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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’
Increasing the ionic strength is reflected not only by temperature transconformation higher but also by increasing the rigidity of the gels obtained. Thus, Hermansson (1989) showed that increasing the concentration of KCl leads to greater rigidity of gels without altering the overall characteristics. Furthermore, synergistic effects have been described in the case of transfer of ions of different nature from that of ion-cons (Hermansson, Eriksson & Jordansson, 1991). Thus, κ-carrageenan as Na+ at a concentration of 1% form gels in the presence of 250 mM NaCl and 20 mM KCl. A small amount of Ca2+ (60 mM) also allows gelation of κ-carrageenan as sodium. Similarly, synergistic effects were observed during the gelation of K+ κ-carrageenan in the presence of calcium ions: the gels obtained are much more rigid only in the presence of K+ ions.
2.3 POLYSACCHARIDE MIXTURES
2.3.1 Phase Behavior
Understanding the nature of interactions and mechanisms involved in mixtures of polysaccharides have been numerous studies (Morris, 1998; Stephen, 1995; Tolstoguzov, 2003a, b, c; Zasypkin, Braudo & Tolstoguzov, 1997). The bases that determine the thermodynamic behavior of mixtures of biopolymers in aqueous medium are the same as those of mixtures of synthetic polymers in a solvent. There is no fundamental difference between the two. When two polymers are mixed in solution, two types of behavior can be observed.
The total miscibility is characterized by a stable homogeneous environment; both polymers are co-solutes “indifferent” one relative to another, forming a single phase. This situation can occur when the system is much diluted, occurs very rarely.
The phenomenon is often observed phase separation. The polymers are distributed unevenly in the mixture and two phases coexist within the system. The phase separation can be “associative” or “segregated”. The phenomena of demixing mixtures depend strongly on the nature of interactions existing between the biopolymers. The attractive interactions can be specific (ionic) or nonspecific (Van der Waals, hydrogen, hydrophobic, ionic). The repulsive interactions, nonspecific, may be of electrostatic nature and / or related to a mechanism of excluded volume.
The associative phase separation or “complex coacervation” results in separation of two liquid phases: the two polymers are combined in the same phase, while the other phase contains mainly the solvent. It results in the formation of soluble or insoluble complexes that can precipitate. This type of phase separation typically occurs within a mixture of polyelectrolytes on oppositely charged such as a protein (a pH below its isoelectric point) and an anionic polysaccharide.
The phase separation “segregated” is a rather commonly found in mixtures of polysaccharides has been studied for many systems: dextran/methylcellulose (Albertsson, 2006) Amyloid/dextran(Kalichevsky, Orford & Ring, 1986), amylose/amylopectin (Kalichevsky & Ring, 1987), dextran/agarose (Medin & Janson, 1993), LBG/dextran (Garnier, Schorsch & Doublier, 1995), pectin/pectin (MacDougall, Rigby & Ring, 1997), guar/amylopectin (Closs, Conde-Petit, Roberts, Tolstoguzov & Escher, 1999), amylose/κ-carrageenan (Tecante & Doublier, 2002), guar-dextran (Simonet, Garnier & Doublier, 2000, 2002), hydroxypropyl Starch/κ-carrageenan (Chaudemanche, 2007). This list is not exhaustive but reflects the number of mixtures of polysaccharides imcompatibles that have been studied.
The low entropy of mixing resulting from the macromolecular nature and the endothermic enthalpy of mixing explain the inconsistency between these entities. Two polymers are thus more difficult to combine their respective molecular weights are high. The effect of ionic strength and pH were determined in the case of polyelectrolytes. The presence of ionic groups on one of the components reduces the tendency to phase separation by increasing the area of compatibility.
