©Copyright Ben Rotter 2012
Musts high in titratable acidity (TA) are often encountered in winemaking, especially when using fruit with a high acid content or using a high proportion of acidic non-grape fruits (with high concentrations of juice in the must). The following articles list methods that can be used to reduce acidity. (TA is quoted as tartaric acid in this article.)
Note that musts sourced from non-grape ingredients, the success of the deacidification method will depend on the type(s) of acid present in the must, which may be influenced by where the fruit variety and the location it is grown. For information on the predominant acid typically found in certain fruits, see this website's article on Common Sugar and Acid Levels of Fruits
2 pH Considerations
It is important that shifts in pH are considered in conjunction with shifts in TA when deacidifying. This is because pH has significant influence the perception of acidity, astringency, biological stability, free SO2, colour hue, and potentially other wine attributes. Chemical deacidification can seriously alter pH and should be conducted with an awareness of the induced pH changes.
Winemaking procedures such as fruit skin maceration, malolactic fermentation and even fermentation itself tend to cause a small rise in pH. This can be helpful in reducing the perceived acidity in a wine. (See the “Influences of pH” article for these rises.)
3 Natual Changes in Acidity During Winemaking
The TA of wines naturally increases during fermentation due to the formation of such acids as succinic, lactic, gluconic, phosphoric, sulphurous, carbonic and acetic. This increase is generally between approximately 1 g/l and 2.5 g/l.
In grape wines, with their predominantly tartaric acid composition and reasonably high level of potassium, this increase is offset by a reduction in acidity due to potassium bitartrate precipitation (as the alcohol content increases during fermentation, the solubility of potassium bitartrate decreases and it is precipitated). The decrease this causes is usually equal to the increase in acidity that occurred during fermentation, resulting in a net increase in TA between 0 and 0.5 g/l.
The acid profile of non-grape fruits, however, is usually naturally dominated by malic and citric acids. Provided that no significant portion of tartaric acid has been added to a non-grape must and the predominant acids are malic and citric, a significant acid reduction due to potassium bitartrate precipitation will not occur. Thus, it can be expected that a non-grape must with a relatively natural acid profile will undergo a TA increase of around 2 g/l to 2.5 g/l due to fermentation. The pH will also drop accordingly. This fact should be considered in parallel with deacidification methods to best arrive at the desired TA in the final wine.
4 Deacidification Methods
4.1 Water Dilution/“Amelioraton”
Dilution of the must is the most common method used for the deacidification of non-grape musts. It is, however, considered undesirable in grape winemaking. Dilution is not recommended unless absolutely necessary due to the inevitable dilution of other compounds in the wine (i.e., aroma/flavour, tannin, sugar, etc).
Acidity is inversely proportional to volume; thus, if the volume of must is doubled then the acidity is halved.
4.2 Chemical Deacidification
Chemical deacidification is generally conducted when considered absolutely necessary.
Deacidifying before fermentation is encouraged as post fermentation chemical deacidification tends to result in a wine with a less integrated acid profile.
Acids are titrated in order of the dissociation constants (pKa) of their respective available hydrogens, i.e. in order of the "most available" proton donor. This means that tartaric acid will be titrated before malic acid, and generally all tartaric acid will be titrated before any malic is titrated.
4.2.1 Calcium Carbonate (CaCO3)
Calcium carbonate is generally used for aggressive deacidification and reduces acidity through precipitation.
Calcium carbonate reduces acidity by precipitating tartrate anions and leaving the much weaker carbonic acid which, in turn, then dissipates as carbon dioxide and water, leaving no remaining acidity.
Approximately 0.67 g/l of calcium carbonate will reduce the TA by 1 g/l (2.53 g reduces 1 US gal. by TA of 0.1% (1 g/l); 3.03 g reduces 1 Imperial gal. by TA of 0.1 %) and raise pH by approximately 0.3 units.
A dosage of 2 g/l to 3 g/l is usually the maximum dosage recommended, after which chalky flavours become apparent and pH increases beyond acceptable levels.
It is usually added to a portion of the batch, carbon dioxide is given off while the portion is well mixed, and cold stabilisation is conducted several days later. The deacidified liquid can then be racked off the salt lees.
Calcium malate (the salt formed by deacidiying malic acid with calcium carbonate) has a higher solubility than calcium tartrate (the salt formed by deacidifying tartaric acid with calcium carbonate) and may not fully precipitate from the must/wine, rendering a salty taste. For this reason, the "double salt" method (see below) is often adopted when conducting deacidification with calcium carbonate to force dissociation of the malic acid and induce precipitation of the salt.
4.2.2 Potassium Carbonate (K2CO3) and Potassium Bicarbonate (KHCO3)
Potassium carbonate is generally used for a less aggressive deacidification than calcium carbonate and reduces acidity through precipitation (about 30% of potential acid reduction occurs through precipitation during cold stabilisation) and neutralisation.
