# Non-Thermal Methods for Ensuring the Microbiological Quality and Safety of Seafood
## 3 Ultrasound--US  
### General Description of US  
Ultrasound (US) is another thriving non-thermal method with increased interest in the food industry. US is a minimal process technology that can be implemented in several types of foods to inactivate foodborne pathogens and spoilage microorganisms without affecting the desired quality attributes of the product [102]. Ultrasonic waves used in the food industry are divided into low-energy ultrasounds, with high frequencies in the range of MHz (>1000 kHz), and high-energy ultrasounds, with low frequencies from 20 to 100 kHz [103]. Another parameter that influences US performance is the intensity of the treatment, where the application of low- and high-energy ultrasounds are generally performed at intensities lower or higher than 1 \(W\)/cm\({}^{2}\), respectively. The US technique is safe for human hearing, while it cannot detect frequencies lower than 20 Hz or higher than 20 kHz [104]. Whatever type of commercial system is used to apply high ultrasound to foods, it will consist of three necessary parts: generator, transducer, and the reactor to which it is coupled [103]. Epigrammatically, typical ultrasonic systems are the ultrasonic bath, the ultrasonic probes, the parallel vibrating plates, and the radial vibrating systems [103] and the airborne acoustic ultrasound systems first developed by [PERSON] et al. (1978) [105].  
Low-energy ultrasounds are used as a powerful tool at the laboratory and industry level for non-invasive analysis and monitoring of several food materials to improve the qualitative characteristics of high-quality foods and ensure safety [106]. Low-energy ultrasound has also been used to evaluate in a non-destructive way many measuring factors such as the concentration, viscosity, and composition of raw and fermented meat products [107; 108], fish and seafood [109; 110; 111], and poultry [112]. On the other hand, high-energy ultrasound can disrupt the physical systems, cause different chemical or biochemical reactions, and lead to mechanical effects on food properties [103]. Ultrasonication is also aimed at microbial inactivation as a pasteurization method for liquid foods and a decontamination method of solid products [102]. The central mechanism of US is to inactivate microorganisms previously described by many authors [113; 114; 115] and is mainly due to cavitation phenomena leading to the breakdown of cell walls, disruption, and thinning of cell membranes caused by the generated shockwaves after the bubble collapsing, DNA damage through free radicals' production, and local heating.  
Over the past few years, US has been applied for large-scale applications in the food industry, leading to a strategic advantage in the various stages of processing [116]. The effects of high ultrasound as an alternative to thermal pasteurization of liquid foods and as a method to inactivate microorganisms from solid products such as meat, poultry, fish, and seafood have been extensively studied in the last years. High ultrasound as a single-use technology cannot achieve the 5-log reduction in compliance with the FDA (2004) [117] requirements and is mainly used with mild thermal treatments. Therefore, high ultrasound is combined with other technologies to achieve a more effective bacterial log reduction and its mechanical, chemical, or biochemical effects as shown analytically in Table 2. Currently, US is considered an emerging and promising applicable technology in the food processing industry [118].  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Products** & **Treatment Conditions** & **Combination of Methods** & **Mechanical Effects** & **Chemical/Biochemical Effects** & **References** \\ \hline Red seabream (Pagras _major_) fillets & 40 kHz, 200 W, 10 °C & Ultrasound and thawing at 0 °C & No free water changes and improved physicochemical properties of proteins, actin had better thermal stability & - & [119] \\  & & & &