Mixed gels of fine-stranded and particulate networks of gelatin and whey proteins

Mixed gels of gelatin and whey protein concentrate were investigated, as well as their pure systems, by tensile tests and by dynamic oscillatory measurements. The systems were studied for homogeneous particulate whey protein gels at pH 5.4 and for inhomogeneous particulate whey protein gels at pH 4.6. The influence on the systems of the Bloom number of the gelatin component has also been investigated. Results of the fracture properties, such as stress and strain at fracture, indicate a transition in rheological properties. Results of the elastic modulus, obtained by tensile measurements, as well as the storage modulus, obtained by dynamic oscillatory measurements, both agree with predictions for phase inversions from the Takayanagi models as modified by Clark, which are in agreement with the fracture properties. The transition points are different for the different mixed gel series but take place between 1 and 3 wt% gelatin and 8 wt% whey protein concentrate, depending on factors such as the microstructure of the whey protein concentrate. Dynamic oscillatory measurements showed that gel formation of whey protein concentrate is unaffected by the presence of gelatin, which is in agreement with light microscopy results. Light microscopy revealed that the mixed gel systems were bicontinuous and that the whey protein network structure was unaffected by the presence of gelatin. It is postulated that the predicted phase inversions of the mixed gels are due to a shift in rheological properties without any phase inversions in the microstructure.     

Aerated whey protein gels as a controlled release system of creatine investigated in an artificial stomach

A controlled creatine-release system has been developed from whey protein-based gels. Their functionalization was carried out by aeration and sodium ions induced “cold gelation” processes. The effect of protein concentration in the aerated whey protein gels at pH 7.0 and 8.0 was analyzed. Physicochemical properties of the aerated gels were evaluated. It was possible to obtain the ions induced whey protein aerated gel with well distributed creatine and different microstructure as well as rheological properties. Different protein concentrations and pH enabled obtaining gels with different rheological properties, texture, air fraction, diameter of air bubbles, microstructure and surface roughness. An increase in the protein concentration enhanced the hardness of the samples, regardless of their pH. The mechanical strength of gels prepared at pH 8 were higher than those obtained at pH 7, as was manifested by the smaller storage modulus of the latter. The former gel exhibited a microstructure between particulate and fine-stranded. A stronger gel matrix produced smaller air bubbles. Aerated gels produced at pH 7.0 had higher roughness than those obtained at pH 8.0. Optimal conditions for inclusion of air bubbles into the gel matrix were: 9% protein concentration at pH 8.0 and this aerated gel was selected for digestion in the artificial stomach. There is a small conversion of creatine to creatinine in the artificial stomach digestion process (9.6% after 6 h). The diffusion of creatine crystals from the aerated gel matrix was the mechanism responsible for the release process. Aerated whey protein gels can be used as matrices for time extended releasing of creatine in the stomach.     

肉桂醛添加方式对乳清浓缩蛋白凝胶特性的影响

利用乳液和微胶囊的包载改善肉桂醛的亲水性,并将其添加至浓缩乳清蛋白溶液中,利用热诱导形成乳清浓缩蛋白凝胶,表征凝胶的流变特性、持水性、质构及微观结构,探究肉桂醛添加方式对所得凝胶特性的影响。结果表明,添加肉桂醛后,蛋白质凝胶的持水力显著增加。以乳液形式添加时,肉桂醛含量与凝胶的黏弹性、持水力、质构特性和网络结构的致密性呈正相关;以微胶囊形式添加时,肉桂醛可显著增强凝胶的黏弹性、持水力、质构特性和网络结构的致密性,但肉桂醛含量变化对凝胶特性的影响较小。对比两种添加方式,肉桂醛以微胶囊形式添加更能有效地调控蛋白凝胶的持水性和质构特性。     

Viscoelastic and fragmentation characters of model bolus from polysaccharide gels after instrumental mastication

