Food Lipids: Chemistry, Nutrition, and Biotechnology

Food emulsions always contain droplets that have a range of sizes, and so it
is usually important to characterize both the average size and the size distribution
of the droplets. The droplet size distribution is usually represented by a plot of droplet
frequency (number or volume) versus droplet size (radius or diameter). Some of the
most important experimental techniques for measuring droplet size distributions are
included in Table 2.*
Light-scattering and electrical conductivity techniques are capable of providing
a full particle size distribution of a sample in a few minutes. Since, however, these
techniques usually require that the droplet concentration be very low (� < 0.001),
samples must be diluted considerably before analysis. Optical and electron microscopy
techniques, which provide the most direct measurement of droplet size distribution,
are often time-consuming and laborious to operate, and sample preparation
can cause considerable artifacts in the results. In contrast, recently developed techniques
based on NMR and ultrasonic spectroscopy can be used to rapidly and nondestructively
measure the droplet size distribution of concentrated and optically
opaque emulsions [1a]. These techniques are particularly useful for on-line characterization
of emulsion properties.
C. Microstructure
The structural organization and interactions of the droplets in an emulsion often play
an important role in determining the properties of a food. For example, two emulsions
may have the same droplet concentration and size distribution, but very different
properties, because of differences in the degree of droplet flocculation. Various forms
of microscopy are available for providing information about the microstructure of
food emulsions. The unaided human eye can resolve objects that are farther apart
than about 0.1 mm (100 �m). Most of the structural components in food emulsions
(e.g., emulsion droplets, surfactant micelles, fat crystals, ice crystals, small air cells,
protein aggregates) are much smaller than this lower limit and cannot therefore be
observed directly by the eye.
Optical microscopy can be used to study components of size between about
0.5 and 100 �m. The characteristics of specific components can be highlighted by
selectively staining certain ingredients or by using special lenses. Electron microscopy
can be used to study components that have sizes down to about 0.5 nm. Atomic
force microscopy can be used to provide information about the arrangements and
interactions of single atoms or molecules. All these techniques are burdened by
sample preparation steps that often are laborious and time-consuming, and subject
to alter the properties of the material being examined. Nevertheless, when carried
out correctly the advanced microscopic techniques provide extremely valuable information
about the arrangement and interactions of emulsion droplets with each
other and with the other structural entities found in food emulsions.
D. Physical State
The physical state of the components in a food emulsion often has a pronounced
influence on its overall properties [1a]. For example, oil-in-water emulsions are par-
*A comprehensive review of analytical methods for measuring particle size in emulsions has recently
been published [31].
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.