Effects of Gelation Rate on the Rheological Properties of Polysaccharides

Y.Nitta, S.Gao, R.Takahashi and K.Nishinari Graduate School of Osaka City University, Sumiyoshi, Osaka 558-8585, Japan

Phone:06-6605-2818 Facimile:06-6605-3086 e-mail: nisinari@life.osaka-cu.ac.jp

It has been shown that the saturated elastic modulus of gels increased with decreasing rate of gelation for gelatin solutions. However, there have been contradictory reports on the gelation of agarose. The gelation rate of these cold-set thermoreversible gels is determined mainly by the temperature.1 For the gelation of gelatin, gelation proceeds faster at lower temperatures than at higher temperatures for the isothermal gelation. For the gelation of agarose, Morris and his coworkers2 showed that the elastic modulus of slowly formed gels was higher than those fast formed. On the other hand, Norton and his coworkers3 reported that the stronger gels were formed when solutions were annealed at lower temperatures, and that the solutions showed the phase separation when kept at higher temperatures even though it is lower than the helix-coil temperature. One of the difficulties in the study of this problem is occurrence of the slippage during the dynamic viscoelastic measurements in shear mode.

Zhang et al4 showed that the maximum observed in the oscillatory shear measurements to detect the storage and loss shear moduli should be attributed to the slippage because they did not observe any maximum in the measurement of compressive force during gelation of konjac glucomannan. Nitta et al also showed that it was possible to prevent the slippage using the serrated geometry in the oscillatory shear measurements for gellan solutions. Nitta et al showed that the storage shear modulus increased with decreasing gelation rate for gellan solutions by controlling the cooling rate. Gao et al prepared the konjac glucomannan with different degrees of acetylation, and the gelation rate was slow for higher degrees of acetylation in the presence of alkali sodium carbonate. They found that the saturated elastic modulus became higher with increasing degree of acetylation (Poster Ipa17 by Gao).

In the present poster presentation, effect of gelation rate on the rheological properties of gellan and konjac glucomannan will be discussed based on these experimental results in comparison with previous works on the gelation of agarose and gelatin. 1.C. Michon et al, Int J Biol Macromol, 20, 259(1997). 2. M.Mohammed et al, Carbohydr Polym 36, 15(1998). 3. P. Aymard et al, Biopolymers, 59, 131(2001). 4. H. Zhang et al. Biopolymers. 59, 38 (2001).

Effects of Gelation Rate on the RheologicalProperties of Polysaccharides

Yoko Nitta, Shanjun Gao, Rheo Takahashi, Katsuyoshi Nishinari

Department of Food and Human Health SciencesGraduate School of Human Life Science

Osaka City Universitynisinari@life.osaka-cu.ac.jp

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Entangled Chains

Increase Solvation

Decrease Inherent Chain and Polyelectrolytic Solubility

Produce Nuclei

Gel Formation

Figure 1. Schematic diagram of a proposed gelation mechanism for KGM.

Gelation for KGMOH¯

Deacetylation

Aggregation

Williams et al Biomacromolecules, 2000

Percolation

Gelation for gellan

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Disordered ChainsCooling

Helix formationAggregation

Gel formation

Figure 2. Schematic diagram of a proposed gelation mechanism for gellan.

The aim of this study

• The degree of acetylation of KGM• The cooling rate for gellan• The curing temperature

will influence gelation rate of the polysaccharide-water systems.

The effect of these factors on the rheological and related properties was investigated in order to elucidate relation between gelation kinetics and gel properties of polysaccharides.

Table 1. Effect of amount of pyridine and temperature on extent of acetylation and the results of viscosity measurements of the products.

9.709.9111.010.39.8810.910.112.0Mv × 10-5

480487524500486520493557[� ] a (cm3 g-1)

0.420.310.300.230.190.180.160.05DS

10.157.577.405.854.824.474.131.38DA (%)

3222222Time (h)

40805050404040Temp (oC)

102.52.521.510.5Pyridine (mL)

Ac-DAc6Ac5Ac4 Ac3Ac2Ac1RsSample

* DA (amount of acetyl-substituted residues in the KGM backbone) was examined by a modified Eberstadt method including saponification and successive titration, which is based on the ASTM volumetric method used to determine acetyl content in cellulose acetate. DS (the degrees of substitution)

The intrinsic viscosity measurements of KGM cadoxen solutions were carried out at 25 ± 0.02 oC by using an Ubbelohde type viscometer. The viscosity-average molecular weights (Mv) of the KGM samples were calculated according to the [� ] = 3.55×10-2 � M0.69 (Nishinari et al., 1993).

