In Student Articles

By Geena Whiting.

Going through university we come across many different study techniques, and one of the most recommended ways of indicating your true knowledge of a subject is to try and explain it to someone. I have a massive test coming up and between you and me I am very nervous about writing it, it is on Malolactic fermentation and it is not my strong suit, so please allow me to try and explain it to you so that I can pass my test.

Malolactic fermentation is the breakdown of malic acid to lactic acid by bacteria. There are three metabolic groups that the bacteria fall under: Obligate homofermentitive, Obligate hetereofermentative and Facultatively heterofermentitive. The major bacteria O.oeni we use belongs to the obligate heterofermentative group, they posse the phosphoketlase enzyme and ferment glucose/fructose to lactate, ethanol, acetic acid and CO2.

The other common bacteria is L.plantarum species, these belong to the facultatively heterofermentative groups. It ferments glucose/fructose via the EMP pathway; however glucose represses the enzyme of the phosphoglyconate pathway and therefore doesn’t increase the volatile acidity of the wine.

Enough about the tiny details (That’s a microbiology joke because bacteria are small)

How does malic acid fermentation impact wine sensory attributes?

Aroma: there is an improved fruitiness, butteriness and a reduced vegetative character.

Taste: it’s less acidic than the original, a creamier and fuller palate and more complex flavours are produced.

Mouthfeel: there is better structure and a more balanced wine.

Colour: The impact on a heavy wine is limited but malic acid on cool climate wines or thin skinned grapes will cause a slight colour loss.

The aromas that the MLF produced can be described as buttery, lactic/yoghurt, nutty, yeasty, oaky, vanilla, fruity, caramel and toffee.

Where do the wine aromas come from?

Mainly the degradation of citric acid, citric acid exists in lower concentrations in wine (0.2-0.6g/l), the degradation begins when 2/3 of the malic acid degradation is complete.  Citrate is broken down by citrate lyase to oxaloacetate – this reaction also produced acetate which results in the increase of volatile acidity. Oxaloacetate is degraded to pyruvate and this gives rise to acetate, D-lactate, Diacetyl and 2,3-butandiol.

Diacetyl in low levels gives rise to toasty, nutty and yeasty characteristics, at medium levels you will smell buttery or butterscotch and adds complexity, high levels give you rancid butter and masks the fruity and vegetative aromas. This is what we want to see in a tasty Chardonnay.

2,3 – Butandiol at threshold levels of 600mg/l will give you fruity , buttery and creamy aromas.

Esters something that I don’t really think about in red wine, however MLF is a major process in making red wine… well, drinkable. Ethyl lactate is the most important ester; it provides the fruity, buttery and creamy mouthfeel. Ethyl hexonate gives off the brandy cherry flavour. Those beautiful floral aromas of some white wines are caused by ethyl octanoate. We all know that banana flavour in some pinotages, well that is caused by isoamyl acetate.

There are some downsides to MLF – this is the higher alcohol this can give rise to Isobutanol which gives off solvent and bitterness. Isoamyl alcohol which will give off that malty burnt flavour.
2- phenyl ethanol will bring those beautiful honey, spice, rose and lilac flavours.

The Strains also differ. O.oeni will increase the buttery aroma and will produce some VA. L.plantarum increases the fruitiness.

You also have to take into account when you inoculate: co-inoculation is when you inoculate during fermentation, this is great because the free sulphur and ethanol is low. You can use the temperature from fermentation. It will increase the fruitiness, less diacetyl is produced, the wine is less acidic. Sequential inoculation is when you inoculate after fermentation, this can be a problem with stuck fermentation because of alcohol, N sources are depleted and lysozyme are produced which kill the bacteria. There is no increase in Volatile acidity. It produces more buttery notes and the fermentation is easier to control.

Well thanks for your help, I feel much more confident about my test and I hope you have learnt something too!

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