European Food Research and Technology

, Volume 234, Issue 2, pp 305–313

Rheological characterization and activation energy values of binary mixtures of gellan

  • Rafael Emilio González-Cuello
  • Emma Gloria Ramos-Ramírez
  • Alfredo Cruz-Orea
  • Juan Alfredo Salazar-Montoya
Original Paper

DOI: 10.1007/s00217-011-1626-2

Cite this article as:
González-Cuello, R.E., Ramos-Ramírez, E.G., Cruz-Orea, A. et al. Eur Food Res Technol (2012) 234: 305. doi:10.1007/s00217-011-1626-2

Abstract

The present study determined the flow behavior and activation energy of high (HA) and low (LA) acyl gellan dispersions (0.2%) and their mixtures as a function of preparation temperature (25 and 90 °C) and of the presence or absence of Ca2+ (30 mM). Heated gellan mixtures containing calcium were acidified with δ-gluconolactone to obtain gels and determine linear viscoelasticity using the Kelvin–Voigt model. The studied dispersions showed non-Newtonian shear-thinning behavior. HA dispersions (with and without Ca2+) showed the highest activation energy values, 88.60 and 51.18 kJ/mol. Whereas, LA dispersions showed the lowest activation energy values, 3.73 and 9.19 kJ/mol. With respect to the rheological studies, it was observed that the relationships between HA and LA gellan did not affect the recovery percentages because similar values were obtained (86.90–90.00%), and this behavior along with the mean viscosity values obtained in the gel mixtures could indicate that the hydrogen bond formation between both gellan helix (HA, LA) is possible. These results can contribute to possible industrial applications of gellans in the development of new alimentary products.

Keywords

High and low acyl gellan Viscoelasticity Kelvin–Voigt model Activation energy 

Introduction

Polysaccharides have been used in the alimentary industry as ingredient substitutes, mainly to produce hypocaloric foods [1]. Gellan gum is an anionic extracellular heteropolysaccharide produced by the bacterium Sphingomonas paucimobilis and consists of repeating units of a tetrasaccharide (1,3-β-d-glucose; 1,4-β-d-glucuronic acid; 1,4 β-d-glucose; and 1,4-α-L-rhamnose) [2]. At low concentrations, gellan is able to modify the rheological characteristics of some foods. Therefore, it is currently being used at industrial level [3]. Native gellan is known as high acyl (HA) gellan because it presents both an acetate group (C6) and a glycerate group (C2) in its glucose residue [4]. When HA gellan is exposed to strong alkali treatment at high temperatures, the acyl groups are hydrolyzed and low acyl (LA) gellan is obtained [5].

The structural differences between HA and LA gellan allow great diversity of its textural properties [5]. HA gellan gum forms soft, elastic gels; whereas LA gellan gum forms strong, brittle gels [6]. Mixtures of the two gellans can produce gels with a wide variety of intermediate properties depending on the HA to LA gellan proportions [7, 8].

The mechanisms of gelation of HA and LA gellan have been studied [9, 10]; moreover, spectroscopic characterization [6], light dispersion [2], structural changes by circular dichroism analysis, nuclear magnetic resonance [11], and rheological properties of liquid systems containing gellan [12, 13, 14] have been determined; nevertheless, the viscoelastic behavior and activation energy values of HA and LA gellan have not been determined yet. The determination of the activation energy parameter is useful at industrial level, for example, to select covering materials for microencapsulation [15]. Rheological studies are key considering that they provide information about the gelation process and may be used to improve the textural attributes provided by gellan gum to food products [16].

The creep recovery curves are obtained in terms of compliance J(t), which is the quantitative relationship between the stress applied to the sample and the resulting deformation [17]. Creep recovery defines the recovery level of the system. The mathematical representation of the stress/recovery relationship employs different models including springs and dashpot. When these elements are lined up in series or in parallel, they determine viscoelastic properties of solids and fluids. There are mathematical equations that relate the applied stress with the resulting deformation [18]. The Maxwell, Kelvin–Voigt, and Burgers are the main models employed for viscoelastic characterization. The present study used the Kelvin–Voigt model, which describes the behavior of a viscoelastic solid [19]. The aim of this study was to determinate the rheological characterization of binary mixtures of HA and LA gellan along with the determination of the activation energy of their dispersions and the viscoelasticity of their gels.

