Thermostability of Hen Egg Yolk Granules: Contribution of Native Structure of Granules

Exploring non-equilibrium processes and spatio-temporal scaling laws in heated egg yolk using coherent X-rays

The soft-grainy microstructure of cooked egg yolk is the result of a series of out-of-equilibrium processes of its protein-lipid contents; however, it is unclear how egg yolk constituents contribute to these processes to create the desired microstructure. By employing X-ray photon correlation spectroscopy, we investigate the functional contribution of egg yolk constituents: proteins, low-density lipoproteins (LDLs), and yolk-granules to the development of grainy-gel microstructure and microscopic dynamics during cooking. We find that the viscosity of the heated egg yolk is solely determined by the degree of protein gelation, whereas the grainy-gel microstructure is controlled by the extent of LDL aggregation. Overall, protein denaturation-aggregation-gelation and LDL-aggregation follows Arrhenius-type time-temperature superposition (TTS), indicating an identical mechanism with a temperature-dependent reaction rate. However, above 75 °C TTS breaks down and temperature-independent gelation dynamics is observed, demonstrating that the temperature can no longer accelerate certain non-equilibrium processes above a threshold value.

Physical Characteristics of Egg Yolk Granules and Effect on Their Functionality

Eggs are among the most nutritious foods in the world, a versatile ingredient in many food applications due to their functional attributes such as foaming, emulsifying, and coloring agents. Many studies have been reported on egg yolk fractionation and characterization in the last decade because of its nutritional and health benefits, especially egg yolk granules. This has led to the development of new food products and packaging materials. However, the influence of their physical characteristics during processing significantly impacts the functionality of yolk granules. In this overview, the egg yolk, the granule fraction's separation, fractionation, components, and molecular protein structure are first presented. Secondly, recent studies on egg yolk granules published over the past decade are discussed. Furthermore, the application of the granules in different industries and current specific scientific challenges are discussed. Finally, it simplifies the changes in the physical characteristics of the granules during different treatment methods and the impact on the functionalities of the resulting products in the food (emulsifiers, edible films), pharmaceutical, and health (encapsulation systems and biosensors) sectors.

The Quality Characteristics Formation and Control of Salted Eggs: A Review

Salted egg, a traditional characteristic processed egg product in China, is popular among consumers at home and abroad. Salted egg quality characteristics formation primarily includes the hydration of egg white, the solidification of egg yolk, the unique color and flavor of salted egg yolk, and the formation of white, fine, and tender egg whites and loose, sandy, and oily egg yolks after pickling and heating. The unique quality characteristics of salted eggs are mostly caused by the infiltration dehydration of salt, the intermolecular interaction of proteins, and the oxidation of lipids. In recent years, to solve the problems of salted eggs having high salinity, long production cycle, and short storage period, the pickling technology for salted egg has been improved and researched, which has played a significant role in promoting the scientific production of salted eggs. This paper summarizes the mechanisms of salted egg quality characteristics formation and factors influencing quality, with a perspective of providing a theoretical basis for the production of high-quality salted eggs.

Impact of Ultra-High Pressure Homogenization on the Structural Properties of Egg Yolk Granule

Ultra-high pressure homogenization (UHPH) is a promising method for destabilizing and potentially improving the techno-functionality of the egg yolk granule. This study’s objectives were to determine the impact of pressure level (50, 175 and 300 MPa) and number of passes (1 and 4) on the physico-chemical and structural properties of egg yolk granule and its subsequent fractions. UHPH induced restructuration of the granule through the formation of a large protein network, without impacting the proximate composition and protein profile in a single pass of up to 300 MPa. In addition, UHPH reduced the particle size distribution up to 175 MPa, to eventually form larger particles through enhanced protein–protein interactions at 300 MPa. Phosvitin, apovitellenin and apolipoprotein-B were specifically involved in these interactions. Overall, egg yolk granule remains highly stable during UHPH treatment. However, more investigations are needed to characterize the resulting protein network and to evaluate the techno-functional properties of UHPH-treated granule.

