Food Lipids: Chemistry, Nutrition, and Biotechnology

flow rate, decreasing the size of the gap between the disks, and increasing the surface
roughness of the disks. Colloid mills are more suitable than most other types of
homogenizer for homogenizing intermediate or high viscosity fluids (e.g., peanut
butter, fish or meat pastes), and they typically produce emulsions with droplet diameters
between 1 and 5 �m.
3. High Pressure Value Homogenizers
Like colloid mills, high pressure valve homogenizers are more efficient at reducing
the size of the droplets in a coarse emulsion premix than at directly homogenizing
two separate phases [26]. The coarse emulsion premix is forced through a narrow
orifice under high pressure, which causes the droplets to be broken down because
of the intense disruptive stresses (e.g., impact forces, shear forces, cavitation, turbulence)
generated inside the homogenizer [27]. Decreasing the size of the orifice
increases the pressure the emulsion experiences, which causes a greater degree of
droplet disruption and therefore the production of smaller droplets. Nevertheless, the
throughput is reduced and more energy must be expended. A food manufacturer must
therefore select the most appropriate homogenization conditions for each particular
application, depending on the compromise between droplet size, throughput, and
energy expenditure. High pressure valve homogenizers can be used to homogenize
a wide variety of food products, ranging from low viscosity liquids to viscoelastic
pastes, and can produce emulsions with droplet sizes as small as 0.1 �m.
4. Ultrasonic Homogenizers
A fourth type of homogenizer utilizes high intensity ultrasonic waves that generate
intense shear and pressure gradients. When applied to a sample containing oil and
water, these waves cause the two liquids to intermingle and the large droplets formed
to be broken down to smaller ones. There are two types of ultrasonic homogenizer
commonly used in the food industry: piezoelectric transducers and liquid jet generators
[28]. Piezoelectric transducers are most commonly found in the small benchtop
ultrasonic homogenizers used in many laboratories. They are ideal for preparing
small volumes of emulsion (a few milliliters to a few hundred milliliters), a property
that is often important in fundamental research when expensive components are used.
The ultrasonic transducer consists of a piezoelectric crystal contained in some form
of protective metal casing, which is tapered at the end. A high intensity electrical
wave is applied to the transducer, which causes the piezoelectric crystal inside to
oscillate and generate an ultrasonic wave. The ultrasonic wave is directed toward the
tip of the transducer, where it radiates into the surrounding liquids, generating intense
pressure and shear gradients (mainly due to cavitational affects) that cause the liquids
to be broken up into smaller fragments and intermingled with one another. It is
usually necessary to irradiate a sample with ultrasound for a few seconds to a few
minutes to create a stable emulsion. Continuous application of ultrasound to a sample
can cause appreciable heating, and so it is often advantageous to apply the ultrasound
in a number of short bursts.
Ultrasonic jet homogenizers are used mainly for industrial applications. A
stream of fluid is made to impinge on a sharp-edged blade, which causes the blade
to rapidly vibrate, thus generating an intense ultrasonic field that breaks up any
droplets in its immediate vicinity though a combination of cavitation, shear, and
turbulence [28]. This device has three major advantages: it can be used for contin-
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