# Pasteurization of Foods with Ultrasound: The Present and the Future
## 2 The Science behind Ultrasound  
Ultrasound has a very diverse range of applications in different scientific areas. In the case of food processing and preservation, ultrasound can have physical and chemical effects on the material when it is treated in the range of 16 to 100 kHz, known as low frequency, high intensity, or power ultrasound. The power intensity for low-frequency ultrasound goes from 10 to 1000 W/cm\({}^{2}\)[10]. Most of the research has been conducted with devices operating from 18 to 30 kHz. Opposite that, high-frequency ultrasound works in frequencies higher than 1 MHz, and it is utilized more for nondestructive uses such as quality examination of products. The power intensity for high-frequency ultrasound is \(<\)1 W/cm\({}^{2}\)[10]. There is a range of specific applications for sonochemistry reactions, from 300 to 600 kHz. In this range of frequencies, the production of free radicals is promoted to the maximum [11]. For example, the generation of OH\({}^{\bullet}\) radicals is higher at 358 kHz than at higher frequencies [12]. The production of free radicals has an antimicrobial effect during processing, acting directly on the DNA of the microorganism [13]. Power ultrasound is the one that has been used for pasteurization. Then, the term ultrasound will be used in this manuscript to refer to low frequency, high-intensity, or power ultrasound. The primary mechanism of ultrasound is based on acoustic cavitation, the generation of thousands of bubbles or cavities when the sound waves travel through a liquid medium. Cavitation can have a different energy source, like heat, pressure, or shear [14]. The term cavitation in this manuscript will refer to acoustic cavitation. The physical effects of cavitation are the formation of shockwaves and turbulence inside a liquid [1]. The bubbles from cavitation will create micro-storms and micro-currents with critical temperature and pressure increases. Although cavitation is the primarily visible phenomenon during sonication, there are additional effects during microbial inactivation. The rise in temperature and pressure, the generation of free radicals, recombining of these radicals with other molecules, and the breakdown of macromolecules are associated with the antimicrobial effect [2]. All factors will affect the microbial cells and vegetable and animal tissue differently. Cavitation can be classified as stable and transient. In Figure 1, there is a comparison between both types of cavitation. During stable cavitation, bubbles grow and dissolve in the liquid. There is no collapse of bubbles. However, small eddies are formed in the medium that has a rubbing effect on microbial cells. This effect can create pores in the cell membrane. During transient cavitation, bubbles grow and collapse violently. There is the generation of sonoluminescence. In this cavitation, there are drastic changes in temperature and pressure, reaching extreme values (i.e., 5500 \({}^{\circ}\)C, 50 MPa) and creating zones known as \"hot spots.\" Free radicals are formed from the sonolysis of water (i.e., OH\({}^{-}\) and H\({}^{+}\)) and the breakdown of other macromolecules [13]. The bubble collapse will release potential energy stored in the bubbles; this collapse occurs in picoseconds [14]. Transient cavitation is responsible for the damage to cell membranes, creating not only pores but also disrupting and breaking down cellular structures. The additional antimicrobial effect of temperature, pressure, luminescence, and free radicals are part of ultrasound's inactivation mechanism. The size of bubbles during cavitation is affected by the ultrasound frequency. As the frequency decreases, the bubble size increases, promoting more violent cavitation. Hence, low frequency or power ultrasound is used for microbial inactivation. At higher ultrasound frequencies, the bubble size decreases, and cavitation is less violent [13; 14].