Model bolus from polysaccharide gels was investigated by the stress-relaxation tests and particulate size analyses. Using two gelling agents, gellan gum and a composite of gellan/psyllium seed gums, gels with different physical properties (i.e., elastic gellan single gels and plastic composite gels) and gel hardness were prepared. Gels were masticated instrumentally in the presence or absence of artificial saliva to prepare model bolus. Data from the stress-relaxation tests analyzed by 5-element mechanical model showed that difference between two Maxwell-bodies in the elasticity for the composite gels was generally smaller than that for gellan single gels when compared at equivalent gel hardness and was less influenced by the addition level of saliva. For each gel sample, cumulative particulate size distribution of model bolus was reduced logarithmically with a normal curve regardless of the addition level of saliva. Mean particulate size of model bolus from the composite gels was generally larger than that for gellan single gels when compared at equivalent gel hardness and was less influenced by the addition level of saliva. Based on the particulate size distribution of model bolus, coefficients of skewness and kurtosis calculated for the composite gels tended to be larger than those for gellan single gels when compared at equivalent gel hardness. Results indicated higher structural homogeneity of model bolus from the composite gels, which is related to higher miscibility with saliva. Structural homogeneity should be the key for texture design of nursing-care foods, particularly for dysphagia.     

Microstructure of Whey Protein Isolate Gels Containing Emulsified Butterfat Droplets

The effects of concentration and droplet size of anhydrous butterfat globules on the microstructure of heat-induced whey protein isolate gels (pH 4.60) were studied by scanning and transmission electron microscopy (TEM). All fat globules were emulsified with whey protein isolate and incorporated into the system prior to gelation. Protein aggregates became more closely packed as whey protein concentration was increased from 8 to 15% by weight in gels without added fat. There was no notable change in overall gel microstructure upon addition of fat globules, up to 25% by weight, when viewed by scanning electron microscopy. However, it appeared fat globules were intimately associated with the gel protein matrix. A twofold difference in fat globule size was obvious by TEM. Clusters of droplets became more predominant as butterfat content increased.     

EFFECTS OF OIL DROPLET AND AGAR CONCENTRATION ON GEL STRENGTH AND MICROSTRUCTURE OF O/W EMULSION GEL

The effects of oil droplet size and agar concentration on gel strength and microstructure of emulsion gels were investigated by compression test and cryoscanning electron microscope (Cryo-SEM). At all agar concentrations, the compressive stress values of emulsion gels were lower than those of the oil-free gels. Compressive stress and energy were significantly affected by oil droplet size and agar concentration, but compressive strain was not. SEM observation revealed that the overall volume of void spaces decreased and strand compactness increased with increasing agar concentration. Gels containing oil droplets had some void spaces between the gel network and the oil droplets. The strands of emulsion gels did not cover the oil globules completely, a phenomenon which was also observed for the gel with high agar concentration.     

Effects of added whey protein aggregates on textural and microstructural properties of acidified milk model systems

Non-fat milk model systems containing 5% total protein were investigated with addition of micro- or nanoparticulated whey protein at two levels of casein (2.5% and 3.5%, w/w). The systems were subjected to homogenisation (20 MPa), heat treatment (90 °C for 5 min) and chemical (glucono-delta-lactone) acidification to pH 4.6 and characterised in terms of denaturation degree of whey protein, particle size, textural properties, rheology and microstructure. The model systems with nanoparticulated whey protein exhibited significant larger particle size after heating and provided acid gels with higher firmness and viscosity, faster gelation and lower syneresis and a denser microstructure. In contrast, microparticulated whey protein appeared to only weakly interact with other proteins present and resulted in a protein network with low connectivity in the resulting gels. Increasing the casein/whey protein ratio did not decrease the gel strength in the acidified milk model systems with added whey protein aggregates.     

