RHEOLOGY OF EGG YOLK

The rheological behaviour of egg yolk over the temperature range 5° to 60°C was studied and found to fit the power law. For egg yolk of different moisture content it was found that the data gave a better fit to the power law than to an exponential equation.     

Thermal behaviour of lyophilized egg yolk and egg yolk fractions

Despite its importance from a technical point of view, the influence of temperature on rheological behaviour of lyophilized egg yolk and egg yolk fractions has not yet been studied. In this work, the effect of temperature on rheological properties of these products have been analysed by means of stationary and oscillatory measurements. Firstly, the Arrhenius equation was employed to assess the influence of temperature on viscosity. In addition to temperature effect, solid content showed to be determinant on viscosity, while particle size, water holding capacity and wettability influence on rheological behaviour of samples were not critical. Secondly, rheological changes during heat-induced gelation of the samples were also investigated by means of dynamic tests and the critical network exponents at the gelation point were calculated. An analysis of variance (ANOVA) was also performed using the Statgraphics Plus to evaluate experimental data. Results show that proteins of egg yolk fractions possess a similar network structure before and after gelation, while egg yolk structure is clearly modified by gelation process. Composition and structure is the main responsible of differences between samples.     

SELECTED PHYSICAL PROPERTIES of LIQUID EGG PRODUCTS AT PASTEURIZATION TEMPERATURES

The influence of temperature on the rheological properties and density of liquid egg product at pasteurization temperatures was investigated. the density of all these products decreased with increasing temperature: however, the changes were different depending on the product. the change in density may be represented by simple linear equations. All the products showed pseudoplastic flow behavior that may be described by the Power-law model. Egg white had the lowest power law index (n=0.552). Salted egg yolk had the highest n value (>0.8), thus it is closer to Newtonian fluid in its behavior. This index did not vary greatly with temperature for whole egg and egg white but decreased with temperature for egg yolk and salted egg yolk the lowest consistency coefficient (m=54.52mPa.sⁿ) was observed for whole egg. Salted egg yolk had the highest m value (269.9mPa.sⁿ). Them value increased with temperature for egg yolk and egg white, but decreased with temperature before showing an increase for salted egg yolk and egg white. the calculated Generalized Reynolds number was below 2100 for all the products. These results show that laminar flow exits during typical pasteurization of liquid egg products and therefore the minimum residence time is half the average resident time.     

RHEOLOGICAL BEHAVIOUR OF KIWI FRUIT JUICE CONCENTRATES

The rheological behaviour of diluted kiwi concentrates (22.5–63.0 °Brix) over the temperature range 4 to 70C, was studied. The values below 55.7 °Brix fit the power law while above 55.7 °Brix fit the Herschel-Bulkley equation. Maximum and minimum consistency index values were 62225 and 16 mPa. s ⁿ . Concentrates showed a yield stress at 55.7 °Brix, from 5 to 11 Pa, and 63.0 °Brix, from 19 to 53 Pa. Yield stress was correlated with temperature by means of a linear equation. Arrhenius activation energies were in the range of 28.97 to 33.89 kJ/mol. The average value of the flow behaviour indexes varied from 0.85 (sample at 22.5 °Brix) to 0.59 (sample at 63.0 °Brix). Simple equations are proposed to describe the combined effect of temperature and soluble solids content on consistency index and rheological behaviour index.     

Rheology of spray-dried egg yolk-stabilized emulsions

This paper deals with the influences that concentrations of oil fraction (65–77.5% w/w) and egg yolk (1–5% w/w), as well as temperature (5–35 °C), exerted on the droplet size distribution and rheological functions of concentrated oil-in-water food emulsions that were stabilized by a spray-dried egg yolk product. This work must be considered as a preliminary study concerning the use of low-in-cholesterol egg yolk as emulsifier. In order to achieve this aim, steady-state flow and dynamic viscoelasticity tests were done with emulsions processed with a rotor-stator turbine. The same processing conditions were always maintained. All of the emulsions prepared showed a high stability. An increase in oil fraction yielded higher values for the rheological functions, and larger droplet diameters. The influence of processed egg yolk concentration was more complex. Thus, an increase in egg yolk content yielded lower values for the rheological functions, but, after a particular concentration, a further increase in emulsifier content yielded higher values for the rheological functions. The experimental results have been discussed taking into account the close relationship between rheology and emulsion structural parameters.     

