Topic 4- Technological aspects of yeast fermentation Flashcards
What are the desired fermentation properties of a selected yeast strain?
- rapid initiation of fermentation
- high osmotolerance
- efficient conversion of sugars to ethanol
- high ethanol tolerance
- able to ferment at low temperatures (white wines)
What are the desired Sensory characteristics of a selected yeast strain?
- low VA, acetaldehyde and higher alcohols
- low production of sulfur compounds-H2S, thiols, DMS.
- high glycerol production
- enhanced autolytic activity
- glycosidase activity
- modified esterase activity.
What are the desired production properties of a selected yeast strain?
- resistance to drying and high SO2
- genetic stability
- reduced sulfite binding
- low foamability
- highly flocculant- compact sediment
- killer activity
- low nitrogen demand
- proteolytic activity.
What are the main advantages of using cultured yeast?
- Predictability
- lower incidence of spoilage.
- can choose desired sensory characteristics.
How does the preparation of active dried wine yeasts (ADWY) impact on fermentation performance?
- H2S- propagated with high levels of intracellular nitrogen so are less likely to produce H2S.
- Propagated under aerobic conditions so have high levels of sterols and unsaturated fatty acids to confer ethanol tolerance.
- SO2 resistance- propagated in the presence of SO2 to increase resistance.
- Resistance to dehydration- related to trehalose (disaccharide) content. preserves membrane integrity during drying and rehydration.
What are the advantages of using ADWY rather than wet cultures?
- less equipment required
- less expertise required
- less time required.
What temperature is ideal for the rehydration of ADWY? what is the result of lower temperatures?
When should they be transferred to must?
40C. Maximises chances of successful re-establishment of cell membrane function.
Lower temps result in excessive loss od cytoplasmic contents leading to poor viability and performance.
Should be transferred to must or have juice added quickly to ensure intracellular nutrient reserves are not expended.
What is the inoculation rate of viable yeast cells into must?
5E6 cells/ml
What is the basic procedure for preparing a wet starter culture?
Why should they not be kept for a long time once scaled up?
- transfer yeast from agar slant to 100 ml sterile grape juice. adjust ph to 3.5-4. no SO2 present. 25C for 24 hours.
- 5-10 fold volume increase into sterile grape juice in stainless steel aseptic system.3:1 height to width.
- increases in volume until sufficient volume reached. Aerated with filtered air for sterol production.
- Ethanol never at an inhibitory level and vitamins, nitrogen and sugar never limiting.
Should be used relatively quickly to avoid contamination and genetic change- e.g. yeast my go from low H2S production to high production. Also, yeast cells may become exhausted in essential cellular components
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What are the three broad reasons for a sluggish or stuck fermentation?
- Insufficient nutrients.
- Presence of inhibitory compounds
- Wine production techniques
Deficiencies of what nutrients can cause a sluggish or stuck fermentation?
- Nitrogen-essential for growth, building proteins etc
- lipids-sterols and unsaturated fatty acids for ethanol tolerance.
- Trace nutrients- vitamins and minerals (maintenance of osmotic balance)
What inhibitory compounds can cause a sluggish or stuck fermentation?
- High sugar levels- over 200g/l- osmotic pressure. increase lag phase and rate of fermentation. can lead to higher Acetic acid production.
- Ethanol-toxic as it reduces nitrogen and sugar transport across cell membrane.
- Acetic acid- causes low cellular pH which interferes with glycolytic enzyme function.
- killer toxins- produced by killer yeasts. Affect sensitive strains. K2 toxin is the only one produced by S. cerevisiae active at wine pH. Killer yeasts are immune to the toxin.
- Medium-chain fatty acids and their ethyl esters- synthesised by yeast. Yeast hulls can remove them and stimulate fermentation (survival factor)
- SO2- high pH inside the cell leads to a sulfur trap where bisulfite is the predominate form in the cell.
How is SO2 toxic to yeast?
sulfite trap mechanism leads to large amounts of bisulfite forming in the cell (pH near neutral)
- cleaves disulfide bridges in proteins- lack of functionality
- destruction of thiamine and NAD+
- exhausts ATP by inhibiting glycolytic enzymes.
- forms cross-links between proteins and nucleic acids.
What wine production techniques can cause a sluggish or stuck fermentation?
- Temperature- temp shock of starter culture or ferment. temperatures above 35C can lead to a stuck ferment.
- Insufficient agitation- leads to settling of yeast. exacerbated by very flocculent yeast.
- Excessive clarification of juice- low solids content that are nutrient sources-sterols, fatty acids. more problematic in white ferments.
