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The Appearance of Foods. FDSC400. Goals. Color perception Natural colorants Synthetic colorants. The Dimensionality of Color Perception. Lab Colorspace. Also CIE colorspace. Color. L. a. b. Artificial light. spectrometer. Reflected spectrum. Incident spectrum. Intensity. - PowerPoint PPT Presentation
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The Appearance of Foods
FDSC400
Goals
• Color perception• Natural colorants• Synthetic colorants
The Dimensionality of Color Perception
Lab Colorspace
Also CIE colorspace
SampleInte
nsity
Wavelength
Artificial light
Incident spectrum
Inte
nsity
Wavelength
Reflected spectrum
spectrometer
Color
L a b
Color Perception
• Light source• Reflectivity of surface (diffuse vs. specular)• Pigments at surface (if the pigment absorbs
it will not be seen leaving the complementary color)
• Response of the eye• Interpretation of that response by the brain
The Chemical Basis of Color
• Photons interact with the electronic structure of the color. If the energy of the photon corresponds to the energy (=hf) of a transition between quantum states then it is absorbed.
• If that wavelength is absorbed then it is not reflected and cannot be seen.
• The chemical group in the molecule that absorbs visible photons is the chromophore.
Myoglobin
• the oxygen storage/transport protein of muscle• the heme group is the chromophore• Binds oxygen in an active site in the heme
group. The bond changes the color.• heme contains Fe2+ readily oxidized to Fe3+
States of Myoglobin
Natural States• OxyMb: FeII:O2, bright red
• deoxyMb: FeII:H2O, purple
• metMb: FeIII:H2O, brown
Also nitrosyl Mb: bright pink
Professor to his Cook: “You are a little opinionated, and I have had some trouble in making you understand that the phenomena that take place in your laboratory are nothing other than the execution of the eternal laws of nature, and that certain things which you do without thinking, and only because you have seen others do them, nonetheless derive from the highest scientific principles”
-Brillat-Savarin
The Physiology of Taste, 1825
Isoprenoid derivatives(Carotenoid pigments)
•Lipid soluble, yellow-red plant pigments•4 isoprene molecules form a subunit, two subunits form a carotenoid.•More conjugated db, color shifts from red to yellow. •Conjugated double bond sequence is the chromophore.
ISOPRENECH2=C(CH3)-CH=CH2
Benzopyran derivatives(anthocyanins)
• Very pH dependent
• R+ + H2O = ROH + H+
• i.e., color stronger at low pH
+OHO
OH
OH
R1R2
R3
blue-red plant pigments