3
Acids

3.1 Introduction

Organic acids are weak acids bearing a carbon chain and at least one acidic carboxyl group, –COOH (Figure 3.1, HA). They may contain other functional groups such as alcohols, ketones, or double bonds. The lower molecular weight organic acids (C₁–C₄) are highly water‐soluble but as the carbon chain length increases water solubility is reduced. Organic acids are widespread in the plant kingdom, where they are involved in primary metabolic pathways such as energy production and amino acid biosynthesis, as well as non‐fundamental roles like response to osmotic stress and discouraging predation of fruit. In wine, organic acids have two critical roles: (i) they are a major determinant of wine pH, which affects the appearance, microbial stability, and chemical stability of wine, and (ii) they have direct impacts on taste, particularly sourness.

Figure 3.1 Dissociation reaction involving a weak monoprotic organic acid (HA) forming its conjugate base (A⁻)

3.2 Organic acids in wine

3.2.1 Major organic acids

Six acids represent >95% of the total organic acids found in wine (Table 3.1). These compounds can be extracted from the grape, formed by microbial metabolism, or may be added exogenously by the winemaker [1,2]. With the exception of acetic acid, they are non‐volatile, and may cumulatively be referred to as “fixed acids” in older literature. Also, many of these acids are polyprotic; that is, they contain more than one –COOH group and thus have more than one H⁺ equivalent per mole.

Table 3.1 Structures, indicative concentration ranges, and sensory properties, of major organic acids important to wine [1–11]

Acid a (pKa in water)	Structure	Concentration (g/L, typical)	Sensory notes b	Common sources c
Acetic (4.76)		0.1–0.5 Legal limit in US: 1.4 g/L (red), 1.2 g/L (white)	Volatile – vinegar, pungent aroma (200 mg/L threshold)	Y, LAB, AAB
Citric (3.13, 4.76, 6.40)		0.1–0.7	Sour, astringent	G, Y
l‐Lactic d (3.86)		0–3	Sour, astringent	LAB
L‐Malic d (3.40, 5.11)		2–7 (higher possible when under‐ripe fruit is used)	Sour, astringent	G
Succinic (4.21, 5.64)		0.5–1.0	Sour, salty, bitter	Y
l‐Tartaric d (2.98, 4.34)		2–6	Sour, astringent	G

^a Common names are presented rather than systematic (IUPAC) names. Multiple pK_a values are reported for polyprotic acids with multiple –COOH groups (i.e., pK_a1 , pK_a2 , etc.).

^b Sensory descriptors refer to perception of acid in isolation in simple matrices (water or hydroalcoholic solution).

^c G = grape, Y = yeast, LAB = lactic acid bacteria, AAB = acetic acid bacteria. In many wine regions, tartaric, malic, and citric acid may be legally added by the winemaker (Chapter 27).

^d Stereochemistry of the dominant stereoisomer is shown for chiral compounds. For lactic acid, d‐lactic acid may be formed by lactic acid bacterial metabolism of sugars and can serve as a marker for LAB spoilage.

Tartaric and malic acids are generally at the highest concentration in wines following alcoholic fermentation. Both of these compounds are formed in grapes early in the growing season, but their behavior during grape maturation and winemaking is dissimilar:

• Malic acid is present at very high concentrations (>20 g/kg) prior to veraison, but will be actively metabolized during berry ripening. Concentrations are generally lower in warmer regions and more mature grapes. Malic acid may also be metabolized during fermentation, with conversion to lactic acid being of particular importance to winemaking (Chapter 22.1).
• Tartaric acid is formed during initial berry cell division and is stable throughout berry ripening: its concentration is generally constant on a per berry basis. Tartaric acid is not metabolized during winemaking, but it can be lost through physiochemical mechanisms like precipitation (Chapter 26.1).

Citric, succinic, and acetic acid will be formed as typical products of alcoholic fermentation (Chapter 22.1). Acetic acid production will be higher under conditions of high osmotic stress (Chapter 22.1) and can also increase during malolactic fermentation (Chapter 22.5), and very high concentrations of acetic acid may indicate either lactic acid or acetic acid bacterial spoilage (Chapter 22.5).

