WINE PREPARATION

 

Preparation of Wine by Fermentation

Aim: To prepare wine from grape juice by alcoholic fermentation using Saccharomyces cerevisiae.

Principle: Wine is prepared by alcoholic fermentation of sugars present in fruit juice by yeast (Saccharomyces cerevisiae) under anaerobic conditions. The yeast converts fermentable sugars such as glucose and fructose into ethanol and carbon dioxide.

The lab-scale production of wine is a study of anaerobic metabolism.

·         Inoculum: While wild fermentation is possible via "bloom" (natural yeast on skins), lab practicals use a standardized pure culture of Saccharomyces cerevisiae (e.g., strain EC-1118) to ensure predictable kinetics.

·         Selective Inhibition: We use Potassium Metabisulfite (K₂S₂O₅). It releases Sulfur Dioxide ($SO_2$), which inhibits wild yeasts and acetic acid bacteria while the inoculated S. cerevisiae (which is sulfite-tolerant) takes over.

·         Substrate Chemistry: The "Must" must be balanced. If the sugar concentration is too high (hypertonic), it can cause osmotic stress on the yeast, leading to a "stuck" fermentation.

 

Chemical Reaction: C₆​H₁₂​O₆​→2C₂​H₅​OH+2CO₂​+Energy

Requirements: Materials:

• Fresh grapes – 1 kg
• Sucrose 20 g
• Distilled water – as required
• Active dry yeast (Saccharomyces cerevisiae) – 2–5 g
• Potassium metabisulphite – 0.1% (optional)

Equipment:

• Beakers, Conical flask, Measuring cylinder, Muslin cloth, Fermentation bottle with airlock, pH paper. Incubator (25–30°C)

Procedure:

1. Wash the grapes thoroughly with distilled water.
2. Crush the grapes to extract juice 500 ml. (must).
3. Filter the juice using muslin cloth.
4. Add a small quantity of potassium metabisulphite and allow to stand for 30 minutes to reduce contamination.
5. Activate yeast in water for 5–10 minutes.
6. Inoculate the 50 ml of yeast culture (10%) and 20 ml of sucrose into the grape juice and mix well.
7. Transfer the mixture into a fermentation bottle fitted with an airlock.
8. Incubate at 25–30°C for 7–14 days under anaerobic conditions.
9. After completion of fermentation (no bubble formation), filter and transfer the clear wine into a clean bottle for aging.

Observations:

• Formation of CO₂ bubbles during fermentation
• Alcoholic odor detected
• Sediment formation at the bottom
• Decrease in pH (acidic nature)

Result:

Wine was successfully prepared from grape juice by alcoholic fermentation using Saccharomyces cerevisiae.