Materials and methods

Materials

Food-grade biopolymers were used. HA (Kelcogel®LT100, CAS No: 71010-52-1) and LA gellan (Kelcogel®F, CAS No: 71010-52-1) were purchased at Kelco Biopolymers (Monsanto, USA). The CaCO3 (CAS No: 471-34-1) and δ-gluconolactone (C6H10O6), (CAS No: 90-80-2) were obtained at Sigma-Aldrich (St. Louis, USA).

Dispersions

Gellan dispersions were prepared at 0.2% which is recommended for use in food products [20] HA, LA, and mixtures 25HA/75LA, 50HA/50LA, 75HA/25LA were employed for this study. Biopolymers were dissolved by constant shaking on a Cimarec-3 hot plate stirrer with a temperature control system (Thermolyne, Cole Palmer. USA). The mentioned dispersions were prepared at two different temperatures (25 and 90 °C) with the presence or absence of Ca2+ (30 mM). The mixtures were then cooled to room temperature and subsequently refrigerated (4 °C) for 24 h before flow behavior determination [21].

Flow behavior determination of dispersions

The flow behavior of the HA and LA gellan dispersions was determined at 25 °C with a LS 100 low-stress rheometer (Paar-Physica, Germany) with torques at 1 and 2 mNm for dispersions of the HA gellan and the mixtures 25HA/75LA, 50HA/50LA, and 75HA/25LA that were prepared at 90 °C. For mixtures prepared at 25 °C, torques of 0.1–0.5 mNm were applied using the DG-1 concentric cylinder geometry with diameter of 48 mm and length of 36 mm for shear rate and shear stresses ranging from 1.220 × 10−4 to 2.450 × 10−3 s−1 and from 6.37 × 10−3 to 6.37 × 101 Pa, respectively. The rheological parameters were calculated with the Paar Physica ver. 2.06-E software according to the Ostwald-de-Waele model (Eq. 1)
$$ \sigma = K\gamma^{n}. $$
(1)

Activation energy determination

The viscous flow of polymeric materials may be envisaged as a thermal process [22] in which, the molecules have to exceed an energy boundary in order to move an adjacent spot. As temperature rises, the thermal energy of the molecules also increases. When a liquid flows, the liquid layers of molecules slide over each other and overcome the intermolecular forces that cause resistance to flow. It was under this premise that Eyring [23] first stated the possibility of modeling the relationship between viscosity and temperature using the Arrhenius equation:
$$ \eta = Ae^{{E{\text{f}}/RT}} , $$
(2)
where η is material viscosity (Pa.s), T is temperature (°K), A is the pre-exponential factor, Ef is the flow activation energy, and R is the universal gas constant (8.314 × 10−3 kJ/mol °K), [24, 25, 26, 27, 28] by plotting ln η versus 1/T, a straight line is obtained with the slope Ef/R. Viscosity values were determined at different temperatures (25, 35, 45, and 55 °C).

Dynamic rheological study of gellan gels

Heated gellan dispersions added with Ca2+ underwent acidification with δ-gluconolactone (up to pH 4.5) to release the Ca2+ ion by substitution reaction of the calcium salt; furthermore, the rheological changes of the gels were investigated. All tests were performed with an LS 100 low-stress rheometer (Paar-Physica, Germany) at 25 ± 0.1 °C. A parallel plate geometry (PP20) of 20 mm of diameter was used to apply shear rates between 5.000 × 10−5 and 1.000 × 103 s−1 and shear stresses between 6.370 × 10−1 and 6.370 × 103 Pa. An amplitude scan was performed with torques from 1 × 10−3 to 1 × 101 mNm to determine the linear viscoelasticity zone (LVZ), which is obtained when the storage modulus (G′) and the loss modulus (G″) are independent of amplitude. A frequency scan was subsequently performed to determine the frequency-related behavior of the viscous and elastic components.

