Module 3 - Wine Acids & Acidity Flashcards
What is pH important to?
- microorganism growth
- antimicrobial activity and aroma of sulfur dioxide
- red wine colour intensity
- phenolic compound oxidation
- enzyme activity
What happens to microbial growth as pH decreases?
It becomes progressively inhibited as the pH decreases, i.e. growth conditions become more difficult and therefore more selective.
What happens to red wine colour intensity as pH decreases?
It increases as the pH decreases.
What happens to antimicrobial activity and aroma of sulfur dioxide as pH decreases?
They increase as the pH decreases.
What happens to phenolic compound oxidation as pH decreases?
It occurs more slowly as the pH decreases.
What are pH and titratable acidity relevant to?
- Understanding acidity changes during ripening and winemaking: dissociation of acids is indicated by pH and concentrations of acids by titratable acidity. Evidently both are important to an understanding of wine behaviour. Still more desirable is determination of concentrations of individual acids;
- Potassium bitartrate stability: this depends on the extent of dissociation of tartaric acid and its concentration.
- Acid taste: low pH and high titratable acidity both reinforce the sensation of acid taste.
What type of acids are wine acids and why do they act as acids? Are they weak or strong acids?
- Carboxylic acids;
- They act as acids because they are able to dissociate (or ionise) and donate protons to water.
- The extent or degree of dissociation (or ionisation) is very small, for this reason they are weak acids.
What is the Henderson-Hasselbalch equation?
- When considering the effect of pH on acid behaviour, the acid dissociation constant is often better represented by its negative logarithm, pKa (pK).
-This value can be directly related to pH. - The relationship of pH and pK is given by the Henderson‑Hasselbalch Equation
- The equation expresses mathematically the balance between the undissociated acid, the salt, the pK and the pH of a solution.
What are diprotic or dicarboxylic acids?
What are triprotic or tricarboxylic acids?
- Although some wine acids possess only one carboxyl functional group per molecule, many important ones have two, and citric acid has three.
- Acids with two carboxyl groups (called diprotic or dicarboxylic acids) or with three carboxyl groups (called triprotic or tricarboxylic acids) can undergo ionisation at the second (or third) functional group also.
Why are higher pH conditions necessary for the second ionisation of a dicarboxylic acid to occur?
The first ionisation of the acid form gives its conjugate base, the hydrogen tartrate (or bitartrate) ion (the conjugate base of an acid is what the acid becomes when it loses a H+).
- This conjugate base is itself capable of behaving as a weak acid since it still contains an ionisable hydrogen atom at the second carboxylic acid group. -
- Ionisation of the hydrogen tartrate ion, acting as a weak acid, gives the conjugate base to the hydrogen tartrate ion, the tartrate ion.
- This second ionisation is more difficult than the first because it imposes a negative charge on an already negatively charged ion.
Why is tartaric acid particularly biologically stable?
Certain lactic acid bacteria can convert it to acetic acid and other components but under correct winemaking conditions this is very unlikely to arise as the bacteria cannot thrive below pH 3.65 and are extremely sensitive to free sulfur dioxide.
Why is tartaric acid preferenced as an additive if acid addition is required?
The biological stability and acid strength of tartaric acid favour its use as an additive if acid addition is required.
How do losses to tartaric acid levels occur during winemaking?
Through precipitation of potassium hydrogen tartrate (potassium bitartrate).
What forms can tartaric acid exist in wine?
In addition to the undissociated acid, tartaric acid can exist in the bitartrate and tartrate forms.
Why do levels of malic acid in the grape berry vary more than tartaric acid vintage to vintage?
Its level in the grape berry is more variable from vintage to vintage than that of tartaric acid because it is more easily biologically degraded
In what forms can malic acid exist in a wine?
It can exist in a half neutralised bimalate form and a completely neutralised malate form.
What acid can be expected in cool-cold ripening conditions and warm-hot ripening conditions?
- Under cool‑cold ripening conditions, grapes have mainly malic acid contributing to high titratable acidity; the predominance of this weaker acid helps to avoid unduly low pH conditions.
- Conversely, although titratable acidity is low in warm‑hot ripening conditions, tartaric acid is the main contributor; this stronger acid helps to reduce the rise of pH under these conditions.
What type of acid is citric acid?
Why is it a better source of titratable acidity than tartaric or malic acids?
It is a triprotic acid, i.e. it has three carboxyl functional groups, so for a given weight it is a better source of titratable acidity than either tartaric or malic acids.
