Physical Characteristics of Calcium Induced K-Carrageenan Networks
Physical Characteristics of Calcium Induced K-Carrageenan Networks
Physical Characteristics of Calcium Induced K-Carrageenan Networks
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Abstract
The presence of an optimum counter-ion concentration in calcium-induced k-carrageenan gels at low polymer concentrations of 5 and
10 g/l is observed. At approximately the stoicheometric molar ratio of 1 calcium per carrageenan sulphate, a gel with high elastic modulus,
high optical clarity and fine network structure is observed. On further increase of counter-ion concentration beyond this optimum, elastic
modulus decreases significantly associated with sharp increase in the gels turbidity together with a network characterised with coarse and
large-pore mesh.
The quite complete characterisation of the various gel networks both mechanically by ways of oscillatory and static rheology and optically
by turbidimetry and cryo-SEM shows that the extensive structural charge neutralisation of the polysaccharide by divalent calcium ions is
responsible for a marked aggregation of the polymer strands reminiscent of precipitation. At lower counter-ion to polymer ratios, onset of
gelation might prevent such phase separation.
q 2003 Elsevier Ltd. All rights reserved.
Keywords: k-carrageenan; Calcium; Rheology; Turbidity; Young’s modulus; Cryo-SEM; Gelation; Aggregation; Phase separation
concentrations studied, whether or not the polymer Samples were prepared by adding volumes of desired salt
concentration was high enough to form a gel. In the case solution to samples of carrageenan completely dissolved in
of relatively high polymer concentration (21.6 g/l) these water and the solution were mixed under heating.
authors found that the elastic modulus was not affected by
divalent salt concentration above a certain threshold. They 2.2. Apparatus
also propose that the addition of cations beyond this critical
value contributes only to some localised heterogeneity Dynamic rheology measurements were carried out on a
within the gels without affecting the overall viscoelastic Rheometric Scientific SR2000 stress controlled rheometer
properties of the systems. with parallel plate geometry. Plate diameter was 40 mm and
Lai, Wong, & Lii (2000) also studied the effects of the gap was set to 0.2 mm. Measurements at small
increased calcium concentrations, albeit on un-purified amplitude oscillation were carried out at 1 rad s21 fre-
Eucheuma cottonii sourced polysaccharide. A decrease in quency and 1 or 3 Pa stress, for 5 and 10 g/l gels,
gel strength was reported for quite concentrated gels respectively. Samples were gelled to 5 8C for 30 min before
(15 g/l) above a calcium concentration ranging from 0.1 testing. Dynamic frequency sweeps and dynamic stress
to 0.5 M. This decrease was suggested to be due to sweeps were carried out to ensure that the frequencies and
syneresis. A recent study (Takemasa, Chiba, & Date, stresses used were within the linear viscoelastic domain. G0
2001) on 20 g/l systems comparing sodium, potassium values were ascertained from small amplitude oscillation
and calcium-form k-carrageenans, showed that upon measurements of gels by frequency sweeps in the plateau
gelation the calcium-form system became the most turbid region at 5 8C. Dynamic temperature ramp tests were used
and was characterised by the highest elastic modulus to ascertain the melting and setting temperatures of the gels.
even if its gelling temperature was relatively low The melting and setting points were defined as the crossover
(, 10 8C). This system was also characterised by a points of G0 and G00 curves. The temperature ramps were run
comparatively low strain-optical coefficient, which indi- at 1 8C per minute.
cates low anisotropy of polymer chain and little polymer Compressive stress experiments were carried out on an
chain reorientation caused by mechanical deformation of Instron 5544 (Instron Corp., Canton, MA, USA) fitted with a
the gel network. These results indicate that calcium 500 N load cell. The plunger was made of aluminium and
induces a high increase in branching number during had a contact surface diameter of 3.5 cm. Samples for test
gelation of k-carrageenan, probably due to further helical were moulded in inverted 3.5 cm polystyrene petri dishes
aggregation upon cooling. and were kept refrigerated at 5 8C for 18 h before testing.
In this study our objectives were to investigate the The samples were allowed to equilibrate to room tempera-
physico-chemical characteristics of k-carrageenan gels in ture (23 8C) for 1 h before being compressed at a crosshead
the semi-dilute regime as affected by calcium concen- speed of 2 mm/min. Young’s Modulus calculations were
trations, and to study the relationship between thermal carried out on the force and compression results at low
stability, microstructure and rheological properties. deformation (, 5%).
Turbidity measurements were made on a temperature
controlled Pharmacia LKB (NJ, USA) Ultrospec 3 UV-
2. Materials and methods Visible spectrometer at 450 nm maintained at 8 8C by a
circulating water bath.
2.1. Samples Cryogenic Scanning Electron Microscopy was carried
out on a Jeol 5410 (Akishima, Japan) scanning electron
k-Carrageenan was obtained from Quest International microscope with and Oxford Instruments (Oxford, UK)
(batch no. S9904186) under the trade name Deltagel 379. cryo-preparation chamber. The samples were loaded as gels
The pure sodium form was obtained according to a and then frozen in liquid nitrogen slush, fractured under
procedure derived from Ramzi et al. (1999). 1.5 g Of vacuum and sublimed at 2 80 8C for up to 45 min before
Deltagel 379 was dissolved in 300 ml of a hot 500 mM sputter coating with gold and examined under an accelera-
NaCl, 50 mM NaOH and 50 mM EDTA solution to remove tion voltage of 10 kV.
calcium and potassium ions and balance to pH7. This
solution was twice redissolved in NaCl solution with
precipitation in 2-propanol followed by washing in 2- 3. Results and discussion
propanol-water mix and final drying with pure 2-propanol.
