Yeast And Acidity In Wine

Question 1

Yeast and pH (3)

Answer 1

• Yeast will ferment sugar to alcohol over a very large pH range
• Winemaking pH range is typically 3.0-4.0
• Changing initial pH generally has little effect on fermentation kinetics or products, or final alcohol levels

Question 2

What is a very low pH level and what does it do to yeast

Answer 2

Very low pH (<3) will impede yeast

Question 3

What is a high pH level and what does it do to yeast?

Answer 3

Higher pH >4 will favour bacteria and other competing organisms (Acetobacter)

Question 4

What happens to pH during fermentation?

Answer 4

pH changes slightly during normal fermentation (just little increases due to salification of tartaric acid with Ca and K)

Question 5

what is a VERY high pH and what does it do to yeast?

Answer 5

Very high pH >4.5 will favour other pathways of sugar catabolism (reduced alcohol production)

Question 6

Role of pH and SO2

Answer 6

pH will affect role of any SO2 present as action of SO2 is pH dependent

Question 7

Role of acid in fermentation

Answer 7

Most acids do not take significant part in fermentation metabolism

Question 8

Tartaric acid in fermentation

Answer 8

Tartaric acid may precipitate as tartrate salt (loss of acidity)

Question 9

Malic acid in fermentation

Answer 9

Malic acid may be metabolised to lactic acid (loss of acidity) by yeast or MLF bacteria

Question 10

What is produced in a faulty fermentation

Answer 10

Excess in Acetic Acid

Question 11

What changes slightly due to production of alcohol

Answer 11

Acidity and pH may change slightly due to production of alcohol (changes buffer capacity)

Question 12

Variation in organic acids (affecting pH and taste)

Answer 12

• Overall TA may increase or decrease

Question 13

How does tartaric acid perform/precipitate

Answer 13

Tartaric acid is stable to microbial action but can precipitate mainly as potassium hydrogen tartrate

Question 14

What are produced via normal alternative pathways

Answer 14

Increase in Succinic acid and acetic acid

Question 15

Role of yeast strains and Malic acid

Answer 15

Some yeast strains may produce malic acid, more may convert some of malic acid to lactic acid (increase or decrease)

Question 16

Malo-alcoholic fermentation (2 things)

Answer 16

1. deacidification of the must prior to alcoholic fermentation
2. deacidification of the wine after a. fermentation

Question 17

De-acidification of must

Answer 17

It allows fermentation of musts by replacing Saccharomyces cerevisiae with Schizosaccharomyces

Question 18

Most important aspect of Malo-alcoholic ferm

Answer 18

One of the drawbacks of deacidification, is the variable degree of malic acid degradation, depending on the characteristics of the musts.
- It is necessary to know how deacidification varies according to the conditions of the medium, so that the process can be predicted and controlled.