3.2.2 Minor volatile fermentation‐derived acids and other organic acids

Several other volatile aliphatic organic acids have been identified in wine (Table 3.2), the most plentiful (mg/L concentrations) being even numbered, straight‐chain fatty acids. These are produced as byproducts of fatty acid metabolism, and are common to all alcoholic fermentations. Short branched‐chain fatty acids (isovaleric, isobutyric) are also found at slightly lower concentrations. Yeast production of these volatile acids is sensitive to several physiological factors (nutrient status, oxygen availability, temperature), and these are discussed in detail in Chapter 22.2.

Table 3.2 Structures, indicative concentration ranges, odor thresholds, and odor descriptors of minor microbially‐derived volatile fatty acids important to wine [1–11]

Acid (pKa )	Structure	Typical range (mg/L)	Detection threshold (mg/L) a	Odor descriptors a
Butanoic (butyric) (4.83)		0.4–5	0.2	Rancid, sweat, cheese
Decanoic (4.90)		0.06–0.8	1	Fatty, rancid
Hexanoic (caproic) (4.85)		0.8–4	0.4	Fatty, rancid, cheese
3‐Methylbutanoic (isovaleric) (4.77)		0.3–1	0.03	Rancid, sweat, cheese
2‐Methylpropanoic (isobutyric) (4.84)		0.4–2	2	Rancid, butter, cheese
Octanoic (caprylic) (4.89)		0.6–5	0.5	Fatty, rancid
Propanoic (propionic) (4.87)		Up to 100	8	Pungent, rancid

^a Threshold and descriptor data determined in multiple matrices, including water, hydroalcoholic solution, beer, white wine, and red wine.

Several other acids will be discussed in later chapters:

• Hydroxycinnamic acids and their derivatives (Chapter 13, 23.3)
• Pyruvic acid, α‐ketoglutaric acid, and other α‐keto acids (Chapter 9, 22.1, 22.3)
• Ascorbic acid (Chapter 24)
• Sorbic acid (Chapter 18).

3.3 Organic acids, pH, and wine acidity

The major importance of organic acids to wine is, unsurprisingly, wine acidity. There are several metrics associated with acidity that are used to describe wine, as summarized in Table 3.3.

Table 3.3 Summary of acidity‐related metrics important to wine and juice

Measurement	Calculated as	Typical values in dry wine	Relevance
pH	– log [H+] a	3.0‐3.7 b	Higher pH results in decreased microbial stability and effectiveness of SO2 (Chapter 17), decreased anthocyanin pigment color (Chapter 16), and decreased rate of acid‐catalyzed reactions (Chapter 25)
Titratable acidity (TA)	[H+] + [COOH] c	67–107 mEq/L (5–8 g/L as tartaric equivalents)	Correlated with perceived sourness
Total acidity	[COOH] + [COO−]	100–150 mEq/L	Correlated with buffer capacity
Buffer capacity	OH− concentration necessary to raise pH by 1 unit	25–75 mEq/pH unit	Lower value predicts a larger change to pH following a given change to TA

^a [H⁺] is the concentration of free protons.

^b Typically, a higher pH range (3.3–3.7) and lower TA values (5–6.5 g/L) are observed in red wines.

^c In practice, the TA is determined by titration of a wine sample to endpoint (pH 8.2 in the US and Australia, pH 7.0 in the European Union).

3.3.1 pH, pK_a , and simple solutions of acids

In aqueous solutions like wine, weak organic acids will be partially dissociated, as shown in Figure 3.1. pH is a measure of the free proton concentration of a solution, calculated as pH = – log [H⁺]. The degree of dissociation for a given acid is described by the acid dissociation constant (K_a ), where a larger K_a is associated with greater dissociation and a stronger acid (Equation (3.1)). For the weak organic acids in wine, K_a is < < 1, and values are reported as pK_a (i.e., – log K_a ).

(3.1)

pK_a values determined in water for major wine acids are shown in Tables 3.1 and 3.2. Polyprotic acids, with multiple –COOH groups, have one pK_a for each group. The pK_a values in these tables and elsewhere in the literature are usually for dilute solutions in pure water at 20 °C, but these values will be slightly different in real wines. The major factors affecting pK_a values are [12]:

