Study Questions Flashcards
Which of the following is true of tartaric acid? It is:
A. abundant in many plants other than vines
B. partially converted to potassium salts during grape ripening
C. used as an energy source in the berry through respiration
D. a weaker acid than malic acid
B. partially converted to potassium salts during grape ripening
Which of the following indicates the action of ascorbic acid?
A. It has the same antioxidant activity as sulfur dioxide
B. It possesses antiyeast activity
C. It has both antiyeast and antioxidant activity
D. It can react with dissolved oxygen
D. It can react with dissolved oxygen
Which of the following is true of the effects of temperature on enzymatic processes? The processes:
A. proceed more slowly as the temperature rises
B. stop above a certain temperature
C. continuously double in rate for every 10°C rise of temperature
D. respond in a variable and erratic way to temperature
B. stop above a certain temperature
In the aspiration-oxidation method of sulfur dioxide analysis, what is the role of
phosphoric acid? It:
A. converts involatile forms of free sulfur dioxide to a volatile form
B. is formed by reaction of volatile components with H2O2
C. avoids oxidative loss of sulfur dioxide prior to its aspiration
D. accelerates the liberation of free sulfur dioxide from all bound forms
A. converts involatile forms of free sulfur dioxide to a volatile form
Which of the following best indicates the inhibitory action of bound sulfur dioxide on bacteria?
A. Bound sulfur dioxide has some weak activity that may be due to equilibrium traces of free sulfur dioxide.
B. Bound sulfur dioxide has no activity.
C. Bound sulfur dioxide is able to cross cell membranes
D. Bound sulfur dioxide has almost as much activity as free sulfur dioxide.
A. Bound sulfur dioxide has some weak activity that may be due to
equilibrium traces of free sulfur dioxide.
Which statement best defines a bisulfite addition product?
A. A chemical that has been added to a solution of bisulfite.
B. A molecule that results from a chemical combining with HSO3
C. A molecule that can be combined with HSO3
D. Any sulfur-containing chemical found in wine.
B. A molecule that results from a chemical combining with HSO3
Consider a wine that contains some free and bound sulfur dioxide in equilibrium. If air contact leads to some loss of that free sulfur dioxide, what will happen in the wine?
A. There will be no release of free sulfur dioxide from that which is bound
B. There will be return to the original level of free sulfur dioxide
C. There will be release of free sulfur dioxide only from the weakest binding agent; this may or may not be sufficient to return to the
original level of free sulfur dioxide
D. There is some release of free sulfur dioxide from all bound sulfur dioxide forms, but insufficient to return to the original free sulfur
dioxide level.
D. There is some release of free sulfur dioxide from all bound sulfur dioxide forms, but insufficient to return to the original free sulfur dioxide level.
Flavonoid phenolic materials found in red wine predominantly are sourced from:
A. grape pulp
B. free run juice
C. additives
D. grape skins, stems and/or seeds
D. grape skins, stems and/or seeds
What does a pKa1 of 1.9 indicate about the reaction between sulfur dioxide and water?
A. Below pH 1.9 only molecular sulfur dioxide exists.
B. Water does not react with sulfur dioxide until the solution pH is above pH 1.9
C. At pH 1.9 the rates of reaction between all free sulfur dioxide forms is equal.
D. Equimolar amounts of molecular sulfur dioxide and bisulfite ion exist at pH 1.9
D. Equimolar amounts of molecular sulfur dioxide and bisulfite ion exist at pH 1.9
Which best describes a quinone?
A. An open-chain ketone.
B. A naturally occurring bitter component of gapes.
C. A compound that can be produced from phenolic materials.
D. A compound that is oxidised if juice or wine is exposed to oxygen.
C. A compound that can be produced from phenolic materials.
At which of the following stages of wine production is air contact likely to be most detrimental?
