Pernilla Walkenström

Microstructure and rheological behaviour of alginate/pectin mixed gels

Abstract The synergistic interaction between alginate and pectin was systematically investigated using samples of different chemical compositions. Pectin samples with high and low degrees of esterification (DE) and amidated pectin (LA) were mixed with alginate of high and low M/G (mannuronic acid/guluronic acid) ratio. The microstructure of the gels was characterised by TEM (transmission electron microscopy) and the rheological properties by dynamic oscillatory measurements. The TEM images of the mixed gels revealed a coarse, strand-like network with pores in the range of microns, independent of the ratio and the composition of the samples. A comparison with the microstructure of a pure pectin gel showed that the pectin network was composed of thinner strands and smaller pore sizes than the mixed network. The strongest synergism was found between alginate with low M/G ratio and pectin with a high DE. These gels show the highest G ′ (storage modulus) and the fastest kinetics of gel formation. Lower G ′ and slower kinetics were found for gels based on alginate with a high M/G ratio and pectin with a low DE, or LA pectin. The nature of the pectin sample affected the network density and the strand characteristics. In contrast, no influence was found of the alginate sample. Gels based on pectin with a high DE showed a dense network composed of highly branched strands, whereas the LA-pectin based gels showed a sparse, open network, composed of long, straight strands. A relation close to 1:1 for low-G alginate and pectin with a high DE resulted in gels with the highest G ′. In contrast, for LA-pectin based gels, the highest G ′ was found for mixtures of alginate dominant ratios. For the overall network properties, the homogeneity in the microstructure decreased with alginate content, independent of the pectin sample.

Effects of shear on pure and mixed gels of gelatin and particulate whey protein

Abstract Microstructural and rheological effects of shear on pure whey protein (WPC) gels and on mixed gels of gelatin and WPC have been investigated at pH 5.4. The shear was performed just before the gel formation of the WPC, using shear rates of up to 300/s for times of up to 600 s. The microstructure was investigated by light microscopy (LM) and quantified by image analysis. The behaviour of the storage modulus (G′) upon shear was analysed according to fully factorial experimental designs, where the shear rate and time were used as design variables. Pure WPC suspensions, sheared at ∼2/s for ∼20 s, formed gels which showed an extremum in G′. In the vicinity of the extremum, the G′ showed a value twice that for a gel formed from an unsheared suspension. Image analysis on LM micrographs at different magnifications revealed that an inhomogeneous WPC network was formed from the suspensions sheared at ∼2/s for ∼20 s. Heavily sheared WPC suspensions (92/s for 240 s) formed gels which showed a weaker G′ than the gels formed from unsheared suspensions. The behaviour of G′ of mixed gels upon shear was similar to that of the pure WPC gels. The G′ for the mixed gels proved to be less sensitive to variations in the shear conditions than the pure WPC gels. During cooling after the gel point of both pure and mixed gels, the loss modulus (G″) showed a pronounced peak for samples sheared in the vicinity of the extremum. Mixed suspensions sheared in the vicinity of the extremum formed inhomogeneous WPC networks with large domains of gelatin. The mean pore size of the WPC network, estimated by image analysis, increased from 40 000 μm3, for the unsheared mixed sample, to 120 000 μm3 for the sheared mixed sample. Results from image analysis at different magnifications further confirmed that suspensions sheared in the vicinity of the extremum formed an inhomogeneous WPC network.

High-pressure treated mixed gels of gelatin and whey proteins

Abstract Microstructural and rheological properties of high-pressure treated mixed and pure gels of gelatin and whey protein concentrate (WPC) were studied at pH 7.5 and 5.4. The microstructure was studied using light microscopy and transmission electron microscopy, and the rheological properties using dynamic oscillatory measurements and tensile tests. The results showed that the high-pressure treatment induced a higher degree of aggregation for the pure WPC gels, compared with a conventional heat-treatment, showing a network that consisted of larger aggregates and pores, leading to a weaker gel strength. The pure gelatin networks were unaffected by the high-pressure treatment. Differences in rheological properties were, however, found between pressurized and unpressurized gelatin gels. At pH 5.4, the high-pressure treated mixed gels formed a phase-separated network with a gelatin continuous phase and a discontinuous WPC phase. The rheological properties of the mixed gels followed those of pure gelatin independently of the WPC concentration. At pH 7.5, the rheological properties of the high-pressure treated mixed gels indicated a higher degree of gelatin continuity compared with the heat-treated mixed gels. The microstructural studies showed a dense network in which neither the gelatin nor the WPC network could be identified.

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

Abstract 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.

Fine-stranded mixed gels of whey proteins and gelatin

Rheological properties of mixed and pure gels of gelatin and whey protein concentrate (WPC) have been investigated by means of tensile tests and dynamic oscillatory measurements. The microstructure of the system has been evaluated by transmission electron microscopy. The pH values chosen are within the range where the WPC forms a fine-stranded network structure, i.e. pH 7.5 and 3.0. When the ratio between the polymers was varied at pH 7.5, a shift in rheological properties was observed. The shift took place around 10% WPC addition to 3% gelatin. Below the shift, the mixed gels followed the behaviour of gelatin, and above, they followed the behaviour of WPC. Gel formation studies showed that the components gel individually, suggesting a phase-separation of the polymers. The gel formation of the WPC was independent of the presence of gelatin, while that of gelatin was shown to be dependent on the presence of WPC. At concentrations below the shift the mixed gels were remeltable and the system was interpreted as gelatin-continuous. At concentrations above the shift, the microstructure of the mixed gels suggested that a phase-separated, bicontinuous system was formed. The WPC network structure seemed to be unchanged in the presence of gelatin. No microstructural phase inversion took place. At pH 3.0 the gel formation of the WPC was strongly affected by the presence of gelatin, i.e. a stronger gel with an earlier gel point was formed. The microstructure of the system showed that an inhomogeneous, aggregated mixed gel, containing large pores, was formed.

