Heat-Induced Aggregation of Egg White Proteins as Studied by Vertical Flat-Sheet Polyacrylamide Gel Electrophoresis

Heat-induced aggregation of proteins in egg white was investigated by a vertical flat-sheet polyacrylamide gel electrophoretic method. The fractional and step-wise aggregation of egg white proteins was caused by heating. Even with a heating time of 120 min, ovalbumin and globulins Al and A2 failed to aggregate in egg white (pH 7 and 9) at 70°C, and ovotransferrin and ovomucoid also did not aggregate in egg white at 60°C (pH 9) and 76°C (pH 7 and 9), respectively. The ovoinhibitor was much more unstable than ovomucoid under heat-treatment, and the time dependency of heat-induced aggregation of flavoprotein was greater than those of the other proteins in egg white.     

Heat-Induced Aggregation and Denaturation of Egg White Proteins in Acid Media

Heat-induced aggregation and denaturation of egg white proteins adjusted to pH 5.5, 4.5, 3.5 and 2.5 were investigated by vertical flat-sheet polyacrylamide gel electrophoretic and differential scanning calorimetric methods. The fractional and step-wise aggregation of egg white proteins was caused by heating. As the acidity was increased from pH 5.5 to 2.5, ovotransferrin, ovomacroglobulin, globulin G3A, globulins A1 and A2, and ovalbumin became much more unstable to heat. However, ovomucoid and ovoinhibitor did not aggregate in the acidified egg white under heat treatments of 3 min at 90°C or 20 min at 74°C. The heat-induced aggregation of flavoprotein was slightly greater at pH 4.5 and 3.5 than at pH 5.5 and 2.5.     

Protein Denaturation,Rheology, and Gelation Characteristics of Radio-Frequency Heated Egg White Dispersions

Gel properties of radio frequency (RF) heated egg white dispersions at 27.12 MHz were studied as function of concentration (2.5–2.5 kg/100 kg sample), pH (3–11) and heating time (60–180 s). Egg white dispersions demonstrated a gradual liquid-solid transformation as they denatured and gelled during RF treatment. The critical concentration and heating period for egg white protein denaturation and gelation were found to be 7.5% (w/w) and 150 seconds. The elastic modulus (G′) of RF-heated samples increased with concentration and heating period (temperature), whereas complex viscosity (η*) increased exponentially with concentration. In an alkaline condition, the egg white dispersion did not produce a gel; however, in acidic condition it resulted in a strong gel with significantly (P?<?0.05) higher G′. This could be attributed to the high dielectric constant (ε′) and loss factor (ε″) values of acidified samples as compared to the alkaline and control egg white dispersion. Effect of heating rate (1, 5, 10, and 20°C/min) in situ on rheometer plate significantly affected gel rigidity; the RF treated sample rigidity was comparable to samples heated at the rate of 5 and 10°C/min. Differential scanning calorimetry, dielectric measurement, and sodium dodecyl sulfate (SDS) PAGE electrophoresis results were used to confirm gelation behavior during both conventional and RF heating conditions.     

The allergenicity of ovomucoid and the effect of its elimination from hen's egg white

In order to remove the ovomucoid from hen's egg white, chitin and hydrazide polystyrene beads were used as affinity ligands with 8.9 and 7.1 mg trypsin g^?1 ligand respectively. Ovomucoid was successfully depleted using the trypsin affinity column without hydrolysation of the other egg white constituents. The components of the egg white were purified by high‐performance liquid chromatography, and then the allergenicity of each of these components was compared with that of pooled human serum derived from patients who are allergic to hen's eggs. The importance of using pure protein for studies of the allergenicity of egg white is highlighted, and it was determined (using an enzyme‐immunosorbent assay) that ovomucoid and ovalbumin are major allergenic proteins in egg white. The ovomucoid‐eliminated egg white preparation exhibited significantly less IgE‐binding activity than normal egg white. The ovomucoid‐specific IgE antibodies may have important implications with regard to the egg‐allergic reaction in humans. © 2001 Society of Chemical Industry     

Texture and Microstructure of Cooked Whole Egg Yolks and Heat-Formed Gels of Stirred Egg Yolk

Hardness, cohesiveness, and springiness of heated intact egg yolks were determined by an Instron compression test. Hardness increased as temperature was increased from 75° to 90°C and as time of heating was increased from 10 to 30 min. Cohesiveness and springiness increased as temperature was increased from 75° to 80°C and as time was increased from 10 to 30 min. Egg yolks from cooked shell eggs were lower in all textural parameters than gels formed from stirred egg yolk. Low scores for cohesiveness and springiness were indicative of the crumbliness of yolk from cooked shell eggs; crumbliness was attributed to structural characteristics. The microstructure consisted of adjoining polyhedral grains, which showed no evidence of crosslinking. Grains appeared to be equivalent to yolk spheres, ranging in size from 40 to 100 μm. The structure of a stirred yolk gel consisted of a highly crosslinked protein matrix containing spheres (5–35 μm) and granules (l–2 μm).     

