Who has never had the unpleasant surprise of seeing the precious last glass of a bottle of wine spoiled by the presence of a solid deposit? In the worst case, the person did not realize it in time and sees his mouth filled with a granular liquid that is frankly not pleasant to drink.
I would like to reassure you right away that this deposit is in no way harmful; nor is it a sign of a sick wine in the vast majority of cases.
The solid particles present in wine are the result of a chemical reaction that makes certain compounds insoluble. They can have several origins: technical choice of the winegrower, evolution of the wine, tartar crystals or some wine diseases.
The absence of filtration (or light filtration) and fining favours the appearance of deposits in young red wines. Indeed, in its youth (when it is still with our winegrowers), wine is not this clear and smooth product that we consume. During the maturing process, some of the solid particles fall naturally to the bottom of the vats and barrels, this is called sedimentation, we then speak of bourbes and lees. The lees are mainly composed of soil and debris collected during the harvest; the lees are largely the skeletons of the yeasts that transformed the sugar into alcohol during fermentation, which can be of oenological interest during the winemaking process, while the lees are undesirable. The cellar master will then separate the liquid from the solid by making rackings: he transfers the wine from one container to another, taking only the liquid.
However, sometimes the solid parts have not yet had time to fall to the bottom. This is why some winemakers opt for fining followed by filtration. The fining, which is denser than the wine and has coagulant properties, will carry the remaining solid particles with it. Finally, the last residues will be eliminated by passing through a filter. The filtration can be more or less tight, new technologies allow the elimination of minute particles no larger than one micron (tangential filtration).
Wine makers who use filtration are aware that consumers do not appreciate being left with soup in their pretty crystal glass, but they are also subject to certain regulatory constraints when exporting to certain countries. However, a good number of winemakers believe that filtration brings with it certain aromatic molecules, in other words that it impoverishes the wine; or even that it kills the wine by removing a large part of the micro-organisms that make it evolve, making it come alive. This is why, in some young reds (or even whites), a dark deposit appears, and it is not a sign of poor quality, but is just the result of the sedimentation that takes place in the bottle.
But that's not the only cause of a deposit.
Two major constituents of wine are anthocyanins (colour of the wine) and tannins. Over time, chemical reactions take place between oxygen and these components. These reactions contribute to the improvement of wines for laying down, but are also the cause of a brown/orange deposit at the bottom of the bottle. Indeed, over time the oxygen "kills" some of these molecules, causing them to precipitate.
In some extreme cases, mostly when a wine to be drunk young has been kept for far too long, the phenomenon of dilution can occur: i.e. almost all of the wine's compounds have precipitated, the liquid part is then composed mainly of water and alcohol, and is therefore relatively tasteless. In order to avoid the inconvenience of the deposit when tasting our old, carefully preserved bottles, it is then necessary to carry out a decanting process. Some time before consuming the nectar, it is advisable to take the wine out of bed, in other words to move it from its lying position to an upright position in order to isolate the deposit at the bottom of the bottle. A cool temperature will help this process. Once this step has been completed, the liquid part should be poured into a carafe, leaving the deposit in the bottle. To do this, it is advisable to be in a bright room or to use a lamp so that the boundary between solid and liquid can be clearly seen. This step should be done shortly before consuming the wine, as old wines are very sensitive to contact with air. To limit contact with oxygen, the decanter should be corked as soon as possible once the transfer has been completed.
But that's not all. You may also have seen whitish crystals: this is tartaric acid which has precipitated in the heart of the bottle. This can happen to both reds and whites. Keeping your bottles in a cold place accelerates the creation of tartaric acid. Not all wines have this deposit; indeed, to avoid the formation of these crystals the precipitation of tartaric acid can be caused before bottling, this is called tartaric stabilization. The wine is cooled to temperatures close to zero to cause the formation of sediment, then the wine will be filtered.
There are also wines that are disturbed by chemical instabilities, for example, excess copper or iron. This is called wine disease: it is cloudy, it can change colour and a deposit forms. This is known as copper, iron (red wine turns blue) or protein breakage. However, these risks are now under control by oenologists and their highly sophisticated analytical tools, so it is unlikely that you will encounter this problem.
To conclude, the presence of deposits in the wine is therefore completely normal and testifies to the lively aspect of the beverage. It may be due to various chemical reactions within the bottle itself, which will be accelerated by poor storage conditions. So don't forget to decant your wines that have a deposit, and that's it, you just have to taste it.