The resulting k-carrageenan had 0.16 wt% Kþ, 8.98 wt% 3.1. Thermal stability
Naþ and 0.13 Caþ2 wt% by atomic absorption spectroscopy.
The specific viscosity of the polymer was assessed a Thermal stability was assessed at the point of crossover
25 8C in 0.1 M NaCl at 672 ml/g which corresponds to a of G0 and G00 in dynamic temperature ramps with stress
Mw of 415000 according to Rochas, Rinaudo, and control. The results of inverse of melting temperature, TM ;
Landry (1990). and gelling temperature, TG ; are plotted against the log CT in
P. MacArtain et al. / Carbohydrate Polymers 53 (2003) 395–400 397
Fig. 2. G0 variation with changing calcium to carrageenan ratio for 5 g/l (W)
and 10 g/l (X) k-carrageenan concentrations.
3.3.1. Turbidity
Fig. 6 shows the percentage transmittance of the samples
as a function of the calcium to carrageenan molar ratio.
Previously described phase diagrams (Michel et al.,
1997) show an abrupt transition from clear to turbid systems
at 0.02 M added calcium. As can be seen from Fig. 6 our
systems show a gradual decrease in percentage transmit-
tance with added calcium and no clear transition from clear
to opaque systems can be detected. This discrepancy Fig. 6. Turbidity profiles of 5 g/l (W) and 10 g/l (X) gels with changing
between the two studies may be due to the fact that our calcium to carrageenan ratios.
P. MacArtain et al. / Carbohydrate Polymers 53 (2003) 395–400 399
Fig. 7. Scanning Electron Microscopy images of carrageenan networks at: (a) 5 g/l, 1:1 ratio added Ca2þ; (b) 5 g/l, 4:1 ratio added Ca2þ; (c) 10 g/l, 1:1 ratio
added Ca2þ; (d) 10 g/l, 2:1 ratio added Ca2þ.
cooling. The increasing opacity of the samples with added and Swanson (2001). The formation of coarser strands and
calcium is a good reflection of additional polymer aggrega- consequent more open nature of the network at higher
tion due to excess divalent cation. counter-ion concentrations support the view that added
cations add to the localised aggregation of the strands. We
3.3.2. Scanning electron microscopy can therefore draw a very strong parallel between our k-
Further information about the microstructure of the gels carrageenan system and gellan. At low added divalent
was gained from scanning electron microscopy. Cryogenic cation concentration gellan samples show high gel strength,
preparation of samples with sublimation of water from the fine network structure and high water holding capacity,
network at optimum and high calcium concentrations gave whereas excess divalent cation is responsible for polymer
images as shown in Fig. 7. aggregation resulting in low gel strength and coarse
It can be seen that the 5 g/l sample (Fig. 7a) at 1:1 networks with syneresis (Mao et al., 2001; Milas &
calcium:carrageenan ratio has a fine network within the Rinaudo, 1996).
larger pores. The resulting pore sizes are small. This scheme
is mirrored at higher polymer concentrations of 10 g/l (Fig.
7c). At higher calcium/carrageenan ratios, a more open 4. Conclusion
network with thicker strands and wider pores corresponds to
gels that are weak and turbid. The network at 4:1 In this study of the structure of calcium induced k-
calcium:carrageenan ratio (Fig. 7b) at 5 g/l has visible and carrageenan gels a clear optimum calcium to carrageenan
quite defined strands compared with the fine strands seen in ratio is established in terms of elastic modulus, G0 ; and
the low calcium ratio sample. The calcium ratio of 2:1 at Young’s modulus, E: Increasing this optimum concentration
10 g/l polymer concentration shown in Fig. 7d again decreases both G0 and Young’s modulus, an increase in gel
exhibits larger pores when compared to the optimum turbidity and in a coarsening of the network structure. For
network in Fig. 7c. The results presented here are similar this system it appears that the extensive structural charge
in trend to those found in the gellan system by Mao, Tang neutralisation of the polysaccharide by excess divalent
400 P. MacArtain et al. / Carbohydrate Polymers 53 (2003) 395–400
calcium ions is responsible for a marked aggregation of the behaviour of kappa-carrageenan gels. Carbohydrate Polymers, 16,
polymer strands reminiscent of precipitation. At lower 297 –320.
Lai, V. M. F., Wong, P. A.-L., & Lii, C.-Y. (2000). Effects of cation
counter-ion to polymer ratios, onset of gelation might properties on sol-gel transition and gel properties of kappa carrageenan.
prevent such phase separation. Journal of Food Science, 65(8), 1332–1337.
Mao, R., Tang, J., & Swanson, B. G. (2001). Water holding capacity and
microstructure of gellan gels. Carbohydrate Polymers, 46, 365 –371.
Michel, A. S., Mestdagh, M. M., & Axelos, M. A. V. (1997). Physico-
chemical properties of carrageenan gels in presence of various cations.
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Mr Michael Cooney of the Department of Food Science,
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