Ultrasonic Homogenizers

The possibility of using ultrasound to form emulsions was realized in the early part of this century, and ultrasonic homogenizers have been widely used for this purpose in both industry and research since then (McCarthy 1964, Gopal 1968). This type of homogenizer utilizes high-intensity ultrasonic waves which generate intense shear and pressure gradients within a material that disrupt the droplets mainly due to cavitational effects (Section 6.3.1.2).

A number of methods are available for generating high-intensity ultrasonic waves, but only two are commonly used in the food industry: piezoelectric transducers and liquid jet generators (Gopal 1968). Piezoelectric transducers are used in the benchtop ultrasonic homogenizers that are found in many research laboratories. They are ideal for preparing small volumes of emulsion (a few to a few hundred cubic centimeters), which is an important consideration in research laboratories because the chemicals used are often expensive. An ultrasonic transducer consists of a piezoelectric crystal contained within a protective metal casing,

Liquid Crystal Emulsion

Emulsion

FIGURE 6.8 Liquid jet generators are commonly used in the food industry for the continuous production of emulsions.

Emulsion

FIGURE 6.8 Liquid jet generators are commonly used in the food industry for the continuous production of emulsions.

which is usually tapered at the end. A high-intensity electrical wave is applied to the transducer, which causes the piezoelectric crystal to rapidly oscillate and generate an ultrasonic wave. The ultrasonic wave is directed toward the tip of the transducer, where it radiates into the surrounding liquids and generates intense pressure and shear gradients (mainly due to cavitational effects) which cause the liquids to be broken up into smaller fragments and intermingle with one another. The fact that the ultrasonic energy is focused on a small volume of the sample near the tip of the ultrasonic transducer means that it is important to have good agitation in the sample container. In small vessels, this is achieved by the fluid flow induced by the ultrasonic field itself, but in large vessels, it is often necessary to have additional agitation to ensure effective mixing and homogenization of all of the sample. To create a stable emulsion, it is usually necessary to irradiate a sample with ultrasound for periods ranging from a few seconds to a few minutes. 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 the industrial preparation of food emulsions (Figure 6.8). 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 due to a combination of cavitation, shear, and turbulence (Gopal 1968). The major advantages of this device are that it can be used for the continuous production of emulsions, it can generate very small droplets, and it is more energy efficient than high-pressure valve homogenizers (i.e., less energy is required to produce droplets of the same size). Even so, the vibrating blade is prone to erosion because of the ultrasonic field, which means that it has to be replaced frequently. Fluid flow rates between 1 and 5000 l min-1 are possible using this technique.

The principal factors determining the efficiency of ultrasonic homogenizers are the intensity, duration, and frequency of the ultrasonic waves (Gopal 1968). Below a frequency of about 16 kHz, ultrasonic waves become audible and are therefore objectionable to users. Emulsions can be formed using ultrasonic waves with frequencies as high as 5 MHz, but the homogenization efficiency decreases with increasing frequency. For these reasons, most commercial devices use ultrasonic waves with frequencies between about 20 and 50 kHz. The size of the droplets produced during homogenization can be decreased by increasing either the intensity of the ultrasonic radiation or the length of time it is applied.

Was this article helpful?

0 0
Atkins Low Carb Diet Recipes

Atkins Low Carb Diet Recipes

The Atkins Diet is here. Dr Atkins is known for his great low carb diets. Excluding, Dr Atkins carb counter and Dr Atkins New Diet Revolution.

Get My Free Ebook


Post a comment