A GUIDE TO CAROTENOID ANALYSIS IN FOODS

22 A Guide to Carotenoid Analysis in Foods
Absorbance
300 350 400 450 500 550 600
Wavelength (nm)
Figure 17. Visible absorption spectra of violaxanthin ( ___ ) and its
furanoid derivative (– – –).
dation of carotenoids exist during preparation, processing,
and storage of food. Thus, retention of naturally
occurring or added carotenoids in prepared, processed,
and stored foods is an important consideration.
Carotenoids are also subject to isomerization
and oxidation during analysis, and preventative measures
must be taken to guarantee the reliability of
analytic results.
Chemical Reactions of Functional Groups
Xanthophylls undergo group chemical reactions that
can serve as simple chemical tests in the identification
of carotenoids. Many of the chemical reactions,
in extensive use in the late 1960s and early 1970s,
have now been supplanted by mass and nuclear magnetic
resonance spectrometry. However, some reactions
remain useful and can be performed quickly
with only a small amount of the test carotenoid and
are amenable to rapid monitoring by ultraviolet or visible
spectrometry, thin-layer chromatography, or
HPLC.
For example, primary and secondary hydroxy
groups are acetylated by acetic anhydride in pyridine.
Allylic hydroxyls, isolated or allylic to the chro-
Absorbance
350 400 450 500 550
Wavelength (nm)
Figure 18. Visible absorption spectra of canthaxanthin ( ___ ) and
its reduction product (– – –).
mophore, are methylated with acidic methanol. In both
reactions a positive response is shown by an increase
in the silica thin-layer chromatography R F value or
the retention time in reversed-phase HPLC, the extent
of the increase depending on the number of hydroxy
substituents.
Epoxy groups in the 5,6 or 5,6,5´,6´ positions are
easily detected by their facile conversion to the
furanoid derivatives in the presence of an acid catalyst,
reflected by a hypsochromic shift of 20–25 or
50 nm, respectively (Figure 17).
Ketocarotenoids, such as echinenone and
canthaxanthin, and apocarotenals undergo reduction
with LiAlH 4 or NaBH 4 , manifested by the appearance
of the three-maxima spectra of the resulting
hydroxycarotenoid (Figure 18).