2.3.2 Thermodynamic Incompatibility
Most non-ionic mixtures are incompatible in solution at low concentrations (Zasypkin, Braudo & Tolstoguzov, 1997). In thermodynamics equilibrium, mixtures of polymers can be described by their phase diagram. The first phase diagrams of mixtures of polysaccharides in aqueous solution have been established for methylcellulose-dextran systems (Albertsson, 2006). The components of different nature are incompatible from a thermodynamic standpoint. Each of the two phases formed is enriched in one polymer, the solvent being divided between the two. In thermodynamics equilibrium, each phase contains a majority of two polymers and a smaller amount of another polymer.
Figure 2.24 is the phase diagram mixture of hydroxypropyl waxy starch/κ-carrageenan at 25 °C (Chaudemanche, 2007). The phenomenon of separation between polysaccharides has also been treated theoretically in order to establish rules of behavior or predict the experimental phase diagrams of existing (Clark, 2000; Simonet, Garnier & Doublier, 2000). Most of these studies refer to model systems such as aqueous mixtures polyethyleneoxide (PEO) dextran.
Figure 2. 27: Phase diagram at 25 °C mixture of waxy hydroxypropyl starch/κ-carrageenan (Chaudemanche, 2007).
The phenomenon of thermodynamic incompatibility is explained by the fact that biopolymers of different nature cannot occupy the same volume in solution when no specific interaction between them. This usually means that their inconsistency makes them mutually exclusive, and thereby concentrates in each phase (Zasypkin, Braudo & Tolstoguzov, 1997). These effects of excluded volume can lead, when biopolymers the ability to gel, accelerating the rates of gelation, reduction of the critical concentration of gelation, and sometimes to improve the mechanical properties of gels.
2.3.3 Gels based on mixtures polysaccharides
According to the phase behavior of mixtures, gels based on mixtures of polysaccharides can be divided into three categories (Morris, 1998; Zasypkin, Braudo & Tolstoguzov, 1997): ‘ mixed or interpenetrating’ gels, ‘complex’ gels, and ‘filled or separated’ gels.
A mixed or interpenetrated gel consists of two polysaccharides which gel separately, forming two independent networks. These structures cover the entire volume, but there is no specific interaction between them. These gels are also classified as interpenetrating gels.
The complex gels also called coupled gels involve the intermolecular association of the components forming a single network. The description of the mechanisms of association in the gel is still controversial. Many literature reviews describe these mechanism associations for binary polysaccharide gels (Morris, Rees & Robinson, 1980; Stephen, 1995). Some models, for systems established carrageenan-carob and xanthan-carob, assume that the polysaccharides interact and form junction zones specific cooperatives (Dea, Morris, Rees, Welsh, Barnes & Price, 1977).
A filled gel contains a network comprising the gelling polymer that forms the continuous phase with dispersed particles enriched by the other polymer. The dispersed phase may be in the form of solid or liquid particles, which play a role of support or reinforcement that swells the network. The typical example of a gel is filled gel starch grains and legumes. The continuous phase consists of a network of amylose and the dispersed phase by swollen starch grains composed mainly of amylopectin. The influence of botanical origin and the transformation process on the behavior of gels can then be explained by laws of mixtures applied to composites.
18.104.22.168 Rheological properties
Interactions between components are often characterized by synergies represented by the formation of thermo-gel in conditions where the components alone does not gel (Doublier, 1994; Foster & Norton, 2002). Under the selected conditions, a synergistic or antagonistic effect can be demonstrated in relation to the properties of biopolymers alone. The antagonistic effect is reflected in a weakening of the gel compared to the constituents alone. In the case of gels filled, increasing the volume fraction of particles incorporated in the matrix may decrease the elasticity of the gels (Zasypkin, Braudo & Tolstoguzov, 1997).
