## 3 Factors Influencing and Mechanistic Pathways in Egg Yolk Emulsification
### Non-Thermal Treatment
#### 3.4.1 High-Pressure Technologies  
Initially introduced in the dairy industry, homogenization reduced fat globule size, thereby enhancing the physical and chemical stability of milk. This technology was later applied to other liquid foods, including egg products. [PERSON] et al. [46] investigated the impact of high-pressure homogenization (175 and 300 MPa, one and four passes) on the protein structure and emulsifying qualities of egg yolk granules, finding that homogenization enhanced both the water and oil binding capacities of egg yolk granule proteins. While a single pass at 175 MPa had no effect on the water binding, increased pressure and the number of passes caused significant improvements. The oil binding capacity increased after both 175 MPa treatments, with further enhancement observed at 300 MPa, independent of the number of passes. Similarly, [PERSON] et al. [47] reported that egg yolk emulsification abilities might be enhanced through the use of microfluidization, an advanced type of high-pressure homogenization. Emulsifying activity increased at microfluidization pressures of 103 and 138 MPa, but further increases in pressure to 172 and 207 MPa led to a decrease. Likewise, emulsion stability improved at 103 MPa but decreased with further increases in pressure. The application of high-pressure homogenization or microfluidization disrupts egg yolk granules by breaking down phosphoclic bonds within the HDL-phosvitin complex, releasing HDLs. This process induces HDL denaturation, reducing particle size and increasing surface hydrophobicity due to the exposure of hydrophobic domains. Dissociated HDLs subsequently interact, forming intermolecular disulfide bonds, which enhance emulsifying activity and emulsion stability. However, the decrease in emulsifying properties at higher pressures may be attributed to the fact that the resulting polypeptides are too small to stabilize the oil-water interface effectively. Another reason for the reduced emulsifying properties could be the loss of protein solubility due to the high microfluidization pressure [46; 47]. Additionally, [PERSON] et al. [47] reported an increase in particle size at the highest microfluidic pressure of 207 MPa as a result from the increased interaction and subsequent re-coalescence of smaller fat and protein particles in egg yolk, a phenomenon referred to as 'over-processing' [90; 91]. High-pressure processing (HPP) is another non-thermal technology employed in the food industry. This method involves subjecting hermetically sealed food products to extremely high pressures, typically ranging from 100 to 800 MPa, at an ambient temperature. The process utilizes a liquid medium, commonly water, to uniformly transmit pressure throughout the food product. [PERSON] et al. [48] employed HPP (400 MPa for 5 min) followed by ultrafiltration to produce a new emulsifying ingredient from egg yolk. This combined treatment enabled the selective recovery and concentration of phosvitin in the retenate. Emulsions made with the ultrafiltration retenate of plasma from HPP-treated granules were more stable against flocculation and creaming than those made without HPP [48]. Additionally, applying moderate pressure (50-400 MPa, 5 min) as a pre-treatment before thermal pasteurization improved the emulsifying activity (35-52%) and emulsion stability (41-66%) of egg yolk. The combined effects of pressure and thermal treatment also achieved higher microbial inactivation, as pressure pre-treatment reduced the thermal resistance of microorganisms [72]. Another application of high pressure is its use in hyperbaric storage. Storing liquid egg yolk under hyperbaric conditions at 200 MPa maintained its emulsifying activity for up to 28 days [49]. HPP is a highly developed non-thermal technology, with a technology readiness level typically between 8 and 9. While the technology readiness level can vary based on specific applications, HPP machines used for pasteurization-equivalent processing of food and beverages are considered to be at TRL 9.