Malolactic Fermentation and Your Wine
What is Malolactic
'Malolactic fermentation' (ML) describes a fermentation by bacteria (leuconostoc
oenos) that are able to convert malic acid from grapes into lactic acid.
It occurs alongside, and in addition to regular fermentation, and can
be desirable for two reasons:
Reducing excess acidity. By converting the relatively harsh tasting malic
acid into the softer lactic, ML softens the flavour of the wine.
Adding complexity. In addition to converting the acid, malolactic bacteria
can add a component of 'buttery' flavour (diacetyl), along with more complex
flavours and aromas.
Uncontrolled ML is very undesirable. The same bacteria responsible for
reducing acidity are responsible for the production of sauerkraut, whose
flavours and aromas are not what you would expect in a fine wine. In addition,
if malolactic bacteria work in the presence of potassium sorbate (a preservative
in kit wines and some commercial wines) it will produce geraniol, a compound
that smells like a cross between ripe salmon and rotting geraniums.
Which wines should
get malolactic fermentation?
Generally, only wines made from grapes get malolactic treatment. There
are two reasons for this, the first being that the bacteria needs a small
amount of grape pulp and solids to get a foothold in the wine. Second,
it is only really the top quality wines that merit malolactic treatment,
and these are most often sold in the form of grapes.
Wines made from concentrate are generally unsuitable for malolactic treatment.
Not only are the acidity levels usually balanced toward the low side for
early drinking, the levels of solids in concentrate are virtually nil.
In addition, some concentrates may have small amounts of sorbate in them,
causing geraniol problems.
The treatment criteria
for wines are as follows:
The wine should be one where complexity is desired over fruitiness. ML
tends to emphasize fermentation aromas and flavours and reduce the fresh
fruity flavours of a wine. If this doesn't suit your palate, you should
The wine must have an alcohol level below 14%. Malolactic bacteria are
not especially alcohol tolerant, and a wine with a higher alcohol content
will not support it.
The wine must have a pH of 3.3 or greater. If you don't know what the
pH is, it will be necessary to have it tested before attempting malolactic
The wine must have some grape pulp or yeast sediment to promote the growth
of the bacteria. If it is clear, MLF will be very difficult to start.
What form does
the malolactic bacteria come in?
There are two main types of malolactic culture available, refrigerated
liquid culture, and freeze dried powder. They are both perfectly good
choices for inoculating your grapes. They both have to be kept refrigerated
at all times.
They both come with instructions for use, which, while fine for inoculation
in a laboratory setting, are somewhat confusing for the home user.
What are some of
the dangers of malolactic fermentation?
In addition to transforming malic acid into lactic, leuconostoc oenos
also transforms citric acid into acetic acid, which is the acid that gives
vinegar it's distinctive flavour. This usually isn't a problem, unless
the must has been treated with citric acid.
Also, because it reduces fruity flavours, there are few white wines that
will benefit from ML, and some reds are best left fruity as well.
Because ML fermentation
can cause the pH of the wine to rise, it may make it more susceptible
to other forms of bacterial infection. You may need to adjust sulphite
levels to compensate.
Partial ML, or ongoing
malolactic fermentation in a wine that is to be bottled can cause the
wine to carbonate (become fizzy), break the bottle with CO2 pressure,
push out the cork, and spoil the flavour. You must be certain that ML
is complete and/or arrested before bottling (see below).
How do I go about
starting a malolactic fermentation?
Assuming you have purchased good quality grapes, you have tested the acid
and pH, and are confident that going through ML will produce a better
wine for you, the first step is to obtain a malolactic bacterial culture.
These are sold by Wine Cave, and are labeled as 'Leuconostoc Oenos Bacteria'.
If you are only inoculating 23 litres of wine, you don't have to make
a starter culture with either the freeze dried or liquid cultures. With
the liquid culture package, when the grapes are almost finished fermentation
(below) simply match the temperature of the package with the temperature
of the grapes (it should be 20-25°Celsius) and add it. Within two
to three weeks the ML will be well under way, and should be complete within
With the freeze-dried
culture it is necessary to make a sterile suspension and rehydrate the
bacteria. For two grams of culture, prepare 25 ml of distilled water at
25-30° Celsius in a sanitised cup and sprinkle the culture on top.
After 15 minutes, stir with a sanitised spoon and add to your fermenting
must. Again it is important that the wort be below 5°Brix, so as not
to slow the ML.
If you are inoculating
more than 23 litres, you should purchase the culture about two weeks before
your wine is nearly fermented dry, to allow time to make a starter culture.
Judging the point when your wine will be two weeks away from dryness may
seem difficult on first blush, but a good rule of thumb would be to purchase
the culture the week before you receive your grapes.
A culture from liquid
is prepared thus: obtain 2½ litres of fresh grape juice. Choose
a grape juice with less than 15 PPM of SO2, and with less than 20°Brix
of sugar content. If you can't find an available juice with these characteristics,
it is possible to change the pH and sugar content of a less suitable juice
by either diluting with water or adding a pH adjuster. Ask your retailer
for advice. Make sure you use only grape juice: apple juice or grape concentrate
is not suitable.
Add about 1 gram of
yeast energizer to the juice. This will help speed cell growth. Ensure
the juice is between 20-25°C and inoculate with a package of malolactic
friendly yeast (Epernay is ideal) and allow to sit for 2 days, and inoculate
with liquid malolactic culture and maintain a temperature of 20-25°C.
After 12 days the
culture will be ready. 2½ litres is sufficient for 120 litres of
wine. If you have more than 120 litres, you can culture up the amount
you need by simply adding your starter culture to larger amounts of suitable
juice. You will need approximately ½ litre for every 23 litres
When is malolactic
If the wine is kept at relatively warm temperatures (above 20°C)
the fermentation will be complete within two months. The only way to monitor
this is through the use of a malolactic chromatography test kit. They
are available from Wine Cave, and contain enough materials for many tests.
The test itself is somewhat complicated, and takes about 24 hours to conduct.
If you don't want to pay quite this much for a test kit, or are uncomfortable
with laboratory procedures, the RJSpagnol's lab will do testing for a
fee. In any case, if you wish to know that your malolactic fermentation
is complete, an accurate test is the only way to find out.
Once you are satisfied
that your ML is finished, you can stabilize your wine by adding sulphites
to the level of approximately 50 PPM. This will prevent the re-growth
of any leuconostoc bacteria, and will prevent oxidation. It is a good
idea to test the pH and acidity of your wine at this point as well. Because
ML can cause the pH to rise, you may need to adjust it. Space doesn't
permit a full discussion of pH and wine quality, but several winemaking
books contain good sections on pH or you can ask your retailer for advice.