Creep curves

This behavior consists of two phases: creep and recovery. Initially, the application of stress takes place; then, after stress is suspended, the system’s recovery and the characteristic parameters representing this behavior are evaluated [29]. Gel recovery was determined when stress was suspended and the material recovered its original shape. The obtained results are expressed in terms of compliance and calculated by the Kelvin–Voigt model (Eq. 3). The 2.06 E version of Paar Physica software was used to calculate the dynamic parameters that are: zero-shear viscosity (η0), instantaneous elastic compliance J0 (Pa−1), retarded compliance Ji (Pa−1), and relaxation time λrel (s). All the conducted test was done at 25 ± 0.1 °C in a LS 100 low-stress Paar Physica rheometer
$$ J(t) = J_{\rm o} + \sum\limits_{i = 1}^{n} {J_{1} \left( {1 - e^{ - t/\lambda rel} } \right) + t/\eta_{\rm o} .} $$
(3)

Statistical analysis

The data were analyzed with ANOVA (one way) in order to determine statistically significant differences (p < 0.05) among the samples. This was accomplished, employing the software SPSS (version 13.0 for Windows). All the determinations were done in triplicate.

Results and discussion

Flow behavior of HA and LA gellan mixture dispersions

All the studied dispersions showed a non-Newtonian shear-thinning behavior in which viscosity diminished as shear rate increased. The same type of behavior was observed by Lee et al. [30] in dispersions of gellan and gelatin mixtures, and by Miyoshi and Nishinari [31] where they employed 1% gellan samples at temperatures ranging from 5 to 20 °C. The latter authors attribute this behavior to the conformational changes of gellan, which transforms from a spiral to a helix and is easily oriented by the shear flow.

Adjusting the experimental values obtained in this work to the Ostwald-de-Waele model was possible that determinate the rheological parameters such as consistency coefficient (K) and flow behavior index (n); the adjustments were adequate because correlation coefficients between 0.99 and 1.00 were obtained. The flow behavior index is a measure of the pseudoplasticity from one material, while the consistency coefficient let give magnitude to the viscosity; both of these parameters are useful in process design at industrial level [19].

Figure 1 (a–j) shows the effect of temperature increase in the HA and LA gellan dispersions with and without calcium. In general terms, the dispersions with Ca2+ subjected to heating (90 °C) showed higher viscosity (1.48 times) than dispersions without Ca2+. The opposite effect was observed when dispersions were not exposed to heat (90 °C). A possible explanation for this phenomenon is the calcium carbonate crystallization process reported by Butler et al. [32, 33]. Most likely, some gellan helix stay trapped inside the calcium carbonate crystals and these are deposited by gravity into the bottom of the dispersions; therefore, the viscosity system is reduced.
Fig. 1

Viscosity behavior of gellans dispersions at two different temperatures. Dispersions a, c, e, g, i were prepared at 90 °C, while the dispersions b, d, f, h, j were prepared at 25 °C

The viscosity values obtained for heating dispersions were 21.090 Pa.s for HA gellan with Ca2+; whereas HA gellan without Ca2+ showed a viscosity of 13.708 Pa.s. Likewise, viscosities for 75HA/25LA gellan resulted on 12.049 Pa.s and 7.286 Pa.s for gellan with Ca2+ and without Ca2+, respectively. These were significantly different (p < 0.05) from the viscosity values of the other dispersions. Higher HA gellan content produced greater viscosities. Moreover, dispersions with higher LA gellan content subjected to different conditions (heating and calcium presence) showed no significant differences in viscosity (0.004–0.005 Pa.s), which indicates that the viscosity is determined by the HA and LA gellan ratio. This may be because HA and LA gellan present different gelification mechanisms.

