212 Produce Degradation: Reaction Pathways and their Prevention
7.5 BETALAINS
Betalains are purple, red, pink, orange, and yellow water-soluble pigments. They
occur exclusively in 13 families of Caryophyllales (Mabry et al., 1963), an order of
dicotyledonous plants in the relatively small subclass Caryophyllidae characterized
by free-central or basal placentation. Betalains occur also in some higher fungi.
Betalains may be found in various organs of higher plants (Rosendal-Jensen et al.,
1989), including roots, stems, leaves, bracts, flowers, fruits, and seeds. More than
50 naturally occurring betalains are known (Piatelli and Minale, 1964).
7.5.1 CHEMICAL DEFINITION AND STRUCTURE
The term betalains was introduced by Mabry and Dreiding (1968). They are immo-
nium derivatives of betalamic acid (Piatelli, 1981). The chromophore of betalains
is a protonated 1,2,4,7,7-pentasubstituted 1,7-diazaheptamethin system (Piatelli,
1976). Betalain’s general formula is shown on Figure 7.7. There are two subgroups
of betalains: red-violet betacyanins (see Figure 7.8) and yellow betaxanthins (see
Figure 7.9). The structures of betacyanins may be derived from an aglycone betanidin
with a formula in which the R group is –OH. For example, if the R group is –glucose,
the formula represents betanin, or if the R group is –glucose 2-glucoronic acid-
glucose, the formula represents amaranthin. The common names of betalains are
assigned according to their botanical genus. For example, betacyanin obtained from
Beta vulgaris is named “betanin” and betacyanin from Amaranthus tricolor is named
“amaranthin.” In all betacyanins two chiral carbons (C-2 and C-5) are present.
Depending on the configuration on carbon 15, in the case of betanine and amaranthin
the two epimers are betanine and isobetanin or amaranthin and isoamaranthin,
respectively. In betacyanins an aromatic substituent contributes additional conjugation,
FIGURE 7.7Betalains.
NR
NR'HHOOC COOH