Ultrasonics refers to sounds, really pressure waves, generally above the frequency that humans can hear, starting at 20 kiloHertz (20,000 cycles/sec). The energy of ultrasound is inversely proportional to the square of frequency, so "power" ultrasound used for processing is 15–40 kHz, while frequencies greater than 100 kHz are used for imaging, medical diagnostics, and other applications. Humans can hear 15 kHz and some can detect 20 kHz, so hearing protection and sound-absorbent enclosures may be necessary in these ranges.
Ultrasound is generated by an electrical device, called a generator, that can be tuned to create vibrations at the desired frequency. These vibrations, of relatively low amplitude, are transmitted and the amplitude magnified through a second device, called a coupler. Finally, the vibrations are delivered to a third device, called a horn or tool, which actually comes in contact with the material to be treated. Sound waves are attenuated rapidly in air, i.e., they quickly lose their energy, so the horn must be in physical contact with the material, which may be solid or fluid.
Those applications that involve liquids require immersing a tool in the liquid. For small volumes, the liquid may be enclosed in a test tube or small vessel, as for cell disruption in the lab. For larger volumes, the liquid may be pumped through a chamber where it is exposed to ultrasonic energy. As an ultrasonic tool vibrates in a liquid, it generates very high localized shear. This may also generate localized heating by viscous friction. The shear contributes to mixing and emulsification. The heating can accelerate chemical or biochemical reactions. Under some conditions, very small bubbles of vaporized liquid may form and quickly collapse, creating cavitation and very intense, though short-lived, heating. The combination of heating and enhanced mass transfer helps to promote reactions, as well as dispersion and homogenization.
Food Applications and Equipment
One of the first applications for ultrasound was in welding plastic packaging, such as polystyrene clamshells. This works because the horn, which has a blunt face, very rapidly rubs the two sides of a plastic package against each other, generating heat by friction, which melts the thermoplastic and forms a seam. This is an alternative to hot bar or impulse sealing, where the heat is transmitted through the packaging material by contact. For thick materials, a hot bar may be in contact for so long that it damages the outside surface of the seal area, whereas ultrasonic sealing generates the heat just where it is needed.
Ultrasonic sealing also can push aside potential seal contaminants, such as meat fibers or sauces, that might occur in filling pouches, particularly retort pouches. For that application, the sealing tool might have more of an edge and form a narrower seal than when sealing thicker materials.
Another common application is cutting of sticky or multi-textured foods, such as cakes, pies, and pizzas. The rapid vibration of a properly designed ultrasonic tool "saws" its way through the food with less pressure applied than a conventional knife would need. Blades can be fabricated up to 10 in wide, according to Mark Caldwell, National Sales Manager for Sonics & Materials, Inc., Newton, Conn. ((phone 800-745-1105, www.sonics.biz), one of several competing firms that supply ultrasonic components to original equipment manufacturers (OEMs) who assemble the actual cutting devices. Some of the OEMs offer a standard line of conventional cutting machines and can provide ultrasonic cutting as an option.
Typically, ultrasonic cutting tools are made of titanium because it is light, very strong, and does not corrode, says Joe Re, Product Manager for Food at Dukane Corp., St. Charles, Ill. (phone 630-797-4920, www.dukcorp.com). Dukane supplies ultrasonic components and can fabricate cutting tools up to 16 in wide. The weight is important, Re says, because the mass of the tool, or more properly its inertia, consumes the ultrasonic energy. Dukane fabricates prototype tools from aluminum and uses titanium for the final design. Titanium is expensive and in demand for aerospace and defense applications, so ultrasonic cutting can be an expensive option, but for some materials it may be the only choice.
Depending on the arrangement of the tools, different shapes can be produced. For example, a row of relatively narrow ultrasonic knives can slit a ribbon of food. Cutting a sheet of candy, for instance, into long ribbons and then cutting crosswise can yield bars or cubes. Likewise, a sheet of cake can be cut into portions. Kraft has a patent on a hand-held cheese snack that reveals the use of ultrasonic cutting. The tops of pies or wraps can be scored easily with an ultrasonic knife without crushing the pastry shell.
According to Sonics & Materials, among the foods commonly cut by ultrasonics are frozen cakes and pies, frozen fish, snack and nutrition bars, fresh/frozen prepared meats, dough or baked cookies, soft and hard cheeses, fresh/frozen vegetables, candy and confections, and ice cream bars.
Hielscher Ultrasonics GmbH, Teltow, Germany, and Ringwood, N.J. (phone 973-616-9136, www.hielscher.com), an OEM, lists some other interesting food applications for ultrasonics on its Web site: peeling, disintegration of cells, extracting (extract intracellular components or obtain cell-free bacterial enzyme), activation (acceleration) of an enzyme reaction in liquid foods, acceleration of a microbial fermentation, mixing, homogenizing, dispersion of a dry powder in a liquid, emulsifying of oil/fat in a liquid stream, spraying, degassing, inspection, deactivation of enzymes, microbial inactivation (preservation), crystallization, meat processing, stimulation of living cells, and enhanced oxidation.
Among other OEMs offering ultrasonic processing equipment are Matiss, Saint-Georges, Quebec, Canada (phone 888-891-9036, www.matiss.com), Marchant Schmidt, Inc., Fond du Lac, Wis. (phone 920-921- 4760, www.marchantschmidt.com), and FoodTools, Inc., Santa Barbara, Calif. and South Haven, Mich. (phone 800-644-2377, www.foodtools.com).
Ultrasonics has many other applications besides those mentioned above.
• Ultrasonic Imaging. Ultrasonic imaging uses vibrations of significantly lower power and higher frequency (>1 MHz) transmitted to a target by contact to detect differences in density by differences in ability to transmit the sonic energy. Many are familiar with the technology from pre-natal imaging of unborn babies or diagnosis of cardiac conditions. The same concept can be applied to seals in food packages to detect potential leaks. Because of the time it takes to form a useful image, ultrasonic seal testing is typically done off-line, but it does have the advantage of being nondestructive, so that tested-satisfactory packages are not wasted.
Ultrasonic imaging is used to test welds and to detect buried flaws in solids such as metal structures, pipes, and vessels. The response to ultrasound can be correlated with some useful physical properties of food solids, such as texture and strength.
• Material Handling. Conveying of sticky and fragile foods, such as candy bars and snack cakes can be challenging, especially when changing direction or when aligning for packaging. A typical approach uses a deflector plate or rod, which may accumulate soil from products or accumulate products that stick. An ultrasonic tool can be used instead, as very little will stick to it because of the rapid vibration.
Likewise, another traditional trouble spot in conveying is the transition from one belt to another. Usually there is a "dead plate" on which product slides to prevent pieces from dropping through the unavoidable gap between rollers. If this plate is vibrated ultrasonically there is less sticking, Re says.
• Cleaning. Ultrasonic energy can accelerate cleaning of soils from solids and surfaces. This is typically a batch operation applied to relatively small pieces such as jewelry, but one can imagine using it on tools and even ultrasonic horns after they have been used for a while.
• Analysis. Analytical and process control applications of ultrasonics are discussed in the article "Ultrasonic Spectroscopy in Lipid Food Systems" by Silvana Martini in the February 2007 issue of Food Technology.
Ultrasonics can be a specialized and versatile technology with numerous applications in food processing. It has not realized its full potential, in part because the equipment is fairly expensive and is almost always custom fabricated. The full benefits of ultrasonic energy for extracting valuable materials, accelerating reactions, and homogenizing emulsions have not been achieved and offer opportunities to creative engineers.
by J. Peter Clark,
Consultant to the Process Industries, Oak Park, Ill.