Changes in rheological behaviour and functional properties of hen's egg yolk induced by processing and fermentation with phospholipases

Functional changes of hen's egg yolk rheological behaviour as a result of processing (pasteurisation, freeze-drying) and modifying its phospholipids using phospholipase A₁ (PLA₁) and phospholipase A₂ (PLA₂) were studied. It could be shown how fermentation with phospholipases affects the different steps of heat-induced gelation of egg yolk. As a consequence of fermentation of native yolk with 0.1% PLA₁, the first viscosity maximum was shifted from 79 to 97 °C. In fermented pasteurised yolk, the homogenous gel-like consistency from the first viscosity maximum was even stabilised up to 100 °C. The second viscosity ascent, which indicates solid gel structures (caused by aggregation of denatured proteins), was prevented entirely. In contrast to pancreatic PLA₂, fermentation of egg yolk with microbial PLA₁ resulted in a hydrolysis of approximately 78% of phosphatidylethanolamine and a significant change in flow behaviour (of yolk solutions).     

Effect of salt content on the rheological properties of salad dressing-type emulsions stabilized by emulsifier blends

The effects that salt content and composition of emulsifier blends exert on the rheological properties of salad dressing-type emulsions were studied. Binary blends of egg yolk and different types of amphiphilic molecules (Tween 20, sucrose laurate and pea protein), in several proportions, were used to stabilize emulsions. Salt concentration was ranged from 0 to 2.3% w/w. Steady-state flow tests and small-amplitude oscillatory shear measurements within the linear viscoelastic region were carried out. Rheological tests were complemented with droplet size distribution measurements. Rheological properties and physical stability of the emulsions studied were significantly influenced by salt content and the nature of binary emulsifier blends. In general, the values of rheological parameters studied increased with salt content. However, salt affects in much higher extent the properties of emulsions stabilized by high proportions of egg yolk or pea protein in the emulsifier blend, rather than those mainly stabilized by non-ionic low-molecular-weight surfactants, which are less sensitive to changes in the ionic strength. In this sense, the increase observed in the values of viscosity and linear viscoelastic functions of emulsions is more important when a protein is predominant in the emulsifier blend. This effect was explained on the basis of a more apparent increasing interdroplet interactions and viscosity of the continuous medium, both of them induced by salt addition, which lead to the consecution of an extensively flocculated state and improved creaming stability. On the contrary, different blends of pea protein and egg yolk showed a quite similar evolution of the rheological parameters with salt concentration.     

Kinetische Untersuchungen zur Denaturierung der Eigelbproteine durch rheologische Messungen

Kinetic investigations of the denaturation of egg yolk proteins by rheological measurements Egg yolk is owing to its germ content and enzyme only storable to a limited extent. The microbiological stability of egg yolk can be increased by pasteurization, followed by cold storage. At the same time, the egg yolk proteins will be more or less damaged. In this article kinetic results of the thermal denaturation of egg yolk proteins, with and without stabilizing additives, by rheological investigations are presented and discussed. The fact, that salt stabillized the proteins in a better way than comparable concentrations of saccharose is demonstrated.     