A. During harvesting
B. Just after sulfur dioxide addition
C. Just prior to bottling
D. At fermentation inoculation
C. Just prior to bottling
What proportion of total berry phenolic material is usually extracted into a red wine through fermentation on skins?
A. Usually between 25% and 50%
B. About 75%.
C. About 90%.
D. More than 95% usually.
A. Usually between 25% and 50%
Which of the following indicates the action of ascorbic acid?
A. It has the same antioxidant activity as sulfur dioxide
B. It possesses activity against yeast.
C. It has both antioxidant activity and activity against yeast.
D. It can react with dissolved oxygen
D. It can react with dissolved oxygen
Citric acid is “triprotic”. What does this mean?
A. It has three ionisable hydrogen atoms.
B. It has three times the titratable acidity of acetic acid.
C. It has two anionic forms when it is in an aqueous environment.
D. All of the above.
A. It has three ionisable hydrogen atoms.
Which acid is most expected to be a product of the grape berry?
A. Succinic acid
B. Sorbic acid
C. Ascorbic acid
D. Lactic acid
C. Ascorbic acid
In the aspiration-oxidation method of sulfur dioxide analysis, what is the role of
phosphoric acid? It:
A. converts involatile forms of free sulfur dioxide to a volatile form
B. is formed by reaction of volatile components with H2O2
C. avoids oxidative loss of sulfur dioxide prior to its aspiration
D. accelerates the liberation of free sulfur dioxide from all bound forms
A. converts involatile forms of free sulfur dioxide to a volatile form
Which part of the grape berry contains the highest proportion of total grape phenolic material?
A. Seeds
B. Skins of grapes
C. Pulp
D. None of the above. They are all have the same amount.
A. Seeds
Which of the following is true of tartaric acid? It is:
A. abundant in many plants other than vines
B. partially converted to potassium salts during grape ripening
C. used as an energy source in the berry through respiration
D. a weaker acid than malic acid
B. partially converted to potassium salts during grape ripening
When measuring the SO2 levels in a red wine sample using the aspirationoxidation method, the results for the free SO2 are actually higher than the true
level of free SO2 because:
A. compounds other than SO2 in the red wine are aspirated and measured as free SO2.
B. white wines do not react as strongly with the phosphoric acid as red wines.
C. condenser operation is less efficient for red wine determinations than white.
D. some of the free SO2 measured is actually bound in the sample.
D. some of the free SO2 measured is actually bound in the sample.
How many of the following apply to both ascorbic acid and sorbic acid? They are
* naturally occurring in grape juice;
* antioxidants;
* antimicrobial agents;
* only beneficial for wine if used in the presence of sulfur dioxide;
A. None
B. One
C. Two
D. Three
B. One
Which of the following is generally true of fruit development in warmer rather than cooler areas?
A. Less intense, less distinctive fruit flavour
B. Higher titratable acidity levels
C. Lower pH
D. Lower sugar concentrations
A. Less intense, less distinctive fruit flavour
Which of the following would be most unusual during fermentation of red wine on skins followed by post-fermentation skin contact?
A. Colour intensity reaches a maximum but then declines
B. Both anthocyanin and tannin levels rise rapidly within the first few days
C. After a few days, the tannin concentration (as g L-1) is higher than the anthocyanin concentration
D. The tannin concentration declines after two weeks
D. The tannin concentration declines after two weeks
Which one of the following exhibits the strongest buffering action? A mixture of:
A. A weak acid and its salt form.
B. A strong acid and its salt form.
C. A strong acid and a weak acid.
D. A strong acid and a strong base.
A. A weak acid and its salt form.
What does a pKa of 10.0 indicate about the reaction between the phenolic group and water?
A. Below pH 10 only the phenol group will exist and no phenolate form will be present.
B. Water does not react with the phenolic group until the solution pH is above pH 10
C. At pH 10 the phenolic group can accept a hydrogen.
D. Equimolar amounts of phenol and phenolate would exist at pH 10
D. Equimolar amounts of phenol and phenolate would exist at pH 10
In a wine that contains appreciable amounts of free sulfur dioxide (i.e., 35 mg/L), state the products
generated upon the oxidation of phenolic compounds and how sulfur dioxide interacts with these
productions.
