Food Lipids: Chemistry, Nutrition, and Biotechnology

ticularly prone to partial coalescence when the droplets contain a certain percentage
of crystalline fat (Sec. VI.B). Partial coalescence leads to extensive droplet aggregation,
which decreases the stability of emulsions to creaming and greatly increases
their viscosity. In water-in-oil emulsions, such as margarine or butter, the formation
of a network of aggregated fat crystals provides the characteristic rheological properties.
The most important data for food scientists are the temperature at which
melting or crystallization begins, the temperature range over which the phase transition
occurs, and the value of the solid fat content at any particular temperature.
Phase transitions can be monitored by measuring changes in any property (e.g.,
density, compressibility, heat capacity, absorption or scattering of radiation) that is
altered upon conversion of an ingredient from a solid to a liquid (Table 2). The
density of a component often changes when it undergoes a phase transition, and so
melting or crystallization can be monitored by measuring changes in the density of
a sample with temperature or time.
Phase transitions can also be monitored by measuring the amount of heat absorbed
or released when a solid melts or a liquid crystallizes, respectively. This type
of measurement can be carried out by means of differential thermal analysis or
differential scanning calorimetry. These techniques also provide valuable information
about the polymorphic form of the fat crystals in an emulsion. More recently, rapid
instrumental methods based on NMR and ultrasound have been developed to measure
solid fat contents [1a]. These instruments are capable of nondestructively determining
the solid fat content of a sample in a few seconds and are extremely valuable analytical
tools for rapid quality control and on-line procedures. Phase transitions can
be observed in a more direct manner by means of polarized optical microscopy.
E. Creaming and Sedimentation Profiles
Over the past decade, a number of instruments have been developed to quantify the
creaming or sedimentation of the droplets in emulsions. Basically the same light
scattering, NMR, and ultrasound techniques used to measure the dispersed phase
volume fraction or droplet size distributions of emulsions are applied to creaming or
sedimentation, but the measurements are carried out as a function of sample height
to permit the acquisition of a profile of droplet concentrations or sizes. Techniques
based on the scattering of light can be used to study creaming and sedimentation in
fairly dilute emulsions. A light beam is passed through a sample at a number of
different heights, and the reflection and transmission coefficients are measured and
related to the droplet concentration and size. By measuring the ultrasonic velocity
or attenuation as a function of sample height and time, it is possible to quantify the
rate and extent of creaming in concentrated and optically opaque emulsions. This
technique can be fully automated and has the two additional advantages: creaming
can be detected before it is visible to the eye, and a detailed creaming profile can
be determined rather than a single boundary. By measuring the ultrasound properties
as a function of frequency, it is possible to determine both the concentration and
size of the droplets as a function of sample height. Thus a detailed analysis of
creaming and sedimentation in complex food systems can be monitored noninvasively.
Recently developed NMR imaging techniques can also measure the concentration
and size of droplets in any region in an emulsion [9]. These ultrasound and
NMR techniques will prove particularly useful for understanding the kinetics of
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.