# Non-Thermal Technologies in Food Processing: Implications for Food Quality and Rheology
## 3 High-Pressure Processing (HPP)
The soluble calcium and phosphorus levels increased at pressures up to 350 MPa,  
Figure 2: Schematic diagram of high-pressure processing [90]. but decreased at 450 MPa. Additionally, treatment at 450 MPa formed soft protein gels containing casein substructures (15-20 nm diameter) homogeneously dispersed. Serum proteins were significantly denatured at pressures greater than 250 MPa, highlighting the influence of HPP on the stability and texture of milk proteins. Other researchers have reported that the treatment of milk by HPP denatured the whey protein in it, and because of the milk fat globule membrane interaction with protein, a reduction in cream of 70% is achieved [32; 96]. Other foods in which HPP is used include salsas, guacamole, chilled entrees, and smoothies. It is also famous within the seafood industry to preserve oysters and seafood [91]. Overall, HPP can enhance the food shelf life and save food from detrimental changes in their sensory aspects and nutritional values. The study by [PERSON] et al. [97] examined the effects of HPP on cheese made from ultrafiltered milk (UF-cheese) using recombinant chymosin (RC) or bovine renet (BR). The results showed that HPP treatment at 600 MPa for 5 min at 25 \({}^{\circ}\)C extended the shelf life of UF-cheese by reducing proteolysis and microbial counts while maintaining colour stability. The treated cheeses exhibited increased syneresis over time and had a firmer texture compared to the untreated samples. Proteolysis was significantly reduced by 42.8% in RC cheese and 52.5% in BR cheese after one day of storage, indicating the potential of HPP in enhancing whey protein solubility, influencing the cheese texture and moisture retention [97]. It is worth noting that the HPP process cannot be applied to low-acidic foods. This is because HPP can cause a change in the pH of the food and affects the enzymatic reactions by altering the kinetic constants or by causing conformational changes in the food structure. At pH \(\geq\) 4.6, it has limited effectiveness in inactivating spores of pathogenic bacteria such as _Clostidium botulinum_. Thus, HPP is primarily used for acidic foods (pH \(\leq\) 4.6), where the acidic conditions naturally inhibit spore-forming bacteria. However, it is possible to simultaneously raise the temperature (90-120 \({}^{\circ}\)C) during HPP to boost the effectiveness of inactivating spores and pressure-resistant microorganisms. Synergistic effects against spores on several food commodities have been observed when heat (90-120 \({}^{\circ}\)C) and pressure are applied either sequentially or concurrently [98]. In a similar vein, the successive use of other methods such as irradiation and preservatives in conjunction with HPP has demonstrated additive and synergistic effects against spores and pathogens to ensure the safety of food commodities [99; 100]. Additionally, HPP can cause pH shifts in food matrices, influencing enzymatic reactions by altering kinetic constants and inducing conformational changes in enzyme structures. Under a high pressure (100-1000 MPa), enzymes such as polyphenol oxidase and pectin methylesterase may exhibit increased or decreased activity depending on pressure-induced unfolding or aggregation [101]. For example, the pressure denaturation of pectin methylesterase can increase its activity and thereby modify the fruit texture, while the inactivation of polyphenol oxidase prevents enzymatic browning in juices [82]. These changes affect the food stability, texture, and sensory attributes, making the precise control of pressure conditions essential for maintaining quality. HPP enhances the rheological characteristics of foods by modifying their microstructure and increasing their water-binding capacities. This leads to improved solubility and alters the textural properties, such as the firmness of cheese or the viscosity of pures. The process preserves the natural sensory qualities of foods, including the flavour, colour, and nutrients, making them a sustainable and versatile choice for liquid and solid foods.