The synergistic interactions in binary mixtures of polysaccharides are defined by a strengthening of the gel and/or solution viscosity of the mixture over each of the biopolymers alone. These interactions are often regarded as synonyms of intermolecular bonds between the two polysaccharides leading to gels complex (Cairns, Atkins, Miles & Morris, 1991; Stephen, 1995). However, the thermodynamic incompatibility of the constituents can also lead to synergy effects (Zasypkin, Braudo & Tolstoguzov, 1997). In solution, the synergistic effect is due to the effects of excluded volume effects resulting concentration in the mixture. During the formation of a gel, they come from the effects of concentration in the continuous phase rich in gelling agent (Zasypkin, Braudo & Tolstoguzov, 1997)
22.214.171.124 Rheology of blends of starch
126.96.36.199.1 Solution state
The rheological properties of starch suspensions in the presence of polysaccharides have been many studies. The presence of polysaccharides such as xanthan, guar, carob, causing a considerable increase in viscosity of starch (Alloncle & Doublier, 1991; Christianson, Hodge, Osborne & Detroy, 1981). These effects were explained in various ways. It has been suggested, not taking into account the granular phase, which increase the viscosity of the mixture is due to complexation between the molecules of starch and soluble polysaccharide added (Christianson, Hodge, Osborne & Detroy, 1981; Eidam, Kulicke, Kuhn & Stute, 1995). Alloncle et al. (1991) suggested that increasing the viscosity of mixtures (amidonguar, starch-carob, or starch-xanthan) can be attributed to an artificial increase in the concentration of polysaccharide in the continuous phase due to the swelling of grains of starch for the stiffening. However, in mixtures waxy starch-xanthan, increased dynamic modules with the progressive addition of starch to solutions of xanthan can be explained by the effects of concentration in the continuous phase. Some authors have then proposed a mechanism of depletion-flocculation (Abdulmola, Hember, Richardson & Morris, 1996).
188.8.131.52.2 Gel state
The viscoelastic properties of gels based on starch and polysaccharides have also been described. If the properties of starch show significant increases in viscosity, the rheological properties of gels obtained after demotion does not undergo such dramatic changes. These are mainly governed by the volume fraction of the dispersed phase (grains) and that of the continuous phase. Usually the addition of polysaccharides accelerates the formation of starch gels. This phenomenon has been observed for κ-carrageenan but with the addition of ι-carrageenan the opposite effect (Eidam, Kulicke, Kuhn & Stute, 1995). From a general standpoint, the gel-starch polysaccharides may be described as composite gels in which the effects of thermodynamic incompatibility (Alloncle & Doublier, 1991; Eidam, Kulicke, Kuhn & Stute, 1995; Ptaszek, Berski, Ptaszek, Witczak, Repelewicz & Grzesik, 2009; Ptaszek & Grzesik, 2007), association of grain by mechanisms of depletion-flocculation (Abdulmola, Hember, Richardson & Morris, 1996), or the formation of a second network permeating the starch gel (Mohammed, Hember, Richardson & Morris, 1998) may occur.
CHAPTER 3: MATERIALS AND METHODS
Two gelatin samples of bovine origin were purchased from Sigma Aldrich (Kuala Lumpur, Malaysia) and SIM (Penang, Malaysia) with bloom number 160. The first sample was of higher grade than the second one which was an industrial type. Moisture content of the samples was 10–11% (wb).
The κ-carrageenan used was purchased from Sigma Aldrich. It was a commercial sample and was mainly in the K+ form. Its characteristics were provided by the supplier. It contained 191.8 meq/100 g K+ (7.5%), 87 meq/100 g Na+ (2%), 0.6% chloride, 10 meq/100 g Ca2+ (0.4%), and 1.23 meq/100g Mg2+ (0.03%). Its average molecular weight was 780,000 g/mol. The moisture content was 10% (wb).
3.1.3 Acid hydrolyzed hydroxypropylated cassava starch
Acid hydrolyzed hydroxypropylated cassava starch (HHSS) was generated following the methods described Fouladi et al (2013). Four types of dually modified cassava starch were selected for testing based on solubility and economic factors. The characteristics of these samples are described by Fouladi et al (2013). The samples were acid hydrolyzed for 6, 12, 18, and 24 h, and all were hydroxypropylated with 20% propylene oxide.
