Module 3 - Wine Acids & Acidity Flashcards
(56 cards)
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.
