Chemistry Reference
In-Depth Information
the range 0-6%. Infrared methods correlate well with chemical methods
(Adda et al., 1968; Biggs, 1964, 1979a), although a repeatable procedure
must be established for homogenization and dilution, and the instrument
must be calibrated using several samples analysed by standard methods to
cover the range of each component expected.
Both fat globules and casein micelles scatter light, and it is possible to
measure both the scattered intensity at some angle to the incident beam and
the intensity of light transmitted through the sample. Both transmitted and
scattered light measurements form the basis of the measurement of the fat
content of milk and the size distributions of fat globules and casein micelles.
Haugaard and Pettinati (1959) and Goulden and Sherman (1962) described
methods for the determination of fat content, average fat globule size and
homogenization efficiency in dairy products based on light scattering by fat
globules. Instruments for fat determination are based on measurement of
turbidity (i.e., the attenuation of the incident beam caused by scattering),
usually expressed as optical density or absorbance in a way similar to that in
normal absorption photometry. Milk is first homogenized to achieve a uni-
form fat globule size distribution and the interfering scattering due to protein
is eliminated by using an appropriate dissociating agent, e.g., EDTA or
detergents. The commercial version of this method is known as the Milko-
Tester, the results of which compare favourably with other methods for fat
analysis, provided the instrument is frequently checked and calibrated against
standard methods (Grappin and Jeunet, 1970; Shipe and Senyk, 1973, 1975,
1980). Methods for measuring the average size of casein micelles or fat globu-
les in milk using the wavelength dependence of turbidity have been described
(Walstra, 1965, 1967; Holt, 1975).
Nakai and Le (1970) described a simple method for the determination
of protein and fat in milk simultaneously. Acetic acid was used to dissolve
both protein and fat and the protein content determined from the absorbance
at 280 nm. Turbidity due to fat was developed by adding a solution of urea
and imidazole and measured at 400 nm.
Light scattering and absorption properties of milk greatly affect the
appearance of milk (Walstra and Jenness, 1984). The creamy colour of milk is
due to
-carotene in the fat. Casein micelles scatter blue light (short wave-
length) more effectively than red, giving the bluish colour of skim milk. The
most common methods for the determination of colour involve transmission
and reflectance techniques. Light is directed at the sample and the signal
transmitted through, or reflected from, the sample is measured either in a
tristimulus colorimeter or over the visible spectrum, 380-759 nm, using a
spectrophotometer. Generally the results are expressed using the L*, a*, b*
(Hunter) systems. In this system, L* denotes the position of sample on the
dark-light axis, a* on the green-red axis and b* on the blue-yellow axis.