3.2.1 Preparation of solutions
184.108.40.206 Gelatin solutions
Gelatin comes in the granulated form. The gelatin was allowed to swell in deionized water (18 MΩ) for 10 min at room temperature and then placed in a water bath at 60 °C for 45 min under medium agitation and under vacuum to eliminate air bubbles. The concentration was then adjusted by adding water, and the solution was kept at 50 °C for 30 min. The final concentrations expressed on a dry basis (db) ranged from 20 to 30%. This range of concentrations was based on what is used in industrial applications. The measured pH was 5.5.
220.127.116.11 Starch and κ-carrageenan solutions
Modified starch was prepared at concentrations ~20% (db) by dispersing the powder in deionized water (18 MΩcm) with vigorous stirring at room temperature. The dispersion was then placed in a water bath at 90 °C for 30 min. Residual bubbles in the starch were eliminated by slow stirring at 70 °C for 15 min. The suspension then was kept at 55 °C under slow stirring for 30 min.
The κ-carrageenan solutions were prepared by slowly dispersing the powder in deionized water with vigorous stirring at room temperature. The solution was then placed at 80 °C for 45 min. The solubilization was completed and residual bubbles were eliminated by maintaining the solution at 70 °C for 15 min under slow stirring. The solution then was maintained at a temperature of 55 °C for 30 min.
To prepare mixtures of starch/κ-carrageenan, the components were mixed dry and then dispersed in deionized water at room temperature under vigorous stirring. The dispersion was then placed at 90 °C for 45 min to ensure complete solubilization of the constituents. The homogeneous solution was placed at 70 °C for 2 h to remove residual air bubbles, followed by storage at 55 °C for 30 min. The chosen concentration of starch (A) was ~20% (db). This concentration was chosen based on preliminary dipping process experiments intended to produce a dried film with a thickness near that of pharmaceutical hard capsules (0.1 mm). The concentrations of κ-carrageenan ranged from 1 to 0.1%, with corresponding KCl concentrations ranging from 0 to 11.70 mM. The use of κ-carrageenan made it possible to control the temperature of the helix-coil transition by adjusting the total content of K+. Based on the diagram provided by Rinaudo and Rochas (1981), a gelation temperature of about 25 °C was chosen for κ-carrageenan alone (Figure 3.1).
Figure 3.1: Phase diagram of κ-carrageenan representing the variation of transition temperature on cooling and heating according to the total concentration of potassium (Rochas & Rinaudo, 1980)
Whatever the concentration of κ-carrageenan in the range studied (0.1 to 1%), the total content of K+ (CT) was kept constant by the ratio of the KCl added and the polymer concentration through the following relationship:
CT = CS + γ CP Equation 3.1
Where γ is 0.55, CT is total ion concentration, Cs is ion concentration added by salt, and CP is ion concentration provided by the polymer (Lafargue, Lourdin & Doublier, 2007; Rochas & Rinaudo, 1980). The content of K+ κ-carrageenan was 2.36 × 10-3 eq/g (M0 = 424 g/mol). For concentration of 1% κ-carrageenan without salt added, the total ionic concentration is CT = (2.36 × 10-3 eq/g) × (10g/l) × 0.55 =1.3 × 10-2 eq/l.
Table 3.1 lists the various compositions studied.
Table 3.1: Compositions of the starch- κ-carrageenan solution
HHSS κC 1
HHSS κC 0.75
HHSS κC 0.5
HHSS κC 0.25
HHSS κC 0.1
CT constant = 1.3 × 10–2 eq/l, κC: κ-carrageenan
3.2.2 Rheological properties
The rheological measurements were performed using two rheometers: an AR1000 rheometer (TA Instruments Inc., New Cattle, DE, USA) equipped with cone and plate geometry stainless steel (diameter: 40 mm, angle: 2°; truncation: 54 μm) and a CSL100 rheometer (TA Instruments Inc., New Cattle, DE, USA) with cone and plate geometry (diameter: 40 mm, angle: 2°; truncation: 54 μm). The preliminary experiments were carried out with CSL100 rheometer and the final experiment design was carried out with AR1000 rheometer. For both instruments, temperature was controlled by a Peltier thermostat on the lower plate. During measurements, samples were covered with paraffin oil to avoid evaporation.