In the HA and 75HA/25LA gellan dispersions, heating (90 °C) substantially influenced the viscosity and the consistency coefficient (K) of the samples with and without Ca2+, compared with those which were not exposed to heating. The K values were directly proportional to the viscosity results, showing that the highest coefficients for the HA gellan dispersion (22.207 Pa.sn with Ca2+; 10.062 Pa.sn without Ca2+), while K values for LA gellan dispersions were lower (0.003–0.005 Pa.sn). The K value depends on the molecular weight of the biopolymer as reported by Yánez-Fernández et al. [21].

The monomer composition also can affect the K values [34], and this is clear in the binary mixture of gellan (HA and LA), since the acyl group presence hinder the ionic bonds formation between carboxyl groups of gellan (steric hindrance), [35]. It is interesting to mention that this hindrance does not affect the low acyl gellan. These behaviors markedly influence both, the viscosity and the flow behavior index; therefore, they depend on the HA and LA relations in the mixture.

The flow behavior index values of all dispersions were below one unit confirming that dispersion behavior was that of non-Newtonian shear-thinning fluids [19]. The highest index values were those presented by LA gellan dispersions with Ca2+ (0.987) and without Ca2+ (0.981) while all other dispersions showed values from 0.276 (25HA/75LA without Ca2+) and 0.807 (50HA/50LA with Ca2+). The different values of flow behavior index obtained in this work are due to the different gelation mechanisms of both gellan (HA and LA), [36, 37]. The flow behavior index means how close or far is the liquid behavior with respect to the Newtonian fluid.

Activation energy

An estimate of the activation energy (Ea) values of biopolymers and their mixtures provides important information for adequate selection and application of these materials, which are utilized in processes such as microencapsulation of active compounds [15].

The Ea values of the binary gellan mixtures are shown in Table 1. The highest Ea value was obtained for the HA gellan dispersion subjected to a heating (90 °C), (88.60 kJ/mol), followed by the HA gellan dispersion exposed to heating (90 °C) and Ca2+ (51.18 kJ/mol) and the 75HA/25LA gellan mixture also subjected to a heating process and under the presence of calcium (34.48 kJ/mol). It is interesting to mention that these Ea results are higher than those found by Takigawa et al. [12], who reported an Ea value for gellan of 30 kJ/mol; conversely, LA gellan dispersions in general showed lower Ea values (from 3.73 to 9.19 kJ/mol), mainly due to the fact that this gellan does not gelify as a result of heating, and calcium ions are not released from the calcium salt used herein (CaCO3).
Table 1

Rheological parameters of the Ostwald-de-Waele model of binary gellan mixtures and their energy of activation values

Gellan mixtures

Heat 90 °C

Ca2+ 30 (mM)

η (Pa s)

K (Pa sn)

n (–)

R2 (–)

Ea (KJ/mol)

HA (%)

LA (%)

0

100

X

X

0.005c

0.005c

0.886d

1.000

9.19e

0

100

X

0.005c

0.005c

0.893d

1.000

5.73f

0

100

X

0.004c

0.003c

0.987e

0.998

7.28g

0

100

0.004c

0.003c

0.981e

0.998

3.73h

25

75

X

X

0.009c

0.097c

0.664f

0.990

18.87i

25

75

X

0.008c

0.042c

0.409b

0.995

16.27j

25

75

X

0.006c

0.008c

0.745g

1.000

14.46k

25

75

0.007c

0.012c

0.799h

1.000

12.58l

50

50

X

X

1.626f

5.208e

0.303i

0.990

26.65m

50

50

X

1.450g

0.141c

0.611j

0.998

21.61n

50

50

X

0.012c

0.043c

0.807h

0.999

13.15º

50

50

0.012c

0.049c

0.794h

0.999

15.46p

75

25

X

X

12.049g

9.668bf

0.318k

0.999

34.48q

75

25

X

7.286h

7.812f

0.276l

0.991

28.87r

75

25

X

0.024c

0.229c

0.615j

0.999

14.24s

75

25

0.057d

0.587c

0.535m

0.999

10.01t

100

0

X

X

21.090ª

22.207ª

0.421a

0.990

51.18ª

100

0

X

13.708b

10.062b

0.419ab

0.990

88.60b

100

0

X

0.092d

0.371c

0.445c

0.999

44.89c

100

0

0.054d

0.596c

0.427ª

0.999

40.92d

Rows with no common letter showed statistically significant difference (significance level <0.05)