Why is citric acid microbially unstable?
- Lactic acid bacteria readily convert it to acetic acid.
- Citric acid is often absent from red wines as a result of this breakdown during malo‑lactic fermentation.
What are two other grape acids?
Small amounts of ascorbic acid (50‑100 mg L-1) are present in the berry, but are consumed during yeast fermentation.
Significant levels of galacturonic acid (40‑1000 mg L-1) are present in the berry as a result of pectin breakdown.
Why is the pH of grape juice or wine higher than 2.19?
During ripening, there is a progressive part neutralisation of the acids in the grape berry by exchange of potassium ions (K+) for hydronium ions (H3O+).
This generates, for example, the essentially fully ionised potassium hydrogen tartrate (KHTa, a combination of K+ and HTa- ions) from the only slightly ionised or dissociated tartaric acid (H2Ta, with only weak dissociation to a few H3O+ and HTa- ions).
The part neutralisation increases the molar concentration of HTa- ions and decreases the molar concentration of H2Ta, so the ratio of the two, i.e. [HTa-]/[H2Ta], is increased.
The effect is the principal reason that the pH rises during berry maturation.
What acids are produced during microorganism growth?
- Lactic acid
- Succinic acid
- Acetic acid
What are the key features of lactic acid as it relates to wine?
- Lactic acid is a monoprotic acid produced in small amounts (180‑400 mg L-1) during alcoholic fermentation, but in larger amounts (0.1‑3.0 g L-1) from malic acid in the course of malo‑lactic fermentation.
- It has a pK of 3.8, higher than that of malic acid, and is not efficient at maintaining low pH.
- In addition, its single carboxyl group gives it a weaker influence on titratable acidity than citric, tartaric or malic acids, and a relatively weak acid taste.
What are the key features of succinic acid as it relates to wine?
- Succinic acid is a diprotic acid and a product of alcoholic fermentation.
- The levels produced are quite significant (0.5‑1.5 g L-1, but usually below 1.0 g L-1) and very dependent upon the fermentation conditions.
- Its pK1 of 4.2 shows it to be an even weaker acid than other wine acids.
What are the key features of acetic acid as it relates to wine?
- Acetic acid is a monoprotic acid that is weaker, with a pK of 4.75, than the previously mentioned acids.
- It is produced during yeast fermentation (typically 0.2‑0.4 g L-1).
- The amount formed is very dependent upon the yeast strain, the juice composition and the fermentation conditions.
- Acetic acid is always produced during malolactic fermentation.
- The lactic acid bacteria will also produce it from citric acid and from sugars that yeasts are unable to ferment. This can lead to levels of 0.3‑0.5 g L-1 in the final wines.
What are levels of acetic acid that are higher than 0.3‑0.5 g L-1 a sign of?
- Higher levels than this are a sign of the probable influence of undesirable bacteria that may be acting upon reducing sugars, tartaric acid or glycerol.
- Higher levels may also be a sign of the action of acetic acid bacteria. Growth of acetic acid bacteria is probably the most dangerous of wine faults.
- The acid, and its ester ethyl acetate which is also formed by acetic acid bacteria, are readily detected by aroma and taste, and cannot be removed once formed.
Why is acetic acid, and its ester ethyl acetate readily detected by aroma and taste?
Unlike the other acids discussed above, acetic acid has a sufficiently low molecular weight and low polarity (with only one carboxyl group and no hydroxyl group) to be volatile.
What is volatile acidity?
- Acetic acid has a sufficiently low molecular weight and low polarity (with only one carboxyl group and no hydroxyl group) to be volatile.
- Small amounts of other similar fatty acids are also volatile (e.g. formic, propionic and butyric acids).
- The combined quantity of volatile acids can be measured to give a value of ‘volatile acidity’. Generally, acetic acid comprises 90‑95% of volatile acidity.
Why is acidification carried out?
Why is acidification best carried out at as early as possible?
- In warm viticultural areas, natural acid levels of the grape berry are usually insufficient, at optimum ripeness, for either adequate acid taste in the finished wine or adequately low pH for good control of the winemaking.
- Furthermore, during the winemaking, both pH and titratable acidity can change and may require adjustment.
- Acidification is most sensible if carried out at as early a stage as possible in the winemaking process so that the benefits of low pH affect more of the winemaking process.
Why might tartaric acid be preferred when acidifying wines?