The products are hydrogen peroxide and quinone. Sulfur dioxide can reduce hydrogen peroxide and add
on to (or reduce) the quinone.
Define the terms, ‘free sulfur dioxide’, ‘bound sulfur dioxide’, ‘total sulfur dioxide’, ‘molecular sulfur
dioxide’ and ‘bisulfite addition product’.
- ‘Free sulfur dioxide’ is made up of (molecular sulfur dioxide) SO2, (bisulfite or hydrogen sulfite) HSO3
(sulfite) SO32- - ‘Bound sulfur dioxide’ is sulfur dioxide that is in a reversible addition product with a carbonyl compound
or phenolic compound (i.e., anthocyanin).
‘- Total sulfur dioxide’ is free and bound sulfur dioxide combined.
- ‘Molecular sulfur dioxide’ is SO2, as opposed to hydrogen sulfite and sulfite.
‘bisulfite addition products’ are the products formed between the reaction of bisulfite with carbonyl
compounds or anthocyanins.
Describe what would happen if 2.5 mM sulfur dioxide (i.e., 30 mg/L) was added to a solution of 2.5 mM
anthocyanins (230 mg/L) in a solution at pH 3.2. In your answer indicate if the final solution would be
coloured and describe any reactions that would occur.
- Some of the added sulfur dioxide would bind to the anthocyanin and hence lead to a decrease in colour.
-However, some anthocyanin would still remain unbound (i.e., the anthocyanin is not as strong a binder as acetaldehyde) and hence there would still be some colour in solution.
Describe how the phenolic compound concentrations and profiles of phenolic compounds compare for red and white wines. In your answer, describe the how wine/grape processing/production impacts on any variation in the phenolic compounds for red and white wines
- concentration of total phenolic compounds is usually much higher in red wines compared to white wines (i.e., 1-2 g/L vs 0.1-0.2 g/L), red wines contain higher concentrations of skin/seed derived flavonoids, whilst red and white wines may contain similar levels of pulp-derived non-flavonoids.
- Fermentation on skins for red wine enables greater extraction of skin/seed derived flavonoids, whilst
mimimal mixing of juice with skins/seeds in white wine production ensures minimal levels of flavonoid
extraction).
Describe why the addition of potassium metabisulfite to water would make the water solution slightly
alkaline. (Note: your answer must show at least one chemical equation)
K2S2O5 + H20 <> 2KHSO3
KHSO3 + H+ <> SO2 + H2O + K+
The equilibria highlighted above show what happens upon the addition of potassium metabisulfite to a
solution.
If added to water, the second equilibria shows that H+ will be consumed in the process. This means that if H+ is being consumed in neutral water then the water will be made slightly alkaline.
The measured free sulfur dioxide concentration in a red wine (pH 3.60) and white wine (pH 3.30) is 28 and
31 mg/L, respectively, using the aspiration-oxidation method of analysis.
If possible, calculate the molecular sulfur dioxide concentration in each wine and state if they are likely to be microbially stable. If it is not possible to calculate the molecular sulfur dioxide concentration, state why not.
The calculation is possible in the white wine but not possible in the red wine with the information
provided. The measured free sulfur dioxide concentration in a red wine includes the actual free sulfur dioxide concentration as well as sulfur dioxide bound to anthocyanins.
Consequently, the calculation cannot be performed for the red wine unless we know the true free sulfur dioxide concentration in the red wine.
WHITE WINE CALC:
Molecular SO2 = free SO2 x 10(1.77-pH) = 31 x 10(1.77-3.30) = 31 x 10-1.53 = 31 x 0.02951 = 0.91 mg/L → as this answer is greater than 0.8 mg/L, then the white wine would be microbially stable.