η = viscosity; K = consistency coefficient; n = flow behavior index; R2 = correlation coefficients

– = without Ca2+, heating or both; X = with Ca2+, heating or both; Ea = activation energy

(–) = dimensionless

In general terms, the samples exposed to a heating process (90 °C) displayed higher Ea (5.73–88.60 kJ/mol) than those that were not exposed to high temperature (25 °C). These discrepancies were probably due to the different gelation mechanism of the gellans. In the HA gellan, a steric hindrance is presented; hence, the gelation mechanism of that gellan is carried out by hydrogen bonds between gellan helix. Otherwise, the low acyl gellan gelifies by ion calcium interaction with carboxyl group present in the low acyl gellan helix (ionic bonds), [38, 39, 40]; as a result, we observe an augment in Ea with increasing proportions of HA gellan.

The determinations of the Ea of biopolymers play a key role in designing the appropriate microencapsulation procedure. Re [41] states that biopolymers used for microencapsulation should present thermal properties such as low diffusivity and high Ea to protect the active compound during the drying process. Ea is the amount of energy needed to evaporate the water content when drying the biopolymer [15]. Therefore, the HA gellan dispersion prepared at 90 °C shows adequate properties to be used in the microencapsulation process, because of its high Ea value (88.60 kJ/mol).

The Ea values obtained in the present study are lower than those reported by Nickerson et al. [16], which ranged from 106.80 to 146.90 kJ/mol for gellan dispersions in the presence of different sucrose concentrations. Sworn and Kasapis [42] reported an Ea of 87.00 kJ/mol for 0.72% gellan dispersions with 70% co-solutes and of 89.00 kJ/mol for 0.77% gellan dispersions with 75% co-solutes, indicating that an increase in solute concentration leads to an increase in the Ea value [26, 43].

One-way ANOVA of Ea values revealed significant differences (p < 0.05) among the studied gellan dispersions due to the different HA gellan content and probably to the presence of Ca2+, which seems to have a determining effect on this parameter.

Rheological dynamics study

The storage modulus (G′), the loss modulus (G″), and the phase angle (δ) characterize the system in the study of rheological dynamics. G′ is a measure of the energy temporarily stored in a material. G″ is a measure of the energy used to activate a flow, energy that is dissipated and transformed into heat [19]. The third parameter that described the viscoelastic behavior of a material is tan δ, which is also a function of frequency. Tan δ indicates the relationship between the amounts of dissipated and stored energy, i.e., the quantitative relationship between the viscous and the elastic components of a system [29].

Figure 2 shows the behavior of the storage modulus (G′) and the loss modulus (G″) as a function of frequency in HA, LA, and mixed (HA/LA) gellans at 0.2%. The elastic behavior was greater than the viscous behavior (G′ > G″) for all the gels studied in the frequency intervals (0.01–0.18 Hz), and the dependency of the moduli on frequency was minimal. This characteristic behavior indicates that the gel is strong [44]. These results agree with Takigawa et al. [12], who found the same behavior for the dynamic moduli in a 5% gellan system. In addition, Rodriguez et al. [45] reported that 0.005–0.01% and 0.03% gellan systems containing 10 mM CaCl2 showed typical gel behavior (G′ > G″) in which a rise in the concentration of the biopolymer showed an increase in the viscoelastic character of the systems containing gellan.
Fig. 2

Behavior of dynamic moduli G′ and G″ as a function of the frequency of HA and LA gellan gels and their mixtures

Table 2 shows the dynamic parameters of HA and LA gellan gels and their mixtures (HA/LA).
Table 2

Material functions of HA and LA gellan gels and their mixtures

Gellan mixtures

Torque (mNm)

G′ (Pa)

G″ (Pa)

tan δ (–)

HA (%)

LA (%)

