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).     

Texture Profile Analysis, Expressed Serum, and Microstructure of Heat-Formed Egg Yolk Gels

Egg yolk gels were formed by heating previously stirred egg yolk, having various pHs, protein concentrations, and NaCl levels, at various times and temperatures. Texture profiles were obtained by an Instron double-compression test, and serum was expressed from frozen-thawed gels. Microstructures of selected gels were observed by scanning electron microscopy. Gel hardness generally increased with increasing pH, protein or salt concentration, temperature, and length of heating time. The amount of serum expressed from gels decreased as gel hardness increased. Cohesiveness and springiness of gels increased with time and temperature. Gel microstructure consisted of spheres (4–50 μm) and granules (0.3–1.0 μm) dispersed in a continuous matrix. Spheres were physically disrupted by dilution and by addition of salt.     

Correlation of Protein Sulfhydryls with the Strength of Heat-Formed Egg White Gels

A strong, direct correlation (r = 0.97) was found between egg white protein surface sulfhydryl group (-SH) concentration and the gel strength of its heat-induced gels. An inverse correlation (r = - 0.86) was also found between the -SH group concentration of 0.1% SDS-denatured egg white proteins and gel strength. Data indicated that surface - SH groups played an important role in strong heat-induced gel formation in egg whites.     

Texture Profile Analysis of Heat-Formed Gels and Cakes Prepared with Low Cholesterol Egg Yolk Concentrates

Gels were formed by heating water dispersions of egg yolk protein concentrates of low cholesterol content, prepared from yolk by applying different lipid extraction methods. Texture profiles, obtained by applying an Instron double compression test, showed that in general, gels exhibited higher hardness and springiness and lower cohesiveness than those of the control yolk. Cakes prepared with yolk concentrates showed higher hardness and similar cohesiveness and springiness values and, with one exception, harder textures compared to control cakes, as assessed by panelists. Results were related to the differences in protein and lipid composition of yolk samples.     

Heat-induced Egg White Gels as Affected by pH

The functional properties of heat-induced egg white gels were investigated at five pH values. Textural characteristics were determined using the Instron Universal Machine. Hardness, elasticity, cohesiveness, chewiness, and fracturability were maximum at pH 11. Hunter L values were maximum at pH 5 and 7. Microstructure studied with electron microscopy was distinctly different at the five pH values. Alkaline gels showed a fine ordered network that might have contributed to excellent textural characteristics. Water-holding capacity (WHC) was high at alkaline pH, but decreased with addition of 2-mercaptoethanol, suggesting that disulfide bonds were important in egg white gels. Sodium dodecyl sulfate (SDS) improved WHC at pH 7 and 9. No significant correlation was observed between textural profiles and WHC.     

Texture Changes on Heating Spray-Dried Egg White

Computerized texture profile analysis and a computer-simulated model of protein monomer complexation were used to study egg white gelation. The results showed that the parameter Kⁿ, an association equilibrium constant, was large so that the monomer to monomer complex reaction essentially went to completion. Approximately 8 to 10 protein monomer units combined to form the egg white gel. The monomer complex primarily contributed to the hardness of egg white.     

Failure Deformation and Stress Relaxation of Heated Egg White Gels

Stress-strain relationships were determined for heated egg white gels. Compressive strain at failure was independent of protein concentration, while stress at failure increased with protein concentration. Between the length ratios of 1.6 and 2.4 before and after compression, the heated egg white gels followed the Mooney Rivlin model. In stress relaxation, the asymptotic hypothetical equilibrium modulus increased with protein concentration.     

Comparison of Transparent Gels with Turbid Gels Prepared from Egg White: Creep Analysis of Gels

Egg white from which the precipitate occurring during dialysis against water had been removed gave a transparent gel on heating at lower salt concentrations and pH 3.54. The addition of NaCl or a shift of pH from 3.54 to 4.50 brought about the turbid gels. Creep analyses of these transparent and turbid gels were done using a four-element mechanical model. The instantaneous elastic modulus, E₀, Newtonian modulus, ηn and retarded viscoelastic moduli, E₁ and η₁ increased with NaCl up to 125 mM and then decreased with increase in NaCl concentration at pH 3.54. At 125 mM NaCl and pH 3.54, a translucent gel was obtained. E₀ and η₁ decreased with the temperature of the gel, while E₁ and η₁did not change depending of temperature.     

