The Appearance of Foods FDSC400
Goals • Color perception • Natural colorants • Synthetic colorants
Lab Colorspace Also CIE colorspace
Color L a b Artificial light spectrometer Reflected spectrum Incident spectrum Intensity Intensity Sample Wavelength Wavelength
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) ISOPRENE CH2=C(CH3)-CH=CH2 • 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.
Benzopyran derivatives(anthocyanins) • Very pH dependent • R+ + H2O = ROH + H+ • i.e., color stronger at low pH blue-red plant pigments