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.     

Antigenicity of Ovomucoid Remaining in Boiled Shell Eggs

Antigenicity of egg white proteins which remained in heated shell eggs was analyzed quantitatively by using rabbit antibodies specific to egg white proteins and human antibody from patients allergic to egg. The major antigenic component in heated (100°C for 20 min) and coagulated egg white was ovomucoid. A considerable amount of ovomucoid remained immunoreactive even after heating at 100°C for 15 min, though ovomucoid in stored eggs was a little less stable to heating than that of fresh eggs. Even long-time heating (100°C for 45 min) could not completely eliminate the ovomucoid immunoreactivity to human IgE antibody.     

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.     

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.     

Strength of Protein Gels Prepared with Microbial Transglutaminase as Related to Reaction Conditions

Influence of gelling reaction conditions on the strength of several protein gels prepared with microbial transglutaminase (TGase) was investigated. A method was developed to gel proteins and measure gel breaking strength in a micro well plate. Enzyme concentration range for maximum gel breaking strength varied from 10 to 40 units/g protein. Maxima gel breaking strengths were achieved at 50°C for SPI, caseinate and gelatin and 65°C for egg yolk and egg white proteins. Optimum pH resulting in strong gels was pH 9 for SPI, caseinate, and egg yolk, and pH 6 for gelatin and egg white. Adjusting pH was promoted in egg white the formation of ?-(γ-glutamyl)lysine crosslinks and increased its gel breaking strength.     

Factors Contributing to Antilisterial Effects of Raw Egg Albumen

Sensitivity of Listeria monocytogenes strain Scott A and Brie-1 to several factors in raw egg albumen was investigated. A concentration of ca 15% of albumen in trypticase soy broth was listeristatic after 24 hr at 35°C, and listericidal effects were observed at higher concentrations. Supplementation of albumen with iron or biotin did not reverse the inhibition. Preheating of albumen (50–80°C) caused progressive loss of antilisterial effects. Supplementation of broth with lysozyme (>1 mg/mL) produced antilisterial effects that were enhanced at pH 9; conalbumin (>6 mg/mL) suppressed cell growth, while ovomucoid (>2 mg/mL) was inhibitory only at pH 9. Results inferred that antilisterial effects of albumen were caused primarily by lysozyme and were enhanced by ovomucoid, conalbumin, and alkaline pH.     

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.     

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.     

Depletion of glucose in Egyptian egg white before dehydration

Glucose was completely removed from egg white in h by using Streptococcus lactis and 0.2% yeast extract at pH 6.0 and 30 °C. A distinct objectionable odour was developed accompanied by a change in the appearance of egg white. Using Aerobacter aerogenes at pH 7.0 and 37 °C, glucose depletion was completed after 3 to 4 h depending on the initial number of bacteria used. The undesirable changes in odour and appearance of egg white were not observed. Saccharomyces cerevisiae succeeded, in presence of 0.2% yeast extract, in depleting sugar in egg white in 9 h. The optimum pH for the reaction was in the range of 6.0 to 7.5 at 32 °C. Glucose oxidase powder of fungal origin was also used for glucose depletion. Glucose was completely removed after 8 h by adding 3.8 glucose oxidase units/100 ml egg white at pH 7.3 and 14.5 °C. 1.9 and 0.95 glucose oxidase units per ioo ml egg white were not enough for complete glucose removal. No objectionable odour or undesirable changes in egg white were observed.     

Soybean Protein Aggregation by Sonication: Ultracentrifugal Analysis

Ultracentrifugal analysis was used to study aggregation of 7S globulin induced by ultrasonic treatment, of water extracts from defatted soybean flakes. In pH 7.6, 0.5 ionic strength buffer, aggregated 7S proteins sedimented as 40–50S species and represented 25–40% of the total proteins. Aggregates also existed at low ionic strength, but dialysis at 0.5 ionic strength caused additional aggregation of sonically modified 7S proteins. After exposure to 0.5 ionic strength, aggregates were stable to subsequent changes in ionic strength with one exception; some reversal of aggregation occurred when samples were dialyzed against water. Aggregates exhibited highest stability at pH 6.5–6.8 and were stable at room temperature for 9 hr or more. Sonic-induced aggregation of 7S proteins resembles the phenomenon observed on heating water extracts at 80°C.     