• Ethanol – less polar solvents like ethanol result in less ionization and higher pK_a values. As a rule of thumb, pK_a typically increases ~0.25 units for model solutions containing 12% v/v ethanol, e.g. the pK_a1 of tartaric in 12% v/v ethanol increases from 2.98 to 3.23 [13].
• Ionic strength – ionic strength (I) is a measure of the concentration of ions in solution, and higher ionic strength will lead to a decrease in pK_a values. Ionic strength is defined as
  (3.2)

  where i is the ionic species number, n is the total number of different ions, c is the molar concentration of a species, and z is the species charge state (e.g., z = 1 for K⁺, z = 2 for Ca²⁺). In low ionic strength solutions (I < 0.1 M) the most common approach for estimating the ionic strength corrected pK_a ′ is derived from the Debye–Huckel equation:

  (3.3)

  The ionic strength of most wines is estimated to be 0.075–0.085 M [14]. From the Debye–Huckel theory, this will lead to a ~0.1 unit decrease in pK_a for monoprotic acids and pK_a1 of polyprotic acids, and even larger decreases for the formation of multiply charged anions [12].

• Temperature – the degree of dissociation of weak acids will usually (but not invariably) increase with increasing temperature. For weak organic acids these effects are usually small over a typical range of room temperatures (<0.1 unit decrease in pK_a ).

The effects of ethanol and ionic strength will partially offset each other, and a good rule is that the first pK_a value of an organic acid will be 0.10–0.15 units higher in wine than in water, and 0.10–0.15 units lower for second dissociation constants.

The pH of a simple aqueous solution containing only a single monoprotic weak acid can be calculated readily assuming the acid’s concentration ([HA]) and K_a value are known. Because identical amounts of H⁺ and A⁻ will be formed following dissociation, x can be substituted for [H⁺] and [A⁻], and [HA]‐x substituted into the denominator of the acid equilibrium expression (Equation (3.1)). This yields Equation (3.4), which can then be solved by the quadratic formula of Equation (3.5) (only the positive solution from the quadratic formula is used):

(3.4)

(3.5)

A simplified version to calculate [H⁺] is based on the assumption that dissociation of HA is small, that is, K _a is around 1 × 10⁻⁴ or less and ([HA]‐x) ≈ [HA]:

(3.6)

Calculating the pH of solutions containing multiple organic acids and/or conjugate bases (M⁺COO⁻) is more complex. Typically in these situations, the pH calculations are performed numerically by computer. Free software programs are available for these calculations [15].

3.3.2 Buffers and pK_a

Wines are buffer solutions – that is, they contain a mixture of a weak acid(s) and conjugate base(s), which limits (i.e., buffers) the change in pH following addition of a strong acid or base. It is this resistance to pH change that makes a weak acid solution sour when tasted, as unbuffered solutions would have the H₃O⁺ ions neutralized by salts in saliva. As a strong acid (e.g., HCl) is added to a buffer, it dissociates to form H₃O⁺ and Cl⁻. To re‐establish equilibrium, the concentration of added H₃O⁺ must decrease. This occurs by the reaction of H₃O⁺ with A⁻ to form HA, resulting in only a marginal decrease in pH as compared to the original solution. Analogous reactions occur as base is added.

At any pH, the relative concentrations of a weak acid HA and its conjugate base A⁻ can be calculated from the Henderson–Hasselbalch (H‐H) equation:

(3.7)

From the H‐H equation it follows for a monoprotic acid that:

• When pH is 1 unit greater than pK_a , 90% of the weak acid is ionized.
• When pH is equal to pK_a , 50% of the weak acid is ionized.
• When pH is 1 unit less than pK_a , 10% of the weak acid is ionized.

The distribution of acid species in wine is further complicated because the two major acids in juice or following alcoholic fermentation (malic and tartaric) each contain two carboxylic acids groups (H₂A, diprotic). The first dissociation constant (pK_a1 ) for both acids falls within the range of normal wine pH (i.e., 3.0–4.0) and the second dissociation constant (pK_a2 ) falls between pH 4.5 and 5. In a typical wine, these acids will exist primarily in their fully protonated (i.e. H₂A) and associated conjugate base (i.e., HA⁻) forms, and to a smaller extent as the fully ionized acid (i.e., A₂ ⁻, Figure 3.2).

Figure 3.2 Relative concentrations of neutral organic acid and its ionized forms in aqueous solution as a function of pH for (a) tartaric acid (H₂T), bitartrate anion (HT⁻), and tartrate anion (T²⁻), and (b) malic acid (H₂M), bimalate anion (HM⁻), and malate anion (M²⁻)

3.3.3 Titratable acidity

The titratable acidity (TA) is the concentration of titratable protons in a sample.