Results of Acetylation

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0 200 400 600 800 1000 1200

102

103

104

0 50 100 150 200

102

103

104

G' /

Pa

Time / min

G' (

Pa)

Time (min)

Ac1 Ac2 Ac3 Ac4Ac5 Ac6 AcD Rs

Figure 3. Time dependence of G� of 2.0 wt% KGM aqueous dispersions in the presence of Na2CO3 at 50 oC and at a frequency of 1 rad·s-1. The concentration ratio of Na2CO3 to KGM was fixed to 0.4.

Effect of degree of acetylation on Gelation Behavior of KGM

0 2 4 6 8 100

50

100

150

200

250

tcr of Rs

Code DA tcr Rs 1.38 18 Ac1 4.13 35 Ac2 4.47 44 Ac3 4.82 54 Ac4 5.85 69 Ac5 7.4 81 Ac6 7.57 86 Ac-D 10.1 224

t cr, t

* (m

in)

DA (%)

tcr

2 4 6 8 10

1x104

2x104

3x104

4x104

5x104

6x104

7x104

8x104

G'sat of Rs

G' 20

(Pa)

DA (%)

Figure 4. The critical gelation time (tcr) (A) and the G� at 20 h (G�20) (B) obtained from Figure 5 as a function of DA.

A

B

4

0 100 200 300 400 500 600

0.0

3.0x103

6.0x103

9.0x103

1.2x104

G' (

Pa)

Time (min)

Rs Ac1 Ac2 Ac3 Ac4 Ac5 Ac6 AcD

Figure 5. Time dependence of G� of 2.0 wt% KGM aqueous dispersions in the presence of Na2CO3 at 50 oC and at 1 rad·s-1. The ratio of Na2CO3 concentration to DA (CNa / DA) was fixed to 0.1

Gelation behaviors at a fixed ratio of Na2CO3 Concentration to DA (CNa / DA)

0 2 4 6 8 100

50

100

150

200

250

t cr (m

in)

DA (%)

CNa/DA = 0.2 CNa/DA = 0.1 CNa = 0.8 wt%

Figure 6. tcr of KGM samples as a function of DA in the case of CNa / DA as 0.2 from Figure 7 and as 0.1, respectively. The dotted lines represent tcr in the case that Na2CO3 concentration (CNa) was fixed as 0.8 wt% from Figure 5.

The close critical gelation time (tcr) was observed at the same deacetylation rate

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32.80.20

0.967910440 ± 57615.21 ± 3.936.10.18

0.991011726 ± 46811.37 ± 1.2848.10.14

0.997713741 ± 2817.28 ± 0.3866.40.1

0.992712448 ± 6493.25 ± 0.401400.05

rG�sat (MPa)k × 103 (min-1)tcr (min)CNa/DA

0.884112402 ±2363

10.2 ± 1.7540.00.25

0.99829157 ± 599.0 ± 0.355.00.2

0.94157037 ± 3513.38 ± 0.5791.00.15

0.94486701 ± 3062.18 ± 0.361550.1

2680.07

4800.05

rG�sat (MPa)k × 103 (min-1)tcr (min)CNa/DA

Table 2. The critical gelation time (tcr) and parameters of the first order kinetics model for the gelation of 2 wt% Ac1 and Ac-D dispersion at different CNa/DA ratios.

The approximation of evolution of G� vs time at constant temperature corresponding to KGM gelation processes by an equation of first order kinetics: G�(t) = G�sat (1-e-k(t-t0)) where G�sat: plateau value of G� after a long time, k: rate constant of gelation process, and t0: gelation time. (Nishinari et al., F

ood

H

yd

r

ocolloid

s

, 1999)

Effect of deacetylation rate on gelation behavior

Ac-D

Ac1

0.1

40

400

-1.047

-1.525

t cr (m

in)

CNa (DA)

Ac-D Ac1

The power law dependence of critical gelationtime (tcr) on deacetylation rate (CNa / DA)

Figure 7. Double logarithmic representation of CNa / DA dependence of critical gelation time tcr for 2 wt% aqueous dispersion of Ac1 ( ) and Ac-D ( ) obtained from Table 2.

