A GUIDE TO CAROTENOID ANALYSIS IN FOODS

14 A Guide to Carotenoid Analysis in Foods
SOME PHYSICOCHEMICAL PROPERTIES OF CAROTENOIDS
A good understanding of some of the physical and
chemical properties of carotenoids allows analysts to
determine carotenoids with greater ease and reliability.
Solubility
With very few exceptions, carotenoids are lipophilic.
They are insoluble in water and soluble in organic
solvents, such as acetone, alcohol, ethyl ether, chloroform,
and ethyl acetate. Carotenes are readily
soluble in petroleum ether, hexane, and toluene; xanthophylls
dissolve better in methanol and ethanol.
Crystalline carotenoids may be difficult to dissolve in
the above solvents but do dissolve in benzene and
dichloromethane (Schiedt and Liaaen-Jensen 1995).
Solubility of both ß-carotene and the xanthophyll lutein
in tetrahydrofuran was shown to be excellent (Craft
and Soares 1992).
Light Absorption
The conjugated double-bond system constitutes the
light-absorbing chromophore that gives carotenoids
their attractive color and provides the visible
absorption spectrum that serves as a basis for their
identification and quantification. The color enables
analysts to monitor the different steps of carotenoid
analysis. Loss or change of color at any time during
the analysis gives an immediate indication of
degradation or structural modification. The color
permits visual monitoring of the separation of
carotenoids in open-column chromatography, and
mainly for this reason this classical technique is still a
viable option for quantitative analysis of carotenoids.
The ultraviolet and visible spectrum is the first
diagnostic tool for the identification of carotenoids.
The wavelength of maximum absorption (λmax) and
the shape of the spectrum (spectral fine structure)
are characteristic of the chromophore. The structure-spectrum
relationship has been extensively discussed.
The λmax values of common carotenoids,
taken mainly from Britton’s (1995) compilation, are
shown in Table 7 and will be discussed in relation to
the structures by using the absorption in petroleum
ether.
Most carotenoids absorb maximally at three
wavelengths, resulting in three-peak spectra (Figure
8). The greater the number of conjugated double
bonds, the higher the λmax values. Thus, the most
unsaturated acyclic carotenoid lycopene, with 11 conjugated
double bonds, is red and absorbs at the longest
wavelengths (λmax at 444, 470, and 502 nm) (Figure
8). At least 7 conjugated double bonds are needed
for a carotenoid to have perceptible color. Thus, ζcarotene
is light yellow. Being also acyclic, its spectrum
has three well-defined peaks, but these are at
wavelengths much lower than those of lycopene
(λmax at 378, 400, and 425 nm), commensurate with
Absorbance
350 400 450 500 550
Wavelength (nm)
Figure 8. Visible absorption spectra of lycopene ( ____ ), γ-carotene
(- - -), β-carotene (-.-.-.), and α-carotene (....) in petroleum
ether.