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Rheological characterisation of ‘weak gel’ carrageenan stabilised milks
Food Hydrocolloids 14 (2000) 445–454 www.elsevier.com/locate/foodhyd
Rheological characterisation of ‘weak gel’ carrageenan stabilised milks A.B. Rodd a,*, C.R. Davis b, D.E. Dunstan a, B.A. Forrest b, D.V. Boger a a
CRC for Bioproducts, Department of Chemical Engineering, The University of Melbourne, Parkville, Vic. 3052, Australia b Goodman Fielder Ingredients, Private Bag 12, Botany NSW 2019, Australia Recieved 1 March 1999; accepted 20 April 2000
Abstract The use of a controlled stress rheometer to characterise the rheology of weak gel behaviour of milk containing carrageenan is described. The interaction of carrageenan with the protein in milk produces a long-range network structure where the physical properties are dependent on the type of carrageenan used. With the use of a sensitive controlled stress rheometer, it was possible to characterise the rheological properties of these weak gels and differentiate between samples using both large deformation techniques, and small amplitude oscillatory measurements. Large deformation tests enabled the gels to be qualitatively compared based on an apparent yield stress, ‘structure point’, apparent shear viscosity and degree of hysteresis. Small amplitude oscillatory rheometry was applied to make comparisons from measurements that did not exceed the linear viscoelasticity of the system. Both the frequency and time dependence of the rheological properties were measured to characterise final gel properties and the kinetics of network development. Both large deformation rheology and small amplitude oscillatory rheology have been effective in providing quantitative and qualitative comparisons between the rheological properties of samples containing weak gel networks. 䉷 2000 Published by Elsevier Science Ltd. Keywords: Rheology; Carrageenan stabilised milk; Weak gel behaviour
1. Introduction The use of rheological techniques for the characterisation of food products has increased in its application in food and related industries in recent years. Rheological techniques have been applied to characterise the interaction of food stabilising agents, such as carrageenans, with proteins that impart a ‘weak-gel’ structure. Rheometers capable of measuring stresses as low as 1.8 × 10 ⫺3 Pa, are utilised to characterise foods, however, it is important that the experimenter is aware of the characteristic timescale of the materials being tested. Characterisation of the interaction of carrageenan with milk proteins is a good example of such a system, which is close to the experimental limit of the rheological instrument sensitivity. Such evaluation requires precise measurement and a solid understanding of the limitations of the techniques being employed. Carrageenans are linear polysaccharides of alternating b1,3- and a-1,4-linked galactose residues. A variation of the basic structure results from substitutions on the hydroxyl groups of the sugar residues and from the absence of the 3,6-ether linkage. Substituents are chiefly anionic through the presence of sulphate groups, however, pyruvate and methoxyl groups may be present to a lesser extent (Painter * Corresponding author. 0268-005X/00/$ - see front matter 䉷 2000 Published by Elsevier Science Ltd. PII: S0268-005 X( 00)00 024-2
& Stanley, 1990). There are numerous reviews of their chemistry and applications in foods (Stanley, 1990). Many milk proteins are stabilised by the interaction with carrageenans. Snoeren, Payens, Jeunink and Both (1975), using milk protein fractions have proposed a specific electrostatic interaction of kappa carrageenan with kappa caesin at the normal pH of milk to explain the stabilisation. The chemistry of the interaction of carrageenans with micellar proteins in milk appears to be much more complex. Recent work has highlighted the role of carrageenan–carrageenan interactions (Drohan, Tziboula, McNutty & Parker, 1997) and phase separation (Bourrit, Garnie & Doublier, 1999; Langendorff, Cuvelier, Launay & Parker, 1997) in the rheology and stability of carrageenan–milk protein systems. Rheological characterisation is commonly conducted using shear stress versus shear rate ramps, which are used for characterising the shear thinning nature of food stabiliser systems (Da Silva & Rao, 1992). Shear viscosity–shear stress relationships are often applied to samples to characterise them in terms of their yield stress behaviour and solution viscosity under large deformation conditions. Schenz (1997) and Sherman (2000) have postulated the presence of a structure point in weak gel systems, defining it as the stress at which the destruction of a tertiary structure occurs. The concept of a structure point was first introduced by Vliet, Van and Van Hooydonk (1984) in terms of an inflection
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Fig. 1. Viscosity versus shear rate for Caragem 4435 thickened milk product conducted over 30 min at 5⬚C. Varying equilibration times. (W) 1, (A) 10, (K) 15 min.
observed in the shear rate versus shear stress curve for a milk sample. Boomgaard, Van Den, Van Vliet and Van Hooydonk (1987) applied the same structural quantity to determine the minimum concentration of carrageenan required for stabilisation of cocoa particles in milk. The structure point has been measured in terms of a critical stress, which is considered the quantity of primary importance in providing stabilisation for particulate matter. The structure point appears as a break in the continuity of an apparent viscosity versus shear stress curve. Although controlled stress rheometers are of particular use in characterising the physical properties of complex fluids, the limitations of the results obtained must be recognised. When the characteristic time associated with the sample is of a similar order of magnitude to the timescale of the testing, the results from shear rheology experiments must be interpreted with care. This study reports data from preliminary investigations into the ‘weak gel’ in milk stabilised with carrageenans. Due to equilibrium effects in terms of structure development in the carrageenan–milk systems, both steady shear and oscillatory rheometry have been used. The shear viscosities of the samples have been compared in terms of equilibrium structure buildup and comparison of structure points. The characteristic time of the material in terms of equilibrium structure development has been contrasted to the time duration of the experiment. In addition, the linear viscoelastic limit of the various systems has been determined and experi-
ments have been carried out to observe the building of structure with time in a system perturbed within its linear viscoelastic limit.
