6.1.1.a Turbidity

The purpose of clarification is to obtain a clear, bright wine. A wine is described as cloudy when many particles and/or colloids are observed in suspension.
While deposits of colouring matter and tartar are acceptable in older red wines, the clarity of a wine is generally a consumer requirement. Furthermore, suspended particles and macromolecules can cause precipitations, microbiological risks and decrease filtration quality.

A wine is considered clear when the turbidity approaches 1 NTU for whites and 2 NTU for reds.
Clarification is differentiated from stabilization by its purpose for the wine. It is possible to clarify a wine without necessarily enhancing its stability and vice versa. It is also possible for an operation to be both clarifying and stabilizing. These terms are explained and defined later.

The first parameter generally taken into account when clarifying a wine is the turbidity measured in NTU (Nephelometric Turbidity Unit). This corresponds to a liquid's cloudiness, and is caused by the presence of particles in suspension.
Turbidity meters are generally inexpensive and easy to handle. However, turbidity does not identify the quantity, type or size of particles responsible for the cloudiness.

For example, the same turbidity can be caused by either a high number of fine particles or a low number of large particles. The presence of tartar crystals can also distort the turbidity due to a reflective effect. This instantaneous measurement does not reveal colloids responsible for clogging, which makes it insufficient for assessing a wine's filterability.

Other measurements can be used to identify particle types that create turbidity in a wine, such as calculating the percentage of solids, which is easily implemented when the winery is equipped with a centrifuge, or a particle counter. These measurements are not widely used because of their cost and poor interpretability.

Particle types and macromolecules according to their size

A wine's cloudiness can be caused by a wide variety of particles. Plant debris usually accounts for the largest particles with a diameter exceeding 50 µm. These are also the particles that ordinarily would settle quickly because of their weight. Next come various crystals, which are sometimes visible to the naked eye. Finally, micro-organisms such as yeast and bacteria can be differentiated by their diameter (between 10 and 1 µm for yeast and between 1 and 0.5 µm for bacteria). Furthermore, filtration is adjusted to suit the type of particles present in the wine.

Particle diameter	Particles
>50 mm	Plant debris, clusters, flocculants
50 mm >> 10 mm	Crystals, miscellaneous debris
10 mm >> 1.0 mm	Yeast, precipitates
1.0 mm >> 0.5mm	Bacteria
0.5 mm >> 0.1mm	Amorphous particles, colloidal macromolecules
< 0.1 mm	Coloring matter, various colloids

Principales particules du vin (according to E. Peynaud, 1975) Source: Romat H., 2014. Filtration en œnologie. Féret Bordeaux Edition.

Filterability can depend on turbidity

When turbidity is higher than 50 NTU, it is impossible to measure a wine's filterability, i.e. its ability to be filtered without any problems. A wine with an elevated turbidity has a high risk of clogging during the filtration process. It is therefore necessary to reduce turbidity before proceeding with an operation of this type.

When turbidity is less than 50 NTU, it is possible to measure the filterability of a wine using various methods (See The different filterability measurements of a wine ). These tests help to narrow down the choice of the clarification technique(s) to be implemented.

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