Wine microbiology: response of wine yeast to sulfur dioxide (SO₂) How do standard wine yeasts (e.g., Saccharomyces cerevisiae) typically respond to sulfur dioxide levels used during winemaking?

Introduction / Context:
Sulfur dioxide (SO₂) is used in winemaking for antimicrobial and antioxidant protection. Its effect varies among microbes; understanding yeast tolerance guides dosing strategies that suppress spoilage organisms while permitting clean fermentations.

Given Data / Assumptions:

• SO₂ exists in molecular, bisulfite, and sulfite forms depending on pH.
• Winemakers dose SO₂ to inhibit oxidative browning and suppress wild microbes.
• Commercial wine strains are selected for relative SO₂ tolerance.

Concept / Approach:
At wine pH, the antimicrobial activity is driven by molecular SO₂. Many lactic acid bacteria and wild yeasts are more sensitive than robust Saccharomyces strains. Thus, appropriate SO₂ additions can suppress undesired flora while allowing inoculated wine yeast to ferment, especially once fermentation begins and CO₂ and ethanol add further inhibitory effects.

Step-by-Step Solution:

Verification / Alternative check:
Supplier strain sheets and enology texts list “SO₂ tolerance” as a desirable trait; starter cultures commonly initiate fermentation despite standard pre-fermentation SO₂ doses.

Why Other Options Are Wrong:

• Sensitivity at typical doses: inconsistent with routine successful inoculations.
• SO₂ enhancing enzyme production: not an intended or typical effect.
• Sterilization or “none of the above”: SO₂ is not a sterilant at enological doses.

Common Pitfalls:
Overdosing SO₂, especially in high-pH musts, can inhibit even Saccharomyces; dosage must be pH-adjusted and monitored.

Final Answer:
They are relatively resistant to sulfur dioxide compared with many spoilage microbes