2. Experimental 2.1. Carrageenan/milk solutions Goodman Fielder Ingredients provided the dry carrageenan powders. Each carrageenan was first combined on a dry blend basis with dextrose monohydrate to 10% (w/w). This served to allow easy dispersion of the carrageenan into the milk. The carrageenans were all commercially available as follows: Caragem 4217, kappa; Caragem 4230, kappa; and Caragem 4435, lambda. Fresh pasteurised skimmed milk and whole milk mixtures were made to a ratio of 1:1 (w/w). The carrageenan/dextrose blend (1.2 g) was combined with 198.8 g of the milk to achieve a concentration of 600 ppm carrageenan. Each carrageenan/dextrose blend was dispersed by stirring in a 400 ml tall form beaker, using a IKA Euro-St DS 2 stirrer at 500 rpm for 30 s. Each solution was placed in a water bath at 87⬚C where it was stirred at 300 rpm until its temperature reached 82⬚C. Upon reaching the required temperature, each solution was removed from the water bath and placed in a 4⬚C water bath where it was stirred until reaching 6⬚C. All solutions were stored at 4⬚C and tested within 60 h of solution preparation.
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Fig. 2. Viscosity versus shear rate for Caragem 4217 thickened milk product conducted over 30 min at 5⬚C. Varying equilibration times. (W) 1, (A) 10, (K) 15 min.
Fig. 3. Viscosity versus shear rate for Caragem 4230 thickened milk conducted over 30 min at 5⬚C. Varying equilibration times. (W) 1, (A) 10, (K) 15 min.
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Fig. 4. Ideal viscosity versus shear rate curves for carrageenan casein in milk system.
2.2. Rheology The experiments reported were performed on a Carrimed CSL2100 controlled stress rheometer. A cone and plate geometry was used to ensure a constant shear rate in the sample. A 6 cm cone with an angle of 1 0 59 00 and a truncation of 52 mm was used in all the experiments. For a 2 0 cone angle, the variation in shear rate across the gap is calculated to be 0.21% (Drohan et al., 1997). The stress range of the Carrimed rheometer with this measurement system is 1.8 × 10 ⫺3–40 Pa. The measurement setup described above is for measurements of thin liquids. Linear regions of viscoelastic behaviour were determined prior to the measurement of mechanical spectra.
3. Results and discussion Initial characterisation of the three samples, Caragem 4435, 4217 and 4230 involved conducting shear stress ramps for each of the solutions. To compare these results, it was required that each of their shear histories was as similar as possible before testing commenced. For this purpose, each sample was first allowed to equilibrate to 4⬚C before shearing at an applied stress of 6 Pa for a period of 3 min. The preshear ensured reproducibility of results as a consistent initial solution state was obtained. Complications associated with shear induced structure or aggregation were not observed. Each of the samples was allowed to equilibrate after
preshear for 1, 10 and 15 min. The samples were then sheared over 30 min during which the stress was raised logarithmically between approximately 0.1 and 20 Pa and then returned to 0.1 Pa. Figs. 1–3 illustrate the flow curves for the three samples after each equilibration time. All flow curves showed similar behaviour from which comparisons could be made. Fig. 4 shows an ideal flow curve for carrageenan in milk with each of the flow characteristics that were used to compare the samples described. The initial low stress behaviour shows shear viscosity approaching infinity which indicates the presence of an apparent yield stress. The stress at which this yielding was observed was a function of both the type of carrageenan used and the time in which the solution was allowed to equilibrate. The sample then showed shear thinning behaviour characteristic of most biopolymer systems before reaching a stress where the viscosity shear rate relationship plateaus (as seen in Fig. 2 at a stress of approximately 0.7 Pa), which in comparison with recent literature is argued to indicate a ‘structure point’ (Schenz, 1997; Sherman, 2000; Vliet et al. 1984). The ‘structure point’ has been reported in previous literature (Schenz, 1997) as a discontinuity in the shear thinning behaviour of chocolate milk. The effect seen here depicts a more obvious plateau than previous reports, which may be explained in differences of sample preparation and the higher concentrations of biopolymers being investigated. The ‘structure point’ is reported to represent a long range network that has extended before breaking following which normal shear thinning behaviour is observed. Upon ramping the
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Fig. 5. Viscosity versus shear stress after 15 min equilibrium showing relative viscosity and stress at the initial ‘structure point’ (W) Caragem 4230, (A) Caragem 4217, (K) Caragem 4435.