Potassium carbonate can react with malic acid (though as stated above, tartaric takes precedence) to form potassium bimalate. However, potassium bimalate is soluble in wine and will not (or may be difficult to) precipitate out.
Potassium bi/carbonate should be used carefully due to additions increading must/wine pH (by the inevitable presence of increased potassium ions). Bench testing may be conducted on samples to ensure that the desired level of deacidification does not cause a pH shift beyond acceptable levels.
Approximately 0.67 g of potassium bicarbonate is reported to practically reduce the TA by 1 g/l (2.53 g reduces 1 US gal. by TA of 0.1 %; 3.03 g reduces 1 Imperial gal. by TA of 0.1 %).
Approximately 0.92 g of potassium carbonate\ is reported to practically reduce the TA by 1 g/l (3.49 g reduces 1 US gal. by TA of 0.1 %; 4.19 g reduces 1 Imperial gal. by TA of 0.1 %) and raises pH by between 0.2 to 0.25 units.
Either chemical is usually added to the entire batch, carbon dioxide is given off while the batch is well mixed, and cold stabilisation is conducted several days later to encourage precipitation. The deacidified liquid can then be racked off the potassium bitartrate salt lees. This deacidification is often conducted after fermentation, but should be used before cold stabilisation so that full precipitation is achieved.
Potassium malate has a relatively high solubility in must/wine. For this reason, the "double salt" method (see below) is often adopted when conducting deacidification with potassium carbonate to force dissociation of the malic acid and induce precipitation of the salt.
4.2.3 Double Salt Method
Musts with a high pH and high TA cannot be satisfactorily deacidified by potassium bicarbonate (usually because of a high malic acid content). Nor is calcium carbonate effective because almost all the tartaric acid is removed from a must before any malic acid.
The double salt technique is used to avoid these problems because it removes both tartaric and malic acids. A small portion of the must is (seeded with calcium tartrate and calcium malate crystals and) dosed with a large quantity of calcium carbonate to raise the pH above 4.5 (ideally to around 5.1). At this high pH level, enough tartaric and malic ions are in solution to form the double salt calcium tartrate malate. After the treatment, this portion of the must/wine is blended back into the bulk must/wine volume, resulting in a suitable TA and pH having reduced both tartaric and malic acid levels.
Following the treatment the wine should be cold stabilised to precipitate any remaining salts. Precipitation typically requires some months to complete.
4.2.4 Potassium Hydroxide
Potassium hydroxide may be used to deacidify by reacting with tartaric acid to produce (bi)tartrate and water.
4.3 Cold Stabilisation
Cold stabilisation is used to precipitate potassium tartrate salts because the salt’s solubility is greatly reduced at low temperatures. Naturally, the only wines which benefit from this procedure are those with significant amounts of tartaric acid (see “Non-induced TA Changes” above).
Typically, the wine is cooled to -3°C to -1°C (27°F to 30°F) for one to two weeks until the excess potassium bitartrate precipitates as crystals. Alternatively, chilling the wine to 2°C to 5°C (35°F to 40°F) for a month is an option. Beware not to freeze the wine - for dry wines, the wine's freezing point is approximately half the alcoholic percentage by volume in Celsius (i.e., a wine of 12% alcohol/volume freezes at roughly -6°C (21°F)).
If chilling is slow and progressive, there will be formation of large tartar crystals but precipitation will be incomplete. Rapid chilling will cause very fine crystals difficult to separate and precipitation will be complete up to the solubility threshold. Therefore, rapid chilling is preferred.
Crystallisation is easier when crystalline nuclei are available, thus winemakers often “seed” the wine by stirring in a small quantity (approximately 0.5 g/l, 2g/US gal., 2.4g/Imp.gal.) of potassium tartrate (“cream of tartar”) before chilling commences. This often reduces stabilising time by half. Constant stirring and pre-stabilisation filtration or centrifugation also make crystallisation easier.
Calcium tartrate is less sensitive to temperatures.
Cold stabilisation also affects pH. When the pH is above around 3.65 before cold stabilisation, the formation of tartrates will cause a pH increase. However, when the pH is below this value, cold stabilisation causes a decrease in pH. The “magic number” of 3.65 works well as an estimated figure, but the value does vary from wine to wine.
4.4 Carbonic Maceration (CM)
If whole, uncrushed fruit is placed in a carbon dioxide saturated environment, it may undergo anaerobic metabolism by fruit cell enzymes. This can reduce the malic acid content of the wine significantly and therefore represents a possible strategy towards reducing acid levels. Tartaric and citric acids remain unaffected. The decrease in malic acid depends on grape variety/fruit type. Higher temperatures cause increased malic metabolism.
CM is typically conducted by placing the fruit in a sealed vessel and sparging it with CO2. CO2 should be injected again over the first one to two days after the initial CO2 sparging of the fermentation vessel since the fruit will absorb some CO2. Winemakers should be aware of vessel pressure and allow for CO2 release from the vessel. The enzymatic metabolism is intolerant to alcohol and so will only occur a up to around 2% alcohol/volume. Nevertheless, some fruit will be crushed under the weight of the fruit above it. This will undergo alcoholic fermentation. Minimising the crushed fruit in the vessel is therefore desirable when using this technique.
