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Textural properties of gelled dairy desserts containing κ-carrageenan and starch
Food Hydrocolloids 18 (2004) 817–823 www.elsevier.com/locate/foodhyd
Textural properties of gelled dairy desserts containing k-carrageenan and starch Dirk Verbekena,*, Olivier Thasb, Koen Dewettincka a
Department of Food Technology and Nutrition, Faculty of Agricultural and Applied Biological Sciences, Ghent University, Coupure Links 653, Ghent B-9000, Belgium b Department of Applied Mathematics, Biometrics and Process Control, Faculty of Agricultural and Applied Biological Sciences, Ghent University, Belgium Received 8 July 2003; revised 10 November 2003; accepted 23 December 2003
Abstract Pudding desserts were prepared with k-carrageenan, skim milk powder (SMP), native maize starch, sucrose and water. A mixture design was used to study the effects of varying concentrations of carrageenan, SMP and starch, while the amount of water and sucrose was kept constant. Mixtures were heated for 20 min at 90 8C. Small amplitude oscillatory measurements were performed and the gelation temperature Tg and the complex modulus Gp were recorded. Large deformation penetration tests were also carried out to measure the gel strength of the desserts. The size of starch granules was measured using laser diffraction analysis. Second-order Scheffe´ polynomials were fitted to the experimental data and used to make graphs allowing interpretation of the data. In the defined concentration range, the exclusion effect of starch was found to have an important influence on the dessert’s properties. This effect was more pronounced than the effect of the milk protein concentration. q 2004 Elsevier Ltd. All rights reserved. Keywords: Mixture design; k-Carrageenan; Starch; Skim milk powder; Oscillation measurements; Penetration tests
1. Introduction Carrageenans, sulphated polysaccharides obtained from red seaweeds, are frequently used in combination with starch in gelled dairy desserts (Descamps, Langevin, & Combs, 1986; De Vries, 1992; Mleko, 1997; Rapaille & Vanhemelrijck, 1992; Tye, 1988). Starch imparts body and mouthfeel to the product, while carrageenan provides the appropriate texture: firm and brittle with k-carrageenan, soft and elastic with i-carrageenan (Imeson, 2000). The structural organisation and rheological properties of the dessert are a result of the interactions between the different ingredients. k-Carrageenan is frequently used as gelling agent in milk desserts because of its so-called milk reactivity, which has been ascribed to an electrostatic interaction between a positively charged region of k-casein and the negatively charged sulfate groups of k-carrageenan (Snoeren, Payens, Jeunink, & Both, 1975). The absorption of k-carrageenan on casein micelles was proved by * Corresponding author. Tel.: þ 32-9-264-61-98; fax: þ 32-9-264-62-18. E-mail address: [email protected] (D. Verbeken). 0268-005X/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2003.12.007
Dalgleish and Morris (1988), who found that k-carrageenan increases the charge density of casein micelles below the coil –helix transition temperature. More recently however, Bourriot, Garnier, and Doublier (1999) observed phase separation in micellar-casein/k-carrageenan mixtures, both above and below the coil – helix transition, using confocal laser scanning microscopy. The authors stated that the above mentioned electrostatic interaction is very unlikely for electrostatic and steric reasons, since negatively charged k-carrageenan chains would have to penetrate through the equally charged hairy micelle layer to reach the positive region of k-casein. This phase separation was also observed by Hemar, Hall, Munro, and Singh (2002). The influence of milk proteins on k-carrageenan gelation has been the subject of numerous studies leading to contradictory findings. Drohan, Tziboula, McNulty, and Horne (1997) observed that addition of casein micelles could prevent gel formation at low carrageen levels as a result of a decreased carrageenan availability for gelation. Contradictorily, Schorsch, Jones, and Norton (2000) reported a decrease of the critical gelation concentration of k-carrageenan in the presence of casein micelles.
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Furthermore, Tziboula and Horne (1999b) found a decrease in Gp upon casein micelle addition, whereas other authors reported an increase of the dynamic moduli (Langendorff et al., 2000; Schorsch et al., 2000). Hemar et al. (2002) reported an increase of G0 in the presence of skim milk powder (SMP) and milk protein concentrate, and whey proteins have also been found to increase Gp (Tziboula & Horne, 1998; 1999a,b). Much less information is available on the effect of starch on k-carrageenan gelation. The gel properties of kcarrageenan—starch composites are believed to be mainly governed by the exclusion effect of swollen granules, resulting in higher carrageenan concentrations in the continuous water phase (De Vries, 1992; Lai, Huang, & Lii, 1999; Tecante & Doublier, 1999). Limited effects are ascribed to the interference of soluble starch molecules with the gel structure formation (Lai et al., 1999), although phase separation between amylose and k-carrageenan has also been suggested (Tecante & Doublier, 2002). The objective of this work was to determine the influence of composition on the rheological properties of pudding desserts composed of k-carrageenan, native maize starch, SMP, sucrose and water. The concentration of the latter two components, not being directly involved in the structure build-up of the dessert, was kept constant, whereas a mixture design was used to study the influence of varying concentrations of carrageenan, starch and SMP (Depypere, Verbeken, Thas, & Dewettinck, 2003). In any other type of experimental design, these latter ingredients would be varied at the expense of water or sucrose, which are then considered as inert material. This is, however, not the case in these desserts which contain over 20 wt% dry matter and where competition for available water exists between the different components. A change in water or sucrose content can importantly affect the amount of available water and therefore have its own effect on the rheological properties of the desserts. Varying one component at the expense of water or sucrose can thus result in misleading data and wrong conclusions. Using a mixture design, the water and sucrose concentration can be kept constant and thus differences in rheological properties are only caused by variations in the other ingredients. Since the sum of the carrageenan, starch and SMP concentration has to be constant, a change in one component inevitably implies a change in at least one other component.
