## 3 Factors Influencing and Mechanistic Pathways in Egg Yolk Emulsification
### Non-Thermal Treatment
#### 3.4.2 High-Intensity Ultrasound  
Ultrasound is a promising green technology for enhancing the functional properties of food proteins. High-intensity ultrasound (HIU), with frequencies between 16 and 100 kHz and power levels from 10 to 1000 W/cm\({}^{2}\), has a wide range of potential applications in the food industry. When applied to liquid products, HIU can create cavitation bubbles that collapse violently, causing mechanical damage to the proteins. This damage can improve the functional properties of the proteins by altering their interfacial and structural characteristics [95]. For instance, compared to the untreated egg yolk solution, the emulsifying activity of the egg yolk solution treated with HIU was significantly higher. However, there were no notable differences in emulsifying activity among the different ultrasonic power levels. In contrast to emulsifying activity, the emulsion stability index of the egg yolk solution initially increased with rising ultrasonic power but decreased when the power exceeded 225 W [50]. Additionally, as reported by [PERSON] et al. [51], the emulsifying activity of egg yolk granules showed a modest increase with HIU power levels between 90 and 270 W, achieving a 9.54% increase (\(p<0.05\)) at 270 W. However, for the emulsifying stability index of yolk granules, the HIU treatment resulted in some changes, though these were not statistically significant.  
A recent study examined the impact of various HIU power levels (75, 150, and 210 W), temperatures (30, 45, and 60 \({}^{\circ}\)C), and treatment durations (3, 6, and 9 min) on the emulsifying properties of egg yolk. No significant differences in emulsifying activity were found among the groups at 30 \({}^{\circ}\)C. Meanwhile, treatments lasting 6 and 9 min at 45 and 60 \({}^{\circ}\)C, respectively, showed the highest emulsifying activity at an HIU power of 75 W. At 30 \({}^{\circ}\)C, the emulsion stability index of all groups improved with increasing HIU power and treatment time, with the 210 W treatment for 9 min achieving the highest stability. At 45 \({}^{\circ}\)C, the 150 and 210 W treatments exhibited similar trends in emulsion stability index, with the three 150 W treatments for 9 min and the 210 W treatments for 3 and 9 min showing significantly higher stability than other groups. However, at 60 \({}^{\circ}\)C, only the 210 W treatment for 3 min resulted in the highest emulsion stability [52].  
[PERSON] et al. [53] compared the effects of solvent (ethanol) and HIU treatment on the emulsifying properties of egg yolk LDL. They found that solvent treatment decreased the emulsifying properties, while HIU treatment had no significant effect. [PERSON] et al. [96] demonstrated that applying HIU to trypsin-modified egg yolk significantly improved emulsifying activity. The emulsifying activity at HIU treatment (480 W for 30 min) was 36.26% higher. Additionally, while emulsifying activity increased with power, treatment time at 480 W did not significantly affect it.  
HIU treatment broke down yolk granules, releasing components like proteins, calcium, phosphorus, and phospholipids. Additionally, it increased the polarity of the microenvironment and strengthened hydrogen bonding within the granules. LDL molecules within the granules underwent partial rearrangement. These changes collectively altered the zeta potential, surface hydrophobicity, and sulfhydryl group content of yolk granules, and improving emulsifying properties [50; 51].  
However, further comprehensive studies under controlled conditions are needed to better understand the effects of various variables on the emulsifying properties of egg yolk and its components.