0

100

0.003

165

134

0.81

25

75

0.019

2,300

160

0.07

50

50

0.003

2,150

138

0.06

75

25

0.003

464

28.5

0.06

100

0

0.003

41.6

2.24

0.05

G′ = storage modulus; G″ = loss modulus; tan δ = phase angle

The torque amplitude was within the interval from 0.003 to 0.019 mNm. G′ was approximately 18.5 times greater than G″ for HA gellan gels, while for LA gellan gels, the difference was much smaller (1.23 times greater). It is interesting to note that all the gellan mixtures have higher G′ values with respect to the pure gellan, this is likely by a possible interaction between both gellan (HA and LA) by hydrogen bonds formation. Yoshimura et al. [46] showed that G′ is less dependent on the frequency than G″ in corn starch dispersions, which indicates a gel-type behavior where G′ > G″ [44] showing little or no dependency on the frequency. This coincides with results reported by Rodriguez et al. [47] who observed a strong gel behavior in low concentration (0.005–0.500%) LA gellan. In contrast, Sánchez et al. [48] studied the viscoelastic properties of gellan gels at different temperatures (1–60 °C) and found that G″ > G′; this result was probably obtained because they studied dissolved samples with no incorporation of ions.

In the present study, the increase in HA gellan content in the mixtures was inversely proportional to the G′ values, which disagrees with Rodriguez et al. [49], who states that the increment in gellan is directly proportional to an increase in the dynamic moduli for 3–7% corn starch with 0.1–0.3% gellan gum. Besides, values for tan δ < 1 for all gels were found to range between 0.05 and 0.81, being 0.05 the lowest tan δ value that corresponds to HA gellan; as the LA gellan content increased, this value also increased until it reached 0.81, which was obtained in LA gellan gels.

Creep curves

Figure 3 depicts the creep and recovery curves of all gellan mixtures that showed recovery after suspending the application of stress. Thus, the HA and LA gellan gels and their mixtures (HA/LA) at 0.2% behaved as viscoelastic solids. The greatest recovery values were found for HA gellan gels (90.00%) and for the mixture containing 75HA/25LA (88.82%), while LA gellan gels showed lowest mean recovery percentage (86.90%). In general terms, all gels shown recoveries similar to those reported by Jimenez-Avalos et al. [29] for 3 and 5% corn starch (85.25 and 96.67%, respectively) with torques of 0.001 and 0.007 mNm.
Fig. 3

Creep curves of HA and LA gellan gels and their mixtures

Table 3 shows the rheological parameters of the Kelvin–Voigt model for HA and LA gellan gels and their mixtures, as well as the recovery values for each gel, obtained in the linear viscoelastic region (LVR), (0.002–0.004 mNm). HA gellan gels showed viscosity values between 4.344 × 104 and 5.331 × 104 Pa.s, while LA gellans displayed the highest viscosity values (between 8.058 × 105 and 9.942 × 10Pa.s). Gellan mixtures showed viscosity values between 8.470 × 104 and 1.218 × 10Pa.s for HA and LA gels, indicating that HA/LA gellan mixtures have different physical properties than those of gels obtained with either HA or LA gellans alone [5, 6, 50]. This behavior indicates an interaction between both gellan with a possible hydrogen bonds formation although more studies are needed in order to confirm this hypothesis.
Table 3

Rheological parameters of the Kelvin–Voigt model of gellan mixtures

Gellan mixtures

Torque (mNm)

Viscosity (Pa.s)

J0 (Pa−1)

J1 (Pa−1)

λrel (s)

Recovery (%)

HA (%)

LA (%)