2. Materials and methods Low-heat SMP was obtained from Belgomilk (Belgium), native maize starch (Meritena 100) from Tate and Lyle (Amylum, Belgium) and k-carrageenan (Satiagel AMP 45) from Degussa Texturant Systems (Belgium). SMP was dissolved in distilled water and kept overnight at 4 8C allowing milk proteins to become fully hydrated. Appropriate amounts of starch, k-carrageenan and sucrose were
then added to the cold milk and the mixture was heated for 20 min at 90 8C using a hot waterbath. Small amplitude oscillatory measurements were performed in duplicate on a controlled stress rheometer (CVO, Bohlin Instruments, UK) using a cup and bob geometry. Hot mixtures were transferred to the rheometer, which was preheated at 55 8C. An isolated cover was used to minimize temperature variations within the sample, while the atmosphere under the cover was saturated with water to prevent moisture loss. Samples were cooled at a rate of 1 8C/min and kept at 4 8C for 60 min. G0 ; G00 and tan d were recorded at a frequency of 1 Hz and at a constant strain of 0.002 in order not to disturb gelation. Upon cooling the gelation temperature, Tg was defined as the temperature at which a sharp rise in G0 was detected, coinciding with a decrease in phase angle. Large deformation measurements were performed in 5fold on a Texture Analyser TA500 (Lloyd Instruments, UK) equipped with a stainless steel cylindrical probe. Hot mixtures were poured into smooth-edged glass beakers. The beakers were overfilled allowing to cut off the samples after gelation to obtain a perfectly flat surface. The samples were covered to prevent dehydration and kept overnight at 4 8C. Penetration tests were performed at a penetration rate of 10 mm/min over a distance of 20 mm, corresponding to one-third of the height of the samples. The total work for penetration, represented by the surface under the force deformation curve, was defined as the gel strength (Van Camp & Huyghebaert, 1995). The particle size distribution of swollen starch granules was measured by laser diffraction using a Malvern MasterSizer (Malvern, UK) equipped with a 300 mm Fourier cell and a sample dispersion unit. After heating, a small amount of mixture was diluted in hot distilled water (ratio 1:15) to prevent gelation and cooled to ambient temperature. This dilution was then dispersed in the sample unit and pumped to the measuring cell. As refractive index of starch and continuous phase, 1.5295 and 1.3300 were used respectively (Tecante & Doublier, 1999). The volume weighed mean diameter D½4; 3 was used to investigate the influence of dessert composition on starch swelling, while the volume equivalent mean diameter D½3; 0 allowed the calculation of the carrageenan concentration Cp (w/v%) in the continuous water phase of the dessert. Using D½3; 0 of dry starch granules and the granule density, the theoretical amount of starch granules was calculated. Starting from this amount, the total volume occupied by starch granules in the dessert was calculated using D½3; 0 after heating, which then allowed the derivation of the carrageenan concentration in the remaining water phase. An I-optimal three-component mixture design was built using the component ranges given in Table 1. Because of the inconsistency of the multi-constraint design, the coefficients of the fitted model cannot be interpreted as such, leaving only graphical interpretation of the data (Cornell, 1990). An I-optimal design minimizing the maximal prediction error
D. Verbeken et al. / Food Hydrocolloids 18 (2004) 817–823 Table 1 Component ranges in the I-optimal design Component
Minimum (wt%)
Maximum (wt%)
SMP Starch k-Carrageenan
5 0.5 0.05
10 5.5 0.5
Table 2 Derived predictive models for various response variables including the determination coefficient R2 Factor
A B C AB AC BC R2
Coefficient estimates Gp (Pa)
Tg (8C)
Gel strength (J)
D4,3 (mm)
63.17 746.38 55566.70 2119.81 25537.66 24140.04
3.94 3.54 2100.52 0.12 10.69 11.76
21.36 £ 1024 21.44 £ 1024 5.75 £ 1021 1.93 £ 1024 25.19 £ 1022 25.04 £ 1022
3.38 2.69 30.36 6.91 £ 1022 23.37 22.32
0.912
0.752
0.988
0.775
A: SMP, B: starch, C: k-carrageenan.
inside the experimental region provides graphs with the highest reliability, therefore, allowing the best interpretation of the experimental data. The design consists of 18 experiments including six repetitions for the calculation of the pure error and six experiments for the lack-of-fit test. The concentration of water and sucrose was kept constant at respectively 77.45 and 12 wt%. Second-order Scheffe´ polynomials were fitted to the data allowing response prediction in the experimental region. The cation content of the mixtures was kept constant to exclude cation effects on the rheological properties of the desserts. The amount and type of cations present has been shown to importantly affect the gelation of k-carrageenan (Drohan et al., 1997; Hemar et al., 2002; Hermansson, Eriksson, & Jordansson, 1991; Puvanenthiran, Goddard,
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& Augustin, 2001). The cation content of the different dessert ingredients was determined and the dessert composition with the highest cation content in the design was chosen as reference. Salts were added to adjust the cation content to this reference level.