Preparation of Cooked Egg White, Egg Yolk, and Whole Egg Gels for Scanning Electron Microscopy

Cooked egg white, egg yolk, and whole egg gels, fixed with glutaraldehyde or unfixed, were frozen at - 35°C or at - 95°C and freeze-dried. Alternatively, unfrozen gels fixed with glutaraldehyde, os-mium-thiocarbohydrazide-osmium, or osmium-tannic acid-uranyl acetate were dehydrated in ethanol and critical point-dried from carbon dioxide. Egg yolk and whole egg gels were defatted. Freeze-dried and critical point-dried gels were examined by scanning electron microscopy. Freezing and freeze-drying introduced artifacts due to ice crystal damage, with egg white gels distorted most, and egg yolk gels distorted least. Gels fixed only by glutaraldehyde shrank by 50% during critical point drying. Further fixation by osmium tetroxide and uranyl acetate stabilized gels against shrinkage. Removal of fat from egg yolk and whole egg was essential for observation of protein matrices.     

鸭蛋清蛋白凝胶质构特性影响因素研究

以冻干鸭蛋清蛋白粉为原料,以凝胶硬度和弹性为指标,在探究鸭蛋清蛋白凝胶形成条件的基础上,考察金属离子对凝胶质构特性的影响。结果表明,鸭蛋清蛋白凝胶形成的最佳条件为：蛋清蛋白质量分数12.0%、pH 8.0、80 ℃加热40 min。不同浓度钠离子和钙离子对凝胶弹性的提高均有显著作用,高浓度钾离子作用不显著,镁离子、锌离子和亚铁离子的浓度较大时反而有抑制作用。镁离子对凝胶硬度增加有显著作用,钾离子无显著影响,钠离子和钙离子为先促进后稳定的趋势,锌离子和亚铁离子对凝胶硬度增加有抑制作用。     

不同碱处理对蛋清凝胶物理化学变化的影响研究

采用两种碱处理加工方式制成蛋清凝胶制品(蛋清碱诱导凝胶和皮蛋清),以TPA(Texture Profile Analysis)、溶解度、巯基与二硫键含量、表面疏水性、蛋白质组分、微观结构、蛋白质二级结构为考察指标,研究蛋清碱诱导凝胶与皮蛋清凝胶特性的差异及其作用机理,结果表明:各碱处理组蛋清的蛋白质分子高度交联而形成凝胶网络结构,且蛋清碱诱导凝胶组较为致密有序。蛋清碱诱导凝胶比皮蛋清具有较高的硬度(186.73±3.29 g)、内聚性(0.98±0.02)、咀嚼性(176.21±6.13)、溶解度(55.68±1.20%)、表面巯基(60.34±1.01μmol/g)和二硫键含量(29.42±0.57μmol/g),且具有较低的总巯基含量(64.60±2.28μmol/g)和表面疏水性(419.80±17.22)。蛋清碱诱导凝胶蛋白质二级构象以β-折叠为主(31.24%),分子内具有较强的氢键总相互作用;皮蛋清蛋白质二级结构中α+β构象较低(33.65%),β1(21.21%)、T-转角(25.36%)和γ无规则卷曲(19.78%)较大。蛋清蛋白质组分在两种碱处理加工方式下都发生明显变性。     

Determination of Cross-link Density in Egg White Gels from Stress Relaxation Data

The concentration of permanent cross-links in egg white gels was determined using stress relaxation experiments and equations derived from the theory of rubber elasticity. Cross-link densities of 1.2–1.3 covalent cross-links/molecule were observed. The concentration of covalent cross-links, most likely disulfide bonds, was only slightly higher for gels formed in water as compared to those formed in 0.6M NaCl-0.2M phosphate buffer at pH 6.0. These results differ from the 3—4 cross-links/molecule previously reported for ovalbumin gels in 0.6M urea.     

不同热处理方式对卤蛋蛋白的质构影响

通过TPA方法与感官评价方法,研究了卤蛋加工过程中预煮时间、卤制时间、灭菌方式及时间对卤蛋蛋白的质构影响,发现卤蛋蛋白硬度受预煮时间、卤制时间影响显著;弹性、胶着性受卤制时间影响显著;黏性受高压灭菌时间影响显著。并选择出了较优的热处理方式:以15枚鸡蛋为一组,预煮15 min,卤制1 h、放置1 h、二次卤制1 h,121℃灭菌5 min。     