Effect of heat treatment on hen’s egg ovomucoid: An immunochemical and calorimetric study

Egg causes a great number of allergies, being the allergenic proteins mainly contained in the white fraction. Among white proteins, ovomucoid is one of the most allergenic and can be considered as a good marker to detect egg in foods due to its high thermostability. We have studied the effect of heat treatment on ovomucoid immunoreactivity by ELISA and its denaturation in different media by differential scanning calorimetry (DSC). Ovomucoid was subjected to heat treatment in a range from 85 to 95 °C in buffer at pH 7.4. D values were obtained for the process of loss of immunoreactivity at the different temperatures and the Z value was of 15.6 °C. DSC of ovomucoid has been performed in two different buffers, at neutral or basic pH, and also in egg white. It has been found that ovomucoid is more thermoresistant at basic than at neutral pH. Furthermore, the resistance of ovomucoid in egg white is different to that observed in buffer at the same pH.     

Pasteurization of intact shell eggs

The feasibility of pasteurization for destruction ofSalmonella enteritidisin artificially- inoculated intact shell eggs was investigated. Water-bath and hot air ovens were used for pasteurization of intact shell eggs under different conditions using these two heating methods, either individually or in combination. Eggs pasteurized in a 57°C circulating water-bath for 25 min gave reductions inS. enteritidisATCC 13076 of about 3 log cycles whereas heating at 55°C in a hot air oven for 180 min gave a 5 log reduction ofS. enteritidis. A combin?ation of the two methods (water-bath heating at 57°C for 25 min followed by hot-air heating at 55°C for 60 min) produced 7 log reductions inS. enteritidisATCC 13076 in shell eggs. Examination of lysozyme activity and other physical properties of egg white upon heating indicated that the overall functionality of pasteurized shell eggs are acceptable under the heating conditions defined in this study. This process may be used to produce safe, pasteurized shell eggs.sed     

A New Protein Band Appearing in the Electrophoretic Pattern of Egg White Heated at Below 60°C

When egg white was heated to between 50° and 60°C, a new protein band appeared in its polyacrylamide gel electrophoretic pattern. The formation of the protein band in stored egg white was almost the same as that of fresh egg white. This protein band was shown to be induced by the interaction between macroglobulin and lysozyme during the egg white heating. The optimum temperature for its formation was between 54°C and 57°C. This band formation may be used as an indicator for heat treatment of egg white.     

A differential scanning calorimetric study of the stability of egg white to heat denaturation.

The heat denaturation of egg white and its component proteins was studied by differential scanning calorimetry. At a heating rate of 10 °C/min, egg white at pH 7 shows two major endotherms, at 65 °C and 84 °C, produced by the denaturation of conalbumin and ovalbumin, respectively. The conalbumin endotherm is increased to 70 °C by raising the pH to 9.0, or 77 °C by addition of aluminium at neutral pH. Addition of sucrose stabilises all the proteins; at 10%sucrose, all endotherms are shifted 2 °C to higher temperatures. Within experimental error, the enthalpy of denaturation of egg white equals the sum of the enthalpies of denaturation of its component proteins, and is independent of pH over the pH range 7–9.     

Effects of ohmic heating on technological properties of whole egg

The aim of this work was to study the effects of different ohmic heating conditions on color, rheology, foaming, and gelling properties of whole egg. Industrial products treated by conventional heat pasteurization and the corresponding raw materials were also evaluated. Ohmic treatments accomplished in a static cell (65.5 °C × 3 min, 70 °C × 1 min, and 67 °C × 4.5 min) increased whole egg apparent viscosity (up to 190%), but also foam overrun (up to 28%) and gel hardness (up to 15%). The performance improvement was confirmed by treatments carried out in a continuous pilot plant (71 °C × 0.6 min, 68 °C × 1.4 min) and the products resulted stable during storage at 4 °C for 30 days. In conclusion, this study demonstrated that ohmic heating is a suitable alternative to conventional pasteurization. Low temperature treatments are preferable to avoid possible rheological issues due to protein denaturation.Industrial relevanceWhole egg is a protein ingredient with multiple technological properties, used in many foods. Due to safety reasons, food manufacturers often use pasteurized liquid egg products, microbiologically safer and easier to handle with respect to shell eggs. In order to satisfy the required sanitary levels for liquid egg products, thermal pasteurization treatments are needed. However, since egg proteins are very sensitive to high temperatures, attention must be paid to avoid coagulation entailing deleterious effects against egg quality. In this study, different ohmic heating treatments were evaluated as milder alternatives to conventional pasteurization. The lab- and pilot-scale experiments and the subsequent statistical analyses of the obtained results contributed to assess the effects of the different ohmic treatments on technological features (e.g. color, rheology, foaming, and gelling properties) of liquid whole egg. This study demonstrated that ohmic heating is a suitable technology for whole egg treatment, paving the way for new opportunities in order to produce safe food ingredients with improved technological functionalities.     

