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Polygalacturonic acid has been found to inhibit the spontaneous precipitation of calcium tartrate in a model wine solution. The monouronic acids, galacturonic and glucuronic, do not affect the crystallization of calcium tartrate. Using a calcium ion selective electrode, the binding of calcium to polygalacturonic acid was found to reach a maximum at pH 4 with significant complexation occurring between pH 3.0 and 3.5. The polygalacturonic acid also affects the membrane asymmetry potential of the calcium ion selective electrode, and an appropriate correction must be applied. A study of the concentration of polyuronic acids (chain length >8) in commercial wines by the m-hydroxybiphenyl method showed that they are present at sufficient concentration to inhibit the calcium tartrate precipitation in wine. Sparkling wines have a much lower concentration of polyuronic acids than still white table wines, and this difference may be responsible for the calcium tartrate stability of white table wines. Keywords: Calcium tartrate; spontaneous precipitation; polygalacturonic acid; wine; calcium ion selective electrode; membrane asymmetry potential