Effect of protein composition on the rheological properties of acid-induced whey protein gels

Protein dispersions with different ratios of α-lactalbumin to β-lactoglobulin were heat-denatured at pH 7.5 and then acidified with glucono-δ-lactone to form gels at room temperature. Heat treatment induced the formation of whey protein polymers with reactive thiol group concentrations ranging from 1 to 50 μmol/g, depending on protein composition. During acidification, the first sign of aggregation occurred when the zeta potential reached −18.2 mV. Increasing the proportion of α-lactalbumin in the polymer dispersions resulted in more turbid gels characterized by an open microstructure. Elastic and viscous moduli were reduced, while the relaxation coefficient and the stress decay rate constants were increased by raising the proportion of α-lactalbumin in the gel. After one week of storage at 5 °C, gel hardness increased by 12%. The effect of protein composition on acid-induced gelation of whey protein is discussed in relation to the availability and reactivity of thiol groups during gel formation and storage.     

Gelation properties of salt-extracted pea protein isolate induced by heat treatment: Effect of heating and cooling rate

Gel network formation of a salt-extracted pea protein isolate was studied using dynamic rheological measurements. The gelling point was dependent on heating rate and was unaffected by cooling rate. When both the heating and cooling rates were increased (from 0.5 to 4 °C/min) final G′ value decreased, indicative of decreased gel strength. During the heating phase, the storage modulus and loss modulus fluctuated below 1 Pa at almost constant values with the storage modulus smaller than the loss modulus until the gelling point was reached. The rate of cooling has a greater impact on the development of storage modulus than that of heating. Compared to the gel strength of commercial pea protein isolate (PPIc) and soy protein isolate (SPIc) at the same protein concentration, salt-extracted pea protein isolate (PPIs) was much stronger than PPIc but weaker than SPIc. Careful control of the heating and cooling rates enable maximum gel strength for heat-induced pea protein gel, thus enhancing utilisation of pea protein as an additive in meat food industry.     

Effect of whey protein enzymatic hydrolysis on the rheological properties of acid-induced gels

A protein dispersion blend of β-lactoglobulin and α-lactalbumin was heat-denatured at pH 7.5, hydrolyzed by α-chymotrypsin and then acidified with glucono-δ-lactone to form gels at room temperature. Heat treatment induced the formation of whey protein polymers with high concentration of reactive thiol groups (37 μmol/g). The reactive thiol group concentration was reduced by half after 40 min enzymatic hydrolysis. It was further reduced after enzyme thermal deactivation. During acidification, the first sign of aggregation for hydrolyzed polymers occurred earlier than for non hydrolyzed polymers. Increasing the hydrolysis duration up to 30 min resulted in more turbid gels characterized by an open microstructure. Elastic and viscous moduli were both reduced, while the relaxation coefficient and the stress decay rate constants were increased by increasing the hydrolysis duration. After one week storage at 5 °C, the hardness of gels made from hydrolyzed polymers increased by more than 50%. The effect of polymer hydrolysis on acid-induced gelation is discussed in relation to the availability and reactivity of thiol groups during gel formation and storage.     

Effect of acetylation on rheological properties of fish protein isolates during heating

Rheological properties of concentrated dispersions of muscle Pollack proteins acetylated to different degrees as a function of temperature have been studied. The character of viscosity changes of the partially acetylated isolates is almost identical with that of unacetylated protein. However, the temperatures corresponding to viscosity maxima increase proportionally to the degree of acetylation. A total acetylation of the free lysine ?-amino groups of Pollack protein results in a significant increase in the critical concentration and/or temperature of the sol-gel transition.     