• The TA is determined by measuring the concentration of base (usually NaOH) that must be titrated to bring the sample pH to a particular value (endpoint pH) near neutrality.
• The H‐H equation shows that acids with pK_a values more than 1 pH unit above the endpoint pH will dissociate minimally, and thus contribute negligibly to TA.
• Because the endpoint pH for wine titrations is generally either 7.0 or 8.2, carboxylic acid groups (typically, pK_a  = 3–5) will contribute to TA, while polyphenols will not (pK_a  = 9–11, Chapter 11). Thus, TA measures the sum of free H⁺ and undissociated weak organic acid groups (i.e., [H⁺] + [COOH]).
• TA will include both volatile and non‐volatile acids. Volatile acidity (VA) can be determined separately by including a distillation step prior to titration.

Wine TA is generally in the range of 0.05–0.15 moles of titratrable proton equivalents per liter (0.05–0.15 Eq/L). By comparison, at a typical wine pH of 3–4, the concentration of free H⁺ is negligible (0.001–0.0001 Eq/L) and the majority of TA is thus due to undissociated weak organic acids. In practice, it is more common to report wine in g/L equivalents of an acid, often as tartaric acid equivalents (Table 3.4). For example, if a wine contains 0.10 [H⁺] Eq/L, the concentration as tartaric acid (molecular weight = 150 g/mol, 2 [H⁺] Eq/mol) is shown as

(3.8)

Table 3.4 Summary of acidity‐related metrics important to wine and juice

Organic acid	MW (g/mol)	Number of titratable protons (Eq/mol)	g/Eq	Conversion factor to g/L of tartaric equivalents a	Comments
Tartaric	150	2	75	1.0	Common for New World wines
Malic	134	2	67	1.12	Common for ciders
Citric	192	3	64	1.17	Common for citrus wines
Sulfuric	98	2	49	1.53	Common for wines produced in the EU

^a Multiplying factor to convert from reporting as one acid to tartaric equivalents, for example, a solution with a TA of 1 g/L as citric acid has a TA of 1.17 g/L as tartaric acid.

Typical values for white wine are pH 3.0–3.4 and TA of 6–9 g/L as tartaric acid, while those for red wine are pH 3.3–3.7 and TA of 5–8 g/L as tartaric acid. In certain countries sulfuric acid (98 g/mol, 2 [H⁺] Eq/mol) is often used as a reporting acid in European countries.

The importance of TA to winemakers is that it is well correlated with perceived sourness over the range of values typically observed in wines, as shown in Figure 3.3. The pH has a slight effect independent of TA, and Plane and colleagues [16] proposed that sourness was proportional to TA (g/L) – pH. Since the range of TA far exceeds the range of pH in wine, TA dominates this expression. Although TA is an excellent proxy for the sourness of a wine, other components may mask sourness (e.g., ethanol, sugar).

Figure 3.3 Sourness rankings from a sensory panel (higher rank has greater “acid taste”) plotted against (a) pH and (b) titratable acidity (TA) for 12 model wine solutions composed of varying amounts of tartaric acid, malic acid, and potassium hydroxide.

Data from Plane 1980 [16]

3.3.4 Total acidity

As defined by Boulton and colleagues [17], the total acidity of a wine refers to the sum concentration of all carboxylic acid and carboxylate groups [COOH] + [COO⁻], and is expressed in units of molar or tartaric acid equivalents per L. In practice, total acidity can be determined (i) by quantifying and summing individual organic acids using a technique like HPLC or (ii) by treating a wine with strong cation exchange resin prior to titration. The importance of total acidity is that it is well correlated with the buffer capacity of a wine or juice, as described in the next section.

The total acidity is invariably larger than the TA, and the difference between the two (total acidity – titratable acidity) is equal to the fraction of the acid that has been neutralized at a given pH, or equivalent to the concentration of metal cations in the wine [18]. As shown in Figure 3.4, the slope of a plot of TA versus total acidity (dark line) is only 0.74, indicating that on average approximately one‐quarter of protons are missing from organic acids. Adding the molar equivalents of two of the major metal cation species (K⁺, Na⁺) results in both a better correlation and a slope closer to 1 (Figure 3.4, dashed line).

Figure 3.4 Relationship between total acidity, titratable acidity (TA), and other metal cations in wines. The correlation between TA and total acidity is modest and has a non‐unity slope (solid line) because a portion of titratable protons are replaced with K⁺ and other metal cations. The correlation is improved by summing TA with major metal cations (dashed line).

