Yeast Flashcards
What is the basic equation for alcoholic fermentation by yeast?
C6H12O6 glucose/fructose →(yeast)– 2CH3CH2OH (ethanol) + 2CO2 + heat + minor compounds (~3%)
What are the main products of yeast fermentation?
Ethanol, carbon dioxide, and heat.
Why is fermentation more complex than the simplified equation?
It’s a multi-step enzymatic process with several branching metabolic pathways.
Name three enzymes involved in the glycolysis in yeast.
Hexokinase, phosphofructokinase, glyceraldehyde-3-phosphate dehydrogenase.
What is the main energy vector in cellular/yeast metabolism?
ATP to ADP generates energy.
ATP (adenosine triphosphate). (ADP reduced form)
What is the main redox vector (the direction of electron movement in a redox reaction) in yeast metabolism?
NAD+/NADH (nicotinamide adenine dinucleotide).
What is the equation for glycolysis?
1 glucose → 2 pyruvate + 2 ATP + water.
What is the EMP pathway and what is pyruvate?
Embden-Meyerhof-Parnas pathway; the glycolysis pathway for yeast. - pyruvate is a biochemical building block - in yeast the bulk of it forms alcohol
What happens to pyruvate in fermentation?
It is converted to ethanol via acetaldehyde (ethanal) and CO2 and then ethanol and H2O.
What is the function of alcohol dehydrogenase in fermentation?
It reduces ethanal (acetaldehyde) to ethanol.
Why is glyceropyruvic fermentation significant?
It generates glycerol, is energy-neutral, and produces less ethanol. - bad for winemaking
Under what conditions does glyceropyruvic fermentation occur?
Early yeast growth, presence of SO2, or high sugar musts.
What is the Pasteur Effect?
Oxygen inhibits fermentation by promoting aerobic respiration for more ATP.
What is the Crabtree Effect?
High glucose causes yeast to produce ethanol even in the presence of oxygen.
What are the requirements for amino acid synthesis in yeast?
A carbon skeleton (often from pyruvate) and nitrogen (e.g., ammonium). - yeast need amino acids to grown and multiply
What happens during amino acid catabolism in yeast?
Yeast break down excess amino acids and cant produce new amino acids, producing higher alcohols and H2S.
What are higher alcohols and how are they formed?
Alcohols with more than two carbons, with bad organoleptic properties in high levels; formed during amino acid catabolism and sugar metabolism.
How are esters formed in yeast fermentation?
Through reactions between alcohols and carboxylic acids.
Why are esters important in wine?
They contribute aromatic compounds, although they’re not persistent due to hydrolysis.
How is acetic acid formed during fermentation?
From ethanal oxidation or pyruvate. All yeast produce acetic acid.
How can excessive acetic acid production be prevented?
Avoid extremes in pH, high sugar musts, poor nutrition, over-clarification, and high temps.
What causes hydrogen sulphide (H2S) production in yeast?
Sulfur dioxide, Sulphur-containing amino acid catabolism, sulphate.
How can yeast reverse sulphide production?
Under favourable conditions, yeast can reduce sulphide production or re-metabolise it.