Effect of denatured whey protein concentrate and its fractions on cheese composition and rheological properties

The objectives of this study were (1) to assess the effect of a denatured whey protein concentrate (DWPC) and its fractions on cheese yield, composition, and rheological properties, and (2) to separate the direct effect of the DWPC or its fractions on cheese rheological properties from the effect of a concomitant increase in cheese moisture. Semihard cheeses were produced at a laboratory scale, and mechanical properties were characterized by dynamic rheometry. Centrifugation was used to induce a moisture gradient in cheese to separate the direct contribution of the DWPC from the contribution of moisture to cheese mechanical properties. Cheese yield increased and complex modulus (G*) decreased when the DWPC was substituted for milk proteins in milk. For cheeses with the same moisture content, the substitution of denatured whey proteins for milk proteins had no direct effect on rheological parameters. The DWPC was fractionated to evaluate the contribution of its different components (sedimentable aggregates, soluble component, and diffusible component) to cheese yield, composition, and rheological properties. The sedimentable aggregates were primarily responsible for the increase in cheese yield when DWPC was added. Overall, moisture content explained to a large extent the variation in cheese rheological properties depending on the DWPC fraction. However, when the effect of moisture was removed, the addition of the DWPC sedimentable fraction to milk increased cheese complex modulus. Whey protein aggregates were hypothesized to act as active fillers that physically interact with the casein matrix and confer rigidity after pressing.     

Concentrated whey protein particle dispersions: Heat stability and rheological properties

In this work heat stability and rheological properties of concentrated whey protein particle dispersions in different dispersing media are studied. Whey protein particles (protein content ∼20% w/v) having an average size of a few microns were formed using a combination of two-step emulsification and heat-induced gelation. Particles were dispersed (volume fraction of particles ∼0.35) in solutions of Na-caseinate, whey protein isolate or gum arabic at different concentrations. The microstructure, particle size distribution and flow behaviour of the dispersions were analyzed before and after heating at 90 °C for 30 min.     

Effect of sucrose on rheological properties of xanthan gum-locust bean gum mixtures

Food Science and Biotechnology - The effect of sucrose (0, 10, 20, 30, and 40%) on flow and dynamic rheological properties of xanthan gum (XG) mixed with locust bean gum (LBG) at different mixing...     

Study on the rheological properties of heat-induced whey protein isolate-dextran conjugate gel

Effect of glycosylation on the rheological properties of whey protein isolate (WPI) during the heat-induced gelation process was evaluated. Significant changes in browning intensity, free amino groups content and SDS-PAGE profile showed that the conjugate of WPI and dextran (150 kDa) was successfully prepared using the traditional dry-heating treatment. For the conjugate, during the heating and cooling cycle, the curves of G′ and G″ were considerably shifted to lower values and their shapes varied comparing to the corresponding spectra of initial WPI and WPI + dextran mixture. After holding at 25 °C, G' reached a value of about 2200 Pa, only a tenth of the value that obtained in the initial WPI gel. Moreover, frequency sweep measurements revealed that the stiffness of gel was greatly reduced in the conjugate, although a typical elastic gel was still formed. All data showed that the rheological properties of thermal gelation could be modified upon the covalent attachment of dextran.     

黄原胶对罗非鱼蛋白-大豆蛋白混合体系乳浊液稳定性的影响

以罗非鱼和豆粕为原料,采用碱溶-等电点沉淀法制备鱼分离蛋白(fish protein isolates,FPI)和大豆分离蛋白(soybean protein isolats,SPI),固定蛋白浓度0.5%,在pH 4.0和7.0条件下,高压均质(一级压力30 MPa,二级压力4 MPa),实验黄原胶(xanthan gum,XG)的添加对FPI、FPI-SPI(质量比2∶1)、FPI-SPI(质量比1∶1)和SPI乳浊体系粒径分布、微观显微结构和乳析指数的影响。结果表明:随着黄原胶的添加(0~0.09%),乳浊体系平均粒径减小(P0.05);在pH 4.0条件下,乳浊液液滴聚集和絮凝现象明显减少,宏观乳析稳定性提高(P0.05),添加0.06%和0.09%黄原胶的FPI-SPI混合乳浊体系4℃放置10 d都没有明显的分层,而不同比例混合蛋白体系之间的差异不明显(P0.05);在pH 7.0条件下,添加0.06%的黄原胶时,FPI-SPI(质量比2∶1)和FPI-SPI(质量比1∶1)混合乳浊体系微观絮凝现象没有明显改善,宏观乳析稳定性明显提高(P0.05),4℃放置10 d没有明显的分层现象。总体分析,2种蛋白物理混合对乳浊液体系稳定性的改善程度有限,而适量的黄原胶能明显提高酸性条件下体系的乳浊液稳定性。     

