18.1 Fruits 831(18.25)The flavylium cation (I) is stable only at very
low pH. As the pH increases it is transformed
into colorless chromenol (II). Figure 18.6 shows
the decrease in absorption in the visible spectrum
at various pH’s, reflecting these transformations.
Formation of a quinoidal (III) and ionic anhy-
dro base (IV) at pH 6–8 intensifies the color. At
pH 7–8 structure IV is transformed through ring
opening to yellow chalcone (V). At higher pH’s
the color can be stabilized by the presence of mul-
tivalent metal ions (Me: Al^3 +,Fe^3 +). The com-
plexes formed are deep blue (cf. Formula 18.25).
Figure 18.7 illustrates the shift in absorption
maximum from 510 to 558 nm for cyanidin-3-
glucoside over the pH range of 1.9–5.4. Readings
were taken in the presence of aluminium chloride.
At higher pH’s free anthocyanidins (VII, For-
mula 18.26) are degraded via chromenols (VIII)
Fig. 18.7.Absorption spectra of cyanidin-3-glucoside
(35 μmole/l + 830 μmole/lAlCl 3 ) in aqueous buffered
solutions at pH 1.90, pH 3.50, pH 3.90, and pH 5.36.
(According toJurdandAsen, 1966)
andα-diketones (IX) to aldehydes (X) and car-
boxylic acids (XI):(18.26)Addition of SO 2 bleaches anthocyanins. The
flavylium cation reacts to form a carbinol
base corresponding to compounds XII or XIII
(Formula 18.27). The color is restored by acid-
ification to pH 1 or by addition of a carbonyl
compound (e. g. ethanal). Since compounds of
type XIV (R^1 =CH 3 ,C 2 H 5 ) are not affected by
SO 2 , it appears that compound XIII is involved
in such bleaching reactions.