Determination of sulfite in dried garlic by reversed-phase ion-pairing liquid chromatography with post-column detection

A method is described for determining sulfite in dried garlic. Garlic is extracted with an HCl solution to inhibit the formation of allicin, which interferes with the determination of sulfite. After cleanup of the extract on a C18 solid-phase extraction column, sulfite is converted to hydroxymethylsulfonate (HMS) by adding formaldehyde and heating to 50 degrees C. HMS is determined by reversed-phase ion-pairing liquid chromatography with post-column detection. The post-column reaction system consists of the addition of KOH to convert HMS to sulfite ion, followed by the addition of 5,5'-dithiobis(2-nitrobenzoic acid) to produce 5-mercapto-2-nitrobenzoic acid which is detected spectrophotometrically at 450 nm. Background levels in unsulfited dried garlic equivalent to < 20 ppm SO2 were found. Recoveries of HMS from spiked garlic averaged 94.8% with a coefficient of variation of 3.8%. Sulfite was found in 13 of 21 samples of dried garlic produced in China, with sulfite ranging from 114 to 445 ppm. Sulfite was found in 60% of commercial dried garlic products purchased locally. The suitability of the Monier-Williams method for determining sulfite in garlic is discussed.