乳成分对酸奶凝胶微观结构和流变学性质的影响

酸奶凝胶的许多宏观的物理特性与其微观结构和流变学性质密切相关。从酸奶的微结构、流变学性质和质地等方面综述了乳脂肪、蛋白质及调节酪蛋白和乳清蛋白比例对酸奶凝胶的影响。     

Combining protein micro-phase separation and protein–polysaccharide segregative phase separation to produce gel structures

The ability of protein micro-phase separation and protein–polysaccharide segregative phase separation to generate a range of gel structures and textures was evaluated. Whey protein isolate/κ-carrageenan mixed gels were prepared with 13% (w/v) whey protein isolate, 0–0.6% (w/w) κ-carrageenan and 50, 100 or 250 mM NaCl. The microstructure of gels, determined by confocal laser scanning microscopy, varied from homogenous to protein continuous, bicontinuous, coarse stranded or κ-carrageenan continuous, depending on the κ-carrageenan concentration. Microstructure also varied from stranded to particulate (micro-phase separated) depending on the salt concentration. The rheological behavior of mixed gels corresponded to the shift in the continuous phase from protein to κ-carrageenan. At small concentrations of κ-carrageenan, where carrageenan-rich droplets were dispersed in a continuous protein-rich matrix, gel strength (fracture stress) and firmness (G′) increased due to increased local concentration of proteins caused by phase separation. At higher κ-carrageenan concentrations, gels were substantially less firm, weaker and less deformable (fracture strain). The change in the continuous phase from protein continuous to carrageenan continuous explained the major change in mechanical properties and water-holding properties. The shift in microstructure occurred at lower concentrations of κ-carrageenan when whey proteins were under micro-phase separation conditions. The results demonstrated how the combined mechanisms of ion-induced micro-phase separation of proteins and protein–polysaccharide phase separation and inversion can be used to alter gel structure and texture.     

Factors that determine the fracture properties and microstructure of globular protein gels

Protein gel matrices are responsible for the texture of many foods. Therefore an understanding of the chemical reactions and physical processes associated with fracture properties of gels provides a fundamental understanding of select mechanical properties associated with texture. Globular proteins form thermally induced gels that are classified as fine-stranded, mixed or particulate, based on the protein network appearance. The fundamental properties of true shear stress and true shear strain at fracture, used to describe the physical properties of gels, depend on the gel network. Type and amount of mineral salt in whey protein and β-lactoglobulin protein dispersions determines the type of thermally induced gel matrix that forms, and thus its fracture properties. A fine-stranded matrix is formed when protein suspensions contain monovalent cation (Li⁺, K⁺, Rb⁺, Cs⁺) chlorides, sodium sulfate or sodium phosphate at ionic strengths ≤0.1 mol/dm³. This matrix has a well-defined network structure, and varies in stress and strain at fracture at different salt concentrations. At ionic strengths >0.1 mol/dm³ the matrix becomes mixed. This network appears as a combination of fine strands and spherical aggregates, and has high stress values and minimum strain values at fracture. Higher concentrations of monovalent cation salts cause the formation of particulate gels, which are high in stress and strain at fracture. The salt concentration required to change microstructure depends on the salt's position in the Hofmeister series. The formation of a particulate matrix also occurs when protein suspensions contain low concentrations (10–20 mol/dm³) of divalent cation (Ca²⁺, Mg²⁺, Ba²⁺) chloride salts or di-cationic 1,6-hexanediamine at pH 7.0. The divalent cation effect on β-lactoglobulin gelation is associated with minor changes in tertiary structure involving amide—amide interproton connectivities (determined by ¹H NMR) at 40–45°C, increasing hydrophobicity and intermolecular aggregation. The type of matrix formed appears to be related to the dispersed or aggregated state of proteins prior to denaturation. Mixed and particulate matrices result from conditions which favor aggregation at temperatures (25–45°C) which are much lower than the denaturation temperature (~65°C). Therefore, general (e.g. Hofmeister series) and protein-specific factors can affect the dispersibility of proteins and thereby determine the microstructure and fracture properties of globular protein gels.     

