GEL CHARACTERISTICS OF β-LACTOGLOBULIN, WHEY PROTEIN CONCENTRATE AND WHEY PROTEIN ISOLATE

The gelation characteristics of β-lactoglobulin, whey protein isolate and whey protein concentrate at varying levels of protein (6–11%), sodium chloride (25–400 mM), calcium chloride (10–40 mM) and pH (4.0–8.0) were studied in a multifactorial design. Small scale deformation of the gels was measured by dynamic rheology to give the gel point (°C), complex consistency index (k*), complex power law factor (n*) and critical strain (γ_c). The gel point decreased and turbidity increased with increasing calcium level. The denaturation temperature measured by differential scanning calorimetry was measured at higher pH values. Large scale deformation at 20% and 70% compression was measured using an Instron Universal Testing machine. The true protein level had the largest effect on the stress required to produce 20% and 70% compression and on the consistency (k*) of the gels.     

Rheology of Stirred Yogurt as Affected by Added Milk Fat, Protein and Hydrocolloids

The rheological characterization of stirred yogurt with added milk fat, Na caseinate (or micellar casein) and gelatin (4 Bloom strengths), starch or a xanthan gum/LBG 50:50 mixture was carried out. Dynamic and shear values were measured at 8°C and syneresis at 4°C. Consistency (k* and k) and syneresis were more frequently influenced by the composition variables than the power law factors n* and n and the critical strain γ_c. The k* ranged from 15.8 to 576 Pa sn*, n* from 0.038 to 0.220, γ^c from 1.6 to 49.0 10^-3, k from 0.37 to 32.47 Pa sⁿ, n from 0.005 to 0.587, and syneresis from 0.0 to 49.2%.     

Effect of Trichoderma reesei tyrosinase on rheology and microstructure of acidified milk gels

The aim of this work was to study the potential of tyrosinase enzymes in structural engineering of acid-induced milk protein gels. Fat free raw milk, heated milk or a sodium caseinate solution were treated with tyrosinases from Trichoderma reesei (TrTyr) and Agaricus bisporus (AbTyr) and the reference enzyme transglutaminase (TG) prior to acid-induced gelation. TrTyr treatment increased the firmness of raw milk and sodium caseinate gels, but not that of heated milk gels, even though protein cross-linking was detected in heated milk. AbTyr did not cross-link proteins in any of the studied milk protein systems. TG was superior to TrTyr in gels prepared of heated milk. In acidified heated milk and sodium caseinate, TrTyr and TG treatment resulted in a decrease of the pore size. Scanning electron microscopy revealed more extensive particle interactions in the heated milk gels with TG than with TrTyr.     

Effect of sodium caseinate and κ-carrageenan on binding and textural properties of pork muscle gels enhanced by microbial transglutaminase addition

Response surface methodology was used to investigate the main effects and interactions of sodium caseinate (SC, 0–2%), microbial transglutaminase (MTG, 0–0.6%) and carrageenan (CGN, 0–0.8%) on water binding, textural and colour characteristics of pork gels. Higher κ-carrageenan and sodium caseinate content favored hydration properties and thermal stability, yielding lower cooking loss and higher water holding capacity. Their addition also increased hardness of pork gels, but was unable to improve springiness or cohesiveness. Sodium caseinate either alone or in combination with MTG generally has been found to be inferior to κ-carrageenan for functionality in comminuted meat systems. Although MTG had no effect on binding properties it was found to increase strength of the gels at higher SC levels. Addition of SC and ic-carrageenan significantly altered colour, while no significant influence of MTG on gel colour parameters was observed.     

