Food Chemistry

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 bicyclicβ-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 ofπorbitals; consequently, a hyp-
sochromic shift (a shift to a shorter wavelength)
is observed for the major absorption bands
(Fig. 3.47a).

Fig. 3.47.Electron excitation spectra of carotenoids
(according toIsler, 1971).a— Lycopene (IV),−−
−−−γ-carotene (V),······α-carotene (VI),−·−·−
β-carotene (VII);bCanthaxanthin (XII) before — and
after−−−−oxo groups reduction with NaBH 4

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 hex-

ane 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 cis-

configuration, 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 ChemicalProperties

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 slowMaillard

reaction, but primarily to oxidation of capsanthin

(Fig. 3.48) and to some as yet unclear polymer-

ization reactions.

3.8.4.4 Precursors of Aroma Compounds

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