2009_Book_FoodChemistry

3.8 Unsaponifiable Constituents 241
bonds (Table 3.58). The fine structure of the
spectrum is better distinguished in the case of
acyclic lycopene (IV) than bicyclic β-carotene,
since the latter is no longer a fully planar
molecule. The methyl groups positioned on
the rings interfere with those on the polyenic
chain. Such steric effects prevent the total
overlapping of π orbitals; consequently, a hypsochromic
shift (a shift to a shorter wavelength)
is observed for the major absorption bands
(Fig. 3.47a).
Oxo groups in conjugation with the polyene
system shift the major absorption bands to
longer wavelengths (a bathochromic effect) with
a simultaneous quenching of the fine structure
of the spectrum (Fig. 3.47b). The hydroxyl
groups in the molecule have no influence on the
spectra.
A change of solvent system alters the position of
absorption maxima. For example, replacing hexane
with ethanol leads to a bathochromic shift.
Most of the carotenoids in nature and, thus, in
food are of the trans-double bond configuration.
When a mono-cis- or di-cis-compound occurs,
the prefix “neo” is used. When one bond of all
trans-double bonds is rearranged into this cisconfiguration,
there is a small shift in absorption
maxima with a new minor “cis band” shoulder on
the side of the shorter wavelength.
3.8.4.3 Chemical Properties
Carotenoids are highly sensitive to oxygen
and light. When these factors are excluded,
carotenoids in food are stable even at high
temperatures. Their degradation is, however,
accelerated by intermediary radicals occurring
in food due to lipid peroxidationo (cf. 3.7.2).
The cooxidation phenomena in the presence of
lipoxygenase (cf. 3.7.2.2) are particularly visible.
Changes in extent of coloration often observed
with dehydrated paprika and tomato products are
related to oxidative degradation of carotenoids.
Such discoloration is desirable in flours (flour
bleaching; cf. 15.4.1.4.3).
The color change in paprika from red to brown,
as an example, is due partly to a slow Maillard
reaction, but primarily to oxidation of capsanthin
(Fig. 3.48) and to some as yet unclear polymerization
reactions.
3.8.4.4 Precursors of Aroma Compounds
Fig. 3.47. Electron excitation spectra of carotenoids
(according to Isler, 1971). a — Lycopene (IV), −−
−−−γ-carotene (V), ······ α-carotene (VI), −·−·−
β-carotene (VII); b Canthaxanthin (XII) before — and
after −−−−oxo groups reduction with NaBH 4
Aroma compounds are formed during the
oxidative degradation of carotenoids. Such
compounds, their precursors and the foods
in which they occur are listed in Table 3.59.
The mentioned ionones and β-damascenone