## 3 Non-Thermal Technologies in Food Processing
### High-Pressure Processing  
High-pressure processing (HPP) (also called as high pressure (HP) or high hydrostatic pressure (HHP) or ultra-high pressure (UHP)) is a novel, emerging, and promising non-thermal pasteurization technology, the use of which has steadily increased in food processing since the beginning of the twenty-first century [186; 187; 188]. In fact, the application of high-pressure technologies in the food industry is following an exponential growth over the last few years [189]. HPP technology acts as a cold pasteurization process and takes place at pressures ranging between 100 and 1000 MPa and at temperatures from \(-\)20 to 60 \({}^{\circ}\)C, to solid, liquid, packaged, or unpackaged foods, for several seconds or minutes [190]. A liquid, which is water, is usually used as a means of pressure transfer [8], due to safety and cost considerations [191]. Figure 6 displays a high-pressure processing (HPP) vessel.  
The most suitable foods to be processed with the HPP technique are those that have a satisfactory concentration in water and a lack of air gaps. Today, this technique has found wide application, mainly in fruits and vegetables, seafood and shellfish, dairy products, juices and beverages, ready-to-eat meals, and meat products [186]. This technique is unsuitable for the production of low acidity foods, as they require high pressures of 800-1700 MPa in order to inactivate their bacterial spores [193].  
Food safety, food quality, and extending the shelf life of high-value refrigerated foods have been the main focus of HPP technology in recent years [194; 195]. All these can be achieved, as a variety of pathogenic and spoilage vegetative bacteria, yeasts, molds, viruses, and also spores can be inactivated with HPP treatment [196]. Applying the PP technique has been also shown to enhance healthy attributes in foods. It has been found that the immunoglobulin of dairy products is preserved, the content of resistant starch in cereals is increased, and the glycemic index of fruits is reduced. In addition, this technique promotes the extraction of bioactive compounds from food waste [197].  
Key process parameters for this technology include pressure, temperature, and exposure time, but water activity, types of enzymes and/or microorganisms, and the cell growth phase are some extraordinary factors that critically influence the process [198]. The application of pressure can be performed both directly and indirectly [199]. In direct application, a piston moves and causes a change in volume inside the pressure vessel, while in indirect application, the pressure setting in the container varies according to the amount of pressure fluid [186].  
The application of this technology disrupts cell walls and membranes, inactivates enzymes, denatures proteins, and causes gel formation [190]. HPP has been officially approved as a food pasteurization alternative to traditional pasteurization by the U.S. Food and Drug Administration (FDA) and the U.S. Department of Agriculture (USDA) [200]. Isostatic pressure above 300 MPa is applied to hermetically sealed containers containing food [190]. In HHP technology, pressure is uniform and simultaneous in all directions of the container, so the pasteurization effect of HPP is not affected by the size, shape, product composition, and nutrient content of the food [201].