Modification of rheological properties of whey protein isolates by limited proteolysis

Whey protein isolate (WPI) was subjected to limited tryptic hydrolysis and the effect of the limited hydrolysis on the rheological properties of WPI was examined and compared with those of untreated WPI. At 10% concentration (w/v in 50 mM TES buffer, pH 7.0, containing 50 mM NaCl), both WPI and the enzyme-treated WPI (EWPI) formed heat-induced viscoelastic gels. However, EWPI formed weaker gels (lower storage modulus) than WPI gels. Moreover, a lower gelation point (77 °C) was obtained for EWPI as compared with that of WPI which gelled at 80 °C only after holding 1.4 min. Thermal analysis and aggregation studies indicated that limited proteolysis resulted in changes in the denaturation and aggregation properties. As a consequenece, EWPI formed particulated gels, while WPI formed fine-stranded gels. In keeping with the formation of a particulate gel, Texture Profile Analysis (TPA) of the heat-induced gels (at 80 °C for 30 min) revealed that EWPI gels possessed significantly higher (p < 0.05) cohesiveness, hardness, gumminess, and chewiness but did not fracture at 75% deformation. The results suggest that the domain peptides, especially β-lactoglobulin domains released by the limited proteolysis, were responsible for the altered gelation properties.     

加热和冷却速率对大豆蛋白凝胶特性的影响

凝胶性是植物蛋白最重要的性质。采用小变形振荡(动态)流变测试研究大豆分离蛋白凝胶网络结构形成。结果表明:在加热阶段,储能模量(G')和耗能模量(G″)都低于1 Pa,G'G″且基本保持恒定直到达到凝胶点。Tanδ值随着加热的进行逐渐降低,然后急剧下降,冷却阶段时降至最低且保持恒定,说明形成强而稳定的凝胶网络结构。当加热和冷却速率增大时(从0.5~4℃/min)G'值逐渐下降,表明最终形成凝胶强度下降;慢的加热(冷却)速率时(0.5、1、2℃/min)tanδ较小,说明相对于快的加热速率(4℃/min)形成了更好三维网络结构的凝胶;凝胶点和加热速率有关而不受冷却速率的影响。随着蛋白质浓度的增大,其凝胶G'值增大、tanδ值减少、凝胶点下降。因此,控制加热和冷却速率可以最大限度的提高热诱导大豆蛋白的凝胶强度,而不用改变蛋白质的浓度。     

Modulation of rheological properties by heat-induced aggregation of whey protein solution

Heat-induced protein aggregation at low protein concentrations generally leads to higher viscosities. We here report that aggregated protein can yield weaker gels than those from native protein at the same concentration. Aggregated protein was produced by heating a solution of whey protein isolate (WPI) at 3% and 9% w/w. The higher protein concentration resulted in a larger aggregate size and a higher intrinsic viscosity. The protein fraction in native WPI had the smallest size and the lowest intrinsic viscosity. The same trend was observed for the shear viscosity after concentrating the suspensions containing aggregates to around 15% w/w. Suspensions containing aggregates that were produced from a higher concentration possessed a higher viscosity. After reheating the concentrated suspensions, the suspension from the 9% w/w aggregate system produced the weakest gel, followed by the one from 3% w/w, while the native WPI yielded the strongest gel. Reactivity of the aggregates was also an important factor that influenced the resulting gel properties. We conclude that aggregation of whey protein solution is a feasible route to manipulate the gel strength of concentrated protein systems, without having to alter the concentration of the protein.     