Soy protein cold-set hydrogels as controlled delivery devices for nutraceutical compounds

The release of riboflavin from soy protein hydrogels of filamentous or particulate microstructure was investigated under gastric and intestinal conditions in the presence or absence of digestive proteases. Microscopic examination showed riboflavin arranged into crystals dispersed randomly throughout the gels. Rheological analysis revealed that the riboflavin load weakened the gel networks, which led to gel disintegration during hydro-swelling tests at pH 7.5. Dissolution tests showed that riboflavin release was faster at pH 7.5 than at pH 1.2 for both gels. Initial release was faster from particulate gels, due to their higher porosity and eroding granular texture. The release mechanisms involved in riboflavin release were diffusion and matrix degradation for both gels at pH 1.2 and pH 7.5. In the presence of pepsin at pH 1.2, both gel types provided good protection of riboflavin for at least 6 h, while both were digested in the presence of pancreatin at pH 7.5. These results suggest that both gels might be useful for transporting bioactive molecules through the gastrointestinal tract and delivering them in the small intestine. Considering their non-synthetic nature, they should be of great interest to developers of innovative functional foods.     

Structure and physical properties of yogurt gels: effect of inoculation rate and incubation temperature

The effects of inoculation rates and incubation temperatures on the physical properties and microstructure of yogurt gels were investigated. A 2-factor experimental design with 3 replicates was used for data analysis. Yogurt gels were made with 0.5, 1, 2, 3, or 4% (wt/wt) inoculation rates and incubated at 40 or approximately 46 degrees C. Dynamic low amplitude oscillatory rheology was performed to monitor the formation of yogurt gels. Gel permeability and the amount of surface whey were determined. Gel structure was examined by confocal scanning laser microscopy. Higher storage modulus values were observed in yogurt gels made at higher inoculation rates and lower incubation temperatures. Gels made at higher inoculation rates and incubation temperatures exhibited higher yield stress and maximum loss tangent values, respectively. Permeability, pore size, and whey separation of yogurt gels increased with decreased inoculation rate and increased incubation temperature. An increase in the inoculation rate resulted in a decrease in the pH where the maximum in the loss tangent occurs, presumably reflecting less efficient solubilization of colloidal calcium phosphate (which is a slow process) and the need to attain a lower pH to complete the solubilization. Significant positive correlations were observed between whey separation and the value of maximum in loss tangent (r = 0.94) and permeability (r = 0.89). Whey separation was negatively correlated with storage modulus (r = -0.48). It was concluded that rearrangements of casein particles in the gel network and the rate at which the solubilization of colloidal calcium phosphate occurred were important driving forces for whey separation and weak gel.     

Heat-induced Gelation Properties of Salt-Soluble Muscle Proteins as Affected by Non-Meat Proteins

Addition of whey protein concentrate (WPC), whey protein isolate (WPI) or soy protein isolate (SPI) to salt-soluble muscle proteins (SSP) decreased the gel strength. WPI:SSP gels had higher water-holding capacity than SSP, SSP:WPC or SSP:SPI gels. Myosin heavy chain was a principal contributor to gel network formation in SSP, SSP:WPC, SSP:WPI and SSP:SPI systems. The characteristic fibrous network formed by SSP was the dominant feature of the microstructure of SSP:WPC and SSP:WPI gels. SSP:SPI gels had a more aggregated appearance due to the occurrence of clusters of SPI throughout the gel matrix.     

Microstructure, rheology and storage stability of low-fat yoghurt structured by carrot cell wall particles

Plant cell wall particles derived from fruits and vegetables are natural fibre materials with a low calorie content that can be used as a healthy alternative to gum stabilisers and starches for structuring low-fat yoghurt. In this study we investigated the effect of cell wall particle (CWP) addition on the gelation kinetics, viscoelastic properties, microstructure, texture and whey loss of the set yoghurt gels as a function of CWP concentration, particle size and storage time. Three particle sizes of dried carrot CWP (d_0.5 = 34, 71 and 80 ??m) were produced from an industrial carrot pomace. Rehydrated CWP was added to skim milk prior to acidification. The results showed that the addition of carrot CWP accelerated the rate of pH reduction and induced earlier gelation. The gel viscoelastic properties were enhanced with increased CWP concentration. This was accompanied with progressive reduction in the whey loss. The smallest cell wall particles (d_0.5 = 34 ??m) gave better gel strength and lower whey loss compared to the larger CWP particles, possibly due to higher contact between the CWP and casein particles thus contributing to the stronger gel network. The CLSM images of yoghurt gels containing CWP showed that carrot CWP occupied the void space within casein particle network. The enhanced gel strength and reduced whey loss achieved by the addition of CWP were maintained throughout the 28 day storage period. The reduction of fermentation processing by almost 1 h, yet still achieving good gel properties for the yoghurt type product could be a significant benefit from a manufacturing point of review.     