- The biological stability and acid strength of tartaric acid favour its use for acidification.
- Its low pK1 value makes it attractive if the principal aim is to lower pH rather than increase titratable acidity.
Why may the need for removal of potassium hydrogen tartrate be increased following tartaric acid additions?
- Tartaric acid addition always increases the hydrogen tartrate ion concentration, so the need for removal of potassium hydrogen tartrate may be increased and an already potassium hydrogen tartrate stabilised wine may be made unstable again.
What are the benefits of using malic acid and what are the drawbacks when acidifying wines?
- If malic acid is added to increase acid levels, some of the acidity (both in terms of pH and titratable acidity) will be lost if subsequent malolactic fermentation occurs.
- The higher pK values of malic acid in comparison to tartaric acid favour its use if an increase of titratable acidity rather than a decrease of pH is desired, and if malolactic fermentation can be avoided.
Why shouldn’t you use citric acid to acidify a wine?
It should not be used with red wine or any wine that is not adequately stabilised against bacterial action as increased acetic acid levels will result.
What are the challenges of using lactic acid when acidifying a wine?
- Lactic acid has been advocated for acid adjustment and has the advantage of biological stability.
-Unfortunately, it is only a monoprotic acid with a moderately high pK.
- Consequently it is not as effective as the others at raising titratable acidity or lowering pH.
Why is deacidification carried out?
In cold viticultural areas, grape acid levels can be undesirably high and deacidification may be advisable.
Explain the mechanism for simple deacidification with calcium carbonate
- This procedure involves addition of calcium carbonate, preferably to the juice or must.
- Tartaric acid and malic acid are rapidly neutralised by the carbonate ion, with liberation of carbon dioxide and water. That is, the carbonate sequesters the two hydrogen ions (H+) from the organic acids and the intermediate hydrogen carbonate (H2CO3(aq)) formed rapidly degrades to water (H2O(l)) and carbon dioxide (CO2(g)).
- An amount of tartrate ion equal to the amount of neutralised acid is also slowly precipitated as calcium tartrate. Malate ion is not, however, precipitated as calcium malate in simple deacidification because at the typical pH values encountered during this process, there is insufficient concentrations of malate (M2-(aq)) to interact with Ca2+(aq). This is not the case for the more acidic tartaric acid which has reasonable concentrations of tartrate (T2-(aq)) to interact with Ca2+(aq) during the procedure.
Why should care be taken to leave at least 1.0‑1.5 g L-1 of tartaric acid in the juice or must when deacidifying with calcium carbonate?
- Both tartaric acid and malic acid react with carbonate (CO32-(aq)) but only tartrate salts are removed are precipitation with calcium ions.
- Consequently it cannot ameliorate very high bimalate salt levels, and it may give an imbalance between malic and tartaric acids that affects flavour.
Explain the mechanism for double salt deacidification with calcium carbonate?
- It is similar to the simple deacidification but conducted at a higher pH to allow precipitation of both calcium malate and calcium tartrate.
- It involves addition of sufficient calcium carbonate to a portion of juice or wine to pH 4.5‑6.0. Under these conditions, both tartaric and malic acid are converted extensively to their tartrate (T2-(aq)) and malate (M2-(aq)) forms by reaction with carbonate ion.
- There is then precipitation of a double salt containing both tartrate and malate in a calcium bound form. Following precipitation and separation of the crystals, back blending is carried out.
What is the advantage of using double salt deacidification with calcium carbonate?
What are the disadvantages?
- An advantage of this method is that both tartaric and malic acid levels are reduced, so it is useful if extremes of acidity are encountered.
- A disadvantage is that a procedure must be meticulously followed if the portion to be deacidified and the extent of deacidification is to succeed
Why is potassium carbonate (K2CO3)) often used as deacidifying agents instead of calcium carbonate?
- A disadvantage of utilising calcium carbonate is that precipitation of calcium tartrate in some instances can be particularly difficult to fully induce prior to bottling.
- Consequently, there can be the danger that the calcium tartrate will precipitate after bottling the wine and hence produce a deposit in the wine bottle that it seen as a defect by wine consumers.
What is an acid? (Bronsted-Lowry definition)?
A Brønsted-Lowry acid is any species that is capable of donating a proton: H +
What is the difference between a strong acid and a weak acid?
A strong acid is an acid which is completely dissociated (ionized) in an aqueous solution. They are usually inorganic acids.