Caustic-induced gelation of β-lactoglobulin

The gelation kinetics of β-lactoglobulin (βLg) solutions has been determined in the alkaline regime over a wide range of protein concentrations, gelling temperatures and gelation pH (pH_gel), set using NaOH. The behaviour is compared with caustic-induced gelation of whey protein concentrate and the alkaline dissolution of heat-induced whey gels. The gelation time decreases significantly between neutral conditions and pH_gel 9, because of the activation of the free cysteine groups and displacement to the monomeric form, and between pH_gel 10 and 11, due to the base denaturation of βLg. Both transitions are associated with a significant decrease in the activation energy of gelation. At pH_gel >11.5 the gelation time is observed to increase with pH_gel, owing to destruction of interprotein crosslinks. These results are consistent with the recently reported observation that a minimum pH for the dissolution of βLg gels and aggregates exists around 11.6 [ Biomacromolecules 8 (2007) 1162]. This phenomenon has been assigned to the destruction of non-covalent interactions that would inhibit the final percolation of the gel.     

Factors affecting the acid gelation of sodium caseinate

Acid gelation, turbidity and particle size development of dispersions of sodium caseinate and other protein fractions were studied. Sodium caseinate dispersions became particulate prior to the onset of gelation. Casein particles had enhanced stability to gelation in the presence of sodium chloride. Removal of the hydrophilic part of the κ-casein molecule through renneting and acidification of the soluble sodium para-caseinate resulted in increased gelation pH. Removal of most of the κ-casein through ethanol fractionation of sodium caseinate resulted in an α_s1-β-fraction, which was markedly destabilised at higher pH values during acidification in the presence of sodium chloride. Preheated β-lactoglobulin/sodium caseinate dispersions had similar acid gelation profiles in the presence of sodium chloride with or without N-ethylmaleimide, suggesting that secondary thiol-disulphide interchange reactions between κ-casein and pre-heated β-lactoglobulin aggregates did not effect the gel point and final storage modulus in the short time-frame (120 min) of acidification. It was found that κ-casein and sodium chloride played a significant role in both particle development and subsequent stability of sodium caseinate dispersions on acidification.     

Tara gum–bovine sodium caseinate acid gels: Stabilisation of W/W emulsions

Microstructural, rheological, and textural behaviour of tara gum–bovine sodium caseinate aqueous mixtures and their acid gels were evaluated. Acid gels with different microstructures and texture were obtained. These results can be related to a competition between the protein acid gelation process and the segregative phase separation. Depending on the concentration ratio of both biopolymers, a continuous protein gel network or a water‐in‐water emulsion stabilised by acid gelation was observed. These findings may be used to address the development of new food‐grade gels with different textures and also for the obtention of protein microgels to encapsulate hydrophilic compounds.     

SURFACE PROPERTIES OF PROTEIN LAYERS ADSORBED FROM MIXTURES OF GELATIN WITH VARIOUS CASEINS

We report surface tensions and surface shear viscosities of protein layers adsorbed from mixtures of gelatin +α_s11-casein, β-casein, k-casein or sodium caseinate. Under conditions where the two protein components carry opposite net charges, the combined experimental data at 25C and neutral pH are consistent with a two-layer model of the mixed protein film. The surface properties of sodium caseinate lie intermediate between those for the two major individual caseins (α_s1 and β).     

Textural properties of a model aqueous phase in low fat products. Part 2: Alginate/caseinate blends and three-component systems

Summary The second paper in this series completes the investigation of the textural properties of alginate, starch and caseinate preparations. Thus mixtures of alginate and caseinate show liquid-like properties up to 200 p.p.m. Ca²⁺. At higher levels of the counterion, alginate gels form a continuous matrix which supports the liquid-like caseinate inclusions. Caseinate acts mainly as a sequestrant for calcium ions. In the absence of added counterions, the ternary system is characterized by a weak starchy network which supports the liquid alginate and caseinate phases. Addition of Ca²⁺ produces a plethora of textures governed mainly by the alginate phase and the sequestering influence of caseinate. Intelligent manipulation of polymeric composition and levels of calcium resulted in binary systems of caseinate/starch, alginate/starch and caseinate/alginate capable of reproducing the texture of ternary mixtures, thus offering attractive alternatives to the commercial aqueous phases. In conclusion, it is demonstrated that basic understanding of ingredients and their properties in mixtures leads to purposeful formulation of products.     

