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Table 4.3: Viscosity of κ-carrageenan in different concentrations
Concentration of κ-carrageenan (%, w/v)
Figure 4.12: Newtonian behavior of κ-carrageenan in the concentration range of 0.25% to 1% at 50 °C
18.104.22.168 Dually modified cassava starch/κ-carrageenan blends
– Flow properties
The flow properties of starch/κ-carrageenan mixture were also determined at 50 °C. The total concentration of starch/κ-carrageenan was 25%. Figure 4.13 represents the flow curves of the HHSS12-κC0.5 mixture. The flow curves of dually modified starch HHSS12 at 20%, 23% and 25% and κ-carrageenan at 0.5% (κC0.5) are also presented. The mixture HHSS12-κC0.5 (total concentration 20%) behaved similarly to starch alone except that shear thinning was more pronounced. The apparent viscosity of the mixture was much higher than that of the constituents alone. The viscosity of the mixture was also higher than that of 25% starch. These results indicate a strong synergy between starch and κ-carrageenan at 50 °C.
Figure 4.13: Flow curves of the mixture HHSS12-κC0.5 (•), 20%HHSS12 and 0.5% κ-carrageenan, κC0, 5 (×), and starch dispersions HHSS12 20% (□), 23% (○) and 25% (Δ). The temperature was 50 °C
Non-thixotropic and non-Newtonian behavior of combinations of κ-carrageenan and dually modified cassava starches are shown in Figure 4.14.
Figure 4. 14: Flow curve of the HHSS12-κC0.5. Shear rate up 0 to 100 s-1 empty symbols, and down 100 to 0 s-1 filled symbols
– Influence of κ-carrageenan content in starch/κ-carrageenan blends
Figure 4.15 presents the flow curves of mixtures of 25% starch HHSS12 with concentrations of κ-carrageenan ranging from 0.25% to 1% compared with 25% starch HHSS12 alone. Increasing the concentration of κ-carrageenan in the mixture caused a significant increase in apparent viscosity with pronounced increases in shear thinning behavior. The shape of the flow curve of the mixture κC0.25-HHSS12 (0.25% κ-carrageenan) was similar to starch alone, with a slight synergistic effect. At lower concentration of κ-carrageenan (κ-C = 0.1%), the flow curves of the mixtures were comparable with that of starch alone, reflecting the limited influence of κ-carrageenan at this concentration (not shown at the figures).
Figure 4.15: Flow curves of mixtures of 25% starch HHSS12 with κ-carrageenan at different concentrations. Measurements were taken at 50 °C
– Influence of starch with different extents of hydrolysis in starch/κ-carrageenan blends
Figure 4.16 represents the flow curves at 50 °C of mixtures of starches hydrolyzed for various times and κC0.5. In all cases, the apparent viscosity of mixtures is much greater than that of starches alone (shows before), confirming the synergistic effect for all starches. Shear-thinning behavior was more pronounced when starches were less hydrolyzed, meaning that the molecular weights of the starches were higher.
Figure 4.16: Flow curves for 0.5% κ-carrageenan and mixtures of 25% dually modified cassava starches/κC0.5. Measurement temperature was 50 °C.
– Viscoelastic properties
Figure 4.17 shows the mechanical spectra at 50 °C of the mixture of HHSS12-κC0.5, 0.5% κ-carrageenan alone (κC0.5), and 25% HHSS12 alone. The κC0.5 shows a behavior typical of a macromolecular solution with G’α ω2 and G” α ω1. The point of intersection of the modulus is 100 rad/s. At this temperature, κ-carrageenan chains are in disordered conformation. This behavior was observed for all concentrations studied of κ-carrageenan.
The mechanical spectrum of the mixture HHSS12-κC0.5 is quite similar to that of starch alone. For both, G” depended on frequency (G’ α ω0.92) and G” was greater than G’ at high frequencies (10-100 rad/s). Also a typical behavior that reported for starch with G’ which tends to plateau at low frequencies. The point of intersection of the modulus was 0.25 rad/s. The mechanical spectrum of mixture HHSS12-κC0.5 actually resembled that of a 30% solution of starch. This behavior reflects the synergy between the major constituents of the mixture previously observed by viscometric measurements (Lafargue, Lourdin & Doublier, 2007).
Figure 4.17: Mechanical spectrum of κC0.5 (solid lines ■, □), HHSS12 (solid lines ●, ○), and the mixture κC0.5-HHSS12 (■, □). Concentration of HHSS12 alone was 25% and in combination total concentration was 25%. G’: filled symbols, G”: empty symbols. Measurement temperature: 50 ° C. Strain amplitude: 1%
4.2.2 Rheological behavior in sol-gel transitions (from 50 °C to 20 °C)
Viscoelastic modulus G’ and G” between 20 °C and 50 °C for κ-carrageenan at a concentration of 0.5% and for the HHSS12-κC0.5 mixture with total concentration of 25% are shown in Figure 4.18. The heating and cooling rates were 1 °C/min. CT was kept constant at 1.3 × 10-2 eq/l by adjustment of the K+ ion concentration. During the cooling at temperatures above 27 °C, the modulus G” was constant for κC0.5 (Figure 4.18a). In this temperature range, the κ-carrageenan behaves as a macromolecular solution. From 27 °C, G” rapidly increased and finally reached 10 Pa at 20 °C. If, as in the gelatin system, the gelation temperature of the system was defined as G’ = G”, then TGEL of κC0.5 is 25 °C. Without adjusting K+ ion concentration, previous studies showed that the gelation temperature was around 20 °C (Lafargue et al., 2007a).
For the κ0.5-HHSS12 mixture, G” was slightly higher than G’, reflecting the dispersal behavior previously described on the basis of the mechanical spectrum. From about 32 °C, both modulus increased and intersected at a temperature of 28 °C. At 20 °C, G’ was approximately 500 Pa and G” was about 60 Pa. At the beginning of heating ramp (Figure 4.18b), G’ and G” for κC0.5 and for the HHSS12-κ0.5 mixture were significantly higher than values obtained at the end of cooling. G’ and G” were approximately 50 Pa and 15 Pa for κC0.5 and 600 Pa and 80 Pa for the HHSS12-κC0.5 mixture. This difference reflects the evolution of the gel over time at 20 °C (approximately 30 minutes). The TM was 30°C for κC0.5 and 38°C for the mixture. The thermo-reversibility of κ-carrageenan gel-based mixtures was previously observed (Tomsic, Prossnigg & Glatter, 2008).
Figure 4.18: Variation of viscoelastic modulus G’ and G” as a function of temperature for κC0.5 and for the mixture of κC0.5 and HHSS12. a) Cooling from 50 °C to 20 °C. b) Heating from 20 °C to 50 °C. Heating/cooling rate: 1 °C/min. Frequency: 1 rad/s. Strain amplitude: 1%