Review Ultrasonic techniques are finding increasing use in the to,-.M industry for both the analysis and modifw.ation of foods. Low-intensity ultrasound is a non-desOuctive technique that provides information about physicochemical properties, such as composition, structure, physical state and flow rate. High-intensity ultrasound is used to alter, either physically or chemically, the properties of foods, for example to generate emulsions, disrupt cells, promote chemical reactions, in- hibit enzymes, tenderize meat and modify crystallization processes. Animals have utilized ulWasound for the characteriz- ation and modification of foods for millions of years. Bats and dolphins use low-intensity ultrasonic pulses to determine the size, shape and velocity of the insects and fish that they prey on; while certain marine species use high-intensity pulses of ultrasound to stun their victims before capture. Within the past century, humans have begun using ultrasound to characterize and modify foods, albeit with the aid of sophisticated electronic equipment, rather than natural organs. Just as in nature, the applications of ultrasound in the food industry can be divided into two distinct cat- egories, depending on whether they use low-intensity or high-intensity ultrasound. Low-intensity ultrasound uses power levels (typically <1 Wcm -2) that are so small that the ultrasonic wave causes no physical or chemical alterations in the properties of the material through which the wave passes, that is it is non-destructive. The most common appfication of low-intensity ultrasound is as an analytical technique for providing information about the physicochemical properties of foods, such as composition, structure, physical state and flow rate -'-3. In contrast, the power levels used in high-intensity applications are so large (typically in the range 10-1000 W cm-2) that they cause physical disruption of the ma- terial to which they are appEed, or promote certain chemical reactions (e.g. oxidation). High-intensity nitra- sound has been used in research laboratories for many years to generate emulsions, disrupt cells and disperse aggregated materials 4-5. More recently, a number of novel applications have been developed, including the modification and control of crystallization processes, the inactivation of enzymes, the tenderization of meats, enhanced drying and filtration, and the promotion of oxidation reactions e-9. tow-inten~ ultrasonicmeasurements The three parameters that are measured most fre- quently in ultrasonic experiments are the ultrasonic velocity, attenuation coefficient and acoustic impedance (Box I). These parameters are related to the physical D. JuUam McClmne~ is at the Depa~nento[ FoodScience,University of Massachusetts, Amherst, MA 01003, USA (fax: +1-.413-545-1262; e-mail: mcclements@foodsci.umass.edu). Advances in the application of ultrasound in food analysis and processing D. JulianMcClements properties of foods that are of interest to food scienti""""~s, such as composition, stmctme and physical state. Ultrasonic velocity The velocity (c) at which an ultrasonic wave travels through a material depends on its elastic modulus (E) and density (p): 1 p The modulus used in Eqn 1 depends on whether the material being tested is a gas, fiquld or solid, and whether a compression or shear wave is usedt° (Box 1, figure at left). The moduli and densities of materials depend on their structure, composition and physical state; thus, uluasonic velocity measmements can be used to provide information about these proper- ties. The ulWasonic velocity of a material can be deter- mined in one of two ways: either the wavelength of ultra- sound is measured at a known frequency (cffi Af), or the time (0 taken for a wave to travel a known distance (d) is measured (c = d/t). Attenuation coefficient The attenuation coefficient (a) is a ~ of the decrease in ampfitude of an ultrasonic wave as it travels through a material. The major causes of attenuation are adsorption and scattering. Adsorption is caused by physi- cal mechanisms converting enex~j stored x~ ultrasound into heat, for example fluid viscosity, thermal conduc- tion and molecular relaxation. Scattering occurs in heterogeneons materials, such as emulsions, suspen- sions and foams, when an ultrasonic wave is incident on a discontinuity (e.g. a particle) and is scattered in direc- tions other than that of the incident wave. Unlike in the case of adsorption, the energy is still stored as ultra- sound, but it is not detected because its pmpegafion direction and phase have been altered. Measurements of the adsorption and scattering of ulWasound pro- vide valuable information about the physicochemical Trends in Food ,Science& Technology September 1995 [Vol. 6]