Lecture 10: Problem Fermentations. Reading Assignment: Text, Chapter 4, pages 168- 181. This lecture will cover the principle types of fermentation problems that can arise during the alcoholic fermentation: Stuck fermentations and off-character production. Problem Fermentations.
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Text, Chapter 4, pages 168- 181
This lecture will cover the principle types of fermentation problems that can arise during the alcoholic fermentation: Stuck fermentations and off-character production
The main challenge of slow and arrested fermentations is that they are not recognizable until after fermentation rate has changed. At this point it may be too late to change the adaptive response of the yeast.
GOAL: To be able to distinguish a normal from a problem fermentation as soon as possible.
1: lag time; 2: max fermentation rate; 3: transition point; 4: post-transition fermentation rate; 5: overall time to dryness
Glutamate + X -ketoglutarate + N-X
Glutamine + X’ glutamate + N-X’
Alanine + X’’ pyruvate + N-X’’
Where “X” is an intermediate in amino acid/ nucleotide biosynthesis, and “N-X” is an amino acid or nucleotide base.
NH4 + -ketoglutarate glutamate
NH4 + glutamate glutamine
Ratio of K+:H+
Must be at least 25:1
Needs to be adjusted early in fermentation
Probably important in building an ethanol tolerant membrane
Transporters with a high substrate affinity can get “jammed” at high substrate concentrations
Plasma membrane is the most ethanol-sensitive cell structure:
Composition: Protein 50%
Regulation of uptake
Mediates response to environment
Maintains electrochemical gradients
Mediates cell-cell interactions
Impact of ethanol
Perturbs membrane structure at protein:lipid interface
Leads to increased “passive proton flux” and acidification of cytoplasm
Inhibits protein activity
Affects membrane “fluidity”
Adaptation of membrane requires:
Increasing content of sterols
Increasing relative content of proteins
Increasing level of desaturation (number of double bonds) in fatty acid side chains
Modification of phospholipid head groups?
Inositol Serine Ethanolamine Choline
Sterol and fatty acid desaturation are Oxygen-requiring processes
New protein synthesis requires nitrogen be available
Phospholipid head group synthesis requires cofactors (S-adenosyl- methionine) be available
The factors leading to arrest of fermentation are interacting. Limitation for nutrients enhances the toxicity of ethanol as does high temperature and the presence of other toxic substances.
Early (first 2-4 days): due to N imbalance
Late (end of fermentation): due to autolysis, degradation of S-containing compounds
H2S produced early can be driven off by carbon dioxide during active phase of fermentation
Cysteine Cystathionine Homocysteine Methionine
R R R
HCNH C=O HC=O
Amino Acid Alcohol Acid
May be too intense for some wines
Esters form from the reaction of an alcohol and an acyl-CoA molecule
R1-OH + R2-CSCoA R1-O-C-R2
Decarboxylase Vinyl Phenol
Yeast needs are simple, but it can be challenging to keep them happy.