Wines buffer capacity

In addition to the use of malo-lactic fermentation in red wines, it also has been tried in V. vinifera cultivar Chardonnay. In the experiments known to the author, the use of the malo-lactic fermentation in Chardonnay has not proved successful from a sensory point of view. In general, the rise in pH was too great and the buffering capacity of the wine too great to permit adequate adjustment with tartaric acid. However, this work is continuing in conjunction with a number of variations in the local viticultural practices to produce Chardonnay of a lower total acidity. In addition to the use of malo-lactic fermentation for the reduction of the acidity, considerable work has been done in Washington on the use of acid reduction with calcium. Both calcium carbonate and the double salt precipitation, as described by Steele (23, 24), have been utilized. Some very significant successes have been achieved, particularly with the double salt method. 

The total acidity of must or wine takes into account all types of acids, i.e. inorganic acids such as phosphoric acid, organic acids including the main types described above, as well as amino acids whose contribution to titratable acidity is not very well known. The contribution of each type of acid to total acidity is determined by its strength, which defines its state of dissociation, as well as the degree to which it has combined to form salts. Among the organic acids, tartaric acid is mainly present in must and wine as monopotassium acid salt, which still contributes towards total acidity. It should, however, be noted that must (an aqueous medium) and wine (a dilute alcohol medium), with the same acid composition and thus the same total acidity, do not have the same titration curve and, consequently, their acid-alkaline buffer capacity is different. 

The slope of the linear segment of the two neutralization curves differs noticeably. The curve corresponding to the must has a gentler slope, showing that it has a greater buffer capacity than the wine. 

Wines acidobasic buffer capacity is largely responsible for their physicochemical and microbiological stability, as well as their flavor balance. 

For example, the length of time a wine leaves a fresh impression on the palate is directly related to the salification of acids by alkaline proteins in saliva, i.e. the expression of the bnffer phenomenon and its capacity. On the contrary, a wine that tastes flat has a low buffer capacity, but this does not necessarily mean that it has a low acidity level. At a given total acidity level, buffer capacity varies according to the composition and type of acids present. This point will be developed later in this chapter. 

One method that is httle-known, or at least rarely used to avoid this total acidity imbalance, consists of partially or completely eliminating the malic acid by chemical means, using a mixture of calcium tartrate and calcium carbonate. This method precipitates the double calcium salt, tartromalate, (Section 1.4.4, Figure 1.9) and is a very flexible process. When the malic acid is partially eliminated, the wine has a buffer capacity based on those of both tartaric and malic acids, and not just on that of the former. Tartrate buffer capacity is less stable over time, as it decreases due to the precipitation of monopotassium and calcium salts during aging, whereas the malic acid salts are much more soluble. 

Standard acidification and deacidification methods are aimed solely at changing total acidity levels, with no concern for the impact on pH and even less for the buffer capacity of the wine, with all the unfortunate consequences this may have on flavor and aging potential. 

This is certainly due to the lack of awareness of the importance of the acid-alkali buffer capacity in winemaking. Changes in the acid-alkaline characteristics of a wine require knowledge of not only its total acidity and real acidity (pH), but also of its buffer capacity. These three parameters... 

It is now possible to automate plotting a neutralization curve, with access to the wine s initial pH and total acidity, so measuring buffer capacity at the main stages in winemaking should become a routine. 

Theoretically, variations AB and ApH must be infinitely small, as the value of the AB/ ApH ratio at a fixed pH corresponds geometrically to the tangent on each point on the titration curve (Figure 1.4). More practically, buffer capacity can be defined as the number of strong base equivalents required to cause an increase in pH of 1 unit per liter of must or wine. It is even more practical to calculate smaller pH variations in much smaller samples (e.g. 30 ml). Figure 1.4 clearly shows the difference in buffer capacity of a model solution between pH 3 and 4, as well as between pH 4 and 5. 

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. 

Applying Buffer Capacity to the Acidification and Deacidification of Wine... 

It is also necessary to know the wine s acido-basic buffer capacity. Thus, in the case of wines from northerly regions, initially containing 6 g/1 of malic acid after malolactic fermentation, tartrating may be necessary to correct an impression of flatness on the palate. Great care must be taken in acidifying this type of wine, otherwise it may have... 

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. 

Copper casse is specific to white wines. They are not as well protected from oxidation and reduction phenomena as red wines, where phenols have a redox buffer capacity. Furthermore, the colloidal cupric derivative contains proteins, while red wines have a low protein content due to combination reactions with phenols. 

Amino acids have molecular weights below 200, and 32 of them have been identified in must and wine. The most important are listed in Table 5.1. Amino acids contribute to the acidobasic buffer capacity... 

The sensory impression of oxidation or reduction in wine indicates abnormal development. This is linked to the presence of an oxidizing (oxygen) or reducing agent, and is also related to the buffer capacity that protects wines to varying degrees from sharp variations in their oxidation-reduction potential. 

Independently of its disinfectant properties, sulfur dioxide is widely used to protect wines from oxidation (Volume 1, Section 8.7.2). It thus contributes to the oxidation-reduction buffer capacity and prevents an increase in potential that would otherwise occur when oxygen is dissolved. Due to their structure, white wines require a higher dose of SO2 than red wines to ensure effective protection. 

The buffer capacity of wines is largely owing to the buffer capacity of the organic acids they contain. The following data of Kramer and Bohringer (1940) are for 0.5% solutions of four acidic substances and a mixture they show the change in pH with dilution with water ... 

These results show that the pH changes least with a mixture of tartaric acid and potassium acid tartrate. Addition of sugar, alcohol, or gelatine generally raised the pH values and on dilution, even up to 50%, the pH s remained higher. Morani (1930) correctly noted the influence of strong acids on the buffer capacity of wines. Morani and Marimpietri (1930) gave... 

Fig. 2. Titration curve and buffer capacity of a wine. The total acidity is equal to about 107 meq. (after Vergnes from Jaulmes, 1961).

Perhaps the outstanding advances made in this period have been in the greater quantization of data and the welcome biochemical interpretation which has been applied to it. This has been true particularly of the new and better data on a number of the minor constituents. The application of new analytical techniques will certainly aid in future work. Many important gaps exist in our knowledge. The relation between the various acids, cations, and the buffer capacity of musts and wines should be delineated. Data on the substances which contribute to the characteristic odor of many wines are almost completely lacking. And finally, there is a pressing need for more specific and accurate methods for the determination of practically all of the organic constituents of wines— both methods for control purposes and accurate research procedures. 

The dosage liqueur can be acidified with citric acid, if necessary. It also contains the quantity of sulfur dioxide required to eliminate any dissolved oxygen, and may be supplemented with ascorbic acid (50 mg/1). This offsets the sudden oxidative effect of disgorging the redox potential may increase by 150 mV, or even more, depending on the redox buffer capacity of the wine. 

In sparkling wines, ascorbic acid acts not only by its reducing properties but also by its capacity as an oxidation-reduction buffer. Their potential remains stable at 240 mV for several years. In the absence of ascorbic acid, it varies between 200 and 265 mV, according to the effectiveness of corking. This phenomenon clearly affects the organoleptical characters of wine (Makaga and Maujean, 1994). 

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