Data from Boulton 1980 [18]

Viticultural or winemaking factors that result in K⁺/H⁺ exchange, e.g. K⁺ uptake by the grape during ripening or hard pressing, can decrease TA while leaving the total acid nearly constant (Table 3.5). The difference between TA and total acidity (0.029 and 0.037 Eq/L for free run and hard pressed, respectively) is due primarily to an exchange of K⁺ for H⁺ (0.019 and 0.28 Eq/L). The remaining difference arises from other metal cations, for example, Ca²⁺, Na⁺, Mg²⁺ (Chapter 4).

Table 3.5 Relationship of TA, K⁺, and total acidity in a free‐run and hard‐pressed Baco noir juice

Sample	pH	TA in g/L as tartaric (Eq/L)	K+ in g/L (Eq/L)	Total acidity in g/L as tartaric (Eq/L)
Free run	3.50	5.01 (0.066)	0.76 (0.019)	7.10 (0.095)
Hard‐pressed	3.63	4.52 (0.059)	1.10 (0.028)	7.20 (0.096)

3.3.5 Buffer capacity

The buffer capacity (BC) is the ability of a solution to resist changes in pH following addition of a strong acid or base, and is defined as follows:

(3.9)

The buffer capacity in wine is correlated with the concentration of buffers in wine. Since the major buffering compounds in wine are organic acids and their conjugate bases, higher buffering capacity is correlated with higher total acidity. Typical values for buffering capacity in wine are in the order of 25–75 mEq/L (1.9–5.6 g/L as tartaric) per pH unit.

The buffer range is the range of pH values over which buffering can be observed. For a simple system containing HA and A⁻, the buffer range is generally defined as the pH range within ±1 unit of the pK_a , and the maximum buffer capacity will be observed at pH = pK_a . However, a complex system like wine containing a number of conjugate acid–base pairs with multiple pK_a values will have an extended buffer range, so the point of maximum buffering may not be obvious. As seen in Figure 3.5, BC will vary among wines, but is generally constant for a given wine (i.e., linear relationship between pH and TA) over a normal wine pH range of 3–5 [19]. Buffer capacity is of importance to winemakers when acid adjustments are performed: wines with larger buffer capacities will require greater removal or addition of acid to effect a pH change. High buffer capacity can present a problem in certain cases, for example, if the aim of a winemaker is to reduce the pH to decrease the microbial spoilage risk.

Figure 3.5 Titration curves for three wines with addition of a strong base. The endpoint for titration is defined as pH = 8.2, but note that a near identical endpoint would be achieved for pH 7.0. ^aTitratable acidity, g/L as tartaric acid. ^bBuffer capacity, g/L as tartaric acid per pH unit, calculated between pH 3 and 5.

Data from Mattick 1980 [19]

3.4 Acid adjustments

3.4.1 Acid additions

In many regions, organic acids such as tartaric, citric, and malic or their mixtures can be added to either juice or wine, while the addition of mineral acids such as sulfuric is universally prohibited. All of these acids will result in an increase in TA. Beyond this, addition of tartaric acid may result in precipitation of potassium bitartrate, as described below, such that the final change to TA is lower than the amount of acid added.

3.4.2 Neutralization and/or precipitation of organic acids with carbonate salts

Titratable acidity can be decreased and pH increased by addition of carbonate salts. In most winemaking regions, the legally allowable forms of carbonate salts (Chapter 27) are:

• Potassium salts – KHCO₃ or K₂CO₃ are more commonly used.
• Calcium salts – CaCO₃ is less commonly used because of problems associated with calcium tartrate instabilities (Chapter 26.1).

Beyond neutralization, addition of potassium and calcium salts to wine may also result in precipitation of potassium bitartrate (KHT) or calcium tartrate (CaT), both of which have poor solubility at low temperatures in ethanolic solutions (Chapter 26.1). Precipitation can result in further decreases in titratable and/or total acidity. KHT precipitation is more commonly observed than CaT because (i) K⁺ is at higher concentrations in grapes, (ii) K⁺ salts are more commonly used for deacidification, and (iii) the bitartrate (HT⁻) species is usually at higher concentrations than tartrate (T²⁻, Figure 3.2).