3. Results and discussion The coefficient estimates of the second-order Scheffe´ polynomials fitted to the experimental data are shown in Table 2. For Gp ; Tg ; gel strength, and D4;3 predictive models with a high to very high determination coefficient were obtained, indicating that variations in the response variables could be ascribed to a large extent to variations in the dessert composition. As gelling agent, k-carrageenan has a dominant effect on the rheological properties. Therefore, graphs were constructed showing response changes with varying starch/SMP ratio at constant carrageenan concentrations, so-called iso-carrageenan plots. The iso-carrageenan plot for the complex modulus Gp is represented in Fig. 1. It can be seen that higher values for Gp are obtained with increasing carrageenan concentrations. Furthermore, at a constant carrageenan concentration, Gp rises as the starch/ SMP ratio increases. In other words, the substitution of SMP by starch leads to higher Gp values, up to 4500 Pa at 0.5 wt% carrageenan. At the lowest carrageenan concentration, the substitution of SMP by starch initially seems to result in a decrease in Gp : However, additional experiments have shown that this decrease is not genuine and was caused by the model fit. This demonstrates that the iso-carrageenan plots should be interpreted as a whole and that it is not obvious to draw conclusions from small parts of the design region. Thus, for the defined experimental region an increase in starch concentration at the expense of SMP always leads to higher Gp values. This finding does not give information on the separate effects of SMP and starch. For instance, it cannot be concluded that the addition of SMP
Fig. 1. Influence of starch/SMP ratio on the complex modulus Gp at different k-carrageenan concentrations.
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Fig. 2. Influence of starch/SMP ratio on the gelation temperature Tg at different k-carrageenan concentrations.
causes a decrease in Gp : It only shows that the contribution of starch to Gp is larger than that of SMP. For all dessert compositions phase angle values were close to 68, meaning that G0 values were approximately 10 times larger than G00 values. The only exception was the dessert with the minimal amount of carrageenan and starch and the maximal amount of SMP with a phase angle of around 168, indicating more fluid-like behaviour. The substitution of SMP by starch also leads to an increase of the gelation temperature Tg (Fig. 2). Increasing the starch concentration from its lower to its upper limit causes a rise in Tg of 1 – 3 8C, depending on the amount of carrageenan present. This observation is in accordance with previous studies. Although Lynch and Mulvihill (1994) reported an increase in Tg when adding different casein fractions to a 1% k-carrageenan solution, other authors found that milk proteins had no significant effect on the gelation temperature of k-carrageenan (Drohan et al., 1997; Langendorff et al., 2000; Puvanenthiran et al., 2001; Schorsch et al., 2000). On the other hand, the addition of
1% of rice starch to a 2% carrageenan solution was found to cause a significant increase in Tg ; mainly ascribed to an elevation of the carrageenan concentration in the continuous phase (Lai et al., 1999). The combined effect of lowering the SMP concentration and raising the starch concentration thus results in a higher Tg : Increasing the carrageenan concentration also leads to an increase of the gelation temperature. The iso-carrageenan plot for the gel strength, derived from penetration tests, is represented in Fig. 3. It can be seen that the gel strength increases with the carrageenan concentration. Furthermore, the distance between successive curves clearly increases at higher carrageenan concentrations, indicating that the relation between the gel strength and the carrageenan concentration is rather quadratic than linear. Substituting SMP by starch leads to an increase in gel strength, which is more pronounced at low starch/SMP ratios. In literature, little information on the influence of starch on the large deformation behaviour of k-carrageenan gels is available. De Vries (2002) reported a 25% decrease in breaking strength of a 0.2% k-carrageenan milk gel after
Fig. 3. Influence of starch/SMP ratio on the gel strength at different k-carrageenan concentrations.
D. Verbeken et al. / Food Hydrocolloids 18 (2004) 817–823
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Fig. 4. Influence of starch/SMP ratio on the volume weighted mean diameter D4;3 of swollen starch granules at different k-carrageenan concentrations.
