Filtration is one of the many tools winemakers have in their tool box, should they choose to use it. However, there are many types and applications for filtration that can be used at various stages of production, so there are a few questions to ask first about when, where, what, how and why.
Matt Holdstock, a Senior Oenologist at the Australian Wine Research Institute (AWRI), works with Australia’s winemakers and production staff on a daily basis to solve production issues and support them in implementing best practice wine production. Here are his top tips for filtration.
1. When to sterile filter
Know your wine’s composition. If it contains residual sugar, then it should be sterile filtered to reduce the risk of spoilage post-bottling. Using the wine’s history and composition can help you assess the risk and know if sterile filtration is required.
2. Nominal versus absolute
Use absolute membrane filters when sterile filtering. Nominal filters have a wider tolerance to the stated size of the filter and are not appropriate for achieving sterile filtration.
3. Fit for purpose
Choose the correct filtration type for the job at hand. Cross-flow filters are great for preparing wines for sterile filtration (0.45 µm membrane), but are not a replacement for sterile membrane filtration. Typically, sterile filtration in Australia involves 0.45 µm absolute membranes.
4. Storing filtration cartridges
It is common practice is to store filtration cartridges in an acidified potassium metabisulfite solution when not in use. Be aware that the antimicrobial part of this solution, free sulfur dioxide, does degrade and should be routinely checked and replaced.
5. Membrane fouling
This can still occur in cellar bright wines with turbidity <1 NTU due to the presence of macromolecules such as polysaccharides. Filterability testing can tell you whether or not a membrane filter is likely to foul easily.
6. Filterability and NTU
It is important to understand the meaning of these two terms. NTU will measure how much light is scattered, whereas the filterability index (FI) will give you an understanding of the type of material present and whether or not a membrane filter will foul easily. Low NTU does not equate to a wine that will always filter easily.
7. Integrity testing of membrane
When employing sterile membrane filtration, ask the question – have these filters been integrity tested and did they pass?
8. Final note
A well performed filtration of the appropriate grade will not have a negative effect on wine quality.