A new process for preparing transparent alkalised duck egg and its quality

When fresh duck (Anas plotyrhyncus) eggs (pH 8·0–8·5) are heated, their albumen develops a turbid gel. Through appropriate alkalisation (pH 11·5–12·8), the gel's transparency can be increased. The transparency of the heated duck egg-white is affected by pH value, heating temperature, heating rate and salt concentration. This research deals with the process for preparing the transparent alkalised duck egg and the change in its quality when stored. If fresh duck eggs are pickled in a solution of 42 g NaOH+50 g NaCl litre^?1 (25·3°C) for 8 days, removed, put in a water bath and heated at 70°C for 10 min they become transparent, their hardness and penetration increasing with storage. Total bacterial count and volatile basic nitrogen also increase with storage. The total bacterial count and the volatile basic nitrogen were 4·6 × 10⁶ cfu g^?1, 0·32 mg g^?1 when stored at a temperature of under 25°C for 4 weeks, respectively.     

Effect of pasteurisation on the chemical composition of liquid whole egg. III.—Effect of staling and freezing on the protein fractionation patterns of raw and pasteurised whole egg

Two particular fractions of the soluble proteins were consistently reduced by heating liquid egg for 5 min at 150°F. These fractions were quantitatively greater in the soluble proteins from frozen egg. The soluble proteins appeared to be more stable to heat in eggs that had been stored for 15 days at 70°F than in new-laid eggs. Unfrozen new-laid and stale egg and frozen new-laid egg all showed an increase in insoluble material after pasteurisation, and frozen egg contained more insoluble materials than unfrozen egg. Gel filtration showed that the proportion of compounds of high molecular weight in the insoluble proteins was markedly increased by pasteurisation and even more markedly increased by freezing.     

Electrophoresis and Chromatography of Heat-Treated Plain, Sugared and Salted Whole Egg

The effects of heat treatments on the proteins in plain, 10% sugared and 10% salted whole egg were studied by polyacrylamide disc gel electrophoresis and diethylaminoethyl-Sephacel ion exchange chromatography. Electrophoretograms of products heated from 57–87°C for 3.5 min showed a wide range of protein stability. Livetins and some globulins were most heat sensitive, while conalbumin and ovalbumin were most stable. Sugar and salt increased heat stability of proteins by 1.6 and 7.9°C, respectively. Heat sensitive proteins were most stabilized by sugar and salt. Heating whole egg and sugared whole egg changed the chromatograms substantially, while heating salted whole egg caused fewer changes.     

Interaction Between Ovomucoid and Lysozyme

The interaction between ovomucoid and lysozyme was investigated by precipitation experiments, polyacrylamide gel electrophoresis and inactivation experiments on ovomucoid. The ovomucoid-lysozyme mixture produced turbidity at low salt concentrations over the pH range 6–11. Although both ovomucoid and lysozyme solutions (0.2%, pH 8.5) yielded no precipitation when heated separately at 80°C for 10 mm, about 40% of the proteins in the 0.2% solution of the ovomucoid-lysozyme mixture (2:3, in weight; 1:3, in mole) was precipitated by the same treatment. Ovomucoid was found to form complexes with lysozyme, some of which were precipitated by centrifugation (2000 ×g, 20 mm), and it was presumed that ovomucoid and lysozyme molecules were brought close together by the electrostatic attractive force, unfolded by heating, and then aggregated through intermolecular forces such as hydrophobic forces, hydrogen bonds and disulfide bonds.     

Protein Denaturation and Emulsifying Properties of Plasma and Granules of Egg Yolk as Related to Heat Treatment

Solubility, electrophoresis, viscosity and emulsifying properties of heated solutions of yolk, plasma and granules were determined. Plasma and yolk were not affected when heated under 69°C. Above 69°C, protein solubility dropped sharply and apparent viscosity rose sharply because of aggregation of proteins. For granules, protein solubility and apparent viscosity were not modified up to 76°C. The constituents of granules were not denatured. Emulsifying activity of yolk and plasma decreased after heating at 72°C but remained steady for granules. Emulsion stabilization properties of yolk, plasma and granules were not influenced up to 76°C. Results suggested that intact granules withstood more severe heat treatments than egg yolk without lessening their emulsifying properties.     

