## 3 Factors Influencing and Mechanistic Pathways in Egg Yolk Emulsification
### Thermal Treatment  
#### 3.2.1 Need for Thermal Treatment  
The primary source of microbiological contamination in eggs is the eggshell, which can become contaminated with fecal matter containing enteric bacteria such as _Escherichia coli_ and _Salmonella_ spp., both members of the _Enterobacteriaceae_ family. Microorganisms present in the nesting environment, such as _Staphylococcus aureus_, can also contribute to contamination on the shell's surface [66]. When an egg is cracked, these microbes can enter the egg's interior. Eggs, being rich in proteins and nutrients, offer an optimal environment for microbial growth, including foodborne pathogens, which pose a significant risk to food safety [67]. These bacteria can lead to symptoms like nausea, diarrhea, and abdominal pain. Some strains of Shiga toxin-producing _E. coli_ (STEC), particularly _E. coli_ O157:H7, can cause severe foodborne illnesses and are transmitted through the consumption of contaminated food, such as raw or undercooked meat, milk, or raw egg products [68]. According to European Commission Regulation (EC) No. 2073/2005 on microbiological standards for food, the acceptable limit for _Enterobacteriaceae_ in egg products at the end of production is between 10 and 100 CFU/g or ml [69]. The rapid growth of these microorganisms not only raises the risk of illness but also drastically shortens the shelf life of liquid eggs. Therefore, it is crucial to apply treatments that effectively reduce the initial microbial load in egg liquids [70]. In the food sector, pasteurization is the most widely used heat processing technique. The goal of pasteurization is to improve food safety by heating products to kill pathogens, bacteria, and microorganisms. Commercial egg yolks are thermally pasteurized, usually for 2 to 6.2 min at 60 to 68 \({}^{\circ}\)C, to lower the potential risks associated with egg consumption [71,72]. Elevated temperatures cause alterations to the cellular composition and metabolic processes of microbes, which in turn reduces their proliferation and endurance, thereby lowering the likelihood of contamination. Furthermore, pasteurization prolongs the shelf life of liquid eggs, prevents spoilage, and guarantees their safety for consumption within a specified timeframe [8]. Nevertheless, because egg yolk lipoproteins are heat-sensitive, thermal processing above 65 \({}^{\circ}\)C may cause them to denature and aggregate [73,74]. Protein denaturation increases with temperature, showing a slight rise between 60 and 68 \({}^{\circ}\)C, but a significant increase at temperatures above 76 \({}^{\circ}\)C. This trend is linked to the different denaturation temperatures of major egg yolk proteins: g-livetin (69 \({}^{\circ}\)C), LDL and a-livetin (76 \({}^{\circ}\)C), b-livetin (81 \({}^{\circ}\)C), and HDL (84.3 \({}^{\circ}\)C) [75]. Moreover, damage to the LDL structure after thermal treatment above 65 \({}^{\circ}\)C reduces its ability to bind lipids, which accelerates the release of lipids. This leads to a considerable and permanent rise in viscosity and a decrease in the emulsifying qualities through promoting the creation of heavier aggregated clusters with irregular distributions and forms. Additionally, the released lipids undergo oxidation, producing oxidative by-products: hexanal, (E)-2-octenal, (E, E)-2-4-decenal, 1-octen-3-ol, and 2-pentylfuran, that contribute to off-odors in egg yolk [76]. It is important to recall that hazardous and spoilage microorganisms like _Salmonella_, _Escherichia coli_, _Listeria monocytogenes_, and _Staphylococcus aureus_ that have limited heat tolerance are the main targets of pasteurization. On the other hand, bacteria and spores that are thermophilic and heat-resistant, such as _Microbacterium_ and _Enterococcus_, are able to withstand heat in the egg solution [77]. Pasteurization temperatures may not fully inhibit these microbes. [PERSON] et al. [71] found _Enterococci_ in pasteurized liquid eggs, underscoring the limitations of pasteurization as these bacteria can create enzymes that cause spoiling.