Allium Discoloration: Color Compounds Formed during Greening of Processed Garlic

Structures and formation pathways of compounds responsible for blue–green discoloration of processed garlic were studied in model systems. A procedure was developed for isolation of the color compounds and their tentative identification by high-performance liquid chromatography coupled to a diode array detector and tandem mass spectrometry. It was found that the pigment is a mixture of numerous pyrrole-based purple/blue and yellow species. Experiments with isotope-labeled precursors revealed that two molecules of an amino acid are involved in the formation of each color compound. In the purple/blue species (λ_max = 565–600 nm), both amino acid molecules are incorporated into two 3,4-dimethylpyrrole-derived rings linked together by a propenylidine bridge. On the other hand, the yellow compounds (λ_max = 420–450 nm) contain only one N-substituted 3,4-dimethylpyrrole ring, to which the second amino acid is bound via a propenylidine side chain.