stress back down, a large degree of hysteresis was observed which is consistent with unrecovered loss of structure in the sample. The partial rebuilding of the sample structure as seen in Figs. 1–3 is referred to as the ‘second structural point’. The second structural point was observed at a lower stress than the initial structural point, an observation consistent with the complex beginning to reassemble at a stress low enough such that the rate of destruction was less than the rate of structural recovery. It appears that yield type behaviour was occurring after the second structural point. The above-described properties were used to characterise the flow curves of the samples investigated. The development of yield stress behaviour as a sample equilibrates after the application of preshear, may be analysed to make comparisons of the time dependence of structure development between samples. Fig. 1 suggests that Caragem 4435 has limited equilibrium processes with little change in initial yield stress behaviour with equilibrium time. Caragem 4230, as illustrated in Fig. 2 has a significant yield stress behaviour development that is complete after 10 min. Fig. 3 illustrates that Caragem 4217 has a significant yield stress after 1 min equilibration that is enhanced after 10 min equilibration. It should be noted that equilibrium processes are therefore of a timescale of the order of 10 min, which is similar to the timescale of the stress ramps conducted (15 min). The structure point in the results observed in this work is defined as the mid point of the plateau region as is illustrated in Fig. 4. The ‘initial structure’ point for each carrageenan is
observed at a similar stress for Caragem 4435 and Caragem 4217 (0.7 and 0.8 Pa, respectively). By contrast, this structure point was markedly higher for Caragem 4230 (2.0 Pa). Although the structure point occurs at stresses differing by less than an order of magnitude for all samples compared, the viscosity at the structure point for each sample varies considerably. A difference in viscosity at the initial structure point is indicative of the network density and strength. Fig. 5 shows the flow curves for 15 min of equilibrium for each sample analysed. The structure point for samples Caragem 4435, Caragem 4217, Caragem 4230 occur at viscosities 0.04, 0.6 and 2 Pa s, respectively. The difference in viscosity between samples suggests that Caragem 4230 with a slightly higher structure point stress and a considerably higher viscosity has a denser and generally stronger network than Caragem 4217 and Caragem 4435. The return flow curves show a degree of hysteresis for all samples, although there was considerably less hysteresis for Caragem 4435. The limited hysteresis observed suggests that the protein interaction is weak with the lambda carrageenan used in Caragem 4435 and only a small degree of interaction occurs resulting in a relatively sparse network. The structure point on the return curve is most noticeable in Caragem 4230, a kappa type carrageenan, indicating that it has the most resilient network. To study fluids that display a high degree of hysteresis, suggesting significant structural processes, both in terms of development and destruction, it is beneficial to use an experimental method that allows each point to reach a
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Fig. 6. Creep-recovery curves at varying stresses for Caragem 4230 after 3 min preshear at 6 Pa and 1 min equilibration. Constant stress applied for a period of 300 s following which recovery under no applied stress was measured for 300 s. Temperature maintained at 5⬚C. Fill Line 0.3, dashed Line 0.6, dotted Line 0.9 Pa.
true equilibrium. On a controlled stress rheometer, creeprecovery tests involve the application of a constant stress and measurement of the strain response for a period of time. The applied stress is then removed and strain recovery is
measured. The creep recovery curves at varying applied stresses for Caragem 4230 after equilibration for 1 min are shown in Fig. 6. At an applied stress of 0.3 and 0.6 Pa significant recovery was observed suggesting that these
Fig. 7. Torque sweep for determination of linear viscoelastic region for Caragem 4217 measured at 5⬚C. Limit of linearity of strain with applied stress represents linear viscoelastic limit. Torque sweep conducted at 1 Hz.
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Fig. 8. Frequency sweep for Caragem 4230 at 5⬚C over a period of 20 min (X) G 0 (Pa), (B) G 00 (Pa), (O) h ⴱ (Pa s).
Fig. 9. Frequency sweep for Caragem 4217 at 5⬚C over a period of 20 min (X) G 0 (Pa), (B) G 00 (Pa), (O) h ⴱ (Pa s).
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Fig. 10. Frequency sweep for Caragem 4435 at 5⬚C over a period of 20 min (X) G 0 (Pa), (B) G 00 (Pa), (O) h ⴱ (Pa s).
values were below the yield stress of the material. At 0.9 Pa there is no recovery indicating that the sample had yielded and therefore no elastic component of the structure was recovered. The yield stress, as determined by the flow experiments, mentioned earlier, has approximately 1 Pa after 1 min equilibration. The yield stress as determined from creep recovery tests was therefore lower than that indicated by the flow experiments (Fig. 3). The difference in yield stress obtained from flow and creep-recovery experiments may be explained by considering the timescale of the test relative to the timescale of equilibrium processes. The yield stress obtained from flow experiments may have been higher than a true equilibrium yield stress of the material since the equilibrium processes are such that molecular rearrangements may occur in the time the stress ramp changed from 0.6 to 1.0 Pa. If the flow experiment was conducted over a much longer timescale, allowing more molecular rearrangement as the stress ramped from 0.6 to 1.0 Pa, a lower yield stress would be observed. As the timescale of the test approaches large times and the time spent at any one particular stress increases, the stress ramp theoretically approaches a test of identical nature to the true equilibrium test of creep-recovery. The work of Chang, Borger and Nguyen (1998) suggests that the yield stress obtained from flow experiments may be higher than those obtained from creep-recovery tests due to the time-frame of relaxation within the solutions relative to the time-frame of the flow experiment itself, a concept which supports the statement above and the observed results.
