those with large ones. External forces applied to a system have stronger
effects on large particles than on small ones, which has direct consequences
for their separability.
Size Distributions. Since size is important, so is itsdistribution.
Size distributions can be presented in various ways, e.g., cumulative or as a
frequency (which is the derivative with respect to size of the cumulative
distribution); as a number or as a volume distribution; versus diameter or
(molar) mass, etc. Varioustypes of averagescan be defined and calculated,
and their values can differ by more than an order of magnitude if the size
distribution is relatively wide. It depends on the problem involved what type
of average should be taken. Distribution width can be defined as standard
deviation over average; for most food dispersions, it ranges between 0.2 and
1.2. If the width is considerable, the shape of the distribution may vary
significantly. For anisometric particles, characterization of size poses
additional problems.
Determinationof size distributions can be done by several methods,
each having its limitations and pitfalls. Accurate determination is
notoriously difficult. Systematic and random errors are involved. Several
methods are indirect ones, determining some macroscopic property, for
instance, a light scattering spectrum. The conversion of these data to a size
distribution is generally difficult and may lead to considerable error.
BIBLIOGRAPHY
Several aspects briefly touched on in this chapter are far more elaborately discussed
in texts on colloid science. A somewhat outdated but still very interesting book is
K. J. Mysels. Introduction to Colloid Science. Interscience, New York, 1965.
Far more up-to-date is
R. J. Hunter. Foundations of Colloid Science, Vol. 1. Clarendon, Oxford, 1987.
of which Chapter 3 gives an extensive discussion on particle size and shape, including
methods of determination.
Information on the physical structure of several foods and on some of its
implications is in
J. M. Aguilera and F. W. Stanley. Microstructural Principles of Food Processing and
Engineering, 2nded. Aspen, Gaithersburg, MD, 1999.
Diffuse reflection of foods is discussed by
F. M. Clydesdale. Color measurement. In: D. W. Gruenwedel and J. R. Whitaker,
eds. Food Analysis, Vol. 1, Physical Characterization. Marcel Dekker, New
York, 1984, Chapter 3.