# Non-Thermal Technologies in Food Processing: Implications for Food Quality and Rheology
## 3 High-Pressure Processing (HPP)
During HPP treatment, dry-cured hams achieved a surface reduction of around 2 logs and an internal reduction of about 3 logs. Their study demonstrated that HPP treatment at 600 MPa/5 min could provide food safety against _L. monocytogenes_ according to the United States Department of Agriculture (USDA) and European standards [87]. The high-pressure treatment of milk has been found to only disrupt the ionic and hydrophobic interactions of macromolecules (proteins), without denaturing the beneficial proteins it contains [80]. Along with successful microbial inactivation, very little to no impact on the tiny molecules of milk components (vitamins, flavour, and amino acids), colour, and other nutritional components have been reported [88] at pressures of around 100-500 MPa and a moderate temperature (10-50 \({}^{\circ}\)C). [PERSON] et al. [89] investigated the use of HPP treatment to inactivate foodborne pathogens in milk. The results from their study demonstrated that 300 MPa of pressure for 30 min at 25 \({}^{\circ}\)C was the optimal condition for the inactivation of foodborne pathogens such as _Salmonella_, _E. coli_, _Shigella_, and _S. aureus_ in milk. Therefore, different food products may be subjected to temperatures of 0-110 \({}^{\circ}\)C and pressures up to 100-1000 MPa at a suitable time range, depending on the desired balance between the microbial inactivation and product [80; 82]. The high-pressure processor used in the study by [PERSON] et al. [90] used a mixture of distilled water and propylene glycol with 7:3 as the pressure transmitting fluid. The pressure was increased at the rate of 300 MPa/min and depressurised at 25 MPa/s, while the temperature increased 3 \({}^{\circ}\)C per 100 MPa. The HPP was assisted by carbon dioxide (CO\({}_{2}\)) in the study and was shown to inactivate _Escherichia coli_ and _Staphylococcus aureus_ in liquid samples, demonstrating the potential application of this process in the food industry. As shown in Figure 2, the processor had a two-litre pressure chamber, one heating system, one low-pressure pump, four decompression valves, one water tank, and two pressure intensifiers. During HPP processing, baskets of packaged food are introduced in steel vessels full of water, and a piston is used to apply a high pressure. This way, incompressible water is passed through the food uniformly from all surfaces. This results in the reduction of the size of micelles in dairy protein, leading to the denaturation of proteins, increased solubility, and, in some instances, a colour change. In other applications, the ripening of cheese is accelerated by the process of HPP. This is possibly due to changes in the microstructure of fresh curd due to the applied pressure. In liquids, including whole eggs and tomato juices, a 3-5 log reduction in microbiological counts by HPP has been reported [91]. The viscosity of juices is unchanged by the HPP process compared to thermal treatment and untreated juices. This improves the rheology of foods including mango pulp and tomato purees, where viscosity is reduced with or without heating, but the water binding capacity is increased [92; 93; 32; 91]. In the HPP process, protein denaturation takes place in the food; this process is irreversible and the extent depends upon the type of protein and treatment conditions. These changes are responsible for improving the food quality and preservation. The processing of milk using HPP results in changes to the milk's protein structure such as the destabilisation of casein micelles, bringing about coagulation and an improved texture. This can improve the stringiness and meltability of different cheeses and related products [94; 32]. The study by [PERSON] et al. [95] investigated the impact of HPP at 150-450 MPa for 15 min on the structural change of milk proteins. The results showed that the pressure treatment led to the dissociation of calcium phosphate and, therefore, the destabilisation of the casein micelles.