In the previous article on wine disorders, we identified several components that can cause organoleptic changes in taste, color and clarity of wine. One of the main mandates of the consulting oenologist will be to ensure, in collaboration with the laboratory, the stabilization of the wine in order to prevent physical, chemical or microbiological alterations. Since bottling is the culmination of several months of work, it is imperative to manage the risk well before the final packaging of the wine.
Tartaric precipitation is a condition that we frequently encounter in the laboratory due to the Quebec climate and terroir. During a difficult harvest, such as in 2022, it is possible and even necessary for some winemakers to resort to wine deacidification operations under the advice of the oenologist in order to reduce acidity levels that can reach up to 20 g equivalent tartaric acid/L without treatment. However, this operation will increase the risk of crystal formation such as potassium bitartrate and calcium tartrate if high concentrations of calcium carbonate are used. Laboratory testing make it possible to evaluate the probability of potassium bitartrate crystal formation and then estimate whether preventive treatment should be applied. However, it is not currently possible to provoke tartromalate crystal precipitation because this type of precipitation is completely random and can occur several months after bottling. On the other hand, the laboratory can evaluate the calcium concentration in order to assess the risk. A calcium concentration exceeding 60 mg/L for red wines and 80 mg/L for white wines increases the risk of calcium tartrate precipitation.
Among the recommended treatments for tartaric stabilization, we find cold treatment, electrodialysis, cation exchange columns and the addition of inhibitory agents. Cold treatment of wines will make the potassium tartrate less soluble and cause it to precipitate. The wine can then be cold filtered to remove the crystals. In the electrodialysis process, tartric and potassium ions are removed by means of an electric current. Although effective, this method will have an impact on the organoleptic properties of the wine. Ion exchange columns are made up of a resin containing several charged ions. When the wine passes through, potassium ions will be retained in the column or “exchanged” for cations such as H+ ions. The addition of inhibitory agents such as carboxymethylcellulose, mannoproteins or metatartaric acid prevents the formation of potassium bitartrate crystals.
Protein stabilization aims to reduce the risk that proteins in colloidal suspension will precipitate to form an opaque haze under the influence of heat or pH. This precipitation risk is evaluated in the laboratory during bottling analyses. If a risk of haze is observed, the oenologist may proceed, in collaboration with the laboratory, with clarification tests using different fining agents in order to eliminate problematic proteins. Several types of fining agents have been used historically such as gelatin, casein, egg whites and bentonites. These substances all have in common an ease of agglomeration with wine proteins. New fining agents are now available such as pea proteins, yeast bark and wheat proteins. The laboratory determines the effective dose of the fining agent to be used in concert with the oenologist who will judge the effect of the treatment on the wine’s aromas. Following treatment which can last several days, the fining agent will be eliminated by sedimentation or filtration. The clear wine will then be racked.
Microbiological stabilization consists of evaluating not only the risk of bacterial or fungal contamination but also the risk of refermentation in the bottle, particularly if sugar is present. Microbiological stabilization can be carried out by physical techniques such as filtration, tangential microfiltration and heating. Sheet filters come with varying porosities, making it possible to carry out successive polishing, clarifying and sterilizing filtrations. Tangential microfiltration involves filtering wine parallel to a porous membrane using a pressure gradient. The main advantage of using tangential microfiltration is combined wine clarification and stabilization without affecting its organoleptic properties. However, this instrument represents a significant investment for the winemaker. Wines can also be stabilized by heat through pasteurization. During this process, the liquid is heated for a few minutes at a temperature high enough to destroy microorganisms (between 60 and 70oC). The effectiveness of pasteurization depends on the type and quantity of microorganisms present. It may have a detrimental effect on certain wine aromas. To mitigate this problem, it is possible to carry out a “flash-pasteurization” which involves heating the wine to 72oC for about twenty seconds. This method is particularly suitable for sweet wines and red wines containing malic acid and very little sulfites. However, wines must already be heat-stable. The use of high pressures is an innovative but little-used method due to its cost. It involves exposing the wine to cold at pressures of 100 to 600 mega pascals. In this situation, the membranes of microorganisms are weakened, resulting in the inactivation of yeasts and bacteria.
Colorimetric stabilization aims to preserve the color of red wine, which can deteriorate over time due to oxidation or reduction. To do this, several processes can be used: the addition of antioxidants, such as sulfur dioxide or ascorbic acid, which prevent the oxidation of wine pigments; the addition of tannins, which stabilize pigments through complex bonds; micro-oxygenation, which involves introducing oxygen into the wine in a controlled manner to promote the polymerization of tannins and pigments.
In conclusion, wine stabilization is a set of practices aimed at ensuring the quality and durability of wine after bottling. It requires a good understanding of the physico-chemical and microbiological factors that influence wine and techniques adapted to each type of wine and each objective. Do not hesitate to seek the assistance of OENOSCIENCE laboratory and your oenologist in this crucial production step.
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