Abstract :
1 Introduction
Previously identified as either semi-refined or Philippines Natural Grade(PNG) carrageenan, this food additive is now regulatory termed Processed Eucheuma Seaweed (PES). It is defined for regulatory purposes in the Compendium of Food Additive Specifications (FAO, 1998a) as “A substance with hydrocolloid properties obtained from either Eucheuma cottonii or E. spinosum from the Rhodophyceae class of red seaweeds. The functional component of the product obtained from E. cottonii is kappa-carrageenan.” It is this material (INS No 707a) we compare here with conventional kappa-carrageenan, (INS No 407) which is defined in the Compendium (FAO, 1998b) simply as “A substance with hydrocolloid properties obtained from certain members of the class Rhodphyceae (red seaweeds).” The chemical composition, structure and functionality has been described ( and ). The water-soluble component in PES, is kappa-carrageenan, and as such is indistinguishable from the conventional kappa-carrageenan. Nevertheless, based on overall composition, using chemometric methods, these two forms of carrageenan can be readily distinguished, and they form quite distinct clusters if the individual analytical parameters are subjected to multivariate analysis (Jurasek & Phillips, 1998). There is as an insoluble component of cellulose I, normally present in algae cell walls, and this component has been identified by solid state NMR, X-ray diffraction and atomic force microscopy ( and ). The effects of this insoluble component on the gelation mechanism has been studied (Tanaka, Hatakeyama, Hatakeyama, & Phillips, 1996). Here the effect of the insoluble component on the interaction with water is investigated using differential scanning calorimetry (DSC).
2 Materials and methods
The methods of hydration and DSC procedures are a modification of those previously described. ( and ). A Perkin–Elmer DSC II equipped with cooling apparatus was used to measure the phase transitions of the sorbed water in the samples which were cooled from 25 to −90°C at 10°C/min and subsequently heated to 25°C at a rate of 10°C/min. The temperature scale and heats of fusion were calibrated using distilled water and indium as the standard materials.
After DSC measurements, the sample pans were pricked with a pin to remove water from the samples, which were, thereafter, dried under reduced pressure for 30 min at 110°C, then left overnight at room temperature. After weighing the water content was determined and Wc defined as
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It is possible to distinguish between non-freezing water (Wnf), freezing-bound water (Wfb) and free water (Wf) from the individual transitions or shape of the transitions, and these are connected by the relationship
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3 Results and discussion
A representative set of DSC heating curves for kappa-carrageenan (E407) is shown in Fig. 1, and for PES (E407a) in Fig. 2. The Wc values are listed in the figure. The systems are first cooled to −80°C and slowly warmed. At Wc 0.35 for kappa-carrageenan and Wc 0.22 for PES, no transition is observed, indicating that at this water content the water is bound to the hydrophilic groups and is non-freezing. As the value of Wc is increased, both systems show the presence of at least two broad transitions, due to the melting of free water (at temperature Tf) and freezing-bound water (at temperature Tfb). As Wc is increased Tfb shifts towards Tf and eventually overlaps in each of the systems. We have used this value where overlap occurs of the bound water melting temperature with that of free water as an indication of the relative abilities of individual polysaccharides to bind water (Takigami et al., 1994). It is immediately evident by direct observation of Fig. 1 and Fig. 2 that kappa-carrageenan retains water in a bound form more effectively than PES. The task is to quantify this difference.
Immediately following the first heating and cooling cycle, the procedure was repeated, but we did not observe any significant change in the transitions, indicating that both systems can readily form stable and reversible thermodynamic traps for water. The melting of the freezing-bound water is an endothermic process. The freezing process however is exothermic. Fig. 3shows the typical behaviour of both systems on cooling. More than one transition are evident both at water contents 0.58 and 0.6, which are subsequently masked as the water content is increased. It would appear, therefore, that within the broad transitional envelope, there are several metastable states of water, due to the water interacting and being ‘bound’ within various sites within the polysaccharide structure. The unsymmetrical curves, even at high water contents, show that the free- and bound water exist alongside each other.
The overall behaviour can more readily be identified in Fig. 4, which directly compares the behaviour of the two carrageenan systems. Whereas the temperature of freezing of the free water is constant with an increase in Wc (the value of Tfb moves up, to approach that of free water. Following the sorption of the non-freezing water at the hydrophilic groups of the framework polysaccharide structure, as Wc increases, the freezing-bound water builds up as a structured entity and retains its distinctive character from free water. The enthalpy of melting of this structured water remains less than that of free water throughout (due to defects introduced into the freezing-bound ice. The free water, on the other hand, melts near 0°C in the form of hexagonal ice, because in the free form there is no interference by the carrageenan structure.
As was previously shown, it is possible to calculate the amounts of the various types of water at increasing Wc values ( and ), as shown in Fig. 5 and Fig. 6. In comparison with other polysaccharides we have studied, the ability of PES to strongly bind water is low. There is an initial build-up of non-freezing water to 0.5 g water per gram PES, which does not change with water content. This value represents the complete hydration of the sugar skeleton and corresponds to ∼13 moles of water in the non-frozen state per disaccharide unit. This water is bound tightly by the OH and other ionic sites. The amount of bound water, while increasing with water content remains low particularly for PES. At Wc 1.5, for example, kappa-carrageenan bind at least twice the amount of water than PES, and continues to increase beyond Wc 2.8, whereas PES is saturated at Wc 1.5. Table 1shows a comparison of kappa-carrgeenan and PES with other polymer systems.
The gel–sol transition has been compared for kappa-carrageenan and PES (Tanaka et al., 1996), and demonstrates that the presence of cellulose in PES leads to a lower gel–sol transition temperatures. More heat absorption View the MathML source is observed for PES than for kappa-carrageenan View the MathML source Thus, PES requires more heat absorption to form the junction point. It is possible that the kappa-carrageenan molecules in PES are partitioned by cellulose so that it is necessary to introduce more energy to form a stable network. It can now be added that less water is available at any single temperature for PES, making the gel network formation more difficult than for kappa-carrageenan.
It can, therefore, be concluded that the established presence of cellulose exerts a profound influence on the ability of PES to interact with water.