高酰基结冷胶/酪蛋白酸钠复合凝胶粘弹行为的研究

本文研究了酪蛋白酸钠浓度、结冷胶浓度、离子和测试条件对高酰基结冷胶/酪蛋白酸钠复合凝胶粘弹性的影响。结果表明:高酰基结冷胶/酪蛋白酸钠共混体系为典型的切力变稀流体,表观粘度随着酪蛋白酸钠浓度的升高而降低,而随着阳离子浓度的增大出现先增大后减小的变化趋势。压缩速度对复合凝胶硬度几乎无影响,而内聚性和弹性则随着压缩速度的增加而增大。内聚性随着压缩应变的增大出现先增大后减小的变化趋势。复合凝胶的硬度和弹性随着酪蛋白酸钠浓度的增加而下降,但复合凝胶的内聚性对酪蛋白酸钠浓度不敏感。高酰基结冷胶浓度越高,复合凝胶的硬度和弹性越大。相对于一价离子而言,二价离子形成的凝胶更强且用量更少。钾离子的添加对复合凝胶保水性影响较弱,而钙离子的添加则可以提高复合凝胶的保水性。     

Oil-in-water emulsions stabilized by sodium caseinate: Influence of pH,high-pressure homogenization and locust bean gum addition

The effect of pH, addition of a thickening agent (locust bean gum) or high-pressure homogenization on the stability of oil-in-water emulsions added by sodium caseinate (Na-CN) was evaluated. For this purpose, emulsions were characterized by visual analysis, microstructure and rheological measurements. Most of the systems were not stable, showing phase separation a few minutes after emulsion preparation. However, creaming behavior was largely affected by the pH, homogenization pressure or locust bean gum (LBG) concentration. The most stable systems were obtained for emulsions homogenized at high pressure, containing an increased amount of LBG or with pH values close to the isoelectric point (pI) of sodium caseinate, which was attributed to the size reduction of the droplets, the higher viscosity of continuous phase and the emulsion gelation (elastic network formation), respectively. All the studied mechanisms were efficient to decrease the molecular mobility, which slowed down the phase separation of the emulsions. In addition, the use of sodium caseinate was also essential to stabilize the emulsions, since it promoted the electrostatic repulsive interactions between droplets.     

Interactions between milk proteins and gellan gum in acidified gels

The mechanical properties, microstructure and water holding capacity of systems formed from whey protein concentrate (0–3% WPC w/w), sodium caseinate (0–2% w/w), and gellan gum (0.1–0.3% w/w) in the coil or helix conformational state (Coil/Helix), were investigated. This polymer combination resulted in bi-polymeric or tri-polymeric systems, which were slowly acidified to pH 4.0 by the addition of GDL in order to favor electrostatic protein–polysaccharide interactions. The properties of the tri-polymeric systems differed considerably from the bi-polymeric ones. At high polymer concentrations the WPC-gellan samples showed incompatibility and microphase separation, which resulted in weaker and less deformable gels. However, in systems with coil gellan the incompatibility was less intense, which was attributed to the formation of electrostatic complexes between the protein and the polysaccharide during the mixing process. In caseinate–gellan systems, complex formation was observed and an increase in the gel mechanical properties as the caseinate concentration rose, although the water holding capacity decreased at higher gellan concentrations. The caseinate–gellan coacervate was not visualized in the tri-polymeric systems and the incompatibility between the biopolymers was intensified, although the mechanical properties were considerably higher than in the bi-polymeric gels.     

酪蛋白-卡拉胶体系的流变特性及其相互作用研究

本文研究了不同卡拉胶的添加量对酪蛋白-卡拉胶体系粒径以及流变学性质的影响,对维持体系的流变学特性与凝胶特性的分子间作用力进行了探究,揭示了酪蛋白与卡拉胶混合凝胶机理。结果表明:卡拉胶浓度的升高会导致体系粒径增大,浓度为0.5%时,粒径达到1096.8 nm,约为酪蛋白溶液分子粒径的5倍,表明卡拉胶与酪蛋白发生吸附作用;流变学测试表明体系呈现假塑性流体的特征,随着卡拉胶添加量的增加,流动指数n由0.9251下降到0.7270,稠度系数K由0.3796上升到3.4403;添加NaCl与尿素后,体系的凝胶强度有不同程度的下降,当二者浓度为1.0 mol/L时,体系凝胶强度的损失率分别为100%与32.4%,分子间作用力以静电作用为主;红外光谱显示反应前后酪蛋白的二级结构发生改变,进一步证明卡拉胶与酪蛋白分子之间发生交联反应。     

