Food Lipids: Chemistry, Nutrition, and Biotechnology

e adsorbed to the surface of a droplet during a droplet-emulsifier encounter (i.e.,
the adsorption efficiency); (d) the amount by which the emulsifier reduces the interfacial
tension; and (e) the effectiveness of the emulsifier membrane at protecting the
droplets against coalescence.
It is often assumed that small emulsifier molecules adsorb to the surface of
emulsion droplets during homogenization more rapidly than larger ones. This assumption
is based on the observation that small molecules diffuse to the interface
more rapidly than larger ones under quiescent conditions [3]. It has been demonstrated
that under turbulent conditions large surface-active molecules tend to accumulate
at the droplet surface during homogenization preferentially to smaller ones
[23].
C. Homogenization Devices
There are a wide variety of food emulsions, and each one is created from different
ingredients and must have different final characteristic properties. Consequently, a
number of homogenization devices have been developed for the chemical production
of food emulsions, each with its own particular advantages and disadvantages, and
each having a range of foods to which it is most suitably applied [1a]. The choice
of a particular homogenizer depends on many factors, including the equipment available,
the site of the process (i.e., a factory or a laboratory), the physicochemical
properties of the starting materials and final product, the volume of material to be
homogenized, the throughput, the desired droplet size of the final product, and the
cost of purchasing and running the equipment. The most important types of homogenizer
used in the food industry are discussed in the subsections that follow.
1. High Speed Blenders
High speed blenders are the most commonly used means of directly homogenizing
bulk oil and aqueous phases. The oil and aqueous phase are placed in a suitable
container, which may contain as little as a few milliliters or as much as several liters
of liquid, and agitated by a stirrer that rotates at high speeds. The rapid rotation of
the blade generates intense velocity gradients that cause disruption of the interface
between the oil and water, intermingling of the two immiscible liquids, and breakdown
of larger droplets to smaller ones [25]. Baffles are often fixed to the inside of
the container to increase the efficiency of the blending process by disrupting the flow
profile. High speed blenders are particularly useful for preparing emulsions with low
or intermediate viscosities. Typically they produce droplets that are between 1 and
10 �m in diameter.
2. Colloid Mills
The separate oil and water phases are usually blended together to form a coarse
emulsion premix prior to their introduction into a colloid mill because this increases
the efficiency of the homogenization process. The premix is fed into the homogenizer,
where it passes between two disks separated by a narrow gap. One of the disks
is usually stationary, while the other rotates at a high speed, thus generating intense
shear stresses in the premix. These shear stresses are large enough to cause the
droplets in the coarse emulsion to be broken down. The efficiency of the homogenization
process can be improved by increasing the rotation speed, decreasing the
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.