Lactic acid bacteria Flashcards
What are the three main genera of lactic acid bacteria found in wine?
Lactobacillus, Oenococcus, and Pediococcus.
What is the main energy source for LAB growth in wine?
Residual hexose and pentose sugars after alcoholic fermentation.
What are the two main fermentation pathways used by LAB?
Homofermentation and heterofermentation.
Which pathway is dominant in wine LAB?
Heterofermentative pathway.
What are the products of homofermentative hexose metabolism?
2 lactate molecules and 2 ATP.
What are the products of heterofermentative hexose metabolism?
1 lactate, 1 CO₂, and either 1 ethanol or 1 acetate, plus 1–2 ATP.
What is produced when LAB ferment pentose sugars?
Lactate and acetate, with ATP production.
How is malic acid converted during MLF?
Malic acid is directly decarboxylated to lactic acid and CO₂.
What are the co-factors for malolactic enzyme?
NAD+ and Mn²⁺.
How does MLF indirectly produce ATP?
Through proton gradient from lactate and CO₂ excretion, which drives ATP synthesis.
What undesirable compound can result from citric acid metabolism?
Diacetyl.
What are some factors that increase diacetyl production?
Higher citric acid, lower pH, air exposure, lower temperature, and absence of SO₂.
How can SO₂ affect diacetyl levels?
SO₂ binds diacetyl and inhibits microbial growth, stabilising levels.
Why is citric acid metabolism slower than malate metabolism?
It occurs concurrently but continues after malate is fully converted.
What is the risk of tartaric acid degradation by LAB?
Increased VA, pH rise, and potential spoilage.
What harmful compound is produced by glycerol degradation by LAB?
Acrolein, which reacts with phenolics to form bitter compounds.
What amino acid can LAB metabolise to form ethyl carbamate precursors?
Arginine.
What is a health concern from histidine metabolism by LAB?
Production of histamine, a biogenic amine.
What causes histamine formation in wine?
LAB strain, stress (e.g. carb shortage), and high histidine levels from lees aging.
How does histamine production benefit LAB?
Creates a proton gradient that drives ATP synthesis (proton motive force).