Fabrication and characterization of emulsion stabilized by table egg-yolk granules at different pH levels

BACKGROUND

The egg yolk is complex, which makes it difficult to understand why mayonnaise can be stabilized into a high internal-phase emulsion. This study aimed to assess the possibility of developing oil-in-water emulsions through unmodified natural egg-yolk granules (EYGs) at various pH levels, to further understand the precise mechanism of mayonnaise.

RESULTS

Egg-yolk granules were obtained from hen egg yolk by centrifugation. The sizes of the EYGs were characterized using dynamic light scattering (DLS). Zeta potential of EYGs was detected by DLS and its microstructure was observed by microscope and scanning electron microscope (SEM). Oil / water emulsions were made with EYGs and the size distribution and creaming index of those emulsions were measured at different storage times (1 h and 14 days). The interfacial morphology of EYGs was observed using the emulsion polymerization method. Our results suggested that the prepared EYGs were mainly in an aggregated state but individual EYGs displayed spherical shapes, with a size of 1.0 ± 0.2 μm. The emulsion stabilized by EYGs displayed better stability against creaming at acidic pH (<4.0). At the same time, the interfacial morphology and microscopic observation of the emulsions strongly demonstrated that the emulsions were of the Pickering type.

CONCLUSION

The above results are of great importance for an understanding of the mechanism by which mayonnaise is stabilized by egg, together with the applications of egg in food formulations. © 2019 Society of Chemical Industry.

Relative reactivity to egg white and yolk or change upon heating as markers for baked egg tolerance

BACKGROUND

Hen's egg food allergy is frequent in childhood and phenotypically heterogeneous. Some children can tolerate extensively heated egg. We investigated whether individual relative responses could differentiate children who tolerate baked egg.

METHODS

Reactivities to raw, pasteurized or hard-boiled egg (E), egg white (EW), and egg yolk (EY) fractions were tested by skin prick test (SPT) in 54 egg-allergic children. IgE-sensitization to EW and EY was determined by ImmunoCAP and IgE-binding to EW and 8 EW proteins and to EY and 4 EY sub-fractions by ELISA. Population heterogeneity was assessed by hierarchical ascending classification upon individual variations of reactivity and links between classifications and clinical features by analyzing the contingency tables.

RESULTS

All children had positive SPT to raw E and raw EW and 72% to raw EY. Heating decreased SPT-reactivity for some children, pasteurization being less effective than hard-boiling. Children were classed into three classes from relative SPT-reactivity to raw fractions, two from variations of SPT-reactivity with each thermal processing or EW/EY ratio of sensitization, and four from their sensitization pattern. Classifications according to heating were found independent of each other. SPT variations with hard-boiling, IgE-sensitization (ratio or pattern) were linked to allowance by the physicians of egg in baked products.

CONCLUSIONS

Egg-allergic children were often both sensitized to EY and EW, and heterogeneous patterns of relative responses were evidenced. Irrespective of age and level of sensitization, a low EW/EY ratio or SPT getting null with hard-boiling was found in children allowed to eat baked egg.

Determination of the Gelation Mechanism of Freeze-Thawed Hen Egg Yolk

A study of yolks stored up to 168 d at -20 °C was conducted to determine the gelation behavior and mechanism of freeze-thawed yolk. Methods used were rheology, native and sodium dodecyl sulfate polyacrylamide gel electrophoresis (native- and SDS-PAGE), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), particle size analysis, and proton nuclear magnetic resonance ((1)H NMR) spectroscopy for matrix mobility. Results indicate that both constituents of plasma and granules contributed to gelation of yolk under freezing. PAGE analyses suggest that granular proteins participated in aggregation during freeze-thaw. Increasing gel strength and particle size and decreasing water and lipid-water mobility indicate that lipoproteins or apolipoproteins aggregated. At storage times ≥84 d, increased protein and lipid mobility, the detection of smaller particles, and secondarily increased gel strength suggest the liberation of protein or lipoprotein components from previously formed aggregates and further aggregation of these constituents. Disruption of the gelled yolk matrix observed with TEM supported that ice crystal formation was required for gelation to occur. A two-stage dynamic gelation model is thus proposed.

Thermal-aided phosvitin extraction from egg yolk

BACKGROUND

Phosvitin is the principal phosphoprotein in egg yolk and has great potential for use as a functional food ingredient in improving bone health. This study reports a thermal-aided extraction method without using organic solvents or non-food-compatible chemicals.