The linear viscoelastic limit has been determined by conducting a torque sweep and determining the largest amplitude that gives a linear relationship between displacement and torque. A significant drop in the storage modulus was observed at higher strains than this limit. Fig. 7 illustrates the determination of the linear viscoelastic region for Caragem 4217 at 5⬚C, this procedure was also carried out for the other carrageenan samples. The in phase and out of phase response to that oscillatory perturbation give rise to G 0 (storage modulus) and G 00 (loss modulus), respectively. The storage modulus represents a measure of the elastic response of the liquid whilst the loss modulus is a measure of the viscous response. The frequency sweeps obtained for Carragem 4230, Caragem 4217 and Caragem 4435 are shown in Figs. 8–10, respectively. The frequency dependence of the moduli were analysed quantitatively by fitting simple power law relationships: G 0 / vp
1
G 00 / vq
2
where v is the frequency of oscillation and p and q are the storage and loss moduli power law indices, respectively. The frequency sweeps showed that the frequency dependence of G 0 decreased through samples Caragem 4435, Caragem 4217 and Caragem 4230, as indicated by a decreasing value of the power law index p for the samples (0.30, 0.19 and 0.09, respectively).
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Fig. 11. Normalised storage modulus versus time measured at 1 Hz and 5⬚C after preshear of 50 s ⫺1 for 10 min.
In addition, the absolute difference between G 0 and G 00 and the frequency range for which G 0 is greater than G 00 increased in the same order. Gels typically show the following basic features: G 0 is greater than G 00 over a large range of frequencies and G 0 is relatively independent of frequency (Barnes, Hurtton & Walters, 1989). Inspection of the data suggested gel strengths in decreasing order of Caragem 4230, Caragem 4217 and Caragem 4435, which was in agreement with the results from the flow experiments. The absolute value of G 0 may also be used to compare the relative strengths of the three systems with G 0 decreasing in the order mentioned above. Fig. 11 illustrates the build up of normalised G 0 with time for the three samples after a preshear of 6 Pa for a period of 3 min. Caragem 4230 was the only sample with a significant equilibrium effect which was indicated by a long, time dependence in the development of G 0 from a value of 75% of its final value. The time dependence for equilibration of Caragem 4435 and Caragem 4217 was considerably shorter. The flow and oscillatory results show that Caragem 4230 had the strongest gel network, followed by Caragem 4217 and Caragem 4435.
showed characteristic shapes which could be interpreted in terms of the strength of ‘weak gel’ networks. The high degree of hysteresis observed suggested that care should be taken when drawing quantitative conclusions from such time dependent tests. The oscillatory measurements made were seen to be advantageous as they limited perturbations in the system to be within the characteristic linear viscoelastic limit. Further work to be conducted in this area could investigate the effect of using other rheological geometries (i.e. couette cells), the effect of polymer concentration and the type of hydrocolloid applied and further characterisation of the yield stress of the systems.
Acknowledgements The assistance of the Commonwealth Australian Postgraduate Award Scholarship, A.P.A., and the C.R.C. for Industrial Plant Biopolymers, is gratefully acknowledged by Andrew Rodd. Part of this work is supported by Goodman Fielder Ingredients and we would like to extend our gratitude for permission to publish this work.
References 4. Conclusions The use of sensitive, controlled stress rheometers to study the rheological characteristics of weak gel systems is a powerful technique, providing the limitations of the measurement system is recognised. Milk containing carrageenan has been characterised using simple flow, creeprecovery and oscillatory rheology. The measurements show Caragem 4230 to have the strongest gel properties combined with the most rapid equilibration kinetics. The flow measurements, generated reproducible curves that
Barnes, H. A., Hurtton, J. F., & Walters, K. (1989). An introduction to rheology, Amsterdam: Elsevier. Boomgaard, T., Van Den, T., Van Vlet, T., & Van Hooydonk, A. C. M. . (1987). Physcial stability of chocolate milk. International Journal of Food Science and Technology, 22, 279–291. Bourriot, S., Garnie, C., & Doublier, J. -L. (1999). Micellar-casein-K-carrageenan mixtures. I. Phase separation and ultrastructure. Carbohydrate Polymers, 40, 145–157. Chang, C., Boger, D. V., & Nguyen, Q. D. (1998). The yielding of waxy crude oils. Industrial and Engineering Chemistry Research, 37 (4), 1551–1559. Da Silva, J. A. L., & Rao, M. A. (1992). Viscoelastic properties of food
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hydroclloid dispersions. In M. A. Rao & J. F. Steffe, Viscoelastic properties of foods (pp. 285–315). . Drohan, D. D., Tziboula, A., McNutty, D., & Horne, D. S. (1997). Milk protein–carrageenan interactions. Food Hydrocolloids, 11 (1), 101–107. Langendorff, V., Cuvelier, B., Launay, B., & Parker, A. (1997). Gelation and flocculation of casein micelle/carrageenan mixtures. Food Hydroclloids, 11 (1), 35–40. Painter, T.J. Algal polysaccharides. In Aspinall, G.O. (Ed.), The polysaccharides, Vol. 2, 195–285. Schenz, T. W. (1997). Using rheology of weak gels to improve fluid foods. Food Technology, 51 (3), 83.