KHT precipitation will always be accompanied by a decrease in TA because KHT bears a titratable proton that contributes to TA, but the pH of a wine can increase or decrease depending on the pH prior to precipitation. Wines below pH 3.65 will experience a pH decrease as the dominant H₂T/HT⁻ equilibrium shifts right to increase [HT⁻], thereby increasing [H₃O⁺], whereas above pH 3.65 there is a pH increase due to the dominant HT⁻/T²⁻ equilibrium shifting left, to increase [HT⁻] (Figure 3.6).

Figure 3.6 Tartaric acid equilibrium and effect of KHT precipitation on wine TA and pH depending on whether the wine is initially above or below pH 3.65

Unlike KHT precipitation, the precipitation of CaT will not affect TA, since the compound has no titratable proton. However, it will decrease pH because CaT is a base: total acidity and buffer capacity will also decrease.

Winemakers typically wish to prevent precipitation of KHT, CaT, and other less common salts of organic acids in the bottle. The factors that affect stability of these salts – as well as strategies to test for and prevent instabilities – are discussed in more detail in Chapter 26.1.

A less common precipitation used for deacidification during winemaking is of calcium malate (CaM). The pK_a2 of malic acid is relatively high (5.11), and malate will be at negligible concentrations at wine pH. However, in a technique called “double salt acid reduction,” a portion of wine (20–30%) is near‐completely neutralized with CaCO₃. This results in a pH >5 and precipitation of CaM and CaT [17]. The resulting wine can be racked and added back to the unadjusted wine. This process will increase pH and decrease TA.

When calculating the effects of carbonate salt addition on TA, it is useful to consider the neutralization and precipitation steps individually. The change in TA resulting from neutralization will occur quickly and stoichiometrically. For example, a 1 g/L solution of KHCO₃ (0.01 Eq/L) added to wine will neutralize 0.75 g/L (0.01 Eq/L) of TA as tartaric acid. Similar calculations can be used for K₂CO₃ or CaCO₃, requiring 0.69 g/L or 0.5 g/L, respectively, to achieve a 0.75 g/L change. For potassium salts, precipitation of KHT will be non‐quantitative and occur more slowly. Each mol/L decrease in KHT will result in a 0.75 g/L decrease in TA. Thus, if both neutralization and precipitation go to completion, every 1 g/L addition of KHCO₃ will eventually result in a 1.5 g/L decrease in TA.

3.4.3 Biological deacidification

Conversion of malic acid to lactic acid by lactic acid bacteria (“malolactic fermentation,” MLF) is a common approach to decreasing titratable acidity (Chapter 22.5). Lactic acid has only one –COOH group as compared to two for malic, and a complete MLF will result in a TA decrease equal to the initial molar concentration of malic acid; pH will also increase. Even when a wine starts at an appropriate TA, MLF may still be advantageous because a solution containing equal [H⁺] equivalents of lactic and tartaric acid will have a lower pH than a solution containing the same total [H⁺] equivalents of malic acid (Table 3.6). Maloethanolic conversion – in which yeast cells metabolize malic acid to ethanol and CO₂ – can also occur to a limited extent during alcoholic fermentations (Chapter 22.1). Tartaric acid is generally stable throughout wine production, although in rare cases it can be degraded by spoilage lactic strains, a phenomenon called tourné. Citric acid will also be partially or fully metabolized by lactic acid bacteria (Chapter 22.5), but because of the low concentrations of citric acid in wine and because one of the end products is acetic acid, the net effect on pH and TA is low.

3.4.4 Physiochemical approaches to altering pH and TA

Electrodialysis can be used to decrease TA while minimally affecting pH, and cation exchange resins can be used to increase TA and decrease pH. These approaches are discussed in more detail in Chapter 26.1

3.4.5 The balance of pH and TA

Winemakers often have competing goals of trying to keep pH low (to avoid microbial spoilage, among other reasons), while also preventing TA from getting too high (to avoid excessive sourness). TA and pH are inversely correlated in wines, but their correlation is not perfect for two reasons:

• Organic acid groups vary in pK_a . Stronger acids (lower pK_a ) will result in a greater pH decrease for the same contribution to TA. For example, based on the distribution shown in Figure 3.2, addition of one equivalent of tartaric acid to a solution will cause a greater pH decrease (more ionized so increases [H₃O⁺]) than one equivalent of malic acid, although both will cause a similar increase in TA.
• At a given TA, a higher buffering capacity generally corresponds to a higher pH. This buffering capacity is due to the presence of metal cations (e.g., K⁺) and conjugate bases.