Effect of total solids content and temperature on the rheological behaviour of reconstituted whole milk concentrates

This work investigated the combined effect of solids content, heating and storage temperatures on the rheological behaviour of reconstituted whole milk concentrates. The powder was reconstituted at 35 degrees C in a custom built recombination rig to various total solids content (TS) from 10-48% TS. The concentrates were then heated to 45-85 degrees C and stored at the heating temperature. The rheological behaviour shifter from Newtonian behaviour (below 30% TS) to power law (below 40% TS), with the yield stress observed from 40% TS upwards and time-dependent behaviour was noticed above 44% TS. Higher heating temperatures tend to promote non-Newtonian behaviour at lower solids content. The viscosity-solid content curve showed an exponential relationship, while the viscosity-temperature curve exhibited a minimum at 65-75 degrees C above 46% TS. During age thickening, the yield stress and the consistency coefficient increased, while the flow behaviour index decreased with storage time. This indicated that the milk concentrates deviated away from Newtonian behaviour during age thickening.     

From egg yolk/κ-Carrageenan dispersions to gel systems: Linear viscoelasticity and texture analysis

The effect of pH (3.5–6) and polysaccharide concentration (0–0.5 wt%) on the linear viscoelastic behaviour of egg yolk/κ-Carrageenan (EY/κC) mixtures in aqueous solution was studied by using Small Amplitude Oscillatory Shear (SAOS). Native egg yolk containing 45 wt% solids was used for all the samples. Thermally set EY/κC gels were also studied by SAOS and texture analysis. A variety of linear viscoelastic behaviours depending on κC concentration and pH were exhibited by EY/κC dispersions that may be explained in terms of the contributions of electrostatic attractive interactions and an exclusion volume effect between protein and polysaccharide macromolecules. This last effect seems to be dominant as pH shifts towards the isoelectric point (IEP) of egg yolk proteins, whereas, at pH far from the IEP a certain enhancement in the degree of compatibility, and even some κC autohydrolysis, seems to take place. The results obtained either from rheological or textural characterization of gels were consistent with that balance. In any case, the results obtained suggest that the microstructure of gels is governed by the protein ability to form gels, where hydrophobically driven interactions and subsequent cross linking among protein segments play a dominant role.     

Physicochemical properties of leftover egg yolk after livetins removal

Isolation of immunoglobulin Y (IgY) from egg yolk using water-dilution method generates large quantities of leftover pellet as the co-product. Although egg yolk is well known for its great emulsion property, there is a lack of understanding on how livetins removal would affect the emulsion and rheological properties of the pellet. Therefore, the objective of this study was to determine the effect of soluble protein removal on emulsifying and rheological characteristics of the leftover egg yolk. Egg yolk pellet exhibited distinct structural and physicochemical properties after soluble protein removal. Emulsions prepared from pellet were more vulnerable to coalescence instability than that of egg yolk, although both had a similar oil droplet size. Egg yolk displayed a Newtonian behaviour, compared to a shear-thinning behaviour of pellet. Pellet showed a higher apparent viscosity as well as higher viscoelastic moduli than those of egg yolk, probably due to increased hydrogen bond and hydrophobic interactions in the pellet. Therefore, the changes on the emulsion stability and the rheological properties of egg yolk after soluble proteins removal should be considered in food formulation and processing. Further study is needed to improve the emulsion property of pellet for uses in the food industry.     

Rheological characterisation of salad-dressing-type emulsions stabilised by egg yolk/sucrose distearate blends

The overall objective of this work was to fully characterise the rheological behaviour of oil/water emulsions stabilised by egg yolk and a sucrose ester of intermediate hydrophilic-lipophilic balance. The influence of the egg yolk/sucrose ester weight ratio in the emulsifier blend was studied by keeping the total amount of emulsifier constant. With this aim, steady state and transient flow tests, oscillatory measurements within the linear viscoelasticity regime and stress relaxation tests were carried out. Rheological tests were complemented with droplet size measurements. The results demonstrate that the rheological response and droplet size of the emulsions studied clearly depend on the weight ratio of emulsifiers in the binary blends. Steady-state and transient viscosities, as well as the dynamic functions, significantly increase with sucrose distearate concentration in the emulsifier blend. However, the evolution of other rheological parameters related to shear-induced structural breakdown, such as the critical strain for the onset of the non-linear viscoelastic behaviour, the relative deviation of the Cox-Merz rule, the time corresponding to the stress overshoot in transient tests or the damping function indicates a more developed and resistant structural network at higher egg yolk concentrations in the emulsifier blend. These results were discussed on the basis of a gel-like continuous phase formed by the sucrose ester and the highly flocculated state favoured by egg yolk.     

