Carbazole method, galacturonic acid

In all the colorimetric methods, galacturonic acid is liberated during the assay by hydrolysis of polymeric pectin. An application of the carbazole method, after extraction of pectin from various fruits and vegetables, has been described (6), as has been an automated carbazole method for monitoring uronic acid levels in pectin fractions (7). 

The content of galacturonic acid in the fractions, obtained by gel chromatography, was determined by the carbazole method (9), and the content of neutral monosaccharides was determined by the anthron reaction (16). 

Chemical Methods. Determinations of galacturonic acid of pectin usually includes both the free and esterified forms, since strongly acidic media are employed in the colorimetric methods. Procedures which continue to be used widely are modifications of those described early by Dische. One is based upon reaction with cysteine (3) and the other with carbazole (4). One modification of the carbazole method, which gave a doubling in sensitivity, along with an increased stability in color and greater reproducibility, was reported in 1962 (5). Inclusion of borate into the assay medium was responsible for the enhancement of the method. 

Pectin in almond kernels can be determined by extracting ground samples with water (soluble pectins) and then 0.05 M HCL at 100 °C for 90 min (insoluble pectins). Pectins can be determined as uronic acid by the carbazole method, using galacturonic acid as standard (Bitter and Muir 1962). 

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