**2.3 Colour colloidal instability**

Colour instability of some red wines can cause product depreciation. In the last years, excessive precipitation of colouring matter has been observed at the bottom of the storage tanks and even in bottled wines [27]. This precipitation occurs along with the natural evolution of red wine during the storage period and results in a considerable modification of its colour and limpidity that is undesirable in terms of visual perception and loss in taste and flavour [22]. This precipitate may happen after a few months of ageing. It may be sometimes gelatinous and strongly red coloured. If the red wine is aged and bottled, the precipitate may occur later in the form of a thin leaf, lining the inner side of the bottles [28]. A method commonly used to reduce wine turbidity and stabilise the colouring matter is fining by the addition of proteins (albumin, casein, or gelatine) that promotes flocculation or precipitation before bottling [29]. However, in some cases, the precipitate formation can still be observed in the later stages of the ageing process. The addition of arabic gum could prevents wine colour instability [22].