Sherman, P. In J.M.V. Blanchard, Mitchell, J.R. (Eds.), Food structure its creation and structure. Butterworths, London, pp. 417–432. Snoeren, T. H. M., Payens, T. A. J., Jeunink, J., & Both, P. (1975). Electrostatic interaction between K-Carrageenan and K-Casein. Milch wissenschaft, 30 (7), 393–396. Stanley, N. F. (1990). Carrageenans. In P. Harris, Food gels (pp. 79–119). . Vliet, T., Van, T., & Van Hooydonk, A. C. M. (1984). The gel properties of some beverages. . Proceedings of the 9th International Congress on Rheology, 4, 115.
Rheological characterisation of ‘weak gel’ carrageenan stabilised milks A.B. Rodd a,*, C.R. Davis b, D.E. Dunstan a, B.A. Forrest b, D.V. Boger a a
CRC for Bioproducts, Department of Chemical Engineering, The University of Melbourne, Parkville, Vic. 3052, Australia b Goodman Fielder Ingredients, Private Bag 12, Botany NSW 2019, Australia Recieved 1 March 1999; accepted 20 April 2000
Abstract The use of a controlled stress rheometer to characterise the rheology of weak gel behaviour of milk containing carrageenan is described. The interaction of carrageenan with the protein in milk produces a long-range network structure where the physical properties are dependent on the type of carrageenan used. With the use of a sensitive controlled stress rheometer, it was possible to characterise the rheological properties of these weak gels and differentiate between samples using both large deformation techniques, and small amplitude oscillatory measurements. Large deformation tests enabled the gels to be qualitatively compared based on an apparent yield stress, ‘structure point’, apparent shear viscosity and degree of hysteresis. Small amplitude oscillatory rheometry was applied to make comparisons from measurements that did not exceed the linear viscoelasticity of the system. Both the frequency and time dependence of the rheological properties were measured to characterise final gel properties and the kinetics of network development. Both large deformation rheology and small amplitude oscillatory rheology have been effective in providing quantitative and qualitative comparisons between the rheological properties of samples containing weak gel networks. 䉷 2000 Published by Elsevier Science Ltd. Keywords: Rheology; Carrageenan stabilised milk; Weak gel behaviour
1. Introduction The use of rheological techniques for the characterisation of food products has increased in its application in food and related industries in recent years. Rheological techniques have been applied to characterise the interaction of food stabilising agents, such as carrageenans, with proteins that impart a ‘weak-gel’ structure. Rheometers capable of measuring stresses as low as 1.8 × 10 ⫺3 Pa, are utilised to characterise foods, however, it is important that the experimenter is aware of the characteristic timescale of the materials being tested. Characterisation of the interaction of carrageenan with milk proteins is a good example of such a system, which is close to the experimental limit of the rheological instrument sensitivity. Such evaluation requires precise measurement and a solid understanding of the limitations of the techniques being employed. Carrageenans are linear polysaccharides of alternating b1,3- and a-1,4-linked galactose residues. A variation of the basic structure results from substitutions on the hydroxyl groups of the sugar residues and from the absence of the 3,6-ether linkage. Substituents are chiefly anionic through the presence of sulphate groups, however, pyruvate and methoxyl groups may be present to a lesser extent (Painter * Corresponding author. 0268-005X/00/$ - see front matter 䉷 2000 Published by Elsevier Science Ltd. PII: S0268-005 X( 00)00 024-2
& Stanley, 1990). There are numerous reviews of their chemistry and applications in foods (Stanley, 1990). Many milk proteins are stabilised by the interaction with carrageenans. Snoeren, Payens, Jeunink and Both (1975), using milk protein fractions have proposed a specific electrostatic interaction of kappa carrageenan with kappa caesin at the normal pH of milk to explain the stabilisation. The chemistry of the interaction of carrageenans with micellar proteins in milk appears to be much more complex. Recent work has highlighted the role of carrageenan–carrageenan interactions (Drohan, Tziboula, McNutty & Parker, 1997) and phase separation (Bourrit, Garnie & Doublier, 1999; Langendorff, Cuvelier, Launay & Parker, 1997) in the rheology and stability of carrageenan–milk protein systems. Rheological characterisation is commonly conducted using shear stress versus shear rate ramps, which are used for characterising the shear thinning nature of food stabiliser systems (Da Silva & Rao, 1992). Shear viscosity–shear stress relationships are often applied to samples to characterise them in terms of their yield stress behaviour and solution viscosity under large deformation conditions. Schenz (1997) and Sherman (2000) have postulated the presence of a structure point in weak gel systems, defining it as the stress at which the destruction of a tertiary structure occurs. The concept of a structure point was first introduced by Vliet, Van and Van Hooydonk (1984) in terms of an inflection
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Fig. 1. Viscosity versus shear rate for Caragem 4435 thickened milk product conducted over 30 min at 5⬚C. Varying equilibration times. (W) 1, (A) 10, (K) 15 min.