IONIC INTERACTIONS IN ALGIN/CALCIUM/ MYOFIBRILLAR PROTEIN GELS

Algin/calcium/myofibrillar protein gel interactions were investigated by modifying protein basic amino acids to an unreactive state and evaluating Instron texture profile (TP) of gels. Gels analyzed were: 1% algin and 0.075% CaCl₂; 1% algin, 0.075% CaCl₂ and 1.5% porcine myofibrillar protein (A/C/P); and A/C/P with lysine, histidine or arginine modified. The modification of amino acids reduced (P<0.05) gel TP parameters of hardness, hardness 2, gumminess and chewiness. Similar TP data for protein modified gels indicated that each basic amino acid was involved in algin/protein gelation. Results provided evidence that ionic bonds between protein basic amino acids and algin carboxylate groups may be important for algin/myofibrillar protein gelation.     

Gelation of Mixed Gels Containing κ-Carrageenan and Skim Milk Components

ABSTRACT: The gelation characteristics of mixed gels containing κ-carrageenan and skim milk or milk fractions (skim milk permeate or retentate) obtained by ultrafiltration were examined. Increasing the skim milk solids content of mixtures containing carrageenan increased setting temperatures and gel strength. The milk proteins contributed to gel strength but did not influence the setting temperature of mixtures. The binding of denatured whey proteins to casein micelles affected gel network formation of milk-carrageenan mixtures containing 10% milk solids. Network formation in mixed gels containing carrageenan and milk or milk fractions was initiated by the carrageenan component and dictated primarily by the ionic content of the mixtures.     

Sodium Caseinate/Sunflower Oil Emulsion-Based Gels for Structuring Food

Protein gels have attired attention since they allow structuring foods with no trans or saturated fats. The effects of protein concentration and sucrose addition on gelation kinetics and on physical properties of sodium caseinate (NaCas)/sunflower oil emulsion-based gels were studied by two methods: a new application of backscattering of light (BS) using a Turbiscan equipment and by dynamic oscillatory rheology. Structure of gels was also described by confocal laser scanning microscopy (CLSM) and small angle X-ray scattering (SAXS). T _gel values decreased with increasing sucrose or NaCas concentration. BS method sensed early changes in structure, while rheological measurements were less sensitive to those changes. However, tendencies found by rheological measurements were the same as the ones found by BS experiments. CLSM images of gels formed from emulsions containing high sucrose and protein concentrations had big oil droplets that were not present in initial emulsions. Gels with sucrose concentrations between 15 and 30 wt/wt% released oil. SAXS patterns showed that NaCas nanoaggregate sizes in the aqueous phase were smaller with increasing sucrose concentration. Polar groups of protein interacted with sucrose, and therefore, interactions among protein molecules diminished. As a result of weaker protein molecule interactions, nanoaggregates were smaller. However, this effect was beneficial. In the macroscale, rheological properties and visual appearance of gels were improved. The gel formulated with 5 wt/wt% NaCas and 10 wt/wt% sucrose had a smooth surface and was stable to syneresis and oil release. This formulation was a good alternative to trans fat.     

The effect of the glucono-δ-lactone/caseinate ratio on sodium caseinate gelation

The influence of the acidification rate and the final pH on the properties of sodium caseinate gels (2–6%, w/v) prepared by acidification with glucono-δ-lactone (GDL) at 10 °C was investigated. The rheological properties of the systems were analysed under shear at incipient gelation and under uniaxial compression throughout the entire gelation process. The water holding capacity (WHC) of these gels and their amount of soluble protein in different media were also evaluated. Up to ∼50% of the total κ-casein content was soluble in gels with a higher amount of GDL, and this contributed to the high WHC observed under this condition. The acidification rate did not affect the rheological properties measured under shear during gelation, but the GDL/caseinate ratio used had a significant effect on the steady-state properties. Fast acidification resulted in lower Young's modulus and stress at fracture values, but higher residual stress values and relaxation times. In contrast, slow acidification produced a more interconnected network probably because of the extensive reorganization or rearrangement within the segments near the isoelectric point.     

