Wine Microbiology

Question 1

What are THREE (3) spoilage characteristics that can occur in wine as a result of the growth of Brettanomyces/Dekkera yeast? (3 marks)

Answer 1

1. Volatile acidity
   • Caused by the interaction of the Brett bacteria with the alcohol of the wine and oxygen to produce acetic acid.
   • Accopmanying the production of acetic acid is ethyl acetate which produces the sharp vinegary smell
2. ‘Brett’ character
   • Phenol compounds produced – 4-ethylphenol (4-EP) and 4-ethylgluaiacol (4-EC)
   • Associated with undesirable sensory characters
   i. 4-EP = bandaid, horsey, barnyard
   ii. 4-EC = clove, spicy, smoky
   • Both compunds are present together, although in different ratios depending on grape variety
3. Mousy off-flavour
   • Spoilage detected by taste and not smell
   • Not detected by smell because the compund responsible is not volatile in the acidity level of the wine
   • When mixed with saliva, the potency of volatility increases
   • Compound is a combination of N-heterocycles and derives from Brett and Dekkera yeasts along with lactic acid bacteria.

Question 2

b) List 5 (2) steps a winemaker/microbiologist could take to reduce the incidence of Brettanomyces/Dekkera yeast spoilage in a winery (2 marks)

Answer 2

1. General sanitisation
   • Important to prevent build up of unwanted yeast, bacteria or moulds
   • Daily leaning of winery equipment through vintage to keep populations of unwanted microorganisms at a minimum
   • Keeping processing equipment clean to prevent accumulation of organic material
   • Cleaning tanks and barrels regularly to orevent cross contamination when wines moved around
2. Residual sugar
   • Brett growth favoured by presence of residual sugar
   • Ensure primary fermentaion is complete
   • Aerate ferment when it is most active
   • Avoid temperature shock (as to not kill of yeast a nd leaving behind unfermented sugars)
3. Sulfar Dioxide
   • Important wine additive in preventing microbial spoilage
   • SO2 is antimicrobial and amtioxidant, scavenging for available O2 killing bacteria and yeast
   • Added at time of crushing, during primary and malolactic fermentation
   • Effectiveness aligned to pH (more effective whn pH is lower), total number of bacteria cells (more cells, more to kill) and turbidity (maintain low turbidity (cloudiness))
4. pH
   • Brett growth favoured in high pH (due to relationship with SO2)
   • High pH (low SO2) dominates at malolactic fermentation – large dose of SO2 required (rather than many small doses)
5. Barrels
   • Phenolic compunds present oak wood are precursers to the growth of Brett
   • Care in sanisation of barrels, transferring wine (topping wine) from one barrel to another to avoid comtamination

Question 3

Describe one flavour change associated with these spoilage characters (1 mark)

Answer 3

Mousy off flavour

Question 4

How is residual sugar used to reduce incidence?

Answer 4

Brett growth favoured by presence of residual sugar
• Ensure primary fermentaion is complete
• Aerate ferment when it is most active
• Avoid temperature shock (as to not kill of yeast a nd leaving behind unfermented sugars)

Question 5

How is general sanitation used to reduce incidence?

Answer 5

Important to prevent build up of unwanted yeast, bacteria or moulds
• Daily leaning of winery equipment through vintage to keep populations of unwanted microorganisms at a minimum
• Keeping processing equipment clean to prevent accumulation of organic material
• Cleaning tanks and barrels regularly to orevent cross contamination when wines moved around

Question 6

how is sulfur dioxide used to reduce incidence?

Answer 6

Important wine additive in preventing microbial spoilage
• SO2 is antimicrobial and amtioxidant, scavenging for available O2 killing bacteria and yeast
• Added at time of crushing, during primary and malolactic fermentation
• Effectiveness aligned to pH (more effective whn pH is lower), total number of bacteria cells (more cells, more to kill) and turbidity (maintain low turbidity (cloudiness))

Question 7

How is pH used to reduce incidence?

Answer 7

• Brett growth favoured in high pH (due to relationship with SO2)
• High pH (low SO2) dominates at malolactic fermentation – large dose of SO2 required (rather than many small doses)

Question 8

What is volatile acidity?

Answer 8

• Caused by the interaction of the Brett bacteria with the alcohol of the wine and oxygen to produce acetic acid.
• Accopmanying the production of acetic acid is ethyl acetate which produces the sharp vinegary smell

Question 9

What is brett character?

Answer 9

• Phenol compounds produced – 4-ethylphenol (4-EP) and 4-ethylgluaiacol (4-EC)
• Associated with undesirable sensory characters
i. 4-EP = bandaid, horsey, barnyard
ii. 4-EC = clove, spicy, smoky
• Both compunds are present together, although in different ratios depending on grape variety

Question 10

What is mousy off flavour?

Answer 10

Spoilage detected by taste and not smell
• Not detected by smell because the compund responsible is not volatile in the acidity level of the wine
• When mixed with saliva, the potency of volatility increases
• Compound is a combination of N-heterocycles and derives from Brett and Dekkera yeasts along with lactic acid bacteria.