Chemistry Reference
In-Depth Information
electromagnetic radiation are absorbed by chemical species in the sample. In
the process, the energy is transferred to the atom or molecule and causes it to
be promoted to an excited state from the ground state. Absorption of energy
occurs only if the energy of the photon exactly matches the energy difference
between the ground state and excited states of the atom or molecule. With
transmission, the velocity at which the radiation is transmitted through the
medium containing atoms or molecules decreases compared to the velocity in a
vacuum. The lifetime of an atom or molecule in an excited state is limited and it
will rapidly relax back to the ground state. The energy released may be
transformed into electrical or thermal energy or re-emitted as radiation of
lower energy (therefore longer wavelength), known as fluorescence. Scattering
of electromagnetic radiation does not involve transfer of energy, but rather the
radiation is totally re-emitted randomly in all directions. The intensity of
scattering is dependent on the number and size of particles and the difference
in refractive index between the particles and the surrounding medium.
Milk is a complex colloidal dispersion of fat globules, casein micelles and
whey proteins in an aqueous solution of lactose, salts and other compounds.
Thus, milk not only absorbs light at several wavelengths but also scatters light
because of the presence of large particles, e.g., casein micelles and fat globules.
In the visible region, riboflavin in milk absorbs strongly near 470 nm
and emits fluorescent radiation with a maximum at 530 nm. Milk fat contains
-carotene, which absorbs near 460 nm. In the ultraviolet region, aromatic
amino acid residues of proteins (tyrosine and tryptophan) strongly absorb
near 280 nm and a part of the ultraviolet radiation energy is re-emitted as
fluorescence at 340 nm. Measurement of the intensity of ultraviolet fluores-
cence has been used to quantify the protein content of milk (Konev and
Kozunin, 1961; Fox et al.,1963; Bakalor, 1965; Porter, 1965).
In the infrared region, absorptions are due primarily to amide (II)
groups (CONH) of proteins at 6465 nm, hydroxyl groups (OH) of lactose at
9610 nm and ester carbonyl groups (CTO) of lipids at 5723 nm. These
absorption principles have been used to estimate the protein, fat and lactose
content of milk simultaneously (Goulden et al., 1964; Biggs, 1964, 1979a, b;
Gillickson, 1983) and are the basis of official methods of the Association of
Official Analytical Chemists (AOAC, 1995a) and the International Dairy
Federation (lDF, 1990). Several custom-built infrared analysers are available
commercially for this purpose. The use, additionally, of measurement of
the methylene (CH) stretch has been proposed for the measurement of fat
(Clemmensen, 1980; Mills and van de Voort, 1982). The milk is homogenized
before measurement to reduce the size of fat globules to below 1 mm where
light scattering is negligible (Rudzik and W obbecke, 1982). An extremely
narrow sample thickness, typically 0.037 mm, is used, and under these con-
ditions, a linear Beer-Lambert plot is obtained for each milk component in
