STABILIZATION AND BOTTLING

After fermentation, the wine is cloudy due to the presence of colloids, yeast residues (viable and decomposition), precipitated by supersaturation in the medium and the alcoholic waste plant must be removed to obtain a transparent product. Part of these remains are eliminated by natural sedimentation, but the clarification processes are also used by the addition of some agents (bentonite, gelatin, casein, ovalbumin, tannins) and subsequent filtration. The addition of any of these agents may affect the characteristics of the final product in one way or another, therefore, the specific treatment is generally decided based on the results of the wine aliquots. In these aliquots the essential properties (acidity, pH, color, polyphenols) are determined before and after the treatment to decide the ideal protocol in each case.

This is also the right time to evaluate the stability parameters of the wine to avoid the appearance of undesirable failures both before and after the bottling process that cause the appearance of turbidity, precipitates or changes in the clarity or color of the wine after bottling. , called failure, which results in a negative evaluation of the final wine. The most common faults are tartaric, iron, copper, oxide, water and proteins, depending on the cause of the appearance of precipitates.

Tartaric failure involves the appearance of crystals of potassium bitartrate and calcium tartrate. Tartaric acid can bind potassium ions to form a low alcohol solubility compound. At the end of the fermentation process, the levels of tartaric acid, potassium and calcium in the wine are close enough to saturation (or even supersaturation), which can cause the formation of easily visible crystals in the right conditions. These crystals do not affect the organoleptic characteristics of the wine, but represent an important visual defect. The strategy in this case is to reduce the concentration of potassium and calcium (for example, through exchange resins), or to force precipitation by cold treatment of the wine (cryoprecipitation).

Iron is an element that appears naturally in grapes in concentrations between 2 and 4 mg/L, while in wine it is between 4 and 20 mg/L. Concentrations greater than 10-12 mg/L are capable of producing a ferric rupture due to oxidation in the presence of oxygen from Fe²⁺ to Fe³⁺ ions, which form insoluble compounds. In white wines, these compounds are whitish phosphates (white boxes), while in red and rosé wines, iron is part of complexes colored with polyphenols (anthocyanins and tannins), resulting in a precipitated bluish hue (blue failure ). To prevent this, it is recommended to treat with antioxidants, such as citric acid or ascorbic acid in the presence of excess sulphites (both procedures are subject to legal limits in terms of concentration).

Although copper is the most abundant source of phytosanitary treatments performed on grapes (and can reach values of up to 10 mg/L), most of these precipitate copper and are eliminated by depositing stools in the fermenter. The residual value of copper normally varies, therefore, between 0.2 and 0.5 mg/L. The problem arises when the copper concentration increases due to contact with the materials of the containers used in the basement and the risk of copper failure. It is high if its concentration exceeds 0.5 mg/L. In this case, Cu²⁺ is reduced (and therefore assumes the absence of oxygen) to Cu⁺ causing colloidal precipitation of cuprous sulphide, Cu²S. Cuprous sulfide, in turn, can bind to proteins and cause their flocculation, in the form of whitish precipitates with a milky appearance. Since red wines are much poorer in protein, this type of problem occurs particularly in white wines.

Oxidasic failure is a consequence of the use of grapes infected with Botrytis, in which enzymes with polyphenol oxidase properties appear, very particularly, lacasa. These enzymes cause the oxidation of the hydroxyl groups in the ortho position of the polyphenols to quinones, whereby the characteristic red color of the anthocyanins changes to the brown of the quinones produced. The way to avoid it is to increase the concentration of antioxidants, such as sulphites or ascorbic substances, or to treat with deproteinizing substances (bentonite, casein or tannins) in the clarification phase.

Finally, the failure of hydrolysis is manifested by the hydrolysis of anthocyanins, which causes a loss of color through the precipitation of polymers released with anthocyanidins in the process. This precipitation is mainly due to temperature variations in the wine: hydrolysis increases with temperature, forming colloidal precipitates that, when the temperature is lowered, can polymerize and flocculate. It appears especially in young wines of little color (the wooden tannins in old wines are factors that help stabilize anthocyanins) and have a low iron content.

Finally, the breakdown of proteins appears due to the precipitation of natural grape proteins denatured by the effect of acids, acetaldehyde and phenolic compounds produced. To eliminate bentonites (hydrated aluminum silicates), sodium or calcium can often be used to retain a certain number of proteins, but also other components, such as polyphenols or ions, from electrostatic interactions, so their use should be carried out under completely controlled conditions, after checking the expected result in aliquots of the wine to be treated.

In order to correctly establish the following steps, and their correct stable and precipitate-free leader sequence, it is essential to maintain adequate control of all critical components with precise, reliable and precise laboratory tests that allow their bottling safely.