# Novel Techniques for Microbiological Safety in Meat and Fish Industries
## 6 Non-Thermal Preservation Techniques
### Ultrasound  
Ultrasound is a new technology which has a variety of food processing applications. The sound wave could be defined as higher than the waves that human ears perceive [56]. However, US frequencies above 20 kHz are not audible [57]. Ultrasonic treatments (US) are classified as non-thermal and are used between 20 kHz and 1 MHz in food processing. Cavitation bubbles are produced when the US intensity is strong enough to cause rarefaction that is greater than the medium's intermolecular attraction forces [58]. Ultrasound was first used in the meat industry to assess the fat and muscle of live cattle, and this has been going on since the 1950s. Low-intensity ultrasound is now commonly used to improve quality, taste, and tenderness, which are the three most important quality attributes in terms of customer satisfaction. Many recent studies have suggested that high-intensity ultrasound could be used on fresh meat [59]. US use is currently common in the meat industry to improve meat tenderization, bulk emulsification, marination, frostbite, homogenization, crystallization, drying and inactivation of the microorganism. Since both _Escherichia coli_ and _Salmonella_ are susceptible to US, it can be considered a new green technology for tenderizing and decontaminating poultry meat. US improves the physical and chemical properties of meat proteins, resulting in a decrease in the \(\alpha\)-helix in the intramuscular protease complex as well as lower viscosity coefficients. As a result, ultrasonic treatment can be used to improve the texture of chicken meat. In comparison to other traditional techniques, US can also improve drying rates under vacuum. US can be used at a high frequency (>1 MHz) with low intensity (<1 W/cm\({}^{2}\)) or low frequency (20-100 kHz) with high intensity (10 to 1000 W/cm\({}^{2}\)), depending on the application in food processing [44]. US has the ability to modify cell membranes in meat processing, which can aid in curing, marinating, drying, and tenderizing tissues [60]. Ultrasound curing improves brine transfer into meat, decreases processing time, and improves product quality [61]. The use of an ultrasonic bath at 40 kHz and 110 W, as well as sodium chloride (in marinating for longer periods of time) resulted in increased meat hardness, weight loss, and changes in color parameters (lightness and redness), but also reduced WHC, water content, and pH [62]. Chicken meat treated with a US probe (22 kHz,27.6 W/m2) was found to reduce the number of mesophilic bacteria [63]. US treatment may have a lower discoloration effect in poultry as compared to red meat. When compared to control meat, the color of ultrasonic-treated beef meat at an intensity of 22 W/cm2 showed less redness, less brightness, and a more yellow/orange color [64]. The antimicrobial efficiency of high-intensity ultrasound is dependent on several factors, including the duration of contact with the microorganism, the nature of the microorganism, the amount and composition of food, and the treatment temperature [65]. Gram-positive bacteria are more resistant to ultrasound because their cell walls are thicker, due to a coating of peptidoglycans that protects them from sonication. Cell morphology is indicated as a major factor because coccoid cells demonstrate higher tolerance than spores of bacillus bacteria to acoustic activity. Spoilage by spores of _Clostridium_ and _Bacillus_ species are more resistant to ultrasonic treatment than vegetative cells [59]. Oxidative stability, meat color and sensory characteristics are important factors during meat ultrasonication. The use of sound energy can accelerate the process of meat thawing. The quality of the meat is enhanced by the implementation of ultrasound to reduce the time of meat biring [65]. Difficulty in choosing the right parameters for the desired product (i.e., pressure, time, temperature, strength, power and amplitude) and the production of free radicals (adversely affect the product quality because of oxidation) are some of the drawbacks of US treatment [66].