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:
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%)
Grape Grower and Winemaker January 2001 (via web.archive.org)
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Last updated 29.9.2002