adding 2% native maize starch and suggested that carrageenan gelation was negatively affected by starch. Lai et al. (1999) observed a decrease in gel strength, obtained from a compression test, upon addition of various concentrations of rice starch to a 2.0% carrageenan gel. However, Fig. 3 clearly demonstrates that the contribution of starch to the gel strength of the desserts is larger than that of SMP. And since it is very unlikely that the addition of SMP itself would decrease the gel strength, starch seems to have a pronounced positive effect on the gel strength. The size of the swollen maize starch granules was comparable with sizes found in literature, although for most dessert compositions slightly smaller diameters were measured (Rao, Okechukwu, Da Silva, & Oliveira, 1997; Ziegler, Thompson, & Casasnovas, 1993). From Fig. 4, it can be seen that an increase of the carrageenan concentration leads to smaller granule diameters, indicating that starch swelling is restricted by the presence of carrageenan. The substitution of SMP by starch in general also leads to a decrease in D4;3 ; except at the highest carrageenan concentration. This seems to demonstrate that the availability of water is the limiting factor for granule swelling during heating. The more starch is present, the less water is available per individual granule. This finding illustrates the importance of the use of a mixture design in this study and demonstrates that a linear response approach would most likely lead to wrong conclusions. For instance, varying the SMP content at the expense of water could cause considerable changes in starch swelling, the influence of which on the rheological properties of the dessert would then be considered as an effect of SMP. Table 3 shows the Spearman’s correlation coefficient of Gp ; Tg and gel strength with the carrageenan concentration C p (w/v%) in the water phase. All correlations were found to be highly significant ðp , 0:01Þ; but particularly the one with the gel strength is remarkably high. From Fig. 5, it can be seen that there exists an almost perfect quadratic relation between C p
and the gel strength of the dessert. In other words, the gel strength seems to be determined only by the carrageenan present in the water phase of the dessert. In the defined experimental region, neither milk proteins nor swollen starch granules, considered as structural components, appear to have an effect on the behaviour of the dessert when subjected to large deformations. Not much is known about the influence of milk proteins on the large deformation behaviour of k-carrageenan gels. Hemar et al. (2002) observed an increase in the Young’s modulus and Puvanenthiran et al. (2001) an increase in the gel strength upon addition of SMP to k-carrageenan gels. However, in both cases it was not clear whether this increase was caused by milk proteins or by cations present in the added milk powder. Fig. 5 suggests the latter since the presence of SMP does not seem to contribute to the gel strength when the cation content was kept constant. In the case of starch, it was already reported that an exclusion effect, resulting in a concentration of the gelling agent in the water phase, mainly influenced the rheological properties of composite gels (De Vries, 1992; Lai et al., 1999; Tecante & Doublier, 1999). This also explains the levelling off in Fig. 3, observed at high starch/SMP ratios. As more starch is added to the dessert, the increase in volume, occupied by starch granules, is reduced since the size of the individual granules decreases. Therefore, the increase in carrageenan concentration in the continuous water phase and as a result the increase in gel strength of Table 3 Spearman’s correlation coefficient between the carrageenan concentration in the water phase and the complex modulus Gp ; the gelation temperature Tg and the gel strength
Correlation coefficient * p , 0:01:
Gp
Tg
Gel strength
0.692*
0.726*
0.994*
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Fig. 5. Quadratic relationship between the gel strength and the carrageenan concentration C p in the water phase.
the dessert is decreasing at higher starch/SMP ratios, which can be observed as a decline of the slope of the iso-carrageenan curves. This is also the explanation for the levelling off of the gelation temperature at high starch/SMP ratios, as observed in Fig. 2. Although highly significant correlations with C p were also obtained for Gp and Tg ; it is clear that variations in Cp alone cannot explain the observed changes. Correlation analysis showed that milk proteins and starch also significantly contribute to Gp ðp , 0:01Þ and Tg (p , 0:01 and p , 0:05; respectively). In the case of starch, it could not be said whether this was an effect of swollen starch granules acting as fillers and/or leaked out amylose interacting with carrageenan chains. Since the gel strength was not affected, interference of amylose with the carrageenan gelation seems unlikely. Possibly swollen starch granules are easily deformed during uniaxial large deformation tests, but may affect the shear behaviour of the desserts.
4. Conclusions The rheological properties of the pudding-like desserts seem to be mainly governed by the exclusion effect of swollen starch granules, thus concentrating k-carrageenan in the continuous water phase. The carrageenan concentration C p (w/v, %) in the water phase was highly correlated with the gel strength, the complex modulus Gp and the gelation temperature Tg : In fact, milk proteins and starch itself do not appear to have any effect on the large deformation behaviour, since the gel strength could be perfectly predicted by C p : During small deformation tests, the presence of milk proteins and starch did have a significant influence on the measured properties. At any constant carrageenan concentration, the substitution of SMP by starch led to an increase of both Gp and Tg : This does not mean that SMP itself causes a weaker gel and a lower Tg ; but indicates that starch has a more positive effect on these properties.
Acknowledgements The authors wish to thank Geert Maesmans and Koenraad Swinnen (Amylum, Belgium) and Johan L’Ecluse (Degussa Texturant Systems, Belgium) for their scientific input and Benny Lewille for his help in the laboratory.
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D. Verbeken et al. / Food Hydrocolloids 18 (2004) 817–823 Lai, V. M.-F., Huang, A.-L., & Lii, C.-Y. (1999). Rheological properties and phase transition of red algal polysaccharide–starch composites. Food Hydrocolloids, 13(5), 409–418. Langendorff, V., Cuvelier, G., Michon, C., Launay, B., Parker, A., & De Kruif, C. G. (2000). Effects of carrageenan type on the behaviour of carrageenan/milk mixtures. Food Hydrocolloids, 14(4), 273–280. Lynch, M. G., & Mulvihill, D. M. (1994). The influence of caseins on the rheology of k-carrageenan gels. In G. O. Phillips, & P. A. Williams (Eds.), (Vol. 7) (pp. 323–332). Gums and stabilizers for the food industry, Oxford: IRL Press. Mleko, S. (1997). Rheological properties of milk and whey protein desserts. Milchwissenschaft, 52(5), 262 –265. Puvanenthiran, A., Goddard, S. J., & Augustin, M. A. (2001). Gelation of mixed gels containing k-carrageenan and skim milk components. Journal of Food Science, 67(2), 573–577. Rao, M. A., Okechukwu, P. E., Da Silva, P. M. S., & Oliveira, J. C. (1997). Rheological behavior of heated starch dispersions in excess water: role of starch granule. Carbohydrate Polymers, 33(4), 273–283. Rapaille, A., & Vanhemelrijck, J. (1992). Milk based desserts. In R. Early (Ed.), The technology of dairy products (pp. 221 –246). Glasgow: Blackie and Son Ltd. Schorsch, C., Jones, M. G., & Norton, I. T. (2000). Phase behaviour of pure micellar casein/k-carrageenan systems in milk salt ultrafiltrate. Food Hydrocolloids, 14(4), 347– 358.