Standard procedures were established to characterize the rheological behavior of the samples under near industrial processing conditions to simulate the different stages of the development of hard capsules. The linear viscoelastic region (LVR) was determined prior to dynamic viscoelastic studies. This is the first step in measuring the properties of samples at a temperature of 50 °C (temperature at which stock solutions of gelatin are stored). Then, the behavior of gels during cooling and heating were measured in order to determine the key parameters of interest in this study, namely the kinetics of gel formation, the gelling temperature, and melting forces. The following tests were then carried out on each sample:
18.104.22.168 Flow properties
The samples were loaded on the rheometer plate and heated to 60 °C. The flow curves were performed at 50 °C in linear mode (from 0 to 100 s–1 and back from 100 to 0 s–1) and logarithmic mode (100 to 0.01 s-1 with stabilization for 1 min between each point).
22.214.171.124 Viscoelastic properties
The samples were loaded on the rheometer plate and heated to 60 °C. Several tests were carried out on the samples:
• A mechanical spectrum was achieved at 50 °C in the linear area of 100 to 0.1 rad/s with strains of 0.1%.
• A cooling ramp from 50 °C to 20 °C with rates of 1 °C/min was achieved (strain amplitude of 0.1%, frequency measurement 1 rad/s).
• A mechanical spectrum was achieved at 20 °C in the linear area of 100 to 0.1 rad/s to strains of 0.1%.
• A heating ramp of 20 °C to 50 °C with rates of 1 °C/min was then performed (strain amplitude of 0.1%, frequency measurement 1 rad/s).
• Finally, after storage at 60 °C for 1 h, the variation of the modulus G’ and G’’ over time at 20 °C after rapid cooling from 60 °C (30 °C/min) was recorded.
CHAPTER 4: RESULTS AND DISCUSSIONS
4.1 Rheological behavior of gelatin
In this section, the rheological behavior of gelatin was characterized in conditions similar to those used in the process of obtaining capsules in industry. The goal of these experiments was to yield reference data to be used in the context of this research project.
4.1.1 Gelatin solution at 50 °C
The flow properties were determined at 50 °C for gelatin concentrations of 20 to 30% (g/g of water). In this range of concentrations, Newtonian behavior was observed (Figure 4.1) which is in agreement with literature reports (Leuenberger, 1991; Wulansari, Mitchell, Blanshard & Paterson, 1998). The viscosities are shown in Table 4.1.
Figure 4.1: Newtonian behavior of gelatin at 50 °C and 20% concentration.
Table 4.1: Changes in viscosity of gelatin as a function of concentration. Experiments were performed at 50 °C
The mechanical spectrum of 25% gelatin at 50 °C is shown in Figure 4.2. The modulus G” is greater than G’ over the entire frequency range considered. Moreover, a strong dependence of modulus with frequency was observed: G” α ω1 and G’ α ω1.8. This was observed for all gelatin concentrations studied and reflects the behavior the macromolecular solution of gelatin at this temperature.
Figure 4.2: Mechanical spectrum of 25% gelatin solution. G’: filled symbols, G”: empty symbols. Experiments were performed at 50 °C, strain amplitude was 1%
4.1.2 Sol-gel transitions
Variations of GV and G during cooling between 50 and 25 °C are shown in Figure 4.3 at a concentration of 25% gelatin. At 50 °C, G. was greater than G’ and the value of the elastic modulus was low (GG ≈ 5×10-2 Pa and G ≈ 1 Pa). Between 50 and 37 °C, the modulus varied slightly; the modulus increased rapidly between 37 and 25 °C. At a temperature of 32 °C, the moduli G’ and GG intersected. The point of intersection was taken as the gelation temperature (TGEL), marking the transition from one state (solution) to another (gel) state. It is widely acknowledged that it is the variations of tan δ (G /G/) versus temperature at various frequencies of measurement that must be considered; the freezing point (or melting point) is defined as the intersection curves of tan δ. This method was originally proposed for chemical gels (Winter & Chambon, 1986) and has been used for characterization of physical gels such as gelatin, ι-carrageenan (Michon, Cuvelier & Launay, 1993), κ-carrageenan (Núñez-Santiago & Tecante, 2007), and various biopolymers (Doublier & Cuvelier, 2006). The criterion of transition G’ = G” obtained by temperature ramps at a fixed frequency is justified by the rapid changes of modulus G’ and G” during the transition and the similarity of the transition temperatures compared with those obtained by the method of Winter and Chambon (1986).
