Malo-lactic Analysis by TLC
Here are a couple of ciders analysed on a TLC (thin-layer
chromatography)
plate, to check for the malo-lactic fermentation. This is carried
out in cider by specialised bacteria which convert the original malic
acid
to lactic, generally as the cider matures once the yeast fermentation
is
completed. This reduces the total acidity (since lactic acid is a
mono-basic acid whereas malic is di-basic), and carbon dioxide is lost
in the process. So the cider tends to become slightly gassy
again.
Desirable strains of these bacteria also soften and round the cider by
producing new flavour compounds . It is now possible to buy
various
strains of ML bacteria commercially and add them to the cider, rather
than
waiting for them to work of their own accord.
The cider samples are spotted at the bottom of the special silica
plate,
on the pencil line, and they move by capillary action up the plate when
placed in a jar of solvent. After drying, the plate is dipped in
an indicator (bromo-cresol green) and the acid spots are revealed as
pale
yellow on a darker background. Lactic acid moves the furthest (the left
hand track) and malic the least (the right hand track). The blue
spots that stay close to the origin are due to the natural potassium
salts
present in the cider. The pale yellow spots are quite faint and
may not be evident at first glance - but they are there, trust me!
So what you see here is that the left hand sample (one of my own, as
it happens) has gone entirely ML with no malic acid remaining.
The
cider on the right, by comparison, has all its original malic acid with
only a trace (if at all) of MLF conversion.
There are many published variants of the TLC procedure in the
scientific literature with different plates, solvents and indicator
reagents but here is one that I have found speedy and useful:
Technical Details
Plate: Macherey Nagel Polygram Sil G Cat No. 805012
Spot Application: 2 microlitres, dried, and then
applied
again
Elution Solvent: Toluene + Acetic Acid + n-butyl acetate
2:1:1
Indicator: Bromocresol green (sodium salt) in ethanol
(0.2%)
Reference: Australian
Grape Grower and Winemaker January 2001 (via web.archive.org)
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Last updated 29.9.2002