observed in the shear rate versus shear stress curve for a milk sample. Boomgaard, Van Den, Van Vliet and Van Hooydonk (1987) applied the same structural quantity to determine the minimum concentration of carrageenan required for stabilisation of cocoa particles in milk. The structure point has been measured in terms of a critical stress, which is considered the quantity of primary importance in providing stabilisation for particulate matter. The structure point appears as a break in the continuity of an apparent viscosity versus shear stress curve. Although controlled stress rheometers are of particular use in characterising the physical properties of complex fluids, the limitations of the results obtained must be recognised. When the characteristic time associated with the sample is of a similar order of magnitude to the timescale of the testing, the results from shear rheology experiments must be interpreted with care. This study reports data from preliminary investigations into the ‘weak gel’ in milk stabilised with carrageenans. Due to equilibrium effects in terms of structure development in the carrageenan–milk systems, both steady shear and oscillatory rheometry have been used. The shear viscosities of the samples have been compared in terms of equilibrium structure buildup and comparison of structure points. The characteristic time of the material in terms of equilibrium structure development has been contrasted to the time duration of the experiment. In addition, the linear viscoelastic limit of the various systems has been determined and experi-
ments have been carried out to observe the building of structure with time in a system perturbed within its linear viscoelastic limit.
2. Experimental 2.1. Carrageenan/milk solutions Goodman Fielder Ingredients provided the dry carrageenan powders. Each carrageenan was first combined on a dry blend basis with dextrose monohydrate to 10% (w/w). This served to allow easy dispersion of the carrageenan into the milk. The carrageenans were all commercially available as follows: Caragem 4217, kappa; Caragem 4230, kappa; and Caragem 4435, lambda. Fresh pasteurised skimmed milk and whole milk mixtures were made to a ratio of 1:1 (w/w). The carrageenan/dextrose blend (1.2 g) was combined with 198.8 g of the milk to achieve a concentration of 600 ppm carrageenan. Each carrageenan/dextrose blend was dispersed by stirring in a 400 ml tall form beaker, using a IKA Euro-St DS 2 stirrer at 500 rpm for 30 s. Each solution was placed in a water bath at 87⬚C where it was stirred at 300 rpm until its temperature reached 82⬚C. Upon reaching the required temperature, each solution was removed from the water bath and placed in a 4⬚C water bath where it was stirred until reaching 6⬚C. All solutions were stored at 4⬚C and tested within 60 h of solution preparation.
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Fig. 2. Viscosity versus shear rate for Caragem 4217 thickened milk product conducted over 30 min at 5⬚C. Varying equilibration times. (W) 1, (A) 10, (K) 15 min.
Fig. 3. Viscosity versus shear rate for Caragem 4230 thickened milk conducted over 30 min at 5⬚C. Varying equilibration times. (W) 1, (A) 10, (K) 15 min.
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Fig. 4. Ideal viscosity versus shear rate curves for carrageenan casein in milk system.
2.2. Rheology The experiments reported were performed on a Carrimed CSL2100 controlled stress rheometer. A cone and plate geometry was used to ensure a constant shear rate in the sample. A 6 cm cone with an angle of 1 0 59 00 and a truncation of 52 mm was used in all the experiments. For a 2 0 cone angle, the variation in shear rate across the gap is calculated to be 0.21% (Drohan et al., 1997). The stress range of the Carrimed rheometer with this measurement system is 1.8 × 10 ⫺3–40 Pa. The measurement setup described above is for measurements of thin liquids. Linear regions of viscoelastic behaviour were determined prior to the measurement of mechanical spectra.
3. Results and discussion Initial characterisation of the three samples, Caragem 4435, 4217 and 4230 involved conducting shear stress ramps for each of the solutions. To compare these results, it was required that each of their shear histories was as similar as possible before testing commenced. For this purpose, each sample was first allowed to equilibrate to 4⬚C before shearing at an applied stress of 6 Pa for a period of 3 min. The preshear ensured reproducibility of results as a consistent initial solution state was obtained. Complications associated with shear induced structure or aggregation were not observed. Each of the samples was allowed to equilibrate after
preshear for 1, 10 and 15 min. The samples were then sheared over 30 min during which the stress was raised logarithmically between approximately 0.1 and 20 Pa and then returned to 0.1 Pa. Figs. 1–3 illustrate the flow curves for the three samples after each equilibration time. All flow curves showed similar behaviour from which comparisons could be made. Fig. 4 shows an ideal flow curve for carrageenan in milk with each of the flow characteristics that were used to compare the samples described. The initial low stress behaviour shows shear viscosity approaching infinity which indicates the presence of an apparent yield stress. The stress at which this yielding was observed was a function of both the type of carrageenan used and the time in which the solution was allowed to equilibrate. The sample then showed shear thinning behaviour characteristic of most biopolymer systems before reaching a stress where the viscosity shear rate relationship plateaus (as seen in Fig. 2 at a stress of approximately 0.7 Pa), which in comparison with recent literature is argued to indicate a ‘structure point’ (Schenz, 1997; Sherman, 2000; Vliet et al. 1984). The ‘structure point’ has been reported in previous literature (Schenz, 1997) as a discontinuity in the shear thinning behaviour of chocolate milk. The effect seen here depicts a more obvious plateau than previous reports, which may be explained in differences of sample preparation and the higher concentrations of biopolymers being investigated. The ‘structure point’ is reported to represent a long range network that has extended before breaking following which normal shear thinning behaviour is observed. Upon ramping the
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Fig. 5. Viscosity versus shear stress after 15 min equilibrium showing relative viscosity and stress at the initial ‘structure point’ (W) Caragem 4230, (A) Caragem 4217, (K) Caragem 4435.