RESULTS

Egg yolk was two times diluted with water and then extracted by 100 g L(-1) NaCl. Effects of pH and heating temperature on the extract were examined. The phosvitin purity increased from 75.7% at pH 8.0 to 80.1% at pH 5.0 and then started to decrease, but the yield decreased at decreasing pHs. The phosvitin purity increased at increasing temperature up to 90 °C and then started to decrease at 95 °C, while the yield increased from 70 to 80 °C and then started to decline at 85 °C.

CONCLUSION

A purity of 88.0% and a yield of 23.5 g kg(-1) yolk dry matter were obtained at 90 °C. The purity and yield were comparable to or higher than those of previously methods. The method developed in this study is simple, including mainly two steps, i.e. water dilution of egg yolk and NaCl extraction with heating, and can be scaled up for industrial production.

Separation of Phosvitin from Egg Yolk without Using Organic Solvents

The objective of this study was to develop a new method to separate phosvitin from egg yolk without using organic solvents. Phosvitin was extracted from yolk granules using 10% NaCl or 10% (NH₄)2SO₄ (final concentration) and then treated with heat to precipitate the lipoproteins from the extracted solution. The optimal pH for the phosvitin extraction from yolk granules was determined, and the iron-binding ability of the extracted phosvitin (final product) was tested. Adding 10% (NH₄)₂SO₄ disrupted the granules, and the subsequent thermal treatment at 90°C for 1 h precipitated low density and high density lipoproteins, which enabled separation of phosvitin by centrifugation. The phosvitin concentration in the extract was significantly higher when the pH of the solution was adjusted to pH ≥9. The purity and recovery rate of phosvitin at the end of the separation process were approximately 78% and 56%, respectively. The separated phosvitin was confirmed to have ferrous and ferric iron binding ability. The advantages of this new method compared with the traditional methods include no organic solvents and high-priced equipment are needed for the separation. Also, this method is more environment and consumer friendly than that of the traditional methods.

Egg yolk: structures, functionalities and processes

Hen egg yolk is an ideal example of natural supramolecular assemblies of lipids and proteins with different organization levels. These assemblies are mainly due to interactions between proteins and phospholipids, and these interactions are essential in understanding and controlling the production of food made with yolk, and particularly emulsions. Furthermore, these assemblies can be modulated by external constraints among which thermo-mechanical and high-pressure treatments. This review focuses on multi-scale structures present in egg yolk, and their modulation by processes, in relation with their emulsifying properties. Egg yolk is mainly composed of two fractions-plasma and granules-which are natural nano- and micro-assemblies. These two fractions possess different composition, structures and functionalities and exhibit specific behaviour under treatments such as high pressure and temperature. Plasma contains a large quantity of lipids structured as lipoproteins (low-density lipoproteins), whereas granules are mainly composed of proteins aggregated in micrometric assemblies. If plasma is responsible for the important emulsifying properties of yolk, granules bring interesting emulsifying properties when assemblies are in the form of micelles in presence of salts. High-pressure or thermal treatments, applied before or after emulsion fabrication, alter their functionalities and could be used to commercially exploit these fractions.

Efficient analysis of egg yolk proteins and their thermal sensitivity using sodium dodecyl sulfate polyacrylamide gel electrophoresis under reducing and nonreducing conditions

The multiple functional properties of egg yolk are mostly influenced by its complex protein composition. The high lipid content of egg yolk as well as the low solubility of delipidated egg yolk lipoproteins make analysis by conventional chromatographic or electrophoretic techniques a difficult task. This work describes a method to profile egg yolk proteins after delipidation with acetone using sodium dodecyl sulfate polyacrylamide gel electrophoresis on precast 8-18% T polyacrylamide gradient gels. Twenty bands were obtained for the whole egg yolk profile with molecular weights ranging between 5 and 221 kDa. The bands were identified based on their molecular weight and by comparison with isolated egg yolk subfractions. The dissociation behavior under reducing and nonreducing conditions provided additionally helpful information for identification and characterization of the yolk proteins. The method presented is very well suited for assaying the thermal sensitivity of whole yolk and its components and thus for the characterization of heat treatment processes.