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Snoeren, T. H. M., Payens, T. A. J., Jeunink, J., & Both, P. (1975). Electrostatic interaction between k-carrageenan and k-casein. Milchwissenschaft, 30(7), 393–396. Tecante, A., & Doublier, J. L. (1999). Steady flow and viscoelastic behavior of crosslinked waxy corn starch-k-carrageenan pastes and gels. Carbohydrate Polymers, 40(3), 221–231. Tecante, A., & Doublier, J. L. (2002). Rheological investigation of the interaction between amylose and k-carrageenan. Carbohydrate Polymers, 49(2), 177–183. Tye, R. J. (1988). The rheology of starch/carrageenan systems. Food Hydrocolloids, 2(4), 259 –266. Tziboula, A., & Horne, D. S. (1998). Influence of milk proteins on the gel transition temperature and the mechanical properties of weak kcarrageenan gels. In P. A. Williams, & G. O. Phillips (Eds.), (Vol. 9) (pp. 202 –211). Gums and stabilizers for the food industry, Cambridge: The Royal Society of Chemistry. Tziboula, A., & Horne, D. S. (1999a). Influence of milk proteins on kcarrageenan gelation. International Dairy Journal, 9(3–6), 359 –364. Tziboula, A., & Horne, D. S. (1999b). Influence of whey protein denaturation on k-carrageenan gelation. Colloids and Surfaces, 12(3– 6), 299 –308. Van Camp, J., & Huyghebaert, A. (1995). High-pressure induced gel formation of a whey protein and haemoglobin protein concentrate. Lebensmittel-Wissenschaft und-Technologie, 28(1), 111–117. Ziegler, G. R., Thompson, D. B., & Casasnovas, J. (1993). Dynamic measurement of starch granule swelling during gelatinization. Cereal Chemistry, 70(3), 247– 251.
Textural properties of gelled dairy desserts containing k-carrageenan and starch Dirk Verbekena,*, Olivier Thasb, Koen Dewettincka a
Department of Food Technology and Nutrition, Faculty of Agricultural and Applied Biological Sciences, Ghent University, Coupure Links 653, Ghent B-9000, Belgium b Department of Applied Mathematics, Biometrics and Process Control, Faculty of Agricultural and Applied Biological Sciences, Ghent University, Belgium Received 8 July 2003; revised 10 November 2003; accepted 23 December 2003
Abstract Pudding desserts were prepared with k-carrageenan, skim milk powder (SMP), native maize starch, sucrose and water. A mixture design was used to study the effects of varying concentrations of carrageenan, SMP and starch, while the amount of water and sucrose was kept constant. Mixtures were heated for 20 min at 90 8C. Small amplitude oscillatory measurements were performed and the gelation temperature Tg and the complex modulus Gp were recorded. Large deformation penetration tests were also carried out to measure the gel strength of the desserts. The size of starch granules was measured using laser diffraction analysis. Second-order Scheffe´ polynomials were fitted to the experimental data and used to make graphs allowing interpretation of the data. In the defined concentration range, the exclusion effect of starch was found to have an important influence on the dessert’s properties. This effect was more pronounced than the effect of the milk protein concentration. q 2004 Elsevier Ltd. All rights reserved. Keywords: Mixture design; k-Carrageenan; Starch; Skim milk powder; Oscillation measurements; Penetration tests
1. Introduction Carrageenans, sulphated polysaccharides obtained from red seaweeds, are frequently used in combination with starch in gelled dairy desserts (Descamps, Langevin, & Combs, 1986; De Vries, 1992; Mleko, 1997; Rapaille & Vanhemelrijck, 1992; Tye, 1988). Starch imparts body and mouthfeel to the product, while carrageenan provides the appropriate texture: firm and brittle with k-carrageenan, soft and elastic with i-carrageenan (Imeson, 2000). The structural organisation and rheological properties of the dessert are a result of the interactions between the different ingredients. k-Carrageenan is frequently used as gelling agent in milk desserts because of its so-called milk reactivity, which has been ascribed to an electrostatic interaction between a positively charged region of k-casein and the negatively charged sulfate groups of k-carrageenan (Snoeren, Payens, Jeunink, & Both, 1975). The absorption of k-carrageenan on casein micelles was proved by * Corresponding author. Tel.: þ 32-9-264-61-98; fax: þ 32-9-264-62-18. E-mail address: [email protected] (D. Verbeken). 0268-005X/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2003.12.007
Dalgleish and Morris (1988), who found that k-carrageenan increases the charge density of casein micelles below the coil –helix transition temperature. More recently however, Bourriot, Garnier, and Doublier (1999) observed phase separation in micellar-casein/k-carrageenan mixtures, both above and below the coil – helix transition, using confocal laser scanning microscopy. The authors stated that the above mentioned electrostatic interaction is very unlikely for electrostatic and steric reasons, since negatively charged k-carrageenan chains would have to penetrate through the equally charged hairy micelle layer to reach the positive region of k-casein. This phase separation was also observed by Hemar, Hall, Munro, and Singh (2002). The influence of milk proteins on k-carrageenan gelation has been the subject of numerous studies leading to contradictory findings. Drohan, Tziboula, McNulty, and Horne (1997) observed that addition of casein micelles could prevent gel formation at low carrageen levels as a result of a decreased carrageenan availability for gelation. Contradictorily, Schorsch, Jones, and Norton (2000) reported a decrease of the critical gelation concentration of k-carrageenan in the presence of casein micelles.
