Elsevier

Carbohydrate Polymers

Volume 50, Issue 2, 1 November 2002, Pages 109-116
Carbohydrate Polymers

The rheology of K+-κ-carrageenan as a weak gel

https://doi.org/10.1016/S0144-8617(02)00009-7Get rights and content

Abstract

The pure potassium form of κ-carrageenan shows rheological behaviour which is very different to that observed for the commercial (unpurified) sample. The measured rheology at 1% concentration of the purified κ-carrageenan is consistent with weak gel behaviour. The resulting weak gel has a significantly smaller linear viscoelastic region than the unpurified commercial form. Upon large deformation, the gel flows. We report results for the viscosity versus shear rate where the sheared gel demonstrates significant structural recovery with time after shearing. The effects of temperature, polymer concentration and KCl addition on the rheology of the gel have been investigated. The role of both calcium and potassium ions is discussed.

Introduction

κ-Carrageenan is a sulfated galactan extracted from red algae. It is comprised of repeating disaccharide units of 3-linked β-d-galactose 4-sulfate and 4-linked 3,6-anhydro-α-d-galactose. The solution of κ-carrageenan may form strong thermally reversible gels upon cooling in the presence of specific cations such as potassium. The mechanism of gelation is reported to involve a coil–helix transition of the κ-carrageenan molecules, followed by aggregation and network formation (Morris, Rees, & Robinson, 1980).

The rheological properties of κ-carrageenan have been extensively studied using compression techniques (Cairns et al., 1986, Rochas et al., 1990). These investigations assessed the effects of κ-carrageenan concentration, added salt and temperature on the mechanical properties of the gels formed.

κ-Carrageenan is known to exhibit syneresis, which results in significant slip during dynamic shear measurements (Richardson & Goycoolea, 1994). In a previous study with the binary system of κ-carrageenan and locust bean gum (Chen, Liao, Boger, & Dunstan, 2001), preformed gels glued between parallel plates were used to overcome slip.

A few dynamic studies on κ-carrageenan in a state of limited or minimal syneresis have been reported. Meunier, Nicolai, Durand, and Parker (1999) provided data on κ-carrageenan of low concentrations (0.01–0.2%) containing 0.01 M KCl plus 0.1 M NaCl. They found that at least 12 h was needed for the κ-carrageenan to approach equilibrium as measured in the rheological behaviour. Chronakis, Piculell, and Borgström (1996) studied the rheology of κ-carrageenan in mixtures of sodium and cesium iodide. Michel, Mestdagh, and Axelos (1997) found that, in the low salt concentration range (to 0.1 M), the effect of added ions on the G′ of resulting κ-carrageenan gel was in the order of K+>Ca2+=Cu2+≫Na+.

When compared to other food biopolymers such as xanthan and gelatine, the number of dynamic oscillatory studies for κ-carrageenan are still limited in number. Hermansson, Eriksson, and Jordansson (1991) provided a comprehensive study with respect to the effects of potassium, sodium and calcium ions on κ-carrageenan in different ionic forms. Substantial amounts (0.02–0.5 M) of salts were added to the κ-carrageenan so that the gel formed remained in a strong gel regime.

The physical properties of κ-carrageenan gels are notoriously affected by the presence of even trace amounts of ions (Nilsson, Picullel, & Jönsson, 1989). While K+ is very effective in promoting gelation, the rheology of pure K+-κ-carrageenan is yet to be systematically studied. Stanley (1990) commented that κ-carrageenan in the pure potassium form is ‘compliant’ indicating that the pure K+-κ-carrageenan is not brittle. Hermansson et al. (1991) tested a 1% K+-κ-carrageenan with 25 mM added NaCl. The reported storage modulus, G′ was only of the order of 300 Pa. By testing a pure, salt-free K+-κ-carrageenan using a specially designed apparatus with capillary tubes, Morris and Chilvers (1983) showed that the shear modulus, GR, was very low (<100 Pa for the K+-κ-carrageenan at 1%). In contrast, a rigid gel with G′ of approximately 10,000 Pa was observed for a commercial κ-carrageenan at 1% concentration (Chen et al., 2001). The above information indicates that pure K+-κ-carrageenan gel is a lot weaker than that of conventional commercial products which contain mixed cations, usually with some free salts present.

The aim of this study is to characterise the rheology of purified κ-carrageenan in the potassium form without free salts, which can then provide a basis for further studies involving other variables such as KCl addition and increase in polymer concentration.

Section snippets

Materials and methods

The sample of κ-carrageenan (type III, lot 39h1211) from ‘Eucheuma cottonii’ was purchased from Sigma-Aldrich (St Louis, MO, USA), which contained 6.7% K+, 2.3% Ca2+ and 0.7% Na+. This sample contained a small amount of free salts. Notably, the seaweed species ‘Eucheuma cottonii’ is presumably Kappaphycus alvarezii, as Eucheuma cottonii has never been farmed and was never a large component used for commercial extraction. Essentially, the only species farmed and currently used in industry for

Results and discussion

Fig. 1, Fig. 2, Fig. 3 show results for a series of measurements on purified K+-κ-carrageenan samples at 1% concentration with no KCl added. The sample was first cooled from 40 to 20 °C, with a cooling rate of 0.5 °C/min. It was then cured for 12 h in the rheometer after which G′ and G″ measurement over the frequency range 0.01–10 Hz was undertaken. Time and frequency spectra were repeated on the same sample by following the sequential procedure of heating, cooling and curing. In order to melt the

Conclusions

Purified κ-carrageenan in the potassium form with essentially no free salts forms a weak gel at moderate concentrations (0.7–1.4%). The gels formed flow under large strains and their structure is able to recover significantly when re-cured. The addition of small amounts of potassium salts and increasing the polymer concentration greatly effects the measured rheological behaviour of the gels. Strong synergy is observed for the gel system in the presence of both potassium and calcium ions.

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