Each acid, with its unique molecular properties, moves a characteristic distance up the paper and, with the help of acid standards, you can readily identify the acids in the wine sample. A color indicator is used to change the acid spot to yellow distinguishing it from the blue/green background color developed on the paper.
Malic acid will show as a large spot on the paper before MLF but as the bacteria fermentation progresses, the malic spot diminishes and the lactic acid spot increases in size. At some point the malic acid spot is difficult to distinguish and MLF is nearing completion. However, since the paper chromatography procedure can only detect down to 100 mg/L (100 ppm), there could still be trace amounts of malic acid in the wine. As little as 30 mg/L can cause bubbling in your bottled wine. Therefore, one should allow the fermentation to continue until no more characteristic MLF bubbles appear in the neck of the carboy. This may take another week or so.