Potassium bitartrate precipitation

Rhein and Neradt (37) reported an accelerated method for bitartrate removal by chilling wine, adding 4 g/L powdered potassium bitartrate, and filtering the wine ninety minutes later. Conductivity measurements were used to monitor potassium bitartrate precipitation. To the author s knowledge, this method has not yet been used in the North Coast. 

In other work, Moine-Ledoux et al. (1997) reported that the use of Mannostab at doses ranging from 15g/hL to 25g/hL inhibit potassium bitartrate precipitation (Table 5.2) while excess amounts of this additive, that is 30g/hL, are ineffective on potassium bitartrate crystallization (Table 5.2). Within the extracts, compounds responsible for the stabilizing effect observed were found to be highly glycosylated mannoproteins of molecular masses ranging from 30 kDa to 40 kDa possessing a glycosyl-phosphadityl-inositol anchor (GPI) (Moine-Ledoux and Dubourdieu 1999, 2002,2007). 

Examination of the results shows that adding 100 g/hl to a cuvee must or wine only resulted in 10-15% acidification, corresponding to an increase in total acidity of approximately 0.5 g/1 (H2SO4). Evaluating the acidification rate from the buffer capacity gave a similar result. The operation was even less effective when there was a high potassium level, and potassium bitartrate precipitated out when the tartaric acid was added. 

Table 1.19. Inhibition of potassium bitartrate precipitation by various metatartaric acids (Peynaud and Guimberteau, 1961)...

The bitartrate ion can combine with potassium ion, also present in high concentrations in grapes, to form the soluble salt potassium bitartrate (also known as cream of tartar). In water sodium bitartrate is fairly soluble 1 g dissolves in 162 ml of water at room temperatureJ1 In alcohol solution (formed as fermentation of the wine yields ethanol), the solubility of potassium bitartrate is significantly reduced 8820 ml of ethanol are required to dissolve 1 g of the saltJ As a consequence deposits of potassium bitartrate form as the salt precipitates out of solution. 

Sulphates. — Dissolve 1 gm of potassium bitartrate in 20 cc. of water, add 5.0 cc. of nitric acid and barium nitrate solution. No precipitate should form within twelve hours. 

A further consideration of the change in pH is the affect on tartrates. As the pH reaches 3.56-3.60 (the midpoint between the two pKa s for tartaric acid) the precipitation of potassium bitartrate is increased (65, 66). This decrease in tartrate concentration in the wine is beneficial in the overall process of achieving tartrate stability. Although the pH of the 1972 wines did not reach this level, the pH does reach and exceed this range in some years (64). 

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). 

This is precipitated as potassium bitartrate by addition of excess of a potassium salt and the solution then titrated with N/4-alkali. 

Total alkalinity of the ash. This alkalinity, expressed in c.c. of N-alkali per litre is, on the average, ten times the number of grams of ash per litre. A low value of the alkalinity of the ash may indicate some treatment of the wine such as plastering or addition of phosphate of free mineral acid, resulting in decomposition Or precipitation especially of the potassium bitartrate, On which the alkalinity of the ash largely depends. 

Total acidity. This acidity, expressed as tartaric acid, varies from 4-5 to 15-X6 grams per litre. Wines rich in alcohol are relatively less acid than those of low alcohol content, owing to precipitation of the potassium bitartrate by the alcohol. Further, the total acidity of a wine diminishes as the wine ages, in consequence of precipitation Of this salt and also Of the tannin and likewise of decomposition of the malic acid into lactic acid Of one-half the equivalent acidity. On the other hand, the total acidity may be increased indirectly as a result of certain diseases of the wine which increase the volatile acidity. 

The brine solution may be enriched with NaCl or KHT to increase its conductivity and acidified to the same pH of the wine under treatment. It is recirculated between the concentrating compartments of the ED stack and another tank equipped with conductivity and pH automatic controls so as to avoid precipitation of potassium bitartrate onto the membranes by diluting the brine with deionized water and/or discharging more or less aliquots of the brine itself when its conductivity reaches 70-80% of its saturation value (Goncalves et al., 2003). In this way, a waste effluent in the range 10-20% of the wine volume treated is to be disposed of or used to recover KHT crystals (Nasr-Allah and Audinos, 1994). 

Clarification, by the sedimentation of suspended particles and precipitation of salts such a potassium bitartrate, is facilitated by storage in barrels. Their small volume reduces convective phenomena and allows the wine s temperature to cool markedly during the winter, encouraging both phenomena. The precipitation of unstable colloids, that can cause wine turbidity, also occurs during maturation. The precipitates are subsequently removed during racking. 

Unfavorable bottled wine storage conditions may cause the appearance of haze or deposits in white wines. Excessive heating of a bottle of white wine may precipitate protein, causing a haze or cloud excess cold may crystallize potassium bitartrate, creating a layer of small crystals in the bottle. Such haze and precipitates in wines may or may not have negative sensory effects but often affect adversely consumer reaction to the wine. 

Bitartrate Stabilization. Potassium and tartaric acid are natural constituents of the grape. Wine content of these constituents depends on a number of variables, not all well understood variety, vintage, and weather pattern degree of skin contact alcohol level bitartrate holding capacity of phenolic compounds and potassium binding capacity of the wine (30, 35). Most wines after fermentation are supersaturated solutions of potassium bitartrate. This compound is less soluble at lower temperatures, and, thus, lower temperatures will cause precipitation of bitartrate crystals. This lowering of temperature and subsequent removal of crystals by filtration is called cold stabilization. 

Care must be taken when fining a sparkling wine with bentonite in order to preserve its foaming properties. Excessive use of bentonite for the fining of sparkling wine cuv es can produce a finished product that has a large bubble size and a poor bubble stability as a result of a reduction in both protein and peptide contents. Cold stabilization procedures cause both a precipitation of potassium bitartrate crystals as well as proteins because of the downward shift in pH. This precipitation of proteins... 

Clarification of the must in winemaking is made by treatments with silica gel, filtration, centrifugation, or by the use of enzymes. During filtration and centrifugation, oxidation of polyphenols may occur loss of protective colloids occurring with enzymes can promote precipitation of potassium bitartrate affecting the tartaric acid, pH and total acidity data. For determination of organic acids either in skins or in the... 

It has been reported that ultrasound can be used to clarify wines through the precipitation of potassium bitartrate [70], The treatment reduces the precipitation time from 4 to 10 days to 1.5-2 h. 

Partly Consumed by Yeast. Partly precipitated as Potassium Bitartrate Partly give zest, to finished wine,... 

This convention is justified by its convenience, provided that (Section 1.4.2) there are no sudden inflection points in the neutralization curve of the must or wine at the pK of the organic acids present, as their buffer capacities overlap, at least partially. In addition to these somewhat theoretical considerations, there are also some more practical issues. An aqueous solution of sodium hydroxide is used to determine the titration curve of a must or wine, in order to measnre total acidity and buffer capacity. Sodium, rather than potassium, hydroxide is used as the sodium salts of tartaric acid are soluble, while potassium bitartrate would be likely to precipitate out during titration. It is, however, questionable to use the same aqueous sodium hydroxide solution, which is a dilute alcohol solution, for both must and wine. 

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. 

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