17.1 Vegetables 797(17.20)Betanin is the main pigment of red beet. It is a be-
tanidin 5-0-β-glucoside. The betaxanthins have
only the dihydropyridine ring in common. The
other structural features are more variable than
in betacyanins. Examples of betaxanthins are nat-
ural vulgaxanthins I and II, also from red beet
(Beta vulgaris):
(17.21)Betalain biosynthesis starts with dopa by opening
of its benzene ring, followed by cyclization
to a dihydropyridine. The (S)-betalamic acid
which is formed undergoes condensation with
(S)-cyclodopa to betacyanins or with some
other amino acids to betaxanthins (cf. reaction
sequence 17.21).
Red betanin is water soluble and is used to color
food. Its application is, however, limited because
it hydrolytically decomposes into the colorless
cyclodopa-5-0-β-glucoside and the yellow (S)-
betalamic acid. This reaction is reversible. Since
the activation energy of the forward reaction
(72 kJ×mol−^1 ), greatly exceeds that of the back
reaction (2.7kJ×mol−^1 ), a part of the betanin
is regenerated at higher temperatures. Betanin is
also sensitive to oxygen.