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.


Figure 1. Array of equipment needed to determine the filterability index of wine.

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.
The pressure vessel is filled with wine, from where it is transferred, by constant pressure from a nitrogen cylinder through a membrane filter in a filter device. The test can, however, be tedious and require considerable manual labour. Fully automated equipment is available. The measuring
principles, however, remain the same for all equipment. Different definitions for FI consist, which imply that a single so called safe FI does not consist. The most suppliers of filtration equipment usually have their own definitions and consequently values which are used by cellars. The Laurenty definition, which consists since 1972 is an example of it. According to his definition 2 different volumes of a wine are filtered with a 0.45μ membrane filter at constant pressure and the two filtration durations are measured in seconds. It can be formulated as follows:
FI = T400 – 2 x T200 where T400 and T200 are the filtration time to filter 400 and 200 ml of wine respectively .If the wine is suitable for filtration
the ratio between T400 and T200 will be equal to 2:1 and the FI will be 0. Laurenty, however, recommends that it is safe to filter a wine, without blocking the filter if the FI is <20. It is important that the same filter medium (membrane) is used for the FI-reading and the commercial filtration. The F1-reading can, depending on this condition, also be used for sheet filtration. Except for the filter blockages, which can be caused by certain wine components like polysaccharides and polyphenols, certain wine additives like arabic gum, tannins, mannoproteins and carboxymethylcellulose (CMC) can also contribute to a decrease in the filterability of wines. If the prescriptions of CMC-suppliers are followed
the CMC dosing will not cause filtration problems. It implies that the CMC dosing must be done 48 hours to 5 days before the bottling
NTU-readings as such, are thus not a guarantee to prevent filter blockages and must preferably be used together with filterability indexes (FI). (Bowyer et al, 2012) (The FI-determination is actually so basic that cellars should consider doing it themselves)
Bowyer, Paul., Edwards, Greg & Amelia Eyre. 2012. NTU vs wine filterability index-what does it mean for you Grapegrower & Winemaker. October 2012 (585): 76-80

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