加热改性醇法大豆浓缩蛋白凝胶性的研究

通过控制加热温度对醇法大豆浓缩蛋白(ALSPC)改性.主要考察了不同加热温度对其溶解性、凝胶性的影响,以及这些功能性与其游离-SH、分子亚基的关系,并且通过电镜观察其微观结构的变化.结果表明,通过加热改性的ALSPC其溶解性有明显改善,NSI由原来的5%左右提高到50%以上;凝胶性在150℃加热时有明显改善,其凝胶硬度可达到400 g左右.游离-SH的变化以及电泳、电镜的观察从结构上也说明加热对其凝胶性的影响.     

二价离子对黄原胶溶液流变特性的影响研究

详细研究了不同盐(钙与镁)及不同离子强度对黄原胶溶液体系流变特性的影响。通过实验发现,Ca2+及Mg2+影响黄原胶的低剪切黏度、黏度、剪切稀化指数特性以及黏弹性,还对频率扫描及动态黏弹性温度扫描时G’与G"相交点的频率、松驰时间和凝胶点温度有影响。通过动态黏度温度扫描实验,发现盐的浓度影响黄原胶溶液的构象转变温度。实验还表明,在稳态流动扫描、应变扫描、频率扫描及动态黏温扫描中二价离子对黄原胶溶液流变特性的影响明显大于一价离子,同时Ca2+的作用大于Mg2+。     

不同浓度黄原胶溶液的流变特性研究

黄原胶有很高的低剪切黏度,同时具有明显的剪切稀化特性。浓度的改变影响其流变行为。本文通过稳态流动、应变扫描及频率扫描试验详细研究了黄原胶不同浓度对其流变特性的影响,并从微观结构上进行解析。试验得到的各个流变特性参数与浓度之间存在高度相关性,同时,稳态流动试验得到的各参数与黏弹性之间亦存在高度相关性,通过数理统计分析,对上述相关性进行拟合并得到一系列方程。     

Effect of whey and pea protein blends on the rheological and sensory properties of protein-based systems flavoured with cocoa

Whey and pea protein combined in different proportions (100W:0P, 75W:25P, 50W:50P, 25W:75P, 0W:100P) were used to prepare protein-based systems flavoured with cocoa and containing κ-carrageenan or κ-carrageenan/xanthan gum as thickeners. Steady and dynamic shear rheological properties of samples were measured at 10 °C and sensory differences were evaluated. Protein-based systems exhibited a shear-thinning flow behaviour that was fitted to the simplified Carreau model. Samples showed different viscoelastic properties, ranging from fluid-like to weak gel behaviour. For both types of system (with and without xanthan gum) viscosity, pseudoplasticity and elasticity rose on increasing the pea protein proportion in the blend. The sample with only whey protein obeyed the Cox-Merz rule, while in the rest of the samples complex viscosity was higher than apparent viscosity. Regarding sensory properties, the protein blend ratio mainly affected sample thickness, which rose as pea protein proportion increased. However, at the same time, the chocolate flavour and sweetness decreased and the off-flavour increased.     

A comparison of drying operations on the rheological properties of whey protein thickening ingredients

An existing procedure for the alteration of whey proteins into a cold‐set thickening agent was modified by developing a spray‐drying operation to replace the prohibitively expensive freeze‐drying step. The original and the modified derivatization procedures were used with a commercial whey protein concentrate (WPC). The freeze‐dried and spray‐dried derivatized WPC powders, along with polysaccharide thickeners, were reconstituted in water and evaluated by using a range of rheological studies. The effects of temperature, concentration, and shear on viscosity as well as the mechanical spectra were assessed to characterize the ability of the powders to function in food systems. Rheological characterization revealed the modified derivatization procedure yielded an ingredient having the same cold‐set thickening and gelling ability as the original derivatized powder. The modified whey proteins were also able to achieve, at higher usage levels, textural properties similar to several polysaccharide thickeners. Use of a spray‐drying technique created a more economical process for the production of a whey protein ingredient that was suitable for contributing viscosity and texture to a wide range of food systems.