| 1. |
- Walkenström, Pernilla, et al.
(författare)
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Effects of flow behaviour on the aggregation of whey protein suspensions, pure or mixed with xanthan
- 1999
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Ingår i: Journal of Food Engineering. - 0260-8774 .- 1873-5770. ; 42:1, s. 15-26
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Tidskriftsartikel (refereegranskat)abstract
- The effect of prior thermal treatment on radiation sensitization were investigated on proton irradiated Type 304 stainless steel (SS) of initially as-received (AR) and thermal-sensitized (SEN) conditions. The Cr depletion profiles were measured by field emission gun transmission electron microscopy/energy dispersive spectroscopy (FEGTEM, EDS), and were calculated by a radiation induced segregation (RIS) model. The different initial conditions were input in the RIS model calculations. For the as-received condition, the initial Cr profile was modeled by a uniform concentration distribution. Overall, the results show that radiation sensitization is characterized by a very narrow Cr depleted zone.
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| 2. |
- Walkenström, Pernilla, et al.
(författare)
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Effects of fluid shear and temperature on whey protein gels, pure or mixed with xanthan
- 1998
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Ingår i: Food Hydrocolloids. - 0268-005X .- 1873-7137. ; 12:4, s. 469-479
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Tidskriftsartikel (refereegranskat)abstract
- The effects of shear on whey protein isolate (WPI) gels, pure or mixed with xanthan, have been investigated at pH 5.4 by dynamic oscillatory measurements and light microscopy (LM). The shear was performed on the suspensions under a constant stress of between 0.04 and 2.1Pa. Various temperature conditions were chosen in order to describe the effects of shear at the different states of aggregation of the WPI. Shear-sensitive aggregation phenomena were already found around 40°C for the pure WPI samples. Continuous shearing during heating from 20 to 40°C, prior to heat treatment at 90°C, resulted in a gel with a storage modulus (G?) half that of the unsheared gel, independent of the shear stress. Continuous shearing during heating from 20 to 76°C resulted in a further decrease in G?. Inhomogeneities arose in networks formed from continuously sheared suspensions during heating from 20 to 50°C and above. Depending on the shear stress and on the heating range of the shear, the networks showed areas of varied compactness and different classes of pores, ranging from 10 to 200?m. A higher G?, compared to that for the unsheared gel, was found for gels subjected to shear for short periods in the vicinity of the gel point. The presence of xanthan inhibited the aggregation and demixing of the WPI, described as a sterical phenomenon. Under static conditions, the presence of xanthan resulted in a more homogeneous WPI network. Exposing the mixed suspensions to shear generally increased the inhomogeneity of the network structure. Short periods of shearing in the vicinity of the gel point affected the kinetics of the gel formation and resulted in gels with higher G? values than the unsheared gel. Continuous shearing under stresses below 0.09Pa, during heating from 20 to 60°C and above, also resulted in gels with an increased G?. Continuous shear under stresses above 0.9Pa resulted in gels with a decreased G?.
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| 3. |
- Walkenström, Pernilla, et al.
(författare)
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Effects of shear on pure and mixed gels of gelatin and particulate whey protein
- 1998
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Ingår i: Food Hydrocolloids. - 0268-005X .- 1873-7137. ; 12:1, s. 77-87
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Tidskriftsartikel (refereegranskat)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 ?m3, for the unsheared mixed sample, to 120 000 ?m3 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. © 1998 Elsevier Science Ltd. All rights reserved.
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| 4. |
- Walkenström, Pernilla, et al.
(författare)
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Fine-stranded mixed gels of whey proteins and gelatin
- 1996
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Ingår i: Food Hydrocolloids. - 0268-005X .- 1873-7137. ; 10:1, s. 51-62
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Tidskriftsartikel (refereegranskat)abstract
- 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. © Oxford University Press.
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| 5. |
- Walkenström, Pernilla, et al.
(författare)
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High-pressure treated mixed gels of gelatin and whey proteins
- 1997
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Ingår i: Food Hydrocolloids. - 0268-005X .- 1873-7137. ; 11:2, s. 195-208
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Tidskriftsartikel (refereegranskat)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. © Oxford University Press.
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| 6. |
- Walkenström, Pernilla, et al.
