Chapter 4 - Alcoholic Fermentation

Question 1

What role does yeast play in fermentation?

Answer 1

Yeast converts sugar into CO2, ethanol and heat energy

Question 2

What are the requirements for alcoholic fermentation?

Answer 2

• Complex carbohydrates (sugars)
• Aqueous solution (H2O)
• Ambient temperature (18-22°C)
• A vessel
• Yeasts (in particular Saccharomyces cerevisiae

Question 3

Where is natural yeast found in the wild?

Answer 3

Natural yeast is found stuck to microflora (bloom). It has a white frosty appearance on grape skins.

Question 4

What are the traits of wild yeast in the fermentation process?

Answer 4

Pros: Complexity, flavours and “natural”

Cons: Unpredictable, can be slow or “stuck ferment” (yeast colony dies)

Question 5

What are the traits of cultivated yeast in the fermentation process?

Answer 5

Pros: Predictable, fast and known flavours

Cons: Costs money, commercial and mainstream “industrial”

Question 6

How much alcohol is in a finished wine?

Answer 6

Depending on how much sugar was in the grape and how vigorous the yeast was, final Alc is generally between 12% and 15%. Red wines are generally 2-3% higher than whites

Question 7

What is Residual Sugar?

Answer 7

Residual sugar is a product of stopping the fermentation process early.

Question 8

What are some aroma and flavour by-products from fermentation?

Answer 8

1. Acetic acid and acetaldehyde (like vinegar)
2. Hydrogen sulphide (natural gas): rotten egg
3. Glycerol (mouth feel and texture)
4. Volatile esters
5. Lees (dead yeast cells)

Question 9

What is Malolactic Fermentation (MLF)?

Answer 9

The conversion of Malic to Lactic acid by Lactobacillus .

Question 10

Why is Malolactic Fermentation is some wines?

Answer 10

Stability (so it doesn’t happen in the bottle) and for a buttery/creamy taste