T2 The Malolactic fermentation Flashcards
MLF does not produce any energy for the LAB. Why would they do this conversion?
The conversion generates a proton gradient and this is used to drive a membrane-bound ATPase enzyme. producing ATP for the cell.
MLF does not provide carbon for the production of new cells so this must be obtained from sugars.
Explain the mechanism of Malolactic conversion
Malate permease transports L-malic acid into the LAB cell. Malate carboxyl lyase then converts l-malate to L-lactic acid + CO2 (NAD+ and Mn2+ coenzymes) and consumes an H+ in the process.
This proton consumption raises the pH of the cell and creates the ΔH+ (protonmotive force) that drives the ATPase to produce ATP.
L-Lactic acid is then exported from the cell.
What effect does the rise in pH have on glucose consumption?
What effect does it have on Malate conversion?
- has a stimulatory effect on glycolysis.
- Malate carboxyl lyase has optimum activity at pH 6.3, so MLF itself stimulates the conversion of more malate
What effect does ethanol have on the optimum temperature for malolactic activity?
As ethanol rises, the optimum temperature decreases.
- At 0% etOH, 30-35C
- At 10-15% etOH down to 20C.
Why is bound SO2 toxic to LAB?
They can metabolise bound acetaldehyde, releasing deadly free.
What is the result of high extracellular lactate levels on MLF?
This inhibits the efflux of lactate out of the cell, meaning that pH decreases in the cell. This decrease in pH reduces the rate of MLF as the optimum pH for the malate carboxyl lyase is pH 6.3.
what is the difference between constitutive and inducible Malolactive activity?
Constitutive activity does not change during the growth of the population. O. oeni has this sort of MLF enzyme expression.
Inducible means MLF enzyme expression will increase during fermentation as it is expressed more. L. plantarum has this sort of expression.
What are the three main reasons for conducting MLF in a wine?
- Deacidification
- Microbial stability
- Flavour and aroma modification
Describe the benefit of MLF for deacidification of a wine.
- Cool climate wines may have a TA of up to 9 g/l. MLF may reduce this by 1-3 g/l and raise pH by 0.3.
- Warm climate wines may experience acidity drops that are too far and may appear flabby. The pH of these wines (maybe above 3.5) may enable the growth of Lactobaccillus and Pediococcus spoilage organisms.
How does MLF help with the microbial stability of a wine?
By having MLF conducted prior to bottling, there is no risk of it occuring in bottle. MLF in bottle creates a haze and gassiness.
This merely helps with stability and does not ensure it. normal measures such as pH adjustment, SO2, filtration, racking and low temperatures are still required to reduce growth of spoilage organisms.
What flavour modifications do we typically attribute to MLF and what compounds are linked to these descriptors?
- Buttery and nutty- diacetyl.
- oaky
- yeasty.
- smoky and sweaty
- Broadness and fullness- ethyl lactate.
What effect does MLF typically have on fruity and vegetal aromas in wine?
What effect on palate?
- Fruity- Neutral towards fruity character. Strongly aromatic wines cover up the mlf character.
- Vegetal- generally reduced by MLF. handy for overly herbaceous young cabernet.
- Softens palate, increases fullness and lengthens aftertaste.
What extracellular enzymes may be produced by LAB and what may be the effect on aroma?
- beta glucanases, xylanases and beta-glucosidase.
- may release some sugar bound aroma compounds from their ester form.
What are some undesirable effects of MLF?
Reduction in colour intensity of red wines.
Possible mechanisms
- dehydration of citric acid to release H+and reduces colour pigments.
- release of acetaldehyde bound SO2 resulting in bleaching.
- beta-glucosidase activity on wine colour pigments.