The goals of low TA and low pH are most readily achieved when the wine contains proportionally higher concentrations of stronger acids (e.g., tartaric) and lower concentrations of weaker acids (malic, lactic, succinic, acetic) and salts of conjugate bases. As shown in Table 3.6, adjustment using KHCO₃ to a typical target of pH 3.5 will yield different final TA values depending on initial acid composition. A high malic acid red wine will end up with a higher TA value because malic is a weaker acid (higher pK_a , decreases pH less) than tartaric.

Table 3.6 Titratable acidities of model solutions following adjustment to pH 3.5 with either KHCO₃ or tartaric acid, and assuming no precipitation. TA values were calculated using CurTiPot speciation software [15]

Matrix	Concentrations of acids in model wine a			TA after adjusting to pH 3.5
	Tartaric acid	Malic acid	Lactic acid
Typical red, pre‐MLF	0.03 M (4.5 g/L)	0.02 M (2.7 g/L)	0	6.4 g/L
Typical red, post‐MLF	0.03 M	0	0.02 M (1.8 g/L)	5.3 g/L
High malic red, pre‐MLF	0.03 M	0.06 M (8 g/L)	0	10.6 g/L
High malic red, post‐MLF	0.03 M	0	0.06 M (5.4 g/L)	7.2 g/L

^a Model wine containing 1.3 g/L K⁺ and 10% EtOH.

In practice, it is common to use potassium salts for deacidifying high‐acid wines when small adjustments are necessary (1–3 g/L as tartaric acid) and when the pH starts relatively low (<3.4). Larger TA corrections by this approach would result in excessively high pH. Since high TA is usually a result of high malic acid, partial or complete MLF is also commonly used for deacidification, although this approach will change other sensory properties and is thus not appropriate for all wine styles (Chapter 22.5). For wines with very high malic acid, a combination of techniques including calcium malate precipitation (“double salt”), standard carbonate addition, and MLF may be employed.

3.5 General roles of organic acids and pH in wine reactions

All organic acids can by esterified by ethanol and other alcohols during fermentation or wine storage, and some of these esters have important aroma qualities (Chapters 7, 22.2, and 25). Conversely, esters can be hydrolyzed to form organic acids. These equilibria are considered in Chapter 7. Lower pH values will also accelerate acid‐catalyzed reactions in wine as follows (note that the effect is not dependent on TA):

• Hydrolysis and/or rearrangements of glycosidic aroma precursors will occur faster (Chapters 8, 23.1, and 25).
• Ester hydrolysis and formation will be faster (Chapters 7 and 25).
• Several reactions involving red wine polyphenolics will increase, including hydrolysis of polymeric polyphenols and activation of carbonyls to react with flavonoids (Chapters 16 and 25).
• pH will affect the equilibria, reactivity, and sensory effects of acidic compounds like anthocyanins (Chapter 16), SO₂ (Chapter 17), and CO₂.

3.6 Sensory effects of acids

The major sensory effect of acids in wine is to increase perceived sourness. As discussed in Section 3.3, sourness is well correlated with TA and only weakly correlated with pH. Low pH solutions can also induce astringent perceptions [20], likely due to precipitation and functional loss of lubricating salivary proteins [21]. Additionally, some acids are described as having characteristics in addition to sourness, for example, “bitter‐salty” taste of succinic acid (Table 3.1). However, the relevance of these extra‐sour sensory attributes in wine‐like matrices (rather than water–ethanol solutions) has not been well studied.

Volatile acidity (VA) is comprised primarily (>90%) of acetic acid. While acetic acid concentrations approaching the sensory threshold (400 mg/L) are commonly found in wine, higher concentrations are typically a result of bacterial spoilage (Chapter ) [22]. Specific regulations vary among regions and wine types, but typical limits are around 1.0–1.5 g/L. A complication of evaluating the effects of volatile acids on wine aroma is that their concentrations are usually correlated with their corresponding ethyl esters, and the latter are generally more potent. For example, acetic acid has a pungent, vinegar‐like aroma in isolation, but the off‐aromas associated with VA appear to be due primarily to the more potent ethyl acetate, formed by esterification of acetic acid (Chapter 7). Furthermore, although most other volatile acids have cheesy or rancid odors in isolation, their sensory contribution in real wine seems to be a general contribution to vinous character.