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D. Verbeken et al. / Food Hydrocolloids 18 (2004) 817–823
Furthermore, Tziboula and Horne (1999b) found a decrease in Gp upon casein micelle addition, whereas other authors reported an increase of the dynamic moduli (Langendorff et al., 2000; Schorsch et al., 2000). Hemar et al. (2002) reported an increase of G0 in the presence of skim milk powder (SMP) and milk protein concentrate, and whey proteins have also been found to increase Gp (Tziboula & Horne, 1998; 1999a,b). Much less information is available on the effect of starch on k-carrageenan gelation. The gel properties of kcarrageenan—starch composites are believed to be mainly governed by the exclusion effect of swollen granules, resulting in higher carrageenan concentrations in the continuous water phase (De Vries, 1992; Lai, Huang, & Lii, 1999; Tecante & Doublier, 1999). Limited effects are ascribed to the interference of soluble starch molecules with the gel structure formation (Lai et al., 1999), although phase separation between amylose and k-carrageenan has also been suggested (Tecante & Doublier, 2002). The objective of this work was to determine the influence of composition on the rheological properties of pudding desserts composed of k-carrageenan, native maize starch, SMP, sucrose and water. The concentration of the latter two components, not being directly involved in the structure build-up of the dessert, was kept constant, whereas a mixture design was used to study the influence of varying concentrations of carrageenan, starch and SMP (Depypere, Verbeken, Thas, & Dewettinck, 2003). In any other type of experimental design, these latter ingredients would be varied at the expense of water or sucrose, which are then considered as inert material. This is, however, not the case in these desserts which contain over 20 wt% dry matter and where competition for available water exists between the different components. A change in water or sucrose content can importantly affect the amount of available water and therefore have its own effect on the rheological properties of the desserts. Varying one component at the expense of water or sucrose can thus result in misleading data and wrong conclusions. Using a mixture design, the water and sucrose concentration can be kept constant and thus differences in rheological properties are only caused by variations in the other ingredients. Since the sum of the carrageenan, starch and SMP concentration has to be constant, a change in one component inevitably implies a change in at least one other component.
2. Materials and methods Low-heat SMP was obtained from Belgomilk (Belgium), native maize starch (Meritena 100) from Tate and Lyle (Amylum, Belgium) and k-carrageenan (Satiagel AMP 45) from Degussa Texturant Systems (Belgium). SMP was dissolved in distilled water and kept overnight at 4 8C allowing milk proteins to become fully hydrated. Appropriate amounts of starch, k-carrageenan and sucrose were
then added to the cold milk and the mixture was heated for 20 min at 90 8C using a hot waterbath. Small amplitude oscillatory measurements were performed in duplicate on a controlled stress rheometer (CVO, Bohlin Instruments, UK) using a cup and bob geometry. Hot mixtures were transferred to the rheometer, which was preheated at 55 8C. An isolated cover was used to minimize temperature variations within the sample, while the atmosphere under the cover was saturated with water to prevent moisture loss. Samples were cooled at a rate of 1 8C/min and kept at 4 8C for 60 min. G0 ; G00 and tan d were recorded at a frequency of 1 Hz and at a constant strain of 0.002 in order not to disturb gelation. Upon cooling the gelation temperature, Tg was defined as the temperature at which a sharp rise in G0 was detected, coinciding with a decrease in phase angle. Large deformation measurements were performed in 5fold on a Texture Analyser TA500 (Lloyd Instruments, UK) equipped with a stainless steel cylindrical probe. Hot mixtures were poured into smooth-edged glass beakers. The beakers were overfilled allowing to cut off the samples after gelation to obtain a perfectly flat surface. The samples were covered to prevent dehydration and kept overnight at 4 8C. Penetration tests were performed at a penetration rate of 10 mm/min over a distance of 20 mm, corresponding to one-third of the height of the samples. The total work for penetration, represented by the surface under the force deformation curve, was defined as the gel strength (Van Camp & Huyghebaert, 1995). The particle size distribution of swollen starch granules was measured by laser diffraction using a Malvern MasterSizer (Malvern, UK) equipped with a 300 mm Fourier cell and a sample dispersion unit. After heating, a small amount of mixture was diluted in hot distilled water (ratio 1:15) to prevent gelation and cooled to ambient temperature. This dilution was then dispersed in the sample unit and pumped to the measuring cell. As refractive index of starch and continuous phase, 1.5295 and 1.3300 were used respectively (Tecante & Doublier, 1999). The volume weighed mean diameter D½4; 3 was used to investigate the influence of dessert composition on starch swelling, while the volume equivalent mean diameter D½3; 0 allowed the calculation of the carrageenan concentration Cp (w/v%) in the continuous water phase of the dessert. Using D½3; 0 of dry starch granules and the granule density, the theoretical amount of starch granules was calculated. Starting from this amount, the total volume occupied by starch granules in the dessert was calculated using D½3; 0 after heating, which then allowed the derivation of the carrageenan concentration in the remaining water phase. An I-optimal three-component mixture design was built using the component ranges given in Table 1. Because of the inconsistency of the multi-constraint design, the coefficients of the fitted model cannot be interpreted as such, leaving only graphical interpretation of the data (Cornell, 1990). An I-optimal design minimizing the maximal prediction error
D. Verbeken et al. / Food Hydrocolloids 18 (2004) 817–823 Table 1 Component ranges in the I-optimal design Component
Minimum (wt%)
Maximum (wt%)
SMP Starch k-Carrageenan
5 0.5 0.05
10 5.5 0.5
Table 2 Derived predictive models for various response variables including the determination coefficient R2 Factor
A B C AB AC BC R2
Coefficient estimates Gp (Pa)
Tg (8C)
Gel strength (J)
D4,3 (mm)
63.17 746.38 55566.70 2119.81 25537.66 24140.04
3.94 3.54 2100.52 0.12 10.69 11.76
21.36 £ 1024 21.44 £ 1024 5.75 £ 1021 1.93 £ 1024 25.19 £ 1022 25.04 £ 1022
3.38 2.69 30.36 6.91 £ 1022 23.37 22.32
0.912
0.752
0.988
0.775
A: SMP, B: starch, C: k-carrageenan.