Typical maceration before pressing is 8 to 15 days under CM. However, this depends on the style of wine desired. Maceration is commonly conducted for 6 to 8 days at 30°C to 32°C, 10 days at 25°C, or 15 days at 15°C (but these rates may not produce equivalent results). Temperatures above 35°C should be avoided as these may affect metabolism.
CM generally results in wines with less colour, phenolics, alcohol, and glycerol than wines made without using CM. Increased volatile acidity levels may also result, though this depends on the microbiology of the wine. At high temperatures (35°C) the final tannin level can be similar in CM wines compared with wines made with non-CM winemaking methods.
Use of SO2 is encouraged at vessel filling (unless malolactic fermentation is desired) and inoculation of yeast may be preferable at the outset of CM, since CM otherwise provides a perfect environment for native yeast and bacteria to thrive due to the unsulphited, low ethanol environment.
CM produces a different aromatic profile in the wine, and for this the technique is famed. At least in red grape winemaking, the technique results in different ester and higher alcohol formation than fruit not undergoing CM. Specifically, vinyl-benzene, phenyl-2-ethyl acetate, benzaldehyde, vinyl-4-gaiacol, vinyl-4-phenol, ethyl-4-gaiacol, ethyl-4-phenol, eugenol, methyl and ethyl vanillate, and ethyl cinnamate.
4.5 Acid Metabolism by Yeast
Schizosaccharomyces yeasts can also be used to reduce the malic acid content of musts. However, the presence of Saccharomyces ellipsoideus in the must reduces their affect (10% presence reduces Schizosaccharomyces malic acid attack efficiency by a third).
A yeast such as 71B-1122 can metabolise up to 40% of the malic acid in the must during fermentation. The amount metabolised is inversely proportional to pH of the must.
Alternatively, a pre-fermentation biological deacidification of malic acid by Lactobacillus plantarum (such as Chr. Hansen’s Viniflora plantarum culture) has been shown to reduce malic acid by 40 to 60%. This can be acheived without a significant increase in volatile acidity or diacetyl, and with limited citric acid degradation. Additionally, it is claimed to give a rounder, more complex, fuller bodied wine with better acid balance, and increased palate length. (See Pilatte, E. and Prahl, C., Biological Deacidification of Acid Grape Varieties by Inoculation on Must with a Freeze-dried Culture of Lactobacillus plantarum, Chr. Hansen, France, 1997.)
4.6 Malolactic Fermentation
There is no strong tradition of conducting malolactic fermentations on non-grape wines due to (1) the emphasis being placed on fruitiness and, (2) the fact that the predominant acid in the must is often malic, meaning that complete malolactic fermentation may reduce a wine's acidity to the point of serious imbalance if not inhibited at some point (inhibition would require sterile filtration or addition of a malolactic bacteria inhibitor such as lysozyme).
Some wine styles are not suited to malolactic fermentation. However, when acid reduction is necessary, malolactic fermentation provides an attractive solution. Malolactic fermentation tends to result in a pH increase of between 0.1 and 0.3 units.
4.7 Ion Exchange
This method requires ion exchange equipment, in which the tartrate or malate ions are exchanged with hydroxyl ions, thereby removing them from the must/wine.
Acidity may be masked by residual sugar (and possibly a high alcohol content) to balance wines.
Blending a high-acid must/wine with a low-acid must/must either pre- or post-fermentation is often conducted to reduce acidity.
The principal of acidity being inversely proportionate to volume applies. For example, if a winemaker had 20 litres of wine “A” with a TA of 10 g/l, wished to blend it with wine “B” which had a TA of 5 g/l, and desired a final TA of 7 g/l, the following would apply: 20*10 + x*5 = 7*(x+20). Solving for x yields x = 30 l. Thus, adding 30 litres of wine “B” to 20 litres of wine “A” will result in an acidity of 7 g/l.
The pH of must/wine may be reduced without making any change to TA by the use of calcium sulphate (gypsum). The reaction is as follows:
CaSO4 + H2T ---> CaT + SO42- + 2H+
1 g/l gypsum reduces the pH by approximately 0.09 units. This technique can, however, leave a bitter aftertaste and the calcium tartrate can be slow to precipitate.
5 Combined Multiple Strategies
Significant deacidification may be achieved by the used of multiple deacidification strategies. For example, a winemaker might:
Partially chemically deacidify the must with potassium bicarbonate, followed by
Biologically deacidify the malic acid in the must/wine by fermenting with yeast strain 71B-1122 or Lactobacillus plantarum, followed by
Further deacidify the malic acid in the wine by malolactic fermentation
Precipitate acids by cold stabilisation
In this way, chemical deacidification is utilised as a way of beginning the natural (biological) deacidification process in wine by raising pH which in turn precipitates potassium tartrate and encourages malolactic fermentation.