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K+ or high concentrations of polysaccharide, conditions under which the gels may show some syneresis, in connection with a large aggregation dimers propellers.
It is established that it is the ionic form of the polysaccharide and the presence of added salts and nature that determine the properties of gels and their thermoreversibility (Núñez-Santiago & Tecante, 2007). Rochas and Rinaudo (1984) showed the coincidence between the helix-coil transition and the sol-gel transition: during cooling, when the macromolecules are arranged κ-carrageenan as a propeller, the system gels. So for a κ-carrageenan as K+ and presence of KCl, the temperature of gelation and fusion are determined by the parameter CT defined in Equation 2.1. The temperatures of gelation and fusion are derived directly from the phase diagram presented previously (Figure 2.19).
For example, Figure 2.22 shows the evolution of storage and loss modules G’ and G” according to the temperature at a concentration of κ-carrageenan (K+ form) to 1% in the absence of added salt (Fernandes, Gonçalves & Doublier, 1992).
Figure 2. 25: Variations of G’ and G” as a function of temperature for a concentration of 1% κ-carrageenan, Frequency 1 Hz, Tg: temperature of gelation, Tm: melting temperature. Cooling G’ (■), G” (•). Heating G’ (□), G” (◊). (Fernandes, Gonçalves & Doublier, 1992).
The system is cooled to 55 °C to 5 °C and then warmed. The hysteresis shown by the difference between the gelation temperature Tg (for G’ G”) and the melting temperature Tm of the gel (for G’ 10 G”). G’ was constant over a wide frequency range (G’ ≈ 3 × 104 Pa). The weak dependency of G’ on frequency and much higher value of G’ than G” indicate that the system behaves as a gel (Clark & Ross-Murphy, 1987). The same behavior was observed for all gelatin concentrations studied.
Figure 4.5: Mechanical spectrum of 25% gelatin. G’: filled symbols, G”: empty symbols. The temperature was 20 °C. Strain amplitude: 1%.
The slight increase of G’ at low frequency was due to the fact that the gel evolved over time. The highest frequency used was 102 rad/s. The time required to obtain each point of the spectrum varied with the inverse of the frequency. Figure 4.6 illustrates this evolution of gelatin gel over time after rapid cooling from 60 to 20 °C (~30 °C/min). Since in this temperature range G’ is much higher than G”, gel formation was almost instantaneous. G’ increased rapidly over time and in the first 20 min of measurement reached about 3×104 Pa then increased more slowly until a near constant value was attained.
Figure 4.6: Changes in modulus G’ and G” as a function of time for a 27% gelatin gel. Measurement temperature was 20 ° C. Frequency: 1 rad / s. Strain amplitude: 1%.
After 6 h G’ was about 4 × 104 Pa. This aspect of the gelation kinetics of gelatin has been widely described (Ross-Murphy, 1992; te Nijenhuis, 1981, 1997), and research has demonstrated that gelatin gels never reach equilibrium. It has been shown that G’ increases over time due to increased junction zones in the network and the stiffening of the chains that connect them. These phenomena are attributed to greater degrees of helix formation (te Nijenhuis, 1997). It should also be noted that the stability of the gels depends on their thermal history (Michon, et al., 1993, te Nijenhuis, 1981). Increased storage time during cooling and/or a higher temperature storage led to greater stability of helices and consequently an increase in melting temperature of the gel.
Figure 4.7 shows that the modulus G’ varies with the concentration to the power 2.1 (G’ C2.1). However, it is accepted that G’ varies linearly with the square of the concentration over a wide concentration range (Ferry, 1948; te Nijenhuis, 1997). Previously it was observed that G’ did not equilibrate and continually increased over time. For this reason, the relationship between G’ and concentration varies from that observed other studies.
Figure 4.7: Changes in G’ as function of gelatin concentration. Data obtained after 6 h of time sweep measurement at 20 °C. Frequency: 1 rad/s. Strain amplitude: 1%.
In summary the rheological properties of gelatin solution at 50 °C show a Newtonian behavior with viscosities in the range from 0.2 to 1.2 Pa·s at concentrations of 20% to 30%. In this range of concentrations, the gelation temperature, TGEL, is 30 to 33 °C. The gels formed are thermo-reversible with storage modulus (G’) a function of the square of the gelatin concentration. For a concentration of 27% gelatin, the value of the storage modulus G’ at 20 °C was about 104 Pa.
