Effects of Gelation Rate on the Rheological Properties of Polysaccharides

Transcription

1 Effects of Gelation Rate on the Rheological Properties of Polysaccharides Y.Nitta, S.Gao, R.Takahashi and K.Nishinari Graduate School of saka City University, Sumiyoshi, saka , Japan Phone: Facimile: 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 coworkers 2 showed that the elastic modulus of slowly formed gels was higher than those fast formed. n the other hand, Norton and his coworkers 3 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. ne of the difficulties in the study of this problem is occurrence of the slippage during the dynamic viscoelastic measurements in shear mode. Zhang et al 4 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).

2 Effects of Gelation Rate on the Rheological Properties of Polysaccharides Yoko Nitta, Shanjun Gao, Rheo Takahashi, Katsuyoshi Nishinari Department of Food and Human Health Sciences Graduate School of Human Life Science saka City University 1

3 Gelation for KGM Entangled Chains H Increase Solvation Deacetylation Decrease Inherent Chain and Polyelectrolytic Solubility Aggregation Williams et al Biomacromolecules, 2000 Produce Nuclei Gel Formation Percolation Gelation for gellan Figure 1. Schematic diagram of a proposed gelation mechanism for KGM. Disordered Chains Cooling Helix formation Aggregation Gel formation 2 Figure 2. Schematic diagram of a proposed gelation mechanism for gellan.

4 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.

5 3 Results of Acetylation Table 1. Effect of amount of pyridine and temperature on extent of acetylation and the results of viscosity measurements of the products. Sample Rs Ac1 Ac2 Ac3 Ac4 Ac5 Ac6 Ac-D Pyridine (ml) Temp ( o C) Time (h) DA (%) DS [ ] a (cm 3 g -1 ) M v * 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 o C by using an Ubbelohde type viscometer. The viscosity-average molecular weights (Mv) of the KGM samples were calculated according to the [ ] = M 0.69 (Nishinari et al., 1993).

6 4 Effect of degree of acetylation on Gelation Behavior of KGM Ac1 Ac2 Ac3 Ac4 Ac5 Ac6 AcD Rs, t* (min) Code DA Rs Ac Ac Ac Ac Ac Ac Ac-D A 50 of Rs G' (Pa) 10 3 G' / Pa x10 4 7x10 4 6x10 4 5x10 4 DA (%) Time / min G' 20 (Pa) 4x10 4 3x10 4 2x10 4 G' sat of Rs B Time (min) 1x DA (%) Figure 3. Time dependence of G of 2.0 wt% KGM aqueous dispersions in the presence of Na 2 C 3 at 50 o C and at a frequency of 1 rad s -1. The concentration ratio of Na 2 C 3 to KGM was fixed to 0.4. Figure 4. The critical gelation time ( ) (A) and the G at 20 h (G 20 ) (B) obtained from Figure 5 as a function of DA.

7 5 Gelation behaviors at a fixed ratio of Na 2 C 3 Concentration to DA (C Na / DA) The close critical gelation time ( ) was observed at the same deacetylation rate G' (Pa) 1.2x x x10 3 Rs Ac1 Ac2 Ac3 Ac4 Ac5 Ac6 AcD (min) C Na /DA = 0.2 C Na /DA = 0.1 C Na = 0.8 wt% 3.0x Time (min) DA (%) Figure 5. Time dependence of G of 2.0 wt% KGM aqueous dispersions in the presence of Na 2 C 3 at 50 o C and at 1 rad s -1. The ratio of Na 2 C 3 concentration to DA (C Na / DA) was fixed to 0.1 Figure 6. of KGM samples as a function of DA in the case of C Na / DA as 0.2 from Figure 7 and as 0.1, respectively. The dotted lines represent in the case that Na 2 C 3 concentration (C Na ) was fixed as 0.8 wt% from Figure 5.

8 Effect of deacetylation rate on gelation behavior C Na /DA C Na /DA (min) (min) k 10 3 (min -1 ) 3.25 ± ± ± ± 3.9 Ac1 k 10 3 (min -1 ) 2.18 ± 0.36 G sat (MPa) ± ± ± ± 576 G sat (MPa) 6701 ± 306 r r ± ± ± ± 59r ± ± d y s 0.1 H i d C Ac-D Na (DA) Table 2. The critical gelation time ( ) and parameters of the Figure 7. Double logarithmic representation of C first order kinetics model for the gelation of 2 wt% Ac1 and Ac-D dispersion at different C Na /DA ratios. d l lo Na / DA dependence of critical gelation time for 2 wt% aqueous dispersion of Ac1 ( ) and Ac-D ( ) obtained from Table 2. o The approximation of evolution of G vs time o at constant o temperature corresponding to KGM gelation processes by an equation of first order kinetics: G(t) = G sat (1-e -k(t-t 0 ) ) where G sat c : plateau value of G after a long time, k: rate constant of gelation process, and t 0 : gelation time. (Nishinari et al., F o, 1999) 6 (min) 400 The power law dependence of critical gelation time ( ) on deacetylation rate (C Na / DA) Ac-D Ac1

9 CNCLUSINS for KGM 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 C Na / 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 ± 1.1 kj mol -1 was observed.

10 K-gellan gum H CH 2 H C - M + H CH 2 H H CH 3 H H H H H n Figure 8. Chemical structure of gellan gum. The metal content of K-gellan in the present study Na, 0.19%, K, 2.68%, Ca, 0.512%, Mg, 0.146%. The weight and number average molecular weight of the present gellan sample after converting the sample into tetramethyl ammonium form Mw = (by light scattering, kamoto et al., Food Hydrocoll., 1993) Mn = (by osmometry, gawa, Food Hydrocoll., 1993) Gellan gum powder was swollen in distilled water and stirred at 40 overnight. 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.

11 0.5 o C/min Effect of cooling rate on the properties of gellan gels Temperature / C ~15 C/min 1 C/min / min Time / min 0.5 C/min Figure 9. Temperature of a circulator ( ) and a sample (solid line) in cooling process from 70 to 25 C. E 0.5 /min: Pa 1 /min: Pa ~15 /min: 4200 Pa Tm 0.5 /min: /min: 87.6 ~15 /min: 81.1 E / Pa o C/min 1 o C/min ~15 o C/min 1 o C/min ~15 o C/min Figure 10. Time dependence of the storage Young s modulus E for the 1.6 wt% K-gellan gels. Displacement / mm Time / min Lower cooling rate Higher values of E and Tm Temperature / C 9 Figure 11. Results of the falling ball test for 1.6% K-gellan gels.

12 Effect of temperatures on gelation behaviour of gellan G, G / Pa o C 35 o C 40 o C Higher storage temperature Higher values of G at 50h curing Higher values of TmT at 50h curing (data not shown) Time / min Figure 12. Time dependence of the storage and loss shear moduli, G (solid line) and G (dotted line) of 1.6% K-gellan solutions. Interpretation Higher storage temperature or lower cooling rate Longer helix formation and less kinetic trapping Network with higher thermal stability and rigidity Higher values of Tm and the elastic modulus 10

13 CNCLUSINS for GELLAN 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 be due to longer helix formation and less kinetic trapping of helix formation and/or aggregation between helices by network formation.