive displacement pump, such as a simple diaphragm pump for small batches and an impeller pump for larger batches. These pumps work by suction without the need for priming, but are usually not designed to run dry. An interesting alternative is the vacuum-type pump, capable of running dry, which is configured to displace and filter wine under vacuum and therefore limiting the effects of oxidation. Systems with an integrated pump may include a centrifugal-type pump that is more efficient and gentler on the wine, but these come at a premium price. When choosing a pump, always ensure that it is rated as food-grade.

Filter pads

Single-use depth filter pads manufactured from layers of cellulose fibers come in a variety of grades, i.e., nominal retention ratings, and sizes for various applications and volumes of wine to be filtered. Manufacturers may assign an easy-to-remember number, code or intended type of filtration instead of a retention rating; for example, #1 or K800 for rough polishing, and #3 or K100 for higher polishing clarity. You should be familiar with the retention ratings of pads from each manufacturer prior to use to avoid any surprises.

Pad size is also important when assessing filtering efficiency and capacity. Since wine is filtered throughout the thickness of pads, the filtering volume of each pad must be considered, and given that the standard pad thickness is 4 mm (5⁄32-in), the surface area is most often used. Pads with a greater surface area will filter more effectively and more volume than pads with a smaller surface area. The total surface area is the aggregate of surface areas of all pads combined in a system; for example, the total pad surface area in a three 20×20-pad filter system is 3×20×20=1,200 cm2 (186 sq. in.).

Some systems are equipped with a pressure gauge to monitor filtering efficiency. As the gauge nears the maximum pressure, it indicates that the pads are getting clogged and should be replaced; but you should never encounter this situation with a properly processed wine.

And here’s a very important point: Filter pads have a smooth side and a rough side. For proper filtration, wine must be filtered into the rough side of filter pads and out from the smooth side.

Fittings, tubing, etc.

All fittings, connectors and any other materials such as hoses that will come into contact with juice or wine should be food-grade. Particularly, stainless steel fittings are always recommended in winemaking applications; never use brass fittings as they can contribute to some lead contamination.

And if you work with oak chips or suspect that the wine still contains grape solids that may damage the pump, consider attaching an in-line strainer between the carboy or holding tank and the pump.

Filtration efficiency

Filtration can be relatively simple and trouble-free if you understand filtration principles and your wine. Trying to shortcut the process by filtering an “unclean” wine with tight (i.e., low retention rating) pads is asking for trouble. Pads will clog prematurely, causing a heavier strain on the pump and possibly causing equipment failure or damage, and, more importantly, compromising the quality of the wine. Let’s look at important factors impacting filtration efficiency.

Red wines have a much higher concentration of particles than whites and, therefore, reds need to be filtered with coarser pads first. Always coarse filter first to avoid any unpleasant surprises, even when the wine has been racked multiple times and also fined. And beware, finer particles will clog filter pads much faster than coarse particles. For example, white wine can look deceptively crystal clear, but should still be coarse filtered first, unless it has been fined.

The use of enzymes during winemaking can greatly improve filtration efficiency. Enzymes break down colloidal matter that affects clarity and filtration. For example, in red winemaking from grapes, you can add macerating enzymes at crush time to prevent pectin-related problems that could interfere with filtration or, more importantly, cause haze in the bottle. Macerating enzymes help break down cell walls of red grapes for a more gentle phenolic extraction resulting in improved color stability and smoother, rounder mouthfeel. Use pectic enzymes when making white wine from grapes and especially fruit and country wines as these tend to have more pectins that could negatively affect filterability. Similarly, it is recommended to treat white and country wines with bentonite to inhibit protein haze and to improve filterability.

The type of fining agent used for clarification is another important consideration. Not all fining agents act on colloidal matter in the same fashion; some cause the particles to flocculate very efficiently and create a compact layer of sediment at the bottom of the carboy or tank, while others are less efficient, take longer to flocculate particles, cause particles to cling to the wall of the carboy or tank, or create a loose layer of sediment. Isinglass is known to cause particles to cling to the wall of container, and, same as with bentonite and Sparkolloid, these all create a loose layer of sediment. For these then, a longer flocculation period is recommended to maximum sedimentation and compactness. And in all cases, always rack wine before any filtration.

Another factor affecting filterability is tartrate crystals, which can clog up filter pads very quickly. Tartrate crystals form when wine is subjected to cold temperatures in a process called tartrate stabilization; this is to prevent tartrates from forming in the bottle. Tartrates tend to cling to glass in carboys or the inside of tanks and therefore need to be gently stirred back into suspension to allow the crystals to fall to the bottom. The wine is then racked prior to filtering.

Cross-flow filtration

Although cross-flow filtration is strictly the domain of commercial wineries — and only those with the capital to afford the expensive equipment — it is worthwhile mentioning it to complete the picture on filtration technologies, and also because you will likely hear a lot about it.

Cross-flow filtration is a technology and process where juice (or wine) is made to flow tangentially to the filter media — as opposed to perpendicularly across the filter media in clarifying filtration applications — to avoid clogging the media; as such, it is a very effective filtration technology. It is also used to implement reverse osmosis processing for removing alcohol as well as volatile acidity (VA) from a finished wine.

Ready to filter?

Now that you understand the physical chemistry of filtration and your wine, you are ready to filter. If you are just starting out in home winemaking, your only investment is in disposable filter pads; many retailers rent out filter units for a nominal fee. If you intend to process larger quantities of wine on a regular basis, you should then consider investing in an appropriate system for your needs.

Happy filtering.