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The distinct difference in the DA dependence of gelation times and saturated moduli for the KGM samples when the gelation condition was set at a fixed alkaline concentration independent of DA or a fixed ratio of CNa / DA, respectively, suggested that deacetylation rate governed the gelation kinetics of KGM samples.

At a fixed alkaline concentration, the aqueous dispersions of KGM samples with higher DA formed more elastic gels, at least in the DA range of the present work.

For the gelation of KGM, temperature, alkaline concentration, and KGM concentration all affected the gelation kinetics and the elastic modulus of KGM gel.

The apparent activation energy (Ea) for the gelation of KGM samples was found to be independent of DA, and an average Ea of 110.6 ± 1.1 kJ · mol-1 was observed.

CONCLUSIONS for KGM

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K-gellan gum

n

OCH2OH

OH

OH

OCH3

OHOH

O

O

O

OH

COO-M+

OCH2OH

OH

O OOH

OH

The metal content of K-gellan in the present studyNa, 0.19%, K, 2.68%, Ca, 0.512%, Mg, 0.146%.

The weight and number average molecular weight of the present gellansample after converting the sample into tetramethyl ammonium form

Mw = 240000 (by light scattering, Okamoto et al., Food Hydrocoll., 1993) Mn = 50000 (by osmometry, Ogawa, Food Hydrocoll., 1993)

Gellan gum powder was swollen in distilled water and stirred at 40overnight. The 1.6% (w/w) gellan dispersion prepared in this way was heated at 80 for 2 h and at 90 for 10 min and then cooled at various cooling rates to obtain gels.

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Figure 8. Chemical structure of gellan gum.

20 30 40 50 60 70 80 90 10010-1

100

101

0.5oC/min 1oC/min ~15oC/min

0 100 200 300 400 500

104

0.5oC/min

1oC/min

~15oC/min

Figure 10. Time dependence of the storage Young’s modulus E for the 1.6 wt% K-gellan gels.

Effect of cooling rate on the properties of gellan gels

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Temperature / °C

E /

Pa

Dis

plac

emen

t / m

m

Time / min

TTmm

0.5 0.5 /min: 96.6 /min: 96.6

1 1 /min: 87.6 /min: 87.6

~15 ~15 /min: 81.1 /min: 81.1

Lower cooling rate Lower cooling rate Higher values of Higher values of EE andand TTmm

Figure 11. Results of the falling ball test for 1.6% K-gellan gels.

0 20 40 60 80 10020

40

60

80

~15 C/min 1 C/min

0.5 C/min

Time / minTime / min

Tem

pera

ture

/ °C

EE0.5 0.5 /min: 34000 Pa/min: 34000 Pa

1 1 /min: 24000 Pa/min: 24000 Pa

~15 ~15 /min: 4200 Pa/min: 4200 Pa

Figure 9. Temperature of a circulator ( ) and a sample (solid line) in cooling process from 70 to 25 C.

Figure 12. Time dependence of the storage and loss shear moduli, G(solid line) and G (dotted line) of 1.6% K-gellan solutions.

Effect of temperatures on gelation behaviour of gellan

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Higher storage temperature Higher storage temperature

Higher values of Higher values of GG atat 50h curing50h curing

Higher values of THigher values of Tmm at 50hat 50h curingcuring(data not shown)(data not shown)

Time / min

G, G

/ Pa

InterpretationInterpretationHigher storage temperature or lower cooling rateHigher storage temperature or lower cooling rate

Longer helix formation and less kinetic trappingLonger helix formation and less kinetic trapping

Network with higher thermal stability and rigidityNetwork with higher thermal stability and rigidity

Higher values of Tm and the elastic modulusHigher values of Tm and the elastic modulus

102 103101

102

103

104

105

40oC35oC30oC

K-gellan gel formation was controlled by kinetics of gelation.

Higher values of E' or G' and Tm of gels induced by storage at higher temperatures or by lower cooling rates were thought to bedue to longer helix formation and less kinetic trapping of helixformation and/or aggregation between helices by network formation.

CONCLUSIONS for GELLAN

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