NO - Besoin d'assistance

Composition of tomatoes and tomato products in antioxidants (WG1) page 82
__________________________________________________________________________________________
8.5 Analysis
Total carotenoid levels can be estimated through the red color determination. By using the
L*a*b colorimetric system, the red (a) and yellow (b) colors are mesured and the lycopene
level is evaluated.
Total carotenoid levels could be measured by spectrophotometric assessment of a hexane
extract at 450 nm. However such simple methods cannot provide information on the
carotenoid profile since most of these compounds absorb between 440 and 505 nm. Thus
other methods must be used for analysis of plasma and fruits which contains a mixture of
carotenoids. However in red tomatoes in wich lycopene is the predominant carotenoid and β-
carotene and xanthophyll levels are very low, this method could represent a rapid, cheap and
simple alternative to high performance liquid chromatography (HPLC) methods to determine
the pigment concentration. Moreover the maximum absorption of lycopene occurs around 472
nm and that of β-carotene occurrs around 450 nm. Thus shifting the absorbance measurement
to the red radiations would result in sufficient response of lycopene and a very low if not
undetectable response of β-carotene. Hence lycopene from tomato puree was extracted with
hexane containing 0.08% BHT. Optical density of the hexane extract was measured on
spectrophotometer at 502 nm against a hexane blank. For validation purposes, hexane extract
of lycopene was analyzed by HPLC. HPLC data confirmed that quantitative determination of
lycopene by direct absorbance measurement could be an accurate method (Fraile et al. 1998).
However this method should not be recommended for tomatoes which contained both
β−carotene and lycopene.
When a mixture of tocopherol isomers and carotenoids are present in the sample,
chromatographic analyses allowed for progress. The oldest methods described separation of
these analytes on paper, thin-layer and open column (AOAC method) chromatographies.
They were generally time consuming and resulted in low sensitivity. There are numerous
HPLC methods published in literature using normal (Columns with polar solid phase)
(Khachick et al. 1992, Schmitz et al. 1994), reverse (columns with non-polar solid phase)
(Olmedilla et al. 1990, Schierle et al. 1997), and supercritical conditions (Lesellier et al.
1993). Similarly isocratic mobile phase as well as gradient of mobile phase and flow rate were
used. Some of these methods used single UV/Visible, diode array, fluorescence and
electrochemical detectors. It should be noted that the detection limits for β-carotene by