# Non-Thermal Technologies in Food Processing: Implications for Food Quality and Rheology
## 3 High-Pressure Processing (HPP)  
High-pressure processing (HPP), also known as ultra-high and high-hydrostatic-pressure processing, is a processing technology that applies a controlled high pressure to food to extract key bioactive components and inactivate possible microbial contaminants without applying heat. The beauty of this process is that it maintains the flavour, colour, texture, and nutrients of foods. Although, its effect on colour can vary depending on the food matrix and specific processing conditions [80]. This cold pasteurisation process reduces the quantity of microbes and enhances the shelf life of foods. HPP is based on two thermodynamic principles, both of which account for microbial inactivation: [PERSON]'s principle, where a high pressure decreases the volume and affects molecular interactions, and the isostatic principle, which enables consistent pressure applications independent of the food geometry [81]. HPP inactivates microorganisms by breaking down non-covalentbonds and damaging microbial cell membranes [3; 6]. The treatment parameters usually vary depending on the food type, with exposure times ranging from 1 to 20 min at temperatures of 0-110 \({}^{\circ}\)C and pressures of 100-1000 MPa [80; 82], although pressures up to 1000 MPa are rarely used routinely. A high pressure also disrupts the tertiary and secondary micro-molecule structure and proteins within foods. In this process, the breakdown of non-covalent bonds alters the cells' cellular function, leading to their inactivation. For liquid foods like juices and milk, lower pressures (100-400 MPa) and moderate temperatures (20-50 \({}^{\circ}\)C) are typically used to inactivate microbes while preserving the sensory and nutritional quality [51; 80]. The study by [PERSON] and [PERSON] [83] demonstrated that to obtain the optimum conditions for the treatment of the food product, the most resistant microorganism can be selected as the target microorganism. In their study, a 5 min treatment at 400 MPa was sufficient to inhibit _Leuconstoc mesenteroides_, which was determined to be the most resistant spoilage bacteria. Also, at 400 MPa, a six log decrease in _E. coli_ O157:H7 was seen after 10 min of treatment; at 500 and 550 MPa, no survivors were found following the 1 min treatment. Counts were recorded during treatment and throughout subsequent storage for 28 days. Since _E. coli_ O157:H7 was the most resistant pathogenic bacterial strain, the HPP was built based on it. Hence, when there was no growth of _E. coli_ O157:H7 during the whole storage period, the results demonstrated that these procedures were satisfactory [83]. [PERSON] et al. [84] also reported that _E. coli_ O157:H7 and _Staphylococcus aureus_ were inactivated by high-pressure treatment (200-400 MPa/0-40 min/40 \({}^{\circ}\)C) in carrot juice and peptone water, demonstrating the potential of HPP in the development of safe processed foods. In contrast, solid foods such as meats, meat products, and seafoods, require higher pressures (400-1000 MPa) and, in some cases, elevated temperatures (above 50 \({}^{\circ}\)C) to ensure microbial inactivation [80; 82]. Indeed, it has been noted that bacteria show the greatest resilience to pressure at temperatures ranging from 20 to 30 \({}^{\circ}\)C [85]. The study by [PERSON] and [PERSON] [86] revealed a 1 log decimal reduction of _E. coli_ O157:H7 in poultry meat after 15 min of treatment at 400 MPa and 20 \({}^{\circ}\)C. However, a six log reduction was attained when 400 MPa treatments were coupled with 50 \({}^{\circ}\)C temperatures within 15 min. [PERSON] et al. [87] investigated the impact of high-pressure processing on deboned dry-cured hams. They discovered that after 60 days of storage at 4 and 12 \({}^{\circ}\)C, a high pressure of 600 MPa for 5 min inactivated _L. monocytogenes_ at the surface. On the other hand, variations in the dry-cured ham's interior and exterior salt and nitrate contents, moisture content, and water activity demonstrated variations in pathogen inactivation.