The filtration of wine remains a controversial issue, especially as there are an increasing number of ‘natural’ wines on the market. Critics of filtration say that the process strips the character from the wine, although I cannot find any studies that validate this claim. The pro-filtration camp argues that clear, brilliant wines are essential for customer acceptance. The presence of observable solids, including crystals or particulate ‘floaties’, is deemed to detract from the wine’s acceptability. The clarity of the finished wine is something that I have always promoted in my teaching days and this is from the occasional consumer of Cooper’s sparkling ale!
There are different requirements for wine clarification, based on whether white wine or red wine is being produced. For white wine, it is common practice to clarify the juice after crushing and pressing the grapes to give a clear juice prior to fermentation. The extent of clarification depends to some extent on winemaking philosophy and tradition. For new world wines, reasonably clear juice is used to produce a fruitier wine because fermentation on solids can give baggy or wet cardboard aromas. On the other hand, and this is more of an issue in new world winemaking, extensive clarification can reduce the amount of yeast nutrients, which may require supplementation. The benefit of having some solids present in the ferment is wine with greater depth of flavour.
Allowing the juice from white grapes to settle over time can be effective, although it is slow because the juice in the tank must be held at low temperature to stop the onset of fermentation. Centrifugation was popular for some lower-end wines as was the use of diatomaceous earth as a filtration medium. As discussed later, there are issues with the use of diatomaceous earth in wineries. Flotation has come back into favour as a method for rapid clarification. The grape skins or lees are floated to the surface using gas bubbles, usually nitrogen, and the floating lees (FLEES!) are swept off, giving juice of sufficient clarity to commence fermentation. Clarification by flotation can be achieved in hours, rather than days by settling, and juice losses are minimal (bit.ly/2nEQDIF). Pectin from the crushed grapes can make clarification a challenge, although pectolytic enzymes can be added to break down many of the pectic substances. The fermentation of red wine occurs on skins for colour extraction, so clarification prior to inoculation is not required.
After fermentation and removal of yeast lees, the wines are stabilised either in tank or barrel. Further processing, such as malo-lactic fermentation or protein stabilisation in white wines may occur. Prior to bottling, the crunch time for a decision regarding whether and how to filter occurs. When I first started a close involvement in the wine industry, diatomaceous earth was in wide use as a filter medium. Diatomaceous earth is cheap but really only useful for coarse filtration, and unless care is taken in treating the earth before use, iron contamination may occur. There are many other downsides to diatomaceous earth. It is expensive to dispose of, it can remove colour in red wine filtration and it can also affect aroma. Health and safety issues require careful workplace management.
In essence, the choice falls between depth or membrane filtration. In depth (sometimes ‘pad’) filtration, cellulose is probably now the most commonly used or preferred filtration medium (bit.ly/2E6MSpF). The so-called Begerow cellulose filters possess significant advantages over older materials, particularly in relation to physical strength and minimal impact on the wine itself. Essentially, the separation process works by capturing material within the structure of the filter, so it is only possible to claim a nominal pore size with this method.
Membrane filters work by surface trapping of components larger than the declared pore size of the membrane. Some capturing within the membrane itself may also occur. Common materials for the filters are cellulose acetate, polyether sulfone and nylon, and these are generally available in 0.45 µm and 0.2 µm pore size. We have compared these filters in some of our recent work on copper sulfide filtration, about which I will write soon. Cellulose acetate and polyether sulfone are relatively inert with respect to impact on wine. However, nylon, owing to its amino acid structure, can absorb colour and is thus of limited value for red and even rosé wines (see bit.ly/2E6MSpF where there are some great images of before and after filters).Cross-flow, sometimes called tangential-flow, is more effective than perpendicular or depth-flow, especially for wines with high particulate load. While some winemakers claim that cross-flow filtration is also effective for sterile filtration, others question this. Sterile filtration is better left for another day. Establishing a cross-flow filtration system involves considerable capital cost. Pad filtration is cheaper, but as most winemakers will say, it is a pain to set up. A small cross-flow system might involve an outlay of $30 000 or more. Alternatively, mobile units can be leased, making cash flow a little easier to manage.