Thermal Denaturation of Hen Egg White Studied by Chromatographic and Immunochemical Techniques

The course of thermal denaturation of hen egg white at 70°C was studied in detail by fast gel chromatography on Superose 12 and by immunochemical techniques. The mechanism of thermal denaturation differed depending on the pH of the environment. The aggregation of protein at the beginning of heat treatment was typical at pH 5 while at pH 9 the breakdown to smaller fragments prevailed. The chromatographic method for the determination of thermally undamaged (native) ovalbumin correlated well with the immunochemical method.     

Effect of NaCl on thermal aggregation of egg white proteins from duck egg

Thermal aggregation of duck egg white solution (1 mg protein/ml, pH 7) was monitored in the presence of different NaCl concentrations (0–6%, w/w) across the temperature range of 20–90 °C. Duck egg white solution exhibited higher turbidity with coincidental increases in surface hydrophobicity and decreases in sulfhydryl group content as temperatures increased from 70 to 90 °C (p < 0.05). As NaCl concentration increased, the negative charge decreased, with coincidental increases in particle size of aggregate after heating to 90 °C. As visualised by confocal laser scanning microscopy, larger clusters of protein aggregates were observed with increasing NaCl concentrations. Major duck egg white protein with molecular mass of 45 kDa disappeared in the presence of 2–6% NaCl after heating above 80 °C, regardless of concentrations. Therefore, NaCl, especially at high concentrations, could induce thermal aggregation of duck egg white protein, which could determine the characteristics of salted egg white after heating.     

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     

Coagulation of Egg White of Soft-Shell Turtle (Pelodiscus Sinensis) at High Pressure or High Temperature

The coagulations of soft-shell turtle egg white at high pressure or at high temperature were investigated and compared to chicken egg white. Unlike chicken egg white, no coagulation of soft-shell turtle egg white was observed after high-pressure processing (HPP; 200–600 MPa) or heating treatment (75°C). Basic amino acids, histidine, lysine, and arginine were present at higher percentages in soft-shell turtle egg white proteins than in chicken egg white proteins. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis showed that the intensities of soft-shell turtle egg white components were balanced, there were more components with high molecular mass in soft-shell turtle egg white, and that aggregates formed in both soft-shell turtle egg white and chicken egg white. This work revealed some differences in soft-shell turtle egg white compositions. The low protein content in egg white was responsible for the non-coagulation of soft-shell turtle egg white in response to high pressure or temperature.     

Effect of pasteurisation on the chemical composition of liquid whole egg. II.—Comparison of the protein fractionation patterns of raw and pasteurised liquid whole egg

Samples of commercial raw and pasteurised whole egg were re-examined after 41 weeks' storage under frozen conditions. As well as differences between the two samples, the protein fractionation patterns revealed marked differences from the original unstored material. A second comparison of commercial raw and pasteurised egg indicated that considerable differences could occur in the protein fractionation patterns of raw egg. This was confirmed in subsequent experiments. Two experiments on egg white showed that pasteurisation for 2 1/2 min at 135°F caused no change in the fractionation pattern of the proteins, and provided sound evidence for the reproducibility of the fractionation procedure. The separate influences of time and temperature of heating on liquid whole egg were studied in a series of laboratory-scale pasteurisation experiments. It appeared that time was a more important factor than temperature and that a heating time of 5 minutes, which is approximately the time that the bulk of the egg is held at the pasteurising temperature in commercial plants, caused significant changes in the fractionation patterns of both soluble and insoluble proteins. When pasteurisation was carried out for 2 1/2 min at 148–152°F, and also when time was varied at 150°F, the thiol and disulphide concentrations of the soluble proteins were not altered by any of the pasteurisation conditions studied. A short study was made of the distribution of α-amylase activity during the fractionation of the soluble proteins of raw whole egg. Preliminary experiments on gel filtration of the insoluble protein portions indicated that this technique is capable of revealing changes in molecular weight distribution resulting from pasteurisation and gave support to the view that heat treatment of whole egg results in aggregation or association of some of the proteins.     

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.     

