148 R. Marchal and P. Jeandet
observed in white wine during its conservation on lees. Finally, experiments using
carboxymethylcelluloses, polymers ofβ-D-glucose units whose primary and sec-
ondary alcohol groups are etherified by natrium carboxymethyl groups, are under
investigation to evaluate their ability to stabilize wine as regard to tartrate salts
precipitation.
5.2.1 Use of Metatartaric Acid for Stabilization of Tartrate Salt
Precipitation
Metatartaric acid is a polydisperse polymer, that is, a mixture of polymers with
different molecular weights as it consists of a polymerized compound formed
by the intermolecular esterification between the carboxylic group of one tartaric
acid unit and the secondary alcohol group of another molecule of tartaric acid
(Maujean 2000). The legally imposed minimum esterification rate for metatartaric
acid must be 40%. As the esterification rate than can be predicted theoretically is
at equilibrium at 33%, tartaric acid solutions must be heated to 160◦C in a partial
vacuum to obtain higher esterification rates (Maujean 2000). Metatartaric acid con-
tains some impurities such as oxaloacetic acid and, mainly, pyruvic acid (1—6 wt%
of metatartaric acid) which are formed, respectively, by a loss of one molecule of
water and the resulting decarboxylation ofoxaloacetic acid. Metatartaric acid acts,
as do other macromolecules known as “protective colloids”, by opposing the growth
of the submicroscopic nuclei around which crystals are formed. Metatartaric acid is
thus able to inhibit the transfer of tartrate salt units from the bulk (the hydroalcoholic
solution) to the growing crystal.
Though metatartaric acid is perfectly effective, this compound is not stable over
time since a rapid hydrolysis of the ester group may take place, leading to an
increase in acidity by release of tartaric acid units in wine. According to previ-
ous studies (Rib ́ereau-Gayon et al. 1977), a total hydrolysis of a 2% metatartaric
acid solution can be observed within three months at a temperature of 23◦Cand
ten months at 5◦C followed by a decrease by 50% of the esterification rate. As a
consequence, winemakers should prepare a concentrated solution of metatartaric
acid, that is 200 g/L, in cold water just prior to use. Unfortunately, metatartaric acid
was also proven to be unstable in wine, depending on the temperature used for
wine storage. Accordingly, Rib ́ereau-Gayon et al. (1977) have shown that metatar-
taric acid is stable for several years at 0◦C, over 2 years at 10–12◦C, 1 year to
18 months at temperatures that are commonly found in a cellar, that is 10◦Cin
winter and 18◦C in summer, 3 months at 20◦C, 1 month at 25◦C, 1 week at 30◦C
and a few hours at temperatures ranging from 35◦Cto40◦C! This instability is very
detrimental to the efficacy of treatments using metatartaric acid and some prob-
lems commonly occur with champagne wines that may have been stabilized with
this compound. Since metatartaric acid can be removed through fining and since
addition of this compound may induce an increase in wine turbidity, it is recom-
mended to add it to wine, respectively, after the fining step and before the final
clarification.