NaCl、糖类和木瓜蛋白酶对碱诱导蛋清凝胶性的影响

为了研究不同添加物在高温条件下对碱诱导蛋清凝胶硬度和色度所作出的贡献,把NaCl、糖类和木瓜蛋白酶加入到碱诱导的蛋清中并加热,以蛋清凝胶强度和色差等为评价指标,对不同添加剂作用于碱诱导下的蛋清凝胶进行研究。结果表明:1%的Na Cl使凝胶硬度显著上升(p<0.05),透光率显著下降(p<0.05),且显著增加了凝胶的白度(p<0.05),硬度从81.94g上升到144.94g,增加了77%,透光率从34.33%下降到9.10%,持水性随盐浓度增加而上升。蔗糖和麦芽糖的加入使凝胶质构变化较小,高浓度麦芽糖可降低凝胶的硬度,5%浓度的麦芽糖使凝胶的透光率显著下降(p<0.05),从25.3%下降到9.34%,减少了63%,而蔗糖的透光率却增加了26%,凝胶的总体白度都下降。木瓜蛋白酶的添加使凝胶的透光率显著上升(p<0.05),从34.33%上升到54.27%,增加了60%,凝胶的总体白度略微下降,凝胶硬度和咀嚼性下降了30%左右。综上所述,Na Cl、木瓜蛋白酶、蔗糖和麦芽糖对高温条件碱诱导的蛋清凝胶均有不同的影响,可为不同蛋制品的理化和感官需求提供参考依据。     

蛋白质量浓度对鸡蛋热诱导凝胶特性的影响

以蛋清、蛋黄和全蛋液为原料,研究蛋清、蛋黄和全蛋蛋白热诱导凝胶的形成能力以及蛋白质量浓度对蛋清、蛋黄和全蛋蛋白热诱导凝胶特性的影响。结果表明：蛋清、蛋黄和全蛋形成热诱导凝胶的最低蛋白质量浓度分别为50、55、50mg/mL;在50～135mg/mL范围内,随蛋白质量浓度的增加,蛋清、蛋黄和全蛋蛋白的凝胶强度和保水性不断增大,蒸煮损失整体呈下降趋势;蛋白质量浓度对蛋清、蛋黄和全蛋凝胶的弹性也有显著影响。     

Isostrength Comparison of Large-Strain (Fracture) Rheological Properties of Egg White and Whey Protein Gels

The effects of protein concentration and heating conditions on the physical properties of whey protein isolate (WPI) (in 50 mM NaCl, pH 7) and egg white (pH 9) gels were examined. Egg white and WPI gels had similar values for shear stress at fracture (i.e., isostrength), while trends for shear strain at fracture were protein-type specific. The rigidity ratio (R_0.3), ratio of the rigidity at fracture (G_f) to the rigidity at 30% of fracture strain, measured departure from the stress-strain relationship of an ideal Hookean solid. All gels fit master curves of G_f vs R_0.3, which were described by a power law model of R_0.3=A(G_f)-°19, where "A" showed protein type-specific characteristics.     

Elastic Attributes of Heated Egg Protein Gels

The equilibrium stress of egg white (EW) and ovalbumin (OV) gels using the statistical theory of rubber elasticity suggested that 3.9 covalent disulfide cross-links per OV molecule were formed on gelling. DTNB and PCMB reacted with approximately 4 sulfhydryls of OV in 2% SDS and/or at 82°C. Moreover, the equivalent of 3 secondary bonds, calculated from the initial stress and presumably a composite of numerous weak secondary bonds, contributed significantly to the protein gel stress.     

蔗糖共溶质体系中明胶-变性淀粉共混凝胶相行为、质构及微观结构研究

本文探索蔗糖作为共溶质对明胶-变性淀粉共混凝胶相行为、质构、干燥脱水、透明度L*值及微观结构性质的影响。结果表明:与对照样相比,16%浓度蔗糖共溶质场中,所制备未进行干燥的共混凝胶硬度值及粘附性值均显著降低(p0.05),降低幅度分别达到21.47%和13.36%。蔗糖浓度及干燥时间对硬度值和粘附性值改变趋势相异,二者具非协同改变效应。随蔗糖浓度增加,共混凝胶干燥脱水速率(-d Mg/dt)显著性降低,透明度L*值呈递减趋势。增加蔗糖浓度,共混凝胶网络结构中孔隙平均直径显著减小(p0.01),与对照相比,添加16%蔗糖后形成共混凝胶的孔隙平均直径由51.28±1.31μm降为46.51±1.89μm,孔隙分布由不规则分布显示更细密、均一化。共混凝胶L*和粘附性性与孔隙平均直径之间具有较强线性相关关系,相关系数R2分别为0.98和0.97。     

酸奶凝胶的结构特性及其影响因素

本文主要说明了酸奶凝胶的结构,物理特性及其影响因素。酸奶凝胶超微结构呈一种纤维网状立体结构,网状纤维中间形成无数有规则的空隙。均质,热处理等加工因素对其质地有不同的影响。