Effect of thermal treatment on the proteins of amaranth isolates

The effect of thermal treatment of proteins from Amaranthus hypochondriacus was studied. Two protein isolates were obtained from the defatted flour by water extraction at a pH of 9 (A9 isolate) and 11 (A11 isolate), followed by isoelectric precipitation at a pH of 5. Effect of thermal treatment (70 and 90 °C, during 3, 5, 10, 15 and 30 min) on A9 and A11 dispersions were analyzed by differential scanning calorimetry (DSC), polyacrylamide gel electrophoresis, UV spectrophotometry, superficial hydrophobicity and solubility in water. Thermal treatment induced the aggregate formation of high molecular mass stabilized by disulfide and non‐covalent bond. Thermal treatment at 70 °C produced a 30% denaturation in both, while at 90 °C A9 was more denatured than A11 (75% and 55% of denaturation, respectively). An increase in thermal stability was also detected by DSC in A9 treated at 90 °C. The denaturation process was accompanied at short heating times by an increase in UV absorbance and changes in superficial hydrophobicity. A decrease in water solubility (35–50%, depending on time–temperature conditions) was also observed for the A9 isolates. The results suggest that the A9 isolates, enriched in a globulin protein fraction, are more sensitive to thermal treatment than isolates A11 enriched in glutelin protein fraction. The changes shown by both isolates, indeed, could affect their functional properties and could definitely limit their use in food products. Copyright © 2007 Society of Chemical Industry     

UV Inactivation of E. coli in Liquid Egg White

Liquid egg white is currently pasteurized using heat; however, this treatment damages the functional properties of the egg. In this study, a nonthermal ultraviolet light (UV) system was developed to pasteurize liquid egg white. The system consisted of low-pressure mercury bulbs surrounded by UV transparent tubing. Egg white was inoculated with Escherichia coli K12 and pumped through the UV system at a flow rate of 330 ml/min. The effects of treatment time (0 to 160 s), temperature (30 to 50 °C), and egg white pH (7 to 9) on the inactivation of E. coli were investigated. The population of E. coli in egg white was reduced by 4.3 log after being exposed to UV at 50 °C for 160 s. Inactivation was linearly dependent on treatment time and was adequately described using first-order kinetics (r ² of 0.94). The electrical energy of the process was calculated to be 44 J/ml. Inactivation was directly dependent on temperature and inversely dependent on pH. Nonthermal UV processing has the potential to improve the safety and functional properties of liquid egg white. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the US Department of Agriculture.     

Color and gelling properties of dried egg white: Effect of drying methods and storage conditions

Dried egg white is extensively used as a food ingredient due to its unique functional properties and extensive shelf life. This study investigated the effect of drying methods and storage conditions on the color and gelling properties of dried egg white. Egg white was dried with two drying methods; freeze drying and hot-air drying, then stored at 25 and 40°C for 4 months. The result showed that the color of hot-air-dried egg white, especially after storage at 40°C, was darker yellow than for freeze-dried egg whites. The gelling properties of both samples were altered during storage, however, substantial changes in the gelling properties were found in the gel made from stored hot-air-dried egg white. A decrease in the enthalpy of protein denaturation that indicated a partially unfolded protein conformation, an increase in exposed sulfhydryl and a decrease in the total sulfhydryl contents were found, especially in stored samples of hot-air-dried egg whites. The sodium dodecyl sulfate-polyacrylamide gel electrophoresis pattern of proteins in the stored samples showed protein aggregation, and this was related to the increased glass transition temperature. This study revealed that drying egg white using hot-air drying and storage especially at high temperature induced changes in their color and protein conformation. These have contributed to protein aggregation which affected dried egg white’s gel properties.     

Effect of Shell Eggs Storage on Ovomucin Extraction

Ovomucin, one of the major egg white proteins, has wide potential applications in food, functional food and nutraceutical industries due to its unique physicochemical properties and bioactivities. A new two-step method was recently developed to prepare high purity ovomucin; in this study, effect of shell egg storage on ovomucin extraction was investigated. The composition of ovomucin extracts from egg white was determined by gel filtration chromatography. Both storage temperature and time could significantly (p < 0.05) affect the purity and yield of the ovomucin. After 9 weeks of storage at 4 °C, the content of the ovomucin in the extract decreased from 92.5% to 82.4%, and the yield of ovomucin decreased from 214 to 120 mg/100 g of egg albumen (egg white). After 5 weeks of storage at 22 °C, the content of ovomucin extract decreased from 92.5% to 73.0%, and the yield of ovomucin decreased from 214 to 112 mg/100 g of egg albumen. Increase in egg white pH during prolonged storage leading to degradation of ovomucin is very likely responsible for the decreased extractability of ovomucin.