stress back down, a large degree of hysteresis was observed which is consistent with unrecovered loss of structure in the sample. The partial rebuilding of the sample structure as seen in Figs. 1–3 is referred to as the ‘second structural point’. The second structural point was observed at a lower stress than the initial structural point, an observation consistent with the complex beginning to reassemble at a stress low enough such that the rate of destruction was less than the rate of structural recovery. It appears that yield type behaviour was occurring after the second structural point. The above-described properties were used to characterise the flow curves of the samples investigated. The development of yield stress behaviour as a sample equilibrates after the application of preshear, may be analysed to make comparisons of the time dependence of structure development between samples. Fig. 1 suggests that Caragem 4435 has limited equilibrium processes with little change in initial yield stress behaviour with equilibrium time. Caragem 4230, as illustrated in Fig. 2 has a significant yield stress behaviour development that is complete after 10 min. Fig. 3 illustrates that Caragem 4217 has a significant yield stress after 1 min equilibration that is enhanced after 10 min equilibration. It should be noted that equilibrium processes are therefore of a timescale of the order of 10 min, which is similar to the timescale of the stress ramps conducted (15 min). The structure point in the results observed in this work is defined as the mid point of the plateau region as is illustrated in Fig. 4. The ‘initial structure’ point for each carrageenan is
observed at a similar stress for Caragem 4435 and Caragem 4217 (0.7 and 0.8 Pa, respectively). By contrast, this structure point was markedly higher for Caragem 4230 (2.0 Pa). Although the structure point occurs at stresses differing by less than an order of magnitude for all samples compared, the viscosity at the structure point for each sample varies considerably. A difference in viscosity at the initial structure point is indicative of the network density and strength. Fig. 5 shows the flow curves for 15 min of equilibrium for each sample analysed. The structure point for samples Caragem 4435, Caragem 4217, Caragem 4230 occur at viscosities 0.04, 0.6 and 2 Pa s, respectively. The difference in viscosity between samples suggests that Caragem 4230 with a slightly higher structure point stress and a considerably higher viscosity has a denser and generally stronger network than Caragem 4217 and Caragem 4435. The return flow curves show a degree of hysteresis for all samples, although there was considerably less hysteresis for Caragem 4435. The limited hysteresis observed suggests that the protein interaction is weak with the lambda carrageenan used in Caragem 4435 and only a small degree of interaction occurs resulting in a relatively sparse network. The structure point on the return curve is most noticeable in Caragem 4230, a kappa type carrageenan, indicating that it has the most resilient network. To study fluids that display a high degree of hysteresis, suggesting significant structural processes, both in terms of development and destruction, it is beneficial to use an experimental method that allows each point to reach a
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Fig. 6. Creep-recovery curves at varying stresses for Caragem 4230 after 3 min preshear at 6 Pa and 1 min equilibration. Constant stress applied for a period of 300 s following which recovery under no applied stress was measured for 300 s. Temperature maintained at 5⬚C. Fill Line 0.3, dashed Line 0.6, dotted Line 0.9 Pa.
true equilibrium. On a controlled stress rheometer, creeprecovery tests involve the application of a constant stress and measurement of the strain response for a period of time. The applied stress is then removed and strain recovery is
measured. The creep recovery curves at varying applied stresses for Caragem 4230 after equilibration for 1 min are shown in Fig. 6. At an applied stress of 0.3 and 0.6 Pa significant recovery was observed suggesting that these
Fig. 7. Torque sweep for determination of linear viscoelastic region for Caragem 4217 measured at 5⬚C. Limit of linearity of strain with applied stress represents linear viscoelastic limit. Torque sweep conducted at 1 Hz.
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Fig. 8. Frequency sweep for Caragem 4230 at 5⬚C over a period of 20 min (X) G 0 (Pa), (B) G 00 (Pa), (O) h ⴱ (Pa s).
Fig. 9. Frequency sweep for Caragem 4217 at 5⬚C over a period of 20 min (X) G 0 (Pa), (B) G 00 (Pa), (O) h ⴱ (Pa s).