inside the experimental region provides graphs with the highest reliability, therefore, allowing the best interpretation of the experimental data. The design consists of 18 experiments including six repetitions for the calculation of the pure error and six experiments for the lack-of-fit test. The concentration of water and sucrose was kept constant at respectively 77.45 and 12 wt%. Second-order Scheffe´ polynomials were fitted to the data allowing response prediction in the experimental region. The cation content of the mixtures was kept constant to exclude cation effects on the rheological properties of the desserts. The amount and type of cations present has been shown to importantly affect the gelation of k-carrageenan (Drohan et al., 1997; Hemar et al., 2002; Hermansson, Eriksson, & Jordansson, 1991; Puvanenthiran, Goddard,
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& Augustin, 2001). The cation content of the different dessert ingredients was determined and the dessert composition with the highest cation content in the design was chosen as reference. Salts were added to adjust the cation content to this reference level.
3. Results and discussion The coefficient estimates of the second-order Scheffe´ polynomials fitted to the experimental data are shown in Table 2. For Gp ; Tg ; gel strength, and D4;3 predictive models with a high to very high determination coefficient were obtained, indicating that variations in the response variables could be ascribed to a large extent to variations in the dessert composition. As gelling agent, k-carrageenan has a dominant effect on the rheological properties. Therefore, graphs were constructed showing response changes with varying starch/SMP ratio at constant carrageenan concentrations, so-called iso-carrageenan plots. The iso-carrageenan plot for the complex modulus Gp is represented in Fig. 1. It can be seen that higher values for Gp are obtained with increasing carrageenan concentrations. Furthermore, at a constant carrageenan concentration, Gp rises as the starch/ SMP ratio increases. In other words, the substitution of SMP by starch leads to higher Gp values, up to 4500 Pa at 0.5 wt% carrageenan. At the lowest carrageenan concentration, the substitution of SMP by starch initially seems to result in a decrease in Gp : However, additional experiments have shown that this decrease is not genuine and was caused by the model fit. This demonstrates that the iso-carrageenan plots should be interpreted as a whole and that it is not obvious to draw conclusions from small parts of the design region. Thus, for the defined experimental region an increase in starch concentration at the expense of SMP always leads to higher Gp values. This finding does not give information on the separate effects of SMP and starch. For instance, it cannot be concluded that the addition of SMP
Fig. 1. Influence of starch/SMP ratio on the complex modulus Gp at different k-carrageenan concentrations.
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Fig. 2. Influence of starch/SMP ratio on the gelation temperature Tg at different k-carrageenan concentrations.
causes a decrease in Gp : It only shows that the contribution of starch to Gp is larger than that of SMP. For all dessert compositions phase angle values were close to 68, meaning that G0 values were approximately 10 times larger than G00 values. The only exception was the dessert with the minimal amount of carrageenan and starch and the maximal amount of SMP with a phase angle of around 168, indicating more fluid-like behaviour. The substitution of SMP by starch also leads to an increase of the gelation temperature Tg (Fig. 2). Increasing the starch concentration from its lower to its upper limit causes a rise in Tg of 1 – 3 8C, depending on the amount of carrageenan present. This observation is in accordance with previous studies. Although Lynch and Mulvihill (1994) reported an increase in Tg when adding different casein fractions to a 1% k-carrageenan solution, other authors found that milk proteins had no significant effect on the gelation temperature of k-carrageenan (Drohan et al., 1997; Langendorff et al., 2000; Puvanenthiran et al., 2001; Schorsch et al., 2000). On the other hand, the addition of
1% of rice starch to a 2% carrageenan solution was found to cause a significant increase in Tg ; mainly ascribed to an elevation of the carrageenan concentration in the continuous phase (Lai et al., 1999). The combined effect of lowering the SMP concentration and raising the starch concentration thus results in a higher Tg : Increasing the carrageenan concentration also leads to an increase of the gelation temperature. The iso-carrageenan plot for the gel strength, derived from penetration tests, is represented in Fig. 3. It can be seen that the gel strength increases with the carrageenan concentration. Furthermore, the distance between successive curves clearly increases at higher carrageenan concentrations, indicating that the relation between the gel strength and the carrageenan concentration is rather quadratic than linear. Substituting SMP by starch leads to an increase in gel strength, which is more pronounced at low starch/SMP ratios. In literature, little information on the influence of starch on the large deformation behaviour of k-carrageenan gels is available. De Vries (2002) reported a 25% decrease in breaking strength of a 0.2% k-carrageenan milk gel after
Fig. 3. Influence of starch/SMP ratio on the gel strength at different k-carrageenan concentrations.