Mixed gels of gelatin and whey proteins, formed by combining temperature and high pressure

Abstract Mixed and pure gels of gelatin and whey protein concentrate (WPC) were formed by using temperature and high pressure simultaneously. Combining these gel formation methods enables the two polymer networks to set at the same time. The microstructure of the gels was studied by means of light microscopy and transmission electron microscopy, and the rheological properties by means of dynamic oscillatory measurements and tensile tests. The pH values investigated were 5.4, 6.8 and 7.5. The isoelectric point of the WPC is around pH 5.2 and that of gelatin between pH 7.5 and 9. At pH 5.4, the mixed gel formed a phase-separated system, with a gelatin continuous network and spherical inclusions of the WPC. The storage modulus (G) of the mixed gel was similar to that of a pure gelatin gel. At pH6.8, the mixed gel formed a phase-separated system, composed of an aggregated network and a phase with fine strands. The aggregated network proved to be made up of both gelatin and WPC, and the fine strands were formed of gelatin. The mixed gel at pH 6.8 showed a high G compared with the pure gels, which decreased significantly when the gelatin phase melted. At pH 7.5 the mixed gel was composed of one single aggregated network, in which gelatin and WPC were homogeneously distributed. It was impossible to distinguish the gelatin from the WPC in the mixed network. The mixed gel at pH 7.5 showed a significantly higher G than the pure gels. As the gelatin phase was melted out for the mixed gel, a large decrease in G was observed. The pure gelatin gels, formed by a temperature decrease under high pressure, proved to be pH-dependent, showing an increase in aggregation as the pH increased from 5.4 to 7.5. A fine-stranded, transparent gelatin gel was formed at pH 5.4, while an aggregated, opaque gel was formed at pH 7.5. The stress at fracture for the gelatin gels decreased as the aggregation, and consequently the pore size, increased.

Flow Processing and Gel Formation - a Promising Combination for the Design of the Shape of Gelatin Drops

This investigation is a model study on how drops can be structured by a combination of flow processing and gel formation. Different drop shapes were created by subjecting gelatin drops to various flow conditions. At the same time, temperature induced gel formation of the drops fixed the shape. Elongated drops and drops of complex form were created. The flow to shape the gelatin drops was generated in a 4-Roll Mill (4RM) and silicon oil was used as the continuous phase. During processing in the 4RM, the drops were allowed to follow two different streamlines and thereby being subjected to purely elongational and a mixture of shear and elongational flow. The drop size varied between 1.5 and 2.8 mm. The gelatin drops were temperature conditioned before the experiment to 60 °C and the silicon oil to 5 °C. The drops were cooled via the cold oil phase during the flow process, and gel formation was induced. A gel strength strong enough to resist further deformation was achieved at different fixation zones in the 4RM, and this depended on the process parameters of flow type, flow rate, drop size and gelatin concentration. The shape created was directly related to the fixation zone. There was a broad freedom to combine different parameter values to fix a drop in a certain fixation zone. The mechanism behind the various drop shapes is explained in terms of elongation, relaxation, pinching and gel formation in relation to flow pattern and time in the 4RM. Elongation is a major contribution to the mechanism in the case of elongated shapes, while elongation followed by relaxation and pinching are the dominant determinants in the creation of complex shapes.

Microstructure in relation to flow processing

The art of structure formation by process flow control is a growing field of research. The central issue is the understanding that the rheological properties of a food substance are intimately linked to its microstructure. By carefully controlling the flow process environment, the microstructure, and thus the rheological properties, can be crafted to produce new products desired by the consumer market.

Shapes and shaping of biopolymer drops in a hyperbolic flow

Abstract The shaping of drops in a model system based on κ-carrageenan-emulsion drops in the millimetre range in silicon oil has been studied. The drops were shaped by exposing them to drag forces in a hyperbolic flow, while their shape was fixed simultaneously by introducing gel formation of the biopolymer in the drop. The shape and the shaping process were studied and evaluated with image analysis of macrograph sequences of the shaping. The effect of process conditions, flow speed and cooling temperature on the final shape and shape progress was investigated as well as the effect of different κ-carrageenan drop characteristics, such as drop viscosity and gel strength. Drop viscosity was altered by addition of locust bean gum, LBG, and the gel strength was altered by addition of ions. The κ-carrageenan solutions in the drop were characterised by rheological investigations. With the same type of flow, different shapes could be achieved with small process changes and with high reproducibility. The fixation of the characteristic drop features, perimeter, area, Ferets X and Y , does not occur at the same time and position. For the different process parameters investigated, a change in speed affected the process in a similar way to a change in the viscosity ratio. This applies if the viscosity ratio is changed at a constant temperature, but if the change in the viscosity ratio is temperature-induced, the effect is different. The final shape of the produced drops could be graded into three classes, correlated to the position in the flow field where the drops were fixed. A shape map of the different drop shapes obtained was presented.