Effect of Egg Yolk and Egg Yolk + Salt on Rheological Properties and Particle Size Distribution of Model Oil-in- Water Salad Dressing Emulsions

The role of egg yolk and egg yolk + salt in stabilizing oil-in-water (o/w) salad dressing emulsions pre-stabilized with xanthan and propylene glycol alginate was studied using creep tests, steady shear, and particle size analysis. Results show that each combination significantly affects rheological stability of model o/w salad dressing emulsions. All emulsions exhibited viscoelasticity and wcrc characterized by three parameters: η₁, E₁ and η_N. Steady shear measurements showed that both egg yolk and salt increased viscosity radically. Particle size data showed significant changes with both addition of egg yolk and salt to the model system.     

Application of decolourised hsian‐tsao leaf gum to low‐fat salad dressing model emulsions: a rheological study

The effects of mixed gums, including decolourised hsian‐tsao leaf gum (dHG) mixed with propylene glycol alginate (PGA) or xanthan gum (XG), and egg yolk concentration on the rheological properties of low‐fat salad dressing model emulsions were studied. All model emulsions showed pseudoplastic flow behaviour. Model emulsions with PGA addition showed the lowest pseudoplasticity, followed by those with dHG and then XG addition. Increasing the PGA level in the dHG/PGA system reduced the pseudoplasticity. Increasing the XG level in the dHG/XG system did not change the pseudoplasticity significantly but imparted a significant increase in viscosity. Dynamic viscoelasticity measurements indicated that model emulsions with dHG or XG addition could be classified as elastic gels. However, model emulsions with PGA addition essentially belonged to the class of either dilute or concentrated solutions rheologically, depending on the egg yolk concentration. The rheological characteristics of individual gums were found to be confounded in generating the rheological characteristics of model emulsions containing them. Copyright © 2004 Society of Chemical Industry     

Rheological behaviour of concentrated mandarin juice at low temperatures

Several rheological properties of concentrated mandarin juice were investigated, quantifying thixotropic behaviour and fitting experimental data to the Hahn, Weltman, and Figoni–Shoemaker equations. The latter one proved best for modelling the time dependency response. Pseudoplastic behaviour fitted well with the Herschel–Bulkley model in the interval between ?12 and 6 °C, in which yield stress and the flow behaviour index scarcely changed with temperature. The consistency coefficient and apparent viscosity both decreased as temperature increased. Fitting these consistency data to the Arrhenius‐type equation, a value of 33 kJ mol^?1 for activation energy was obtained. Concentrated mandarin juice displayed viscoelastic properties in the temperature range between ?12 and 6 °C, with the elastic character (determined by the juice’s pectin content) predominating over the viscous nature at low frequency values, but with this behaviour being inverted for high values. Furthermore, as the temperature became higher, the elastic character became more important than the viscous nature.     

Rheology of raspberry juices

This paper reports a study of the rheological behaviour of different concentrations of raspberry juice at different temperatures. The behaviour is pseudoplastic at higher concentrations and lower temperatures, although at low concentrations and in some cases at high temperatures the behaviour is Newtonian.The effect of temperature and soluble solids content on the consistency coefficient was also studied: an equation was derived which describes the combined effect of these two variables for two different models and over the range of temperatures and concentrations studied.     

