Wine filtration

Wine filtration passes wine through a material that retains suspended particles. Winemakers use it to improve clarity and, with suitable equipment, remove yeasts and bacteria before bottling. These purposes call for different degrees of filtration. A coarse filter can clear visible solids; microbial stability requires a filter capable of retaining the organisms concerned.

How filters retain material

Depth filters catch particles within a thick, porous bed. Cellulose pads and beds of diatomaceous earth are familiar examples. Retention involves both physical trapping and interactions with the filter material, and these filters can handle relatively large quantities of solids.

Surface filters retain particles at a membrane. In conventional flow, material accumulates against that surface and progressively restricts passage. In cross-flow filtration, wine circulates along the membrane while part of the liquid passes through it. This movement limits the deposit and allows continued filtration of a larger volume. Cross-flow equipment is commonly used to prepare wine for a final membrane filter at bottling.

Clarity and microbial stability

A filter's rating needs interpretation. A nominal rating describes a level of particle retention under specified conditions. An absolute rating defines a maximum opening size. For microbial removal, the filter's demonstrated retention of the target organisms is also relevant. An integrity test checks for leaks or damage that would let wine bypass the intended barrier.

Wine Australia describes an absolute-rated 0.45-micrometre membrane as typical for sterile filtration in Australian wineries.1. Wine Australia, “Top tips for filtration,” 8 July 2016, reporting advice from Australian Wine Research Institute œnologist Matt Holdstock. The final filter works as part of the bottling operation. Clean downstream equipment and prompt packaging protect the filtered wine from renewed contamination.

Residual sugar gives surviving yeast a substrate for renewed fermentation. Removing yeast before bottling is therefore especially useful for sweet wines. The wine's composition and cellar history guide the decision about microbial filtration.

Preparing wine for the filter

Settling and progressively finer filtration reduce the material reaching the final membrane. This protects the membrane from rapid blockage and helps keep the bottling run moving.

Even a visually bright wine may contain large molecules or aggregates that clog a filter. Turbidity measures the scattering of light by suspended material; a filterability test measures how readily wine passes through a test membrane and how quickly flow declines. The two tests answer different practical questions. A low turbidity reading can accompany poor filterability, particularly where polysaccharides contribute to blockage.

Effects on the wine

The sensory effect depends on the wine, filter material, and process. In a research project reported through the American Vineyard Foundation, membrane filtration of Cabernet Sauvignon, Merlot, and a white blend produced limited sensory and chemical changes compared with unfiltered controls. A separate cross-flow comparison in the same project found similar sensory results for the white wine, while the unfiltered red developed an earthy character that the researchers considered possibly microbial.2. American Vineyard Foundation, “Evaluating the Effects of Sterile Membrane and Other Filtration on the Sensory and Chemical Properties of Wine,” research summary, 2015.

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