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

Chlorophyll

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