(författare)
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Mixed gels of gelatin and whey proteins, formed by combining temperature and high pressure
- 1997
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Ingår i: Food Hydrocolloids. - 0268-005X .- 1873-7137. ; 11:4, s. 457-470
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Tidskriftsartikel (refereegranskat)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 pH 6.8, 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 decreasein 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. © Oxford University Press.
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| 7. |
- Walkenström, Pernilla, 1969
(författare)
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Phase Distribution of Mixed Biopolymer Gels in Relation to Process Conditions
- 1996
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The influence of various process parameters on the microstructure and rheological properties of pure and mixed gels of gelatin and whey proteins has been investigated. Gelatin is a cold-set protein and whey proteins are a group of thermo-set globular proteins. Depending on the charge present, whey proteins can form two types of gel structures. Particulate networks, with network strand dimensions in ~.my.m, are formed at pH-values within the isoelectric region (roughly between pH 4 and 6) and fine-stranded networks, with dimensions in nm, are formed above and below the isoelectric region. The microstructure has been characterised by light- and electron microscopy and further quantified by image analysis. The rheological properties have been characterised by viscoelastic measurements and tensile tests. Phase-separated, bicontinuous mixed gels are formed at pH-values within the isoelectric region of the whey proteins, when conventional heating and cooling are used as the gelling technique. The bicontinuous morphology is unaffected by changes in the polymer mixing ratio, while the rheological properties shift, i.e. at a low whey protein concentration, the mixed gels follow the properties of gelatin and vice versa. The bicontinuous morphology is also unaffected by shear in the initial stages of gel formation of the whey proteins, but the homogeneity is affected. A mixed gel with a broader pore size distribution in the whey protein network, and thereby larger domains of the gelatin phase, is created by the shear. When image analysis is used to quantify both pure and mixed networks, it is evident that the shear induces inhomogeneities. In relation to the rheological properties, suspensions sheared under controlled conditions form gels with storage moduli twice as high as those for unsheared gels. Using a combination of temperature and high-pressure processing as the gelling technique, the order of gel formation between the pure polymers is changed compared to that followed when using temperature only. In accordance with the changes in the order of gel formation, the bicontinuous morphology is shifted to a gelatin continuous morphology. Phase-separated bicontinuous mixed gels are formed at pH-values above the isoelectric region, when conventional heating and cooling are used as the gelling technique. As the mixing ratio is varied, a shift in rheological properties takes place, while no corresponding changes are found in the microstructure. When shear is used as a processing parameter for the mixed gels composed of fine-stranded whey proteins, no significant effects are found either in the microstructure or in the rheological properties. When the order of gel formation between the pure polymers is changed, using a combination of high-pressure processing and temperature for the gel formation, the bicontinuous morphology is shifted to a complex coacervate structure, composed of one single network structure. The complex coacervate shows a significantly higher storage modulus than the bicontinuous mixed network.
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| 8. |
- Walkenström, Pernilla, et al.
(författare)
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Shear-induced structuring of particulate whey protein gels
- 1998
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Ingår i: Food Hydrocolloids. - 0268-005X .- 1873-7137. ; 12:4, s. 459-468
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Tidskriftsartikel (refereegranskat)abstract
- The effects of steady shear on particulate whey protein isolate (WPI) gels, at pH 5.4, have been investigated by light microscopy (LM) and dynamic oscillatory measurements. The steady shear was performed on suspensions at constant rates between 0.5 and 126/s. The gel point under static conditions (Tg) was around 78°C and the shearing was performed during heating from 20 to 76 or to 82°C. The gel point was postponed by the shear up to 82°C. Steady shear up to 76°C, at rates less than ~6/s, resulted in a weaker storage modulus (G?), less frequency dependence and a higher stress at fracture compared to the unsheared gel. Steady shear up to 82°C, at rates below ~6/s, resulted in the formation of two different types of network structure. One structure was similar in appearance to the unsheared network, showing pores in the range of 50?m. The other structure was dense, composed of smaller particles than the unsheared network and with pores in the range of 10?m. The gels composed of two structures showed a lower G? and stress at fracture compared to the unsheared gel. A shear rate above 24/s up to 76°C resulted in irregular networks, which were composed of two different types of structures. One was loose and open, similar in appearance to the unsheared network structure. The other structure was dense and compact, and was present as individual aggregates. These gels also showed a weaker G? than the unsheared gel. A shearing up to 82°C at rates above 24/s resulted in a coarse, inhomogeneous network structure. The gels showed a weak G?, indicating aggregate break-up during the steady shear.
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