A weak acid, the degree of dissociation (or ionisation) is very small. Therefore, the dissociation constant (Ka)is small («1).
Which weak acids are in wine?
Acetic, Citric, L-Lactic, L-Malic, Succinic, L-Tartaric are the major organic acids important to wine.
What is the functional group for a carboxylic acid?
The functional group for a carboxylic acid is -COOH.
What is the difference between H+ and H3O+?
Nothing. They are interchangeable.
How is pH calculated?
pH is calculated as pH = -log [H+]
What is the pH of a solution with 0.00025 mol/L H+?
pH = -log [H+]
Therefore,
pH = -log [0.00025] = 3.6
What is titratable acidity?
- Titratable acidity is the concentration of titratable protons (free and bound) in a sample.
- Better measure of perceived acidity on the palate
- The TA is determined by measuring the concentratrion of base (usually NaOH) that must be titrated to bring the sample pH to a particular value near neutrality.
How do we rank the strength of weak acids?
- The value of acid dissociation constants can be used to compare the strength of acids
- Acid strength is proportional to the value of Ka.
- Higher the Ka the stronger the weak acid
- Ka = [ Products ] / [ Reagents ]
How can you work out the distribution of weak acid forms at a given solution pH?
- We use the Henderson-Hasselback equation which links pH of solution to pKa of acid
- pKa = pH - log [A-] / [HA]
- A- = concentration of acid that has released H+
- HA = Concentration of acid that has not released H+
What is a polyprotic acid? What types are in wine?
- polyprotic acids contain more than one -COOH group and thus have more than one H+ equivalent per mole.
- Acetic acid and lactic acid are monoprotic acids.
- Tartaric, Malic, Citric, and Succinic are diprotic acids (can donate 2 protons)
- Citric acid is a triprotic acid (can donate 3 protons)
a. How do we calculate the pH of water after the addition of a weak acid?
b. 1.00 g of citric acid is added to 100.0 mL of water. What is the pH? Citric acid has a Ka1 = 0.00072 M and molecular mass of 192.1 g/mol.
a. [H3O] = square root of (Ka x total acid concentratrion)
- To calculate the pH of water after the addition of a weak acid, use an ICE table with the variable x
- Ka used to signify the change in concentration of the substance due to ionization of the acid.
- Then the Ka expression is used to solve for x and calculate the pH.
b.
Step 1: Calculate the concentration of citric acid.
1.00 g of citric acid / 100.0 mL = 10.0g / 1000 = 10.0 g/L
n = m/MM = 10.0g / 192.1 g/mol = 0.0521 mol / L
Step 2: Determine the pH using the dissociation constant Ka1
= square root of ( 0.00072 x 0.0521) = 0.00612
Step 3: Calculate pH.
PH = - log (0.00612)
Answer = 2.21
What are the mechanisms for deacidification of wine?
- Neutralization and/or precipitation of organic acids with carbonate salts.
- Biological deacidification
a. Theoretically, how much tartaric acid can 1 g/L potassium bicarbonate remove?
b. Theoretically, how much tartaric acid can 1 g/L potassium carbonate remove?
a. H2T + KHCO3 → KHT(↓) + H2CO3
The molar ratio of tartaric acid to potassium bicarbonate in the reaction is 1:1.
1: Convert 1 g/L of potassium bicarbonate to molarity.
Since 1 g/L of potassium bicarbonate corresponds to 0.01 mol/L (because the molar mass of KHCO3 is approximately 100 g/mol):
2: Calculate the amount of tartaric acid that can be neutralized.
Since the reaction is 1:1 between tartaric acid and potassium bicarbonate, the amount of tartaric acid that can be neutralized is also 0.01 mol/L.
3: Convert mol/L to grams per liter.
The molar mass of tartaric acid (is approximately 150 g/mol.
Amountoftartaricacidneutralized = concentration × molarmass
Amountoftartaricacidneutralized = 0.01mol/L × 150 g/mol = 1.5g/L
b. 2H2T + K2CO3 → 2KHT(↓) + H2CO3
What is a pKa value?
pKa = -log Ka
Just provides more convenient scale for weak acids.
Stronger weak acid will have a small pKa and a weaker weak acid = large pKa
How do we determine pKa of polyprotic acids?
Write successive donation of H+ and generate multiple Ka or pKa values
E.g. Tartaric acid (diprotic acid) = H2T
pKa1= 2.98
pKa2 = 4.34
