Potassium Bitartrate and Calcium Tartrate

Brightfield microscope and several slides and cover slips 

H2SO4 (1+3) Carefully add 1 volume of concentrated sulfuric acid to 3 volumes of deionized water. 

Sodium metavanadate (NaV03) (3% w/v) In a 100-mL volume flask, dissolve 3 g NaV03 in hot ( 70°C/158°F) deionized water. Cool and bring to volume. Note NaV03 does not fully dissolve and must be filtered through Whatman 2 (or equivalent) filter paper prior to use. 

Collect a portion of wine sample containing the suspect sediment either by filtration or centrifugation. 

Rinse sediment/crystals with a small volume of deionized water and apply vacuum to remove water. 

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Metatartaric acid is a polyester resulting from the inter-molecular esterification of tartaric acid at a legally imposed minimum rate of 40%. It may be used at doses up to a maximum of 10 g/hl to prevent tartrate precipitation (potassium bitartrate and calcium tartrate) (Ribereau-Gayon etal., 1977). 

Potassium bitartrate and calcium tartrate are responsible for the physical stability of wines. A portion of tartaric acid slowly esterifies with ethanol to form ethyl bitartrate. Malic acid is converted to lactic acid (0-2.5 g/L) during malolactic fermentation, and the taste of wine becomes weaker. Succinic and acetic acids are also formed during fermentation [18,19]. The content of organic acids in vinegar and wines is shown in Table 10.7. 

Calcium tartrate is produced by extracting wine lees (deposited on the bottom of vats containing 19 to 38 % of potassium bitartrate) and grape marc (the residue from pressing grapes), filtering the solution and then adding slaked lime and calcium chloride [31.18]. 

The insoluble salts, potassium bitartrate (KHT) and calcium tartrate (CaT), may be found in bottled wine as crystalline precipitates of varying... 

Tartaric Acid. Quantitative measures of total tartrate are useful in determining the amount of acid reduction required for high acid musts and in predicting the tartrate stability of finished wines. Three procedures may be used. Precipitation as calcium racemate is accurate (85), but the cost and unavailability of L-tartaric acid are prohibitive. Precipitation of tartaric acid as potassium bitartrate is the oldest procedure but is somewhat empirical because of the appreciable solubility of potassium bi-tartrate. Nevertheless, it is still an official AO AC method (3). The colorimetric metavanadate procedure is widely used (4, 6, 86, 87). Tanner and Sandoz (88) reported good correlation between their bitartrate procedure and Rebeleins rapid colorimetric method (87). Potentiometric titration in Me2CO after ion exchange was specific for tartaric acid (89). 

Another method of acid amelioration, used to avoid water amelioration, is the addition of calcium salts for the purpose of substituting calcium for potassium ions. The resulting calcium bitartrate salts, being less soluble, increase the precipitation of bitartrate. This raises technical problems, one being that if the malo-lactic fermentation should take place subsequently, the wine may contain insufficient acidity of any kind. Another is that calcium tartrate precipitates slowly and in more finely divided form, often causing persistent hazes that are hard to remove. Often, too, this precipitation is delayed, leading to the presumption that the wine is tartrate stable. Only after it is bottled, the brilliant and supposedly stable wine may develop a delayed calcium tartrate haze and even a deposit. The calcium salt method in a refined form is used considerably in Germany but rarely here. 

On the other hand, it is a good thing that wines have such low pH values, as this enhances their microbiological and physicochemical stability. Low pH hinders the development of microorganisms, while increasing the antiseptic fraction of snlfnr dioxide. The influence of pH on physicochemical stability is due to its effect on the soln-bility of tartrates, in particular potassium bitartrate but, above all, calcium tartrate and the donble salt calcinm tartromalate. 

In fact, this is a rather simplistic explanation, as it disregards the side-effects of the precipitation of insoluble potassium bitartrate salts and, especially, calcium tartrate, on total acidity as well as pH. These side-effects of deacidification are only fully expressed in wines with a pH of 3.6 or lower after cold stabihzation to remove tartrates. It is obvious from the pH expression (Eqn 1.2) that, paradoxically, after removal of the precipitated tartrates, deacidificafion using CaCOs and, more particularly, KHCO3 is found to have reduced the [salt]/[acid] ratio, i.e. increased true acidity. Fortunately, the increase in pH observed during neutralization is not totally reversed. 

It is important that the wine should really neutralize the CaCOs/CaT mixture and not the reverse, as the formation of the stable, crystalUzable, double tartromalate salt is only possible above pH 4.5. Below this pH, precipitation of the endogenous calcium tartrate occurs, promoted by homogeneous induced nucleation with the added calcium tartrate, as well as precipitation of the potassium bitartrate by heterogeneous induced nucleation (Robillard et al., 1994). 

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