热处理对酶改性蛋黄液乳化性的影响及拉曼光谱分析

蛋黄液对热敏感,64 ℃左右开始变性形成凝胶,加工过程中的热处理会导致其乳化性质下降,限制了其在食品工业中的应用。因此,本实验研究不同热处理条件（60、65、70 ℃均处理4 min）对酶改性蛋黄液乳化性质及蛋白质结构的影响。结果表明：随着热处理温度的升高,蛋黄液的乳化活性呈现下降的趋势,所形成的乳化体系平均粒径表现出增大的趋势。同一热处理条件下,酶改性蛋黄液的乳化活性及乳化稳定性均显著高于未改性蛋黄液（P＜0.05）,酶改性蛋黄液所形成的乳化体系粒径明显减小,粒径分布更加接近正态分布。拉曼光谱分析结果显示,在不同热处理条件下酶改性蛋黄液中蛋白质二级结构发生变化,在65 ℃以下随温度升高α-螺旋结构相对含量显著升高,无规卷曲结构相对含量显著下降（P＜0.05）;其色氨酸残基、酪氨酸残基及脂肪族疏水基团较未改性蛋黄液更倾向于暴露。采用偏最小二乘法建立了基于拉曼光谱的蛋黄液乳化性质模型,结果表明蛋黄液中蛋白质二级结构及构象变化（特别是低密度脂蛋白的构象变化）对蛋黄液乳化性质影响最大。     

蛋清蛋白对豌豆淀粉凝胶化及凝胶特性的影响

为探究蛋清蛋白对豌豆淀粉凝胶化及凝胶特性的影响。分别以0%、3%、6%、9%和12%的蛋清蛋白替代豌豆淀粉,研究蛋清蛋白对豌豆淀粉糊化特性、热特性、静态流变、动态流变、凝胶质构及水分子状态的影响。结果表明：蛋清蛋白以浓度依赖的方式影响豌豆淀粉的糊化特性,峰值黏度、谷值黏度、崩解值、最终黏度、回生值均随蛋清蛋白添加量增加而降低,而糊化温度则升高。蛋清蛋白对豌豆淀粉凝胶化温度影响不显著,但降低了淀粉的凝胶化焓值。添加不同量蛋清蛋白的豌豆淀粉糊均表现为假塑性流体特征,Herchel-Bulkley模型能够很好地拟合其静态流变行为,稠度指数和屈服应力均随蛋清蛋白添加量增加而降低。添加蛋清蛋白降低了豌豆淀粉糊体系的黏弹性及凝胶的硬度、强度和可塑性。蛋清蛋白对豌豆淀粉凝胶中自由水含量影响不显著,但使结合水含量增加而不可流动束缚水含量降低。     

EFFECT OF STORAGE ON THE RHEOLOGICAL BEHAVIOUR OF BABY FOODS

The effect of storage temperature on rheological behaviour of four varieties of baby foods was studied. After 24-month storage at 15C and 25C, the constant A in Weltman model increased significantly for the vegetables, meat and fish samples, and decreased significantly for the fruit samples. After 24-month storage at 5C and 15C there were no significant changes in flow behavior index, consistency index and yield stress of the Herschel-Bulkley model for the vegetables, meat, fish and fruit samples. Increase in storage temperature decreased the consistency index and yield stress of the fruit samples.     

低盐咸蛋的腌制工艺及其品质研究

提出一种两段腌制法制备低盐咸蛋的方法,分别对两段法腌制过程中咸蛋蛋白和蛋黄的水分含量、盐分含量进行实时检测,考察腌制结束时蛋黄的出油率,并且对咸蛋品质进行综合感官评定。结果表明,先使用19%盐水再换用3.5%盐水的两段法腌制工艺,咸蛋产品中蛋白盐分含量为3.64%,比传统法腌制产品蛋白盐分含量降低51.21%,口感适宜;蛋黄含盐量为0.94%,略低于传统法腌制的1.23%;蛋黄脂质渗出率为67.73%,与传统法腌制的69.22%基本相当。该腌制工艺不仅可解决传统咸蛋蛋白盐分过高、口感过咸的问题,同时也可保证蛋黄松沙出油,从而获得高品质的低盐咸蛋。