Mechanisms of gelation of sardine proteins: influence of thermal processing and of various additives on the texture and protein solubility of kamaboko gels

Surimi prepared from freshly caught sardines was mixed with NaCl and other additives and used to prepare kamaboko gels. Protein-protein interactions involved in the setting (at 4 or 37°C) and/or the cooking (at 90°C) gelation steps were investigated (i) by assessment of kamaboko texture as a result of the type and concentration of additive added; (ii) by partial solubilization of kamaboko gels in buffers containing mercaptoethanol, sodium dodecyl sulphate (SDS) and/or urea, followed by determination of the soluble protein constituents by polyacrylamide gel electrophoresis. Cooked gels of high elasticity and of varying rigidity and gel strength were obtained in the 73–80% water range. Adequate gel texture required a NaCl content of 1.7–3.5% and a pH range of 6.4–8.4. Low concentrations of reducing agents (mercaptoethanol, dithiothreitol, cysteine) or of divalent cations (Ca²⁺, Mg²⁺) improved the texture of gels obtained by setting at 37°C with and without subsequent cooking at 90°C. On the other hand, the addition of N -ethyl maleimide or of ethylene diamine tetra-acetate led to texture deterioration after cooking. These data demonstrate the involvement of disulphide bonds and of electrostatic interactions in surimi gelation. Gel solubilization experiments indicate that the aggregation of myosin heavy chains through various types of protein-protein interactions may be responsible for the elastic gel network formed during setting at 37°C (30 min) or 4°C (24h). Strengthening of the gel network after cooking appears to be due to additional disulphide and hydrophobic interactions.     

On the temperature reversibility of the viscoelasticity of acid-induced sodium caseinate emulsion gels

Viscoelastic properties of acid-induced sodium caseinate emulsion gels have been investigated using a controlled shear stress rheometer. Gelation was introduced by addition of acidulant glucono-δ-lactone (GDL) at three different temperatures (5, 25 and 45°C). It was found that the gelation temperature has a significant effect on the rate of gelation and on the dynamic moduli of the emulsion gels. The rheology of these emulsion gels was investigated over the temperature range 5–45°C. The viscoelasticity of the emulsion gel prepared at 45°C was temperature reversible, suggesting that the temperature change only affects the strength of physical bonding within the network and not the gel microstructure. In contrast, the temperature-dependent viscoelasticity of the emulsion gel prepared at 5°C exhibited a highly irreversible character. This implies significant structural reorganization of the network during the heating stage from 5°C. Analogous temperature irreversibility has been observed in emulsion electrophoretic mobility measurements and in solution surface tension measurements of the corresponding caseinate systems at pH values near the isoelectric point of the protein.     

Small and large deformation rheology and microstructure of κ-carrageenan gels containing commercial milk protein products

The rheological behaviour of commercial milk protein/κ-carrageenan mixtures in aqueous solutions was studied at neutral pH. Four milk protein ingredients; skim milk powder, milk protein concentrate, sodium caseinate, and whey protein isolate were considered. As seen by confocal laser microscopy, mixtures of κ-carrageenan with skim milk powder, milk protein concentrate, and sodium caseinate showed phase separation, but no phase separation was observed in mixtures containing whey protein isolate. For κ-carrageenan concentrations up to 0.5 wt%, the viscosity of the mixtures at low shear rates increased markedly in the case of skim milk powder and milk protein concentrate addition, but did not change by the addition of sodium caseinate or whey protein isolate. For κ-carrageenan concentrations from 1 to 2.5 wt%, small and large deformation rheological measurements, performed on the milk protein/κ-carrageenan gels, showed that skim milk powder, milk protein concentrate or sodium caseinate markedly improved the strength of the resulting gels, but whey protein isolate had no effect on the gel stength.