Article by Charl Theron of Wineland Magazine

Only exceptional wine consumers will drink bottled wine if it is not crystal clear. The fact that these sediments or crystals are harmless does not change this consumer requirement. Winemakers often refer to the condition of wine when they evaluate the clarity of wine. Consequently, it is standard practice to apply a final filtration prior to bottling, seeing that it is the last opportunity to comply with the condition required by consumers.

The final filtration prior to bottling is usually a micro filtration, irrespective of whether sheet or membrane filtration is used for it. The blocking of the filters can occur slowly or quickly, depending on the wine which is filtered. It can even occur if the necessary prefiltration is applied by the cellar. A correlation between the clarity of a wine and its filterability should exist. The two measurements which are usually used in this regard are NTU (Nephelometric turbidity units) and filterability index (FI). The basis of the two measurements, do however, differ considerably. NTU is a laboratory measurement which indicates the turbidity degree of wine and FI is a small scale filtration which measures the filtration rate of wine over a certain period.

The turbidity measurements (NTU) is a method to quantify the visual clarity. Yeast, bacteria, amorphous material and crystals are some of the wine components which cause turbidity in wine. Usually a wine with a NTU reading <1,0 prior to bottling is seen as clear enough. If it is however >1,0 another filtration prior to bottling is recommended. A NTU-reading of <1,0 is however not necessarily a guarantee that the filter medium will not be blocked. The contrary is, however, also true that wines with a NTU reading >1,0 can pass the filterability test. The disadvantage of NTU-measurement is that two wines may have the same reading, but the composition or nature of the suspended material in the two wines may differ. Small particles can for example block the filter medium easier than large particles, which remain on the surface of the filter medium but do not block it. The blocking of filters does not necessarily correlate with the measured clarity degree (NTU) of wine. Factors which also play a role in the blocking of filters are temperature and colloids which occur in wine. Blocking occurs easily at low temperatures. Colloidal compounds like polysaccharides, phenols and proteins can block filters easily. These compounds are associated with winemaking practices. Cellars which for example do not use pectolytic enzymes, or leave wine on fermentation lees will have higher pectin and mannoprotein concentrations in their wines, which can cause filters to block easily. Botrytis-contaminated grapes also cause more glucans in wine, which may cause filter blocking.

The filterability index (FI) of a wine is an indication of the time needed to block a specific filter medium during filtration. It is a basic test, seldom applied by cellars themselves and is usually done by service laboratories. The equipment required is basic and cheap. An array of the basic laboratory equipment needed for it is indicated in figure 1.