References

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4. 4. Sowalsky, R.A. and Noble, A.C. (1998) Comparison of the effects of concentration, pH and anion species on astringency and sourness of organic acids. Chemical Senses , 23 (3), 343–349.
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6. 6. Lambrechts, M.G. and Pretorius, I.S. (2000) Yeast and its importance to wine aroma – a review. South African Journal for Enology and Viticulture , 21, 97–129.
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8. 8. Da Conceicao Neta, E.R., Johanningsmeier, S.D., McFeeters, R.F. (2007) The chemistry and physiology of sour taste – a review. Journal of Food Science , 72 (2), R33–R38.
9. 9. Burdock, G.A. (2010) Fenaroliʼs handbook of flavor ingredients , 6th edn, CRC Press, Boca Raton, FL.
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11. 11. Ferreira, V., Lopez, R., Cacho, J.F. (2000) Quantitative determination of the odorants of young red wines from different grape varieties. Journal of the Science of Food and Agriculture , 80 (11), 1659–1667.
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13. 13. Usseglio‐Tomasset, L. and Bosia, P. (1978) Determinazione delle constanti di dissociazione dei principali acidi del vino in soluzioni idroalcoliche di interesse enologico. Rivista Vitic Enol , 31 (9), 380–403.
14. 14. Abguéguen, O. and Boulton, R.B. (1993) The crystallization kinetics of calcium tartrate from model solutions and wines. American Journal of Enology and Viticulture , 44 (1), 65–75.
15. 15. Gutz, I.G.R. CurTiPot. Available from: http://www2.iq.usp.br/docente/gutz/Curtipot.html.
16. 16. Plane, R.A., Mattick, L.R., Weirs, L.D. (1980) An acidity index for the taste of wines. American Journal of Enology and Viticulture , 31 (3), 265–268.
17. 17. Boulton, R.B., Singleton, V.L., Bisson, L.F., Kunkee, R.E. (1999) Principles and practices of winemaking , Kluwer Academic/Plenum Publishers, New York.
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20. 20. Gawel, R. (1998) Red wine astringency; a review. Australian Journal of Grape and Wine Research , 4 (2), 74–95.
21. 21. Siebert, K.J. and Chassy, A.W. (2004) An alternate mechanism for the astringent sensation of acids. Food Quality and Preference , 15 (1), 13–18.
22. 22. Bartowsky, E.J. (2009) Bacterial spoilage of wine and approaches to minimize it. Letters in Applied Microbiology , 48 (2), 149–156.

Notes

1 To summarize, pH describes the acidity of a solution, and pK_a is a property of a compound in that solution. One would not say “the pH of that compound is…,” but rather “the pH of solution containing 10 g/L of that compound is…”

2 More complicated empirical approaches to correcting for ionic strength appropriate for high ionic strength systems may be found in physical chemistry texts.

3 Reporting the acid concentration “as tartaric” is convenient for winemakers, since acid additions are frequently performed with tartaric acid.

4 Note that a solution does not need to contain the reporting acid: writing “this wine contains 7.5 g/L as tartaric” does not mean “this wine contains 7.5 g/L of tartaric acid.” Rather, it means “this wine has the same titratable acid concentration as a 7.5 g/L tartaric acid solution.”

5 Although the concepts are distinguished here, in some texts total acidity is used as a synonym for titratable acidity (TA).

6 An alternate approach to understanding why pH can increase or decrease following KHT precipitation is to consider what would happen if KHT was added instead of removed. Because KHT is amphoteric (can act as both an acid and base), a KHT solution acts like a buffer and at saturation will form a solution with pH ˜ 3.65. What would be the effect if you added this KHT solution to a wine with pH >3.65, assuming nothing precipitates out? The answer is that the pH decreases to a value between 3.65 and the original wine pH, since the KHT solution buffers things to a pH of 3.65. Now, what if you reversed the previous experiment, and magically removed the added KHT? The pH would increase back to the original pH. These effects would be reversed if the original wine has a pH < 3.65.

7 As mentioned in the Introduction chapter, tartaric acid is so stable that it can be found on centuries old archeological samples. Its presence on shards from pottery vessels is considered evidence that the vessel contained wine, as there are no other common sources of liquids that possess tartaric acid.

8  This astringency is readily appreciated by tasting lemon juice (pH 2.2, ~5% citric acid), which causes a gritty feeling in the mouth in addition to intense sourness.