0

100

0.0022

9.942 × 105

6.997 × 10−4

1.000 × 10−6

4.157 × 10−2

87.40

0

100

0.0031

9.373 × 105

6.976 × 10−4

1.000 × 10−6

4.335 × 10−2

88.21

0

100

0.0043

8.058 × 105

5.443 × 10−4

2.520 × 10−4

2.480 × 101

85.09

25

75

0.0183

6.686 × 105

3.780 × 10−4

3.300 × 10−4

4.675 × 101

83.99

25

75

0.0254

7.414 × 105

5.052 × 10−4

1.000 × 10−6

4.443 × 10−2

86.60

25

75

0.0300

6.541 × 105

4.978 × 10−4

1.000 × 10−6

3.626 × 10−2

92.30

50

50

0.0031

7.296 × 105

3.739 × 10−4

3.688 × 10−4

3.163

85.69

50

50

0.0036

1.142 × 106

6.722 × 10−4

1.000 × 10−6

1.280 × 101

89.20

50

50

0.0043

1.218 × 106

6.063 × 10−4

1.000 × 10−6

6.687

88.60

75

25

0.0031

8.881 × 104

8.982 × 10−3

3.607 × 10−4

4.750 × 101

92.07

75

25

0.0036

8.470 × 104

9.063 × 10−3

4.380 × 10−4

4.391 × 101

86.00

75

25

0.0043

1.219 × 105

3.004 × 10−2

1.000 × 10−6

8.270 × 101

88.40

100

0

0.0026

4.344 × 104

2.536 × 10−2

1.000 × 10−6

1.510 × 105

87.90

100

0

0.0031

4.689 × 104

2.564 × 10−2

1.000 × 10−6

8.929 × 102

88.50

100

0

0.0043

5.331 × 104

2.55 × 10−2

1.000 × 10−6

3.270 × 101

93.60

J0 = instantaneous compliance; J1 = retarded compliance; λrel = relaxation time

It should be noted that all gels showed lower values of retarded compliance than instantaneous compliance (J1 < J0), since they need to deform initially before a flow is established, and deform again later by the stress application. The J0 values are inversely proportional to the viscosity values considering that compliance is the opposite of applied stress [29]. The percentages of J0 increase with respect to J1 for HA and LA gellan gels and the 75HA/25LA, 50HA/50LA, and 25HA/75LA mixtures were calculated to be 25.50, 7.63, 5.92, 4.44, and 4.14%, respectively, which shows that values decrease with diminishing HA gellan content.

Relaxation time (λrel) results showed a wide variation ranging from (3.626 × 10−2 to 1.510 × 105 s).

It is important to mention that the relaxation time is the lapse required for the applied stress to diminish 1/e (approximately 63.2%) of its initial value under constant deformation. Relaxation time for HA gellan gels ranged from (3.270 × 101 to 8.929 × 102 s), which were higher than those obtained by LA gellan gels (4.157 × 10−2 to 2.480 × 101 s). Moreover, the higher relaxation times for HA gellan gels are also a great indicative of the higher recovery values.

Conclusions

All gellan dispersions showed non-Newtonian shear-thinning behavior with the adequate adjustment to the Ostwald-de-Waele model. Viscosity results obtained for gellan dispersions showed that greater dependency was exerted by the HA, LA proportions, and exposure to heat than that obtained by the presence of Ca2+. With respect to the activation energy, the HA gellan dispersion is recommended for microencapsulation process because it has the highest values (88.60 and 51.18 kJ/mol); furthermore, the increase in mean activation energy value (30.90%) was proportional to the rise in HA gellan concentration. It is interesting to mention that the relationships between HA and LA gellan did not affect the recovery percentages because were obtained similar values between 86.90 and 90.00%; this behavior along with the mean viscosities values also obtained in gels could indicates that the hydrogen bond formation between both gellan helix (HA and LA) is possible. It should be noted that the results obtained herein can contribute to the development of new products or applications containing functional biopolymers such as gellan.

Acknowledgments

The authors wish to thank CONACyT for the scholarship No. 296286 to REGC and Miguel Márquez for technical support.

Copyright information

© Springer-Verlag 2011

Authors and Affiliations

  • Rafael Emilio González-Cuello
    • 1
  • Emma Gloria Ramos-Ramírez
    • 1
  • Alfredo Cruz-Orea
    • 2
  • Juan Alfredo Salazar-Montoya
    • 1
  1. 1.Department of Biotechnology and BioengineeringCINVESTAV-IPNDF, MéxicoMexico
  2. 2.Department of PhysicsCINVESTAV-IPNDF, MéxicoMexico

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