Topic 2- Alcoholic fermentation Flashcards
2 basic processes in the growth of yeast during alcoholic fermentation
- Catabolic Reactions-Chemical is consumed to produce energy
- Anabolic Reactions- energy generated by catabolism is used for maintenance and generation of cellular components such as proteins and nucleic acids.
Many by-products of these reactions including alcohols, esters and acids
Are catabolic and Anabolic activities coupled? (happen at the same time)
Under optimal conditions YES.
In a wine ferment usually no. Usually, have an exponential growth phase (biomass production) followed by a stationary phase (energy expenditure on maintaining health in harsh environment-coined maintenance energy). during stationary phase reserve materials such as polysaccharides and trehalose are produced for when conditions improve.
What are the fermentable sugars in grape must and what are their concentrations and relative abundance?
What effect do high sugar concentrations have on yeast?
Hexose sugars (glucose and fructose are present in a 1:1 ratio at concentrations of 180-225 g/l (up to 400 g/l for botrytis cinerea affected grapes). A small amount of the disaccharide sucrose present at around 10 g/l which is converted to glucose and fructose.
High sugar concentrations put osmotic pressure on yeast cells. Depending on the yeast strain this can cause a sluggish start or stuck fermentation.
Explain the two systems in S.cerevisiae that transport sugars into the cell.
2 systems- Both mediated by carrier enzymes. One has a low affinity for glucose and fructose and the other has a high affinity.
Low Affinity- Active during the growth phase. decreases in activity during stationary phase as sugar concentrations decrease.
High Affinity- Repressed when sugar concentrations are high, so only active in the later stages of the stationary phase.
Most sugars enter the yeast cell via the low-affinity carrier for this reason.
What does it mean when we say S.cerevisia is a glucophilic yeast?
Both the low-affinity and high-affinity transport systems in S.cerevisiae show a higher affinity for glucose than fructose.
For this reason, in fermentations that do not go to completion, the residual sugar is mainly fructose.
S.cerevisiae fermentation is generally always anaerobic. What pathway is used to consume glucose? Explain this and reference the Crabtree effect
The small amount of dissolved oxygen at the start of fermentation is consumed quickly.
Hexoses are fermented via the glycolytic pathway (glucose+2ADP+2Pi+2(NAD+)–>2 pyruvate +2ATP+2NADH+2(H+)+2(H20)
The Crabtree effect is a mechanism that encourages fermentation rather than respiratory metabolism. Glucose represses respiratory metabolism.
Explain the 2 main steps where ethanol is produced from a hexose sugar.
- Glycolysis- Glucose is converted to pyruvate
2. Conversion of pyruvate to ethanol and carbon dioxide via an acetaldehyde intermediate. NAD+ is regenerated.
What is the overall reaction for the production of ethanol from glucose via the glycolytic pathway?
Does all glucose get metabolised via the glycolytic pathway?
Glucose==>2ethanol + 2CO2 + 2ATP
Not all. Some is channelled through the hexose monophosphate pathway (also called pentose phosphate pathway), which generates NADPH and Pentose sugars. Pentose sugars (e.g. Ribose) are important for the synthesis of nucleic acids.
Explain the Hexose monophosphate pathway ( a.k.a pentose phosphate pathway)
-Some of the glucose 6-phosphate from glycolosis is shunted into the HM pathway. From this, Ribose-5-phosphate (a pentose sugar which is a DNA and RNA building block) and NADPH is generated which helps in cellular antioxidation reactions.
What are the main sources of nitrogen in wine must? How do we express the concentrations of these components?
- Ammonium ions (21-241 mg N/l)
- Amino acids -largest source- (146-339 mg N/l)
- peptides and proteins 21% and 13% of total grape nitrogen
expressed as mg of nitrogen per litre rather than mg/l or ppm as different nitrogen-containing compounds contain different numbers of amine NH3 groups. E.g arginine has 3 NH3 groups while glutamic acid has one.
What are the two main transporter systems for the uptake of amino acids (largest nitrogen source) in S.cerevisiae?
- Specific AA transporter complexes
- Group-specific transporter systems. (GAP-general amino acid permease) These each uptake a group of amino acids. There is generally a pattern of uptake, impacted by the available nitrogen compounds and genetic preference of the strain.
What effect do ammonium ions have on the uptake of amino acids?
suppresses GAP.
this also increases glycerol production as glycolytic intermediated are taken as Amino acid precursors bringing about excess NADPH.
What are the factors that affect the uptake of nitrogen compounds during fermentation?
- Temperature- increased activity of transporter systems with higher temps.
- pH. not affected at wine pH. (optimum is pH6)
- Ethanol- reduces the activity of transporter systems (rely on proton gradient) due to reduced membrane integrity
- Sugar- increased nitrogen demand with higher sugar.
- Dissolved CO2- reduces uptake of keto acid amino acids.
- Aeration- linked to increased membrane integrity which makes transport systems more efficient.
- Yeast strain
What are the effects of nitrogen starvation during alcoholic fermentation?
- sluggish or stuck fermentation- lack nitrogen to produce enzymes for growth. stuck ferments may lose fermentative ability if glucose transport enzymes cannot be produced (irreversible arrest of protein synthesis).
- H2S production
What is the difference between an absolute and relative requirement for a vitamin?
What is the most commonly required vitamin?
If growth is not possible without a vitamin then it is an absolute requirement. One with a relative requirement will cause more rapid growth.
-Biotin is the most commonly required vitamin
