Yeast, Sugar & Gravity: The Science Behind Fermentation
Understanding the science of fermentation
What happens during fermentation?
Yeast fungi (Saccharomyces cerevisiae) convert sugar into ethanol and CO₂:
Sugar → Alcohol + Carbon dioxide C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂
17 g of sugar per liter produces about 1% vol. alcohol.
The hydrometer (Oechsle scale)
The hydrometer measures the density of the must compared to water. The more sugar, the higher the Oechsle value.
Reading it: Place the hydrometer in the must, read the value at the water surface.
Rule of thumb: Oechsle value ÷ 7.5 = approximate alcohol content in % vol.
Example: 90 °Oe ÷ 7.5 = 12% vol. alcohol
Adding sugar (chaptalization)
If the must doesn't have enough natural sugar, you can add sugar:
- 17 g of sugar per liter increases the alcohol by 1% vol.
- You should add a maximum of 3% vol. worth of sugar (otherwise flavors will be missing)
Formula: (target value – actual value) × liters × 17 ÷ 1000 = kg of sugar
Choosing yeast
| Yeast | Characteristics | Suitable for |
|---|---|---|
| Lalvin EC-1118 | robust, high alcohol tolerance | sparkling wine, problematic musts |
| Lalvin 71B | fruity, low acid degradation | young wines, fruit wines |
| Lalvin D47 | aromatic, complex | white wine |
| Lalvin RC212 | powerful, red wine character | red wine |
Fermentation temperature
- Too cold (< 15 °C): fermentation stalls
- Ideal for white wine: 16–20 °C
- Ideal for red wine: 20–26 °C
- Too warm (> 30 °C): yeast dies
Add yeast nutrient if fermentation stalls – besides sugar, yeast also needs nitrogen and minerals.