4.2 Rheological behavior of starch-κ-carrageenan blends
The rheological properties of mixtures of hydrolyzed hydroxypropylated cassava starch (HHSS)/κ-carrageenan were determined under conditions similar to those used previously for gelatin; these conditions are similar to those used for the manufacture of pharmaceutical hard capsules. Four samples of HHSS with different times of hydrolysis (6, 12, 18 and 24 h) were analyzed. The HHSS at concentrations of 20 to 25% were mixed with κ-carrageenan at a concentration less than or equal to 1%. The total ionic concentration of the system (CT) was kept constant at 1.3 × 10-2 eq/l by adjusting the amount of KCl in order to control the temperature of gelation (Materials and Methods Table 3.1).
4.2.1 Rheological behavior at 50 °C
4.2.1.1 Dually modified cassava starch (HHSS)
– Flow properties
After heat treatment at 90 °C, the starch dispersions were transparent, suggesting a good solubilization. The flow properties of hydrolyzed hydroxypropylated cassava starch HHSS6, HH6612, HHSS18 and HHSS24, measured at 50 °C at a concentration of 25% (g/g), are shown in Figure 4.8. Variations of apparent viscosity, η, as a function of shear rate, γ, are represented in logarithmic scales. For all samples, the behaviors were that expected of shear thinning fluid. This behavior was more pronounced when the starch was hydrolyzed for a longer time; longer hydrolysis times led to starch of lower molecular weight. Moreover, the apparent viscosity decreased with decreasing molecular weight of samples. An increase in the apparent viscosity of starches at low shear rates was observed; the phenomenon was more pronounced when the molecular weight of starch was low. This indicates the presence of a threshold in flow. The existence of some type of particle fragments from the starch grains and/or aggregates of insoluble molecules could explain such behavior.
Figure 4.8: Flow curves of hydrolyzed hydroxypropylated cassava starch dispersions at a concentration of 25% (g/g): HHSS6 (●), HHSS12 (■), HHSS18 (o), HHSS24 ((). Measurements were performed at 50 °C
Figure 4.9 represents flow curves for HHSS12 and hysteresis loop between the ramp up and ramp down curves were analyzed for thixotropy. No thixotropy was observed. The same results were obtained for other dually modified starches. Existence of thixotropy could be effects on solution property over times. This is not acceptable for a gelatin alternative especially in dip-molding processing.
Figure 4.9: Flow curves for dually modified cassava starch (HHSS12) dispersions at a concentration of 25% (g/g). Measurement was performed at 50 °C
The flow curves for dually modified cassava starch (HHSS12) shown in Figure 4.10 were measured at 50 °C for concentrations of 20%, 23% and 25% starch. This figure indicates shear thinning as described. As expected, the apparent viscosity of starch increased with concentration.
Figure 4.10: Flow curves of dispersions of hydroxypropyl cassava starch HHSS12 at concentrations of 20% (■), 23% (●) and 25% (▲). Temperature was 50°C
– Viscoelastic properties of dually modified starches
The mechanical spectra at 50 °C for dually modified starches HHSS6, HHSS12, HHSS18 and HHSS24 at concentrations of 25% are shown in Figure 4.11. There was a strong dependence of G” on frequency with the slope close to 1. G'(ω) varied little at low frequencies and could not be measured at frequencies above about 15 rad/s. This is explained by the phase shift between G’ and G”, which is close to 90 degrees, making it difficult to measure G’ for these frequencies.
Figure 4.11: Mechanical spectra of different dually modified cassava starches at concentrations of 25%: a) HHSS6, b) HHSS12, c) HHSS18, d) HHSS24. G’: filled symbols, G”: empty symbols. Measurement temperature was 50 °C and strain amplitude was 1%
The viscoelastic moduli cross at a frequency that varies with the extent of hydrolysis of the starch. This crossover point occurs at frequencies of 0.3 rad/s for HHSS6, 0.9 rad/s for HHSS12, 2.5 rad/s for the HHSS18 and 5.7 rad/s for HHSS24. At lower frequencies, G’ is greater than G” and independent of frequency. This reflects a solid-like behavior with very low values of G’, about 10-1 Pa.
The dependence of G” on frequency (G” ~1) and the transparency of the starch samples suggest that macromolecular solutions were formed. However, the fact that G’ reached a plateau at low frequency likely due to presence of fragments of starch grains. Therefore, the dispersions of starches characterized in this study were assumed to consist largely of macromolecules in solution, but that a small quantity of starch is not fully dissolved and form aggregates. This explains the existence of a threshold in flow and viscoelastic behavior at low frequency.
4.2.1.2 κ-carrageenan
– Flow properties of κ-carrageenan
Newtonian

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