Allium Discoloration: Color Compounds Formed during Pinking of Onion and Leek

Structures
and formation pathways of compounds responsible for
pink discoloration of onion and leek were studied. A procedure was
developed for the isolation and purification of the color compounds
from various model systems and their identification by HPLC-DAD-MS/MS.
In total, structures of 15 major color compounds were tentatively
determined. It was found that the pigment is a complex mixture of
highly conjugated species composed of two N-substituted
3,4-dimethylpyrrole-derived rings linked by either a methine or a
propenylidine bridge. These two-ring units are further modified by
various C1- and C3-side chains. Experiments
with isotope-labeled thiosulfinates revealed that the methine bridge
and C1-side chains originate from the methyl group of methiin,
whereas the C3 units are derived from the propenyl group
of isoalliin.