Acetic acid (AH, pKa1 = 4.75), tartaric acid (TH2, pKa1 = 2.98, pKa2 = 4.34) and citric acid (C
tH3, pKa1 = 3.14, pKa2 = 4.77, pKa3 = 6.39) may all be found in wine. State the most abundant form of each acid at the pH of 3.5 (e.g. for tartaric acid determine whether TH2 or TH- or T 2- would be at the highest concentration).
Also, describe if any acid has issues of microbially instability, potassium precipitate instability and/or contribution to off-aroma in wine.
At pH 3.5, acetic acid will mainly exist as AH, tartaric acid will mainly exist as TH-, and citric acid will mainly
exist as CtH2-
Acetic acid is volatile and can contribute to an off-aroma if its concentrations are sufficiently
high (i.e., vinegar-like), tartaric acid can form a potassium bitartrate precipitate and this can be induced by low temperatures and high ethanol concentration, citric acid is relatively microbially unstable and can be readily converted by lactic acid bacteria to acetic acid.
Sorbic acid may undergo deprotonation to form sorbate and has a pKa value of 4.76. i) Describe if sorbic acid, sorbate or both are efficient against yeast activity. ii) If a wine is changed in pH from 3.0 to 3.8, will the major form of sorbic acid stay the same within the whole pH range or will it change (i.e., sorbic acid or sorbate), and describe why. iii) State which type of wine style is best suited towards the use of sorbic acid and state why particular styles are not suited.
i) Sorbic acid is the form that has activity against yeast,
ii) Below pH 4.76, sorbic acid will dominate
and hence it will dominate from pH 3.0-3.8,
iii) White wines with residual sugar concentrations are
most often suited to sorbic acid addition. These wines will often be wines prepared from botrytis infected grapes that have a high binding capacity for sulfur dioxide. The sorbic acid provides added assistance to sulfur dioxide in preventing the refermentation of sugar present in the wine, whilst also allowing a total sulfur dioxide concentration below the legal limit (i.e., 250-350mg/L, depending on the sugar concentration).
Sorbic acid is not suited to red wine where the true free sulfur dioxide concentration is not sufficiently high to prevent bacteria from converting sorbic acid to an off-odour aroma compound. There is no benefit of adding sorbic acid to dry white wines.
i) Describe the influence of sulfur dioxide, temperature and pH on catechol oxidase activity and laccase activity,
ii) describe the impact of three additional winemaking processes on catechol oxidase activity and laccase activity.
Part i) catechol oxidase,
pH – active at wine/juice pH but loses activity with time, temperature – active at normal processing temperatures but deactivated instantaneously at 75-80oC, sulfur dioxide – majority of activity lost at 40 mg/L sulfur dioxide addition. Laccase,
pH – active at wine/juice pH and little loss of activity with time, temperature – active at normal processing temperatures but deactivated in 5 minutes at 45oC,
sulfur dioxide – laccase is less sensitive to sulfur dioxide in the must compared to the wine and levels of 20-30 mg/L free sulfur dioxide for several days may destroy the enzyme activity.
Part ii)
Tannin addition – decreases catechol oxidase activity but not laccase activity,
pressing – lighter pressing and removal of solids decreases catechol oxidase activity but not laccase
activity, Bentonite addition – decreases catechol oxidase activity but not laccase activity.
What occurs differently in the oxidation of a white wine when ascorbic acid is present?
The oxygen will be scavenged by ascorbic acid rather than phenolic compounds (with metal ions as
catalysts), and the same amount of oxygen will be scavenged much quicker in the presence of ascorbic
acid than in its absence.
The result of ascorbic acid oxidation will be hydrogen peroxide and dehydroascorbic acid. The latter is not normally produced in wine unless ascorbic acid has been oxidised.
Sulfur dioxide is still required to then scavenge hydrogen peroxide and dehydroascorbic acid.