WHEY SEPARATION IN ACID SKIM MILK GELS MADE WITH GLUCONO-δ-LACTONE: EFFECTS OF HEAT TREATMENT AND GELATION TEMPERATURE

The effects of heat treatment and gelation temperature on whey separation in acid milk gels made with glucono-δ-lactone were studied using three empirical methods, two of which were developed specifically to quantify spontaneous whey separation from set acid gels. Gels were made in volumetric flasks and petri dishes and the amount of surface whey produced by gels after 16 h was compared with the amount of supernatant expressed by low speed centrifugation (100 g X 10 min) of gels made in tubes. A central composite experiment design and response surface methodology were used. Using regression analysis a second order polynomial model satisfactorily predicted the effect of heat treatment and gelation temperature on whey separation for gels made in volumetric flasks (R²= 0.89). Whey separation was significantly increased by heat treatment (P < 0.001), gelation temperature (P < 0.01) and the quadratic term for heat treatment (P < 0.01). A significant (P < 0.01) positive correlation (r = 0.71) was observed between whey separation in volumetric flasks and petri dishes. It was suggested that high heat treatments and gelation temperatures favour more rearrangements of the network just after formation, making gels unstable and sensitive to spontaneous whey separation.     

Effect of protein concentration, pH, lactose content and pasteurization on thermal gelation of acid caprine whey protein concentrates

The influence of pH (4.5-6.5), sodium chloride content (125-375 mM), calcium chloride content (10-30 mM), protein concentration (70-90 g/l) and lactose content on the gel hardness of goat whey protein concentrate (GWPC) in relation to the origin of the acid whey (raw or pasteurized milk) was studied using a factorial design. Gels were obtained after heat treatment (90 degrees C, 30 min). Gel hardness was measured using texture analyser. Only protein concentration and pH were found to have a statistically significant effect on the gel hardness. An increase in the protein concentration resulted in an increase in the gel hardness. GWPC containing 800g/kg protein formed gels with a hardness maximum at the pHi, whereas GWPC containing 300 g/kg protein did not form true gels. Whey from pasteurized milk formed softer gels than whey from raw milk. A high lactose content (approximately 360 g/kg) also reduced the gelation performance of GWPC.     

Structural properties of stirred yoghurt as influenced by whey proteins

The effect of whey protein addition on structural properties of stirred yoghurt systems at different protein and fat content was studied using laser diffraction spectroscopy, rheology and confocal laser scanning microscopy (CLSM). The composition of heated milk systems affected micro- and macroscopic properties of yoghurt gels. Particle size increased as a function of increasing whey protein content and decreased as a function of increased fat level. Firmness (elastic modulus) and apparent viscosity of manufactured yoghurt samples increased as a function of increased interparticle interactions, mainly caused by self-aggregation of whey proteins or aggregated whey protein-coated fat globules, respectively. The resistance towards shear-induced disruption of yoghurt gels increased with an increasing proportion of casein protein in the protein mixture, whereas products with high whey protein level revealed lower resistance behaviour towards shear-forces. CLSM images illustrated that the presence of large whey protein aggregates and lower number of fat globules lead to the formation of an interrupted and coarse gel microstructure characterised by large interstitial spaces. The higher the casein fraction and/or the fat level, the less interspaced voids in the network were observed. However, it is evident that the addition of whey proteins reinforces firmness properties of low-fat yoghurts comparable to characteristics of full-fat yoghurt.