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Fig. 10. Frequency sweep for Caragem 4435 at 5⬚C over a period of 20 min (X) G 0 (Pa), (B) G 00 (Pa), (O) h ⴱ (Pa s).
values were below the yield stress of the material. At 0.9 Pa there is no recovery indicating that the sample had yielded and therefore no elastic component of the structure was recovered. The yield stress, as determined by the flow experiments, mentioned earlier, has approximately 1 Pa after 1 min equilibration. The yield stress as determined from creep recovery tests was therefore lower than that indicated by the flow experiments (Fig. 3). The difference in yield stress obtained from flow and creep-recovery experiments may be explained by considering the timescale of the test relative to the timescale of equilibrium processes. The yield stress obtained from flow experiments may have been higher than a true equilibrium yield stress of the material since the equilibrium processes are such that molecular rearrangements may occur in the time the stress ramp changed from 0.6 to 1.0 Pa. If the flow experiment was conducted over a much longer timescale, allowing more molecular rearrangement as the stress ramped from 0.6 to 1.0 Pa, a lower yield stress would be observed. As the timescale of the test approaches large times and the time spent at any one particular stress increases, the stress ramp theoretically approaches a test of identical nature to the true equilibrium test of creep-recovery. The work of Chang, Borger and Nguyen (1998) suggests that the yield stress obtained from flow experiments may be higher than those obtained from creep-recovery tests due to the time-frame of relaxation within the solutions relative to the time-frame of the flow experiment itself, a concept which supports the statement above and the observed results.
The linear viscoelastic limit has been determined by conducting a torque sweep and determining the largest amplitude that gives a linear relationship between displacement and torque. A significant drop in the storage modulus was observed at higher strains than this limit. Fig. 7 illustrates the determination of the linear viscoelastic region for Caragem 4217 at 5⬚C, this procedure was also carried out for the other carrageenan samples. The in phase and out of phase response to that oscillatory perturbation give rise to G 0 (storage modulus) and G 00 (loss modulus), respectively. The storage modulus represents a measure of the elastic response of the liquid whilst the loss modulus is a measure of the viscous response. The frequency sweeps obtained for Carragem 4230, Caragem 4217 and Caragem 4435 are shown in Figs. 8–10, respectively. The frequency dependence of the moduli were analysed quantitatively by fitting simple power law relationships: G 0 / vp
1
G 00 / vq
2
where v is the frequency of oscillation and p and q are the storage and loss moduli power law indices, respectively. The frequency sweeps showed that the frequency dependence of G 0 decreased through samples Caragem 4435, Caragem 4217 and Caragem 4230, as indicated by a decreasing value of the power law index p for the samples (0.30, 0.19 and 0.09, respectively).
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Fig. 11. Normalised storage modulus versus time measured at 1 Hz and 5⬚C after preshear of 50 s ⫺1 for 10 min.
In addition, the absolute difference between G 0 and G 00 and the frequency range for which G 0 is greater than G 00 increased in the same order. Gels typically show the following basic features: G 0 is greater than G 00 over a large range of frequencies and G 0 is relatively independent of frequency (Barnes, Hurtton & Walters, 1989). Inspection of the data suggested gel strengths in decreasing order of Caragem 4230, Caragem 4217 and Caragem 4435, which was in agreement with the results from the flow experiments. The absolute value of G 0 may also be used to compare the relative strengths of the three systems with G 0 decreasing in the order mentioned above. Fig. 11 illustrates the build up of normalised G 0 with time for the three samples after a preshear of 6 Pa for a period of 3 min. Caragem 4230 was the only sample with a significant equilibrium effect which was indicated by a long, time dependence in the development of G 0 from a value of 75% of its final value. The time dependence for equilibration of Caragem 4435 and Caragem 4217 was considerably shorter. The flow and oscillatory results show that Caragem 4230 had the strongest gel network, followed by Caragem 4217 and Caragem 4435.
showed characteristic shapes which could be interpreted in terms of the strength of ‘weak gel’ networks. The high degree of hysteresis observed suggested that care should be taken when drawing quantitative conclusions from such time dependent tests. The oscillatory measurements made were seen to be advantageous as they limited perturbations in the system to be within the characteristic linear viscoelastic limit. Further work to be conducted in this area could investigate the effect of using other rheological geometries (i.e. couette cells), the effect of polymer concentration and the type of hydrocolloid applied and further characterisation of the yield stress of the systems.
Acknowledgements The assistance of the Commonwealth Australian Postgraduate Award Scholarship, A.P.A., and the C.R.C. for Industrial Plant Biopolymers, is gratefully acknowledged by Andrew Rodd. Part of this work is supported by Goodman Fielder Ingredients and we would like to extend our gratitude for permission to publish this work.
References 4. Conclusions The use of sensitive, controlled stress rheometers to study the rheological characteristics of weak gel systems is a powerful technique, providing the limitations of the measurement system is recognised. Milk containing carrageenan has been characterised using simple flow, creeprecovery and oscillatory rheology. The measurements show Caragem 4230 to have the strongest gel properties combined with the most rapid equilibration kinetics. The flow measurements, generated reproducible curves that
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