topic 1 Growth of yeasts during alcoholic fermentation

Question 1

What is the dominant yeast strain on the grape berry?
What other strains are there?
What is the initial concentration of native yeasts per berry?

Answer 1

• Hanseniaspora uvarum
• also present are Candida Debaryomyces, Metchnikowia, Pichia and Rhodotorula
• 10E3 to 10E6

Question 2

What are the main sources of fermentative yeasts?

Answer 2

• Grape Berry
• winery equipment
• inoculated cultures

Question 3

What are the stages of alcoholic fermentation?

Answer 3

• lag phase-2-5 days
• exponential growth phase- biomass production.
• linear growth phase- 50% sugar consumed here and mainly for ethanol production.
• stationary phase- fermentative activity reaches max. viable cells stable or decreasing.
• decline phase-(several weeks) popn decline. slowed consumption of sugars. risk phase for stuck fermentation.

Question 4

Explain the succession of yeast species in primary fermentation

Answer 4

Initial fermentation activity due to Candida, Pichia, Hanseniaspora. as ethanol concentration reaches 3-4% then S.cerevisiae replaces these.
This is regardless of whether an s cerevisiae strain is inoculated.
These other strains will influence the sensory properties of the wine.

Question 5

What are the intrinsic factors that affect the growth of yeast in wine?

Answer 5

• Nutritional status of juice- sugars, nitrogen compounds, vitamins, minerals and dissolved oxygen.
• pH - growth not affected but may influence enzyme activity.
• Fungicide residues
• interactions between microorganisms. can be synergistic or negative.

Question 6

What are some of the interactions between organisms that can take place in fermentation?

Answer 6

• B. cinerea and acetic acid bacteria produce acetic acid which can inhibit S. cerevisiae. can also desiccate berry causing osmotic pressure on favourable yeasts (can lengthen lag phase)
• Organisms such as B. cinerea and lactic acid bacteria can compete for nutrients with S. cerevisiae.
• Killer toxins produced by some strains.
• indigenous sacc and non-sacc strains grow better at low temperatures relative to S. cerevisiae. Nutrient depletion can occur.

Question 7

What are the extrinsic factors that affect the growth of yeast in wine?

Answer 7

-Temperature. 10-20C =3x growth rate. 20-30C =2x growth rate. low temperatures retain fruity aromas. higher temp more subtle flavour and better extraction. >35C =reduced viability. growth of indigenous yeasts stronger from 10 to 15C.

Question 8

What are the processing factors that affect the growth of yeast in wine?

Answer 8

• SO2. ^ lag phase. Reduces oxidation. reduces indigenous yeasts.
• Nutrient supplementation- DAP for reducing h2s.
• vitamin preparations
• yeast hulls- adsorb toxic medium-chain fatty acids, source of lipids and sterols.
• Clarification of juice (enzyme add, cold settling, bentonite fining, filtration etc). removes indigenous yeast. Can remove nutrients such as vitamins and sterols (leading to stuck fermentations).