With the complex relationship between the chemistry and microbiology of wine it is often difficult to know when to filter, what kind of filtration is necessary, and if the filtration process selected will be effective. In this article we hope to shed some light on the complexity of the filtration process.
Firstly, we’ll touch on a “frequently asked question”. I’m ready to bottle, is filtration really necessary?
Well, rare occasions occur where this complex bond between chemistry and microbiology of the wine is in sync and the wine is stable to the point where filtration is not deemed necessary. In these cases, the following criteria is met;
Alcoholic fermentation must be complete, i.e. less than 1g/L of the fermentation sugars glucose and fructose are present.
Brettanomyces and Pediococcus are capable of using the unfermentable sugar pentose. Less than 1g/L of pentoses must be present.
Malolactic fermentation is complete, i.e. less than 0.2 g/L malic acid remains.
Brettanomyces management has been performed and no Brettanomyces populations are present.
The necessary amount of free molecular SO2 is present in correlation with the pH of the wine. Ensure an excess of SO2 is present to account for SO2 loses during and immediately after bottling. Follow the link below if you have any uncertainties about the levels of your molecular SO2 in relation to your pH levels.
The wine is stable in terms of cold stabilization, protein stabilization and pinking in white wines.
All visibly suspended solids have settled out completely, including yeasts and bacteria.
For blended wines, the final blend should have been stable at cellar temperatures for at least six months.
Test bottles of the wine have been stable at room temperature for at least one month.
Unfortunately, if your wine does not meet these requirements filtration will most likely be deemed necessary, leading us to our next point. What are my options in terms of filtration?
Mechanistically, filtration can be divided into the two categories of perpendicular-flow and cross- or tangential flow filtration.
Perpendicular flow filtration includes screen filtration, depth filtration and membrane filtration. The particulate-laden suspension approaches the filter head-on where solids larger than the filters absolute porosity are retained. Filter media can either be diatomaceous earth, or powder, or paper/pads.
Screen filtration: often referred to as pre-filters, are used to reduce levels of large particles en route to pad or diatomaceous earth filters. Their primary application is, therefore, to reduce high solid loads that can cause problems in downstream filtration processes. However, in practice winemakers would generally opt to use conventional gravity clarification or selective fining agents instead of another tier of filtration.
Depth filtration: fibres and/or filter powders comprise this three-dimensional filter matrix that are randomly distributed within the filter. The wine must, therefore, travel through this network of channels and a range of cavities of different sizes where particles in the wine will become trapped. In principle, particles within the wine are being retained by a trapping or sieving effect, as well as, an adsorption effect. It is important to note depth filtration is not an absolute filtration and particles may still be present in the wine. Depth filtration would be considered more of a polishing or pre-filter before sterile filtration.
Membrane filtration: geometrically regular, porous filtration membranes that retain particles on its surface by size occlusion are used when employing membrane filtration. Depending on porosity membrane filtration can be absolute. Yeast removal can be achieved using a 0.6µm to 0.8µm filter, whereas, removal of lactic acid bacteria is achieved using a pore size of 0.45µ Where the microbiological stability of the wine is in question, a 0.45µm pore size is strongly recommended. Membrane filtration is used as a finishing filtration.
Cross- or tangential flow filtration involves filtering the wine across the surface of the filter matrix, thereby significantly reducing the potential of the filter to clog and block. Crossflow filtration is used effectively to remove large quantities of solids in a single filtration and is also an efficient technique for cleaning high sugar wines and juices that the perpendicular filters struggle with. Crossflow filtration is a useful tool in removing larger molecules, like proteins and phenolics, achieving protein and colour stability. Despite the filtration abilities of crossflow filtration, it should be regarded as a separation tool and should be utilized as a nominal and not an absolute filtration.
When deciding which approach to take with filtration measuring the filterability and turbidity is a useful tool. Turbidityis the degree of clarity or haziness of wine. Haziness can be caused by any suspended solids, including yeasts, bacteria, protein or pectin complexes, phenolic complexes and metal precipitates. Turbidity, measured as an NTU (nephelometric turbidity units), readings below 1.0 for white wines and below 2.0 for red wines mean the wine is brilliantly clean.
The Filterability index is useful if the wine will undergo an absolute filtration (0.45µm). Generally speaking, values below 30 mean the wine should not block a 0.45µm membrane filter regardless of the volume filtered. Values below 60 might have some volume limitations, while values higher than 60 will require some form of pre-filtration.
Lastly, a common and important point. My wines won’t filter: where can I look to solve this issue?
Pectins and glucans: These are polysaccharides that can cause both clarification and filtration problems in wine. The addition of pectinases and glucanases throughout the winemaking process will break down these long polysaccharide chains. These enzymes should be added as early on in the winemaking process as possible.
Heavy solid loads: wines that have not had enough time to settle may have large amounts of solid particles resulting in filtration problems. Gauge how dirty the wine is and decide on a pore size that will remove as much of the solid particles as possible. Subsequent filtrations will allow the winemaker to step down the pore size.
Various winemaking practices allow one to minimize filtration. Natural gravity is an age-old tradition but requires time and patience. Racking dislodges the carbon dioxide produced in the fermentation process. This carbon dioxide often holds certain particulate matter in suspension, when dislodged the particulate matter is allowed to settle. Fining agents also result in the removal of suspended solids but may lead to unexpected results and/or damage to wines aroma and mouthfeel. Sanitation reduces risks of spoilage microbes. Wineries free of spoilage microbes may more easily bottle wines with minimal or no filtration.
In summary, filtration is a wonderful tool that previous generations of winemakers were not fortunate enough to have. When implemented correctly, it is not an evil practice used to strip wines of character, finesse and colour but will contribute to the stability and shelf-life of wines.