Rheological Properties of Mozzarella Cheese Filled with Dairy, Egg, Soy Proteins, and Gelatin

The Rheological behavior of mozzarella cheese filled with various proteins (whey protein, caseinate, egg white, soy protein isolate, gelatin) incorporated was determined by uniaxial compression at 10°C and the effect of temperature (10°C?60°C) by dynamic measurement. Mozzarella cheese with whey protein, caseinate, egg white, and soy protein isolate showed significant water retention during heating. Among the proteins, soy protein isolate induced the strongest gel network structure with mozzarella cheese. All proteins altered the viscoelastic properties of mozzarella cheese.     

Hydrocarbons detected in irradiated and heat-treated eggs

Shell eggs were non-irradiated or irradiated at 0.5, 1 and 3 kGy, and boiled, fried or heated in ovens. Hydrocarbons were determined by a sequential procedure of lipid extraction by hexane, Florisil column chromatography and gas chromatography. Hydrocarbons 15:0, 14:1, 17:0, 16:1, 17:1, 16:2, 17:2 and 16:3 were detected in shell eggs irradiated at 0.5 kGy or higher, but not in non-irradiated ones except 15:0 and 17:0. Boiling non-irradiated or irradiated eggs for 40 min little affected detection levels of the hydrocarbons. Frying the egg yolk of the non-irradiated eggs for 5 min produced hydrocarbons 17:1 and 17:2 besides 15:0 and 17:0. Hydrocarbons 15:0, 17:0, 17:1, 17:2, 14:1 and 16:1 were detected in the egg yolk of the non-irradiated eggs while heating it in a cooking oven at 170°C for 60 min. The pattern of hydrocarbons detected in non-irradiated samples when heated was, however, notably different from that of irradiated ones; detection of hydrocarbons in shell eggs or egg products could, thus, identify their post-irradiation.     

Effect of Heat Treatment for Trypsin Inhibitor Inactivation on Physical and Functional Properties of Peanut Protein

Trypsin inhibitor (TI) inactivation in whole peanut kernels exposed to moist heat had a decimal reduction time (D) of 91 min at 100°C and 9.3 min at 120°C. When all processing times were converted to process time at 120°C using Z of 20°C, the composite D at 120°C was 10 min. Solubility decreased with heating to reach a minimum with 98% TI inactivation. Capacities to spontaneously absorb water and to gel were retained better on heating at 120°C than at 100°C for equal TI inactivation. The most functional protein was in kernels heated at 120°C for 10 min. When 20 or 30% meat protein in a meat loaf formulation was replaced with the latter peanut protein, the resulting loaf retained more fat and water and exhibited higher shearing strengths than all meat formulations.     

Estimation of the hydrophobicity modifications in meat proteins upon thermal treatment

A fluorescent probe method using 8-anilino-l-naphthalene sulfonic acid (ANS) and all-trans-retinol (RET) has been used to study the effects of heat treatments on the hydrophobicity of meat proteins. The number of ANS binding sites per unit protein increased from 0.75 for unheated samples to 2.12 for meat proteins heated at 100° C for 30 min. The number of RET binding sites increased from 0.13 to 0.46 with the same heat treatment. This is discussed in terms of % increase in aromatic (182.7%) and aliphatic (253.5%) hydrophobicity for meat proteins heated at 100° C for 30 min. On the other hand, the assay was able to discriminate between samples of meat proteins receiving heat treatments with 100° C intervals (between 40 and 100° C).     

A differential scanning calorimetric study of conversion of ovalbumin to S-ovalbumin in eggs.

The presence of S-ovalbumin, a more heat-stable form of ovalbumin formed on storage of eggs, can be determined by differential scaning calorimetry of egg white. At a heating rate of 10°C/min, at pH 9, the characteristic denaturation temperature of ovalbumin is 84.5°C, that of S, ovalbumin, 92.5°C. The formation of S-ovalbumin proceeds through a previously unrecognised intermediate species having a denaturation temperature of 88.5°C. The kinetics of the conversion on storage of eggs at 4, 22 and 37°C have been determined. Differential scanning calorimetry is a rapid and convenient method of determining the quality of eggs held in storage. Freeze-dried preparations of ovalbumin stored in the cold for 20 years showed partial conversion to the intermediate, but not to S-ovalbumin.