Food Lipids: Chemistry, Nutrition, and Biotechnology

uous production of emulsions; it can generate very small droplets; and it is more
energy efficient than high pressure valve homogenizers (since less energy is needed
to form droplets of the same size).
5. Microfluidization
Microfluidization is a technique that is capable of creating an emulsion with small
droplet sizes directly from the individual oil and aqueous phases [29]. Separate
streams of an oil and an aqueous phase are accelerated to a high velocity and then
made to simultaneously impinge on a surface, which causes them to be intermingled
and leads to effective homogenization. Microfluidizers can be used to produce emulsions
that contain droplets as small as 0.1 �m.
6. Membrane Homogenizers
Membrane homogenizers form emulsions by forcing one immiscible liquid into another
through a glass membrane that is uniform in pore size. The size of the droplets
formed depends on the diameter of the pores in the membrane and on the interfacial
tension between the oil and water phases [30]. Membranes can be manufactured with
different pore diameters, with the result that emulsions with different droplet sizes
can be produced [30]. The membrane technique can be used either as a batch or a
continuous process, depending on the design of the homogenizer. Increasing numbers
of applications for membrane homogenizers are being identified, and the technique
can now be purchased for preparing emulsions in the laboratory or commercially.
These instruments can be used to produce oil-in-water, water-in-oil, and multiple
emulsions. Membrane homogenizers have the ability to produce emulsions with very
narrow droplet size distributions, and they are highly energy efficient, since there is
much less energy loss due to viscous dissipation.
7. Energy Efficiency of Homogenization
The efficiency of the homogenization process can be calculated by comparing the
energy required to increase the surface area between the oil and water phases with
the actual amount of energy required to create an emulsion. The difference in free
energy between the two separate immiscible liquids and an emulsion can be estimated
by calculating the amount of energy needed to increase the interfacial area
between the oil and aqueous phases (�G = ��A). Typically, this is less than 0.1%
of the total energy input into the system during the homogenization process because
most of the energy supplied to the system is dissipated as heat, owing to frictional
losses associated with the movement of molecules past one another [23]. This heat
exchange accounts for the significant increase in temperature of emulsions during
homogenization.
8. Choosing a Homogenizer
The choice of a homogenizer for a given application depends on a number of factors,
including volume of sample to be homogenized, desired throughput, energy requirements,
nature of the sample, final droplet size distribution required, equipment available,
and initial and running costs. Even after the most suitable homogenization
technique has been chosen, the operator must select the optimum processing conditions,
such as temperature, time, flow rate, pressure, valve gaps, rotation rates, and
sample composition. If an application does not require that the droplets in an emul-
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.