After the transition temperature for gelatin in these experiments, the modulus reached values of about 6×103 Pa for G’ and 5 × 102 Pa for G” at 25 ºC. The mechanisms of gelation of gelatin are well known. At sufficiently low temperature, the gelatin chains undergo a conformational transition called a ball-screw and form a thermo-reversible network by the association of helices at junction zones stabilized by hydrogen bonds (Djabourov & Papon, 1983; Ross-Murphy, 1992; te Nijenhuis, 1981, 1997). These areas form a junction structure consisting of triple helices similar to those of collagen (Harrington & Rao, 1970; Veis, 1964).
Figure 4.3: Storage and loss moduli G , G, for a 25% gelatin sample during a cooling ramp. Temperature was ramped from 50 to 20 °C at 1°C/min. Frequency: 1 rad/s. Strain amplitude: 1%
Variations of GV and G during heating of the gel between 25 °C and 50 °C are shown in Figure 4.4. At 25 °C, the value of the modulus was significantly greater than that observed at the end of the ramp cooling. This reflects the evolution of the gel over time when stored at 25 °C for about 30 minutes. At this temperature, G ~ 4×104 Pa and GP ~ 2×103 Pa. During the heating, storage modulus and loss modulus cross at 37 °C, thus defining the melting temperature (TM) corresponding to the transition from gel to sol. The modulus then decrease gradually until 50 °C; the values at this temperature were similar to those obtained before the cooling ramp. This demonstrates the thermo-reversible nature of gelatin gels.
Figure 4.4: Storage and loss moduli G , G as a function of temperature during a heating ramp of a 25% gelatin sample. Temperature was ramped from 20 °C to 50 °C at 1 °C/min. Frequency: 1 rad/s. Strain amplitude: 1%
The temperatures of gelling and melting for different gelatin concentrations studied are given in Table 4.2. There was a slight increase in TGEL from 30 to 33 °C when the concentration was increased from 20% to 30%. The melting temperature TM was greater by several degrees (3 to 5 °C) compared to TGEL and varies in a comparable manner.
Table 4.2: Gelation temperatures, TGEL and melting temperature TM (G’= G”) during cooling from 50 to 25 °C and heating from 25 to 50 °C. The rate of heating or cooling was 1°C/min. Frequency: 1 rad/s. Strain amplitude: 1%.
The state diagrams described by Michon and others (1993) showed that sol-gel temperatures are function of gelatin concentration. A hysteresis was observed (<1 °C at a concentration of 20%) between the transition temperatures and was attributed to the non-equilibrium state of the gel (Michon, Cuvelier & Launay, 1993). Moreover, it has been shown that the phenomenon of hysteresis corresponds to cooperative effects between hydrogen bonds of the triple helices and disruption of these helices (upon heating) that requires more energy than does their creation (upon cooling). Hysteresis also depends upon heating/cooling rates.
4.1.3 Viscoelastic properties of gelatin gels at 20 °C
The mechanical spectrum of gelatin at 20 °C is shown in Figure 5.5. The viscoelastic behavior was dominated by the elastic component G’. Over the entire frequency range, G’ was much higher than G” (with 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
126.96.36.199 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.
– Flow properties of κ-carrageenan