D. Verbeken et al. / Food Hydrocolloids 18 (2004) 817–823
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Fig. 4. Influence of starch/SMP ratio on the volume weighted mean diameter D4;3 of swollen starch granules at different k-carrageenan concentrations.
adding 2% native maize starch and suggested that carrageenan gelation was negatively affected by starch. Lai et al. (1999) observed a decrease in gel strength, obtained from a compression test, upon addition of various concentrations of rice starch to a 2.0% carrageenan gel. However, Fig. 3 clearly demonstrates that the contribution of starch to the gel strength of the desserts is larger than that of SMP. And since it is very unlikely that the addition of SMP itself would decrease the gel strength, starch seems to have a pronounced positive effect on the gel strength. The size of the swollen maize starch granules was comparable with sizes found in literature, although for most dessert compositions slightly smaller diameters were measured (Rao, Okechukwu, Da Silva, & Oliveira, 1997; Ziegler, Thompson, & Casasnovas, 1993). From Fig. 4, it can be seen that an increase of the carrageenan concentration leads to smaller granule diameters, indicating that starch swelling is restricted by the presence of carrageenan. The substitution of SMP by starch in general also leads to a decrease in D4;3 ; except at the highest carrageenan concentration. This seems to demonstrate that the availability of water is the limiting factor for granule swelling during heating. The more starch is present, the less water is available per individual granule. This finding illustrates the importance of the use of a mixture design in this study and demonstrates that a linear response approach would most likely lead to wrong conclusions. For instance, varying the SMP content at the expense of water could cause considerable changes in starch swelling, the influence of which on the rheological properties of the dessert would then be considered as an effect of SMP. Table 3 shows the Spearman’s correlation coefficient of Gp ; Tg and gel strength with the carrageenan concentration C p (w/v%) in the water phase. All correlations were found to be highly significant ðp , 0:01Þ; but particularly the one with the gel strength is remarkably high. From Fig. 5, it can be seen that there exists an almost perfect quadratic relation between C p
and the gel strength of the dessert. In other words, the gel strength seems to be determined only by the carrageenan present in the water phase of the dessert. In the defined experimental region, neither milk proteins nor swollen starch granules, considered as structural components, appear to have an effect on the behaviour of the dessert when subjected to large deformations. Not much is known about the influence of milk proteins on the large deformation behaviour of k-carrageenan gels. Hemar et al. (2002) observed an increase in the Young’s modulus and Puvanenthiran et al. (2001) an increase in the gel strength upon addition of SMP to k-carrageenan gels. However, in both cases it was not clear whether this increase was caused by milk proteins or by cations present in the added milk powder. Fig. 5 suggests the latter since the presence of SMP does not seem to contribute to the gel strength when the cation content was kept constant. In the case of starch, it was already reported that an exclusion effect, resulting in a concentration of the gelling agent in the water phase, mainly influenced the rheological properties of composite gels (De Vries, 1992; Lai et al., 1999; Tecante & Doublier, 1999). This also explains the levelling off in Fig. 3, observed at high starch/SMP ratios. As more starch is added to the dessert, the increase in volume, occupied by starch granules, is reduced since the size of the individual granules decreases. Therefore, the increase in carrageenan concentration in the continuous water phase and as a result the increase in gel strength of Table 3 Spearman’s correlation coefficient between the carrageenan concentration in the water phase and the complex modulus Gp ; the gelation temperature Tg and the gel strength
Correlation coefficient * p , 0:01:
Gp
Tg
Gel strength
0.692*
0.726*
0.994*
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D. Verbeken et al. / Food Hydrocolloids 18 (2004) 817–823
Fig. 5. Quadratic relationship between the gel strength and the carrageenan concentration C p in the water phase.
the dessert is decreasing at higher starch/SMP ratios, which can be observed as a decline of the slope of the iso-carrageenan curves. This is also the explanation for the levelling off of the gelation temperature at high starch/SMP ratios, as observed in Fig. 2. Although highly significant correlations with C p were also obtained for Gp and Tg ; it is clear that variations in Cp alone cannot explain the observed changes. Correlation analysis showed that milk proteins and starch also significantly contribute to Gp ðp , 0:01Þ and Tg (p , 0:01 and p , 0:05; respectively). In the case of starch, it could not be said whether this was an effect of swollen starch granules acting as fillers and/or leaked out amylose interacting with carrageenan chains. Since the gel strength was not affected, interference of amylose with the carrageenan gelation seems unlikely. Possibly swollen starch granules are easily deformed during uniaxial large deformation tests, but may affect the shear behaviour of the desserts.
4. Conclusions The rheological properties of the pudding-like desserts seem to be mainly governed by the exclusion effect of swollen starch granules, thus concentrating k-carrageenan in the continuous water phase. The carrageenan concentration C p (w/v, %) in the water phase was highly correlated with the gel strength, the complex modulus Gp and the gelation temperature Tg : In fact, milk proteins and starch itself do not appear to have any effect on the large deformation behaviour, since the gel strength could be perfectly predicted by C p : During small deformation tests, the presence of milk proteins and starch did have a significant influence on the measured properties. At any constant carrageenan concentration, the substitution of SMP by starch led to an increase of both Gp and Tg : This does not mean that SMP itself causes a weaker gel and a lower Tg ; but indicates that starch has a more positive effect on these properties.
Acknowledgements The authors wish to thank Geert Maesmans and Koenraad Swinnen (Amylum, Belgium) and Johan L’Ecluse (Degussa Texturant Systems, Belgium) for their scientific input and Benny Lewille for his help in the laboratory.
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