Prediction and mastering of wine acidity and tartaric precipitations: the Mextar® software tool

The MEXTAR® software is a mathematical tool able to predict the physicochemical state of a wine. Solving rigorously physicochemical equilibria, including complexation ones, it enables to simulate various operations endured by a wine: tartaric precipitations, acid addition, acid removal, malolactic fermentation. Using only classical chemical analytic determinations, namely, pH, total acidity, tartaric acid and potassium content, MEXTAR® computes the physicochemical state of a wine before and after the endured operation: pH and total acidity, tartaric salts supersaturation and the associated saturation temperature taking into account both the ionic activity coefficients and the complexed fraction of the involved ions, amount of tartaric salts that can be precipitated to obtain either a supersaturation chosen by the user at a given temperature or a chosen final saturation temperature, required amount of acidification or desacification chemical substances to reach a chosen pH or total acidity. The article describes the theoretical background upon which MEXTAR® has been developed and details the computation procedures associated to the simulation of the operations described above. Iterative processes are used to determine the various parameters enounced above. They aim in particular at the verification of the electroneutrality of the sample. However, at first electroneutrality is rarely verified because the wine chemical analytical determination is never exhaustive. Therefore, we introduce the concept of «vinic acid» to compensate the negative ion deficit of usual wine chemical analytical determination. This «vinic acid» is describe as a diacid with a fixed second dissociation constant (pK2 = 5) and a first dissociation constant determined automatically by MEXTAR®. This enables MEXTAR® to simulate accurately experimental tartaric precipitations for several samples. Thanks to the predicting capacity of MEXTAR®, the correlation between the total polyphenol content (IPT) in red wines and the difficulty to stabilise high IPT wine with respect to tartaric salts is confirmed. Finally, malolactic fermentations are also well simulated. A validation of MEXTAR® for the acid addition or removal has yet to be done when reliable experimental data are available.