Module 4 - Sulfur Dioxide Flashcards
What happens when sulfur dioxide dissolves in solution?
When sulfur dioxide dissolves in solution it can equilibrate between molecular sulfur dioxide, bisulfite ion and sulfite ion.
SO2 - Molecular sulfur dioxide or dissolbed gaseous sulfur dioxide
HSO3 - Bisulfite
SO32- - Sulfite
As a result of this equilibrium, the molecular sulfur dioxide concentration will always be less than the concentration of sulfur dioxide added.
What happens when Sulfur Dioxide is added to water?
Solutions of sulfur dioxide in water become acidic. This is due to the weak acid behaviour of sulfur dioxide in water which results from release of hydrogen ions from water.
What are the benefits of using salt potassium metabisulfite?
This is a crystalline solid that is more stable than the bisulfite salt in storage.
The metabisulfite salt can be readily weighed, then dissolved in water and the solutions added to juice or wine.
Under the acidic conditions of the juice or wine, molecular sulfur dioxide is released, and then this will equilbrate between the three different forms of sulfur dioxide.
What happens to the pH of juice or wine when SO2 is added?
Solutions of sulfur dioxide in water are weakly acidic, while in contrast, metabisulfite salts give rise to slightly alkaline conditions when dissolved in water.
In juice or wine, this weak acid or slightly alkaline behaviour is generally swamped by the heavily pH‑buffered environment provided by the juice or wine acids and their acid salts.
Consequently, the juice or wine pH is almost unaltered by added sulfur dioxide or metabisulfite salts, and this pH determines the balance between the various forms of sulfur dioxide.
At wine pH (pH 3-4) what is the predominant free SO2 species?
Bisulfite (>90%)
Why does bisulfite ion bond with aldehyde and ketone compounds to form acetaldehyde and acetone?
The slight positive character of aldehydes and ketones, on their carbon atom involved in the double bond to oxygen, favours their reaction with the negatively charged bisulfite ion.
That is, the reaction is driven by the attraction of a negative charge to a site that is slightly positive in character.
In chemical terms, the ketones and aldehydes are described as being electrophilic (i.e., being attracted to electron-rich compounds), while bisulfite is described as nucleophilic (i.e., being attracted to electron-poor compounds).
a) Bisulfite addition products are called what?
b) The formation of the bisulfite addition product is referred to as what?
a) bound forms of sulfur dioxide
b) the binding of sulfur dioxide
What are the major roles of SO2 in wine?
- Nucleophile. HSO3- can form covalent adducts with aldehydes and other electrophilic wine components
- Reducing agent/ antioxidant. SO2 is one of the most strongly reducing compounds found in wine. SO2 does not directly react with O2, but instead reacts in the form of HSO3-
- Enzymatic deactivation. SO2 can inhibit the activity of many enzymes.
- Antimicrobial. Molecular SO2 inhibits froth of a wine range of microorganisms.
Why is SO2 used in wine?
- Antimicrobial activity
- Antioxidant activity - scavenges oxidation products (quinones, hydrogen peroxide and dehydroasc.
Binds carbonyl compounds (i.e., acetaldehyde)
Why can’t we just dump it in wine?
- If there is not enough we can get microbial growth and oxidation
- If there is too much, then : odour, sulfite sensitive asthmatics will be upset, legal limits
What are the broad categories of SO2 in wine?
- Free sulfur dioxide (molecular sulfur dioxide, hydrogen sulfite / bisulfite and sulfite)
- Bound sulfur dioxide (bisulfite bound to carbonyl compounds like aldehydes and ketones)
In what form is SO2 added to wine?
- As a solid (K2S2O5(s))
- As a liquid (SO2(l))
How does PMS influence the pH of water?
1 molecule of K2S2O5 will give two molecules of SO2
This SO2 can the equilibriate between SO2, HSO3 and SO32-
Overall the pH of the water will increase: as the consumed
Dispense SO2 liquid into water the pH will decrease
Add PMS to water and the pH will increase (more OH- floating around after addition
How does solution pH influence the forms of free SO2?
Relevant to SO2 addition to juice or wine (pH buffered solutions)
As pH increases more HSO3 will form.
As pH decreases more SO 2 will form
If the pH is 1.9 (the pKa1) there will be equal amounts
Bisulfite is the most abundant form at wine pH
Sulfite is the least abundant form at wine pH
Molecular sulfur dioxide (SO2) concentration is very important for active against microbes, only volatile form, aroma at high concentrations.
How do we calculoate molecular SO2?
Molecular SO2 concentratrion (mg/L) = [SO2] = (Free SO2 conc [mg/L]) / (1 + 10 (pH - 1.9))
How much molecular SO2 is required for microbial stability?
0.6 - 0.8 mg/L
How does SO2 interact with red wine pigments?
Pigment bound SO2 < > Young red wine pigment + + HSO3-
Pigment bound SO2 is colourless compound. Therefore, sulfur dioxide ‘bleaches’ colour in young red wines.
The reaction is an equibilirium meaning there’s a dissociation constant = 0.00006 M meaning its a medium strength binder
What is an important difference in the measurement of free SO2 in a young red wine compared to an old red wine (~5 years)?
Measured free SO2 in …
