ects of Cooling Rate and Sodium Chloride on Polysaccharide Gelation

Transcription

1 Food Sci Technol Res, +- (), -/ -/*,,**1 ects of Cooling Rate and Sodium Chloride on Polysaccharide Gelation Hatsue M, Mami T and Kenji K + +, ORITAKA AKAHASHI UBOTA +, Department of Food Science and Nutrition, Showa Women s University, + 1 Taishido, Setagaya-ku, Tokyo +/ 2/--, Japan Department of Biological and Chemical Engineering, Gunma University, + / + Tenjintyou Kiryu-shi, Gunma -10 2/+/, Japan Received August 2,,**0; Accepted May -+,,**1 The e# ects of cooling rate and * + * / sodium chloride (NaCl) on the gelation of * 2 agar, gellan gum and k-carrageenan solutions were investigated based on the storage modulus and by di# erential scanning calorimetry (DSC) Each polysaccharide solution without NaCl had higher values of equilibrium storage modulus at a slow cooling rate than at a rapid cooling rate However, the enthalpy calculated from the area surrounded by a DSC curve and the base line was not influenced by the cooling rate The gelation temperature of each polysaccharide solution was elevated and the e# ect of NaCl addition on the storage modulus of gellan gum and k-carrageenan was intensified by slowing the cooling rate Each aggregated molecule core might grow independently at a rapid cooling rate and the formation of network structure of gellan gum and k-carrageenan at a slow cooling rate might be a# ected by intermolecular repulsion Keywords : cooling rate, gelation, agar, gellan gum, k-carrageenan Introduction al, +332 ; Kusukawa et al, +333 ; Labropoulos et al,,**,), Agar, gellan gum and k-carrageenan are attracting at- gellan gum (Miyoshi et al, +33 a, +33 b, +333; Takahashi et tention as dietary fiber in recent years These polysaccha- al, +333 ; Moritaka et al, +33+, +33,,,**,,,**-; Ogawa et rides have been used in the food industry for a wide al,,**/ a,,**/ b,,**0) and k-carrageenan (Plashchina et al, variety of products Agar extracted from Gelidium (marine +320 ; Hermansson +323 ; Piculell et al, +323; Nishinari et red algae) consists of about 1* agarose and -* agaro- al, +33* ; Lundin and Hermansson +332 ; Lai et al,,***; pectin Agarose is a linear disaccharide composed of Moritaka et al,,***; Puvanenthiran et al,,**,) have been b-d-galactose and -0, -anhydro-a-l-galactose, and plays an published separately However, there are no reports to important role in gelation Agaropectin contains a sulfate our knowledge that compare agar, gellan gum and k- group, glucuronic acid and pirubic acid Gellan gum is a carrageenan with regard to the simultaneous e# ect of microbial polysaccharide produced by aerobic fermenta- cooling rate and sodium chloride on gel structure under tion in Sphingomonas elodea, and is a linear anionic the same conditions However, since these studies are heteropolysaccharide composed of polymeric tetrasaccha- important for the food industry, we investigated the siride ( b-d-glucose, b-d-glucuronic acid, b-d-glucose and a- multaneous e# ects of cooling rate and sodium chloride on L-rhamnose) each with one carboxyl side group per unit the gelation of agar, gellan gum and k-carrageenan The k-carrageenan is a linear sulfated polysaccharides and is extracted from various species of the Rhodophla Materials and Methods (marine red algae) The polysaccharide backbone is a Sample preparation Agar (Geruappu J- +0-*, Lot,+**1), repeating disaccharide ( b-d-galactopyranose and a-d- gellan gum (Kerukogeru, Lot - B*-,-A) and k-carrageenan galactopyranose) each with a sulfate side group (Karaginin CS- /22, Lot -*/-*+) samples were kindly sup- These polysaccharides are known to show a conforwithout plied by San-Ei Gen FFI, Osaka, Japan and were used mational transition from a random coil to a double helix further purification, since the sample remarkably and gels formed by these polysaccharides are composed changes by the further purification Table + shows the of partial bundles of double helical molecular chains The inorganic ion contents of agar, gellan gum and k- gelation of these polysaccharides is influenced by the carrageenan Sodium chloride (NaCl) used was extra-finepolysaccharide concentration, the thermal history and grade reagent (Wako Pure Chemical Industries, Osaka, the presence of additives The gelation of gellan gum and Japan) and the type of water was ultra pure water Agar k-carrageenan is inhibited by the electrostatic repulsion and gellan gum were dispersed by stirring and left to of the carboxyl group or sulfate group respectively Ca- swell at,/ C for 0* min, and k-carrageenan was left to tions shield the electric repulsion or form the ionic bond swell for 3* min Agar and k-carrageenan solutions were Many studies on agar (Watase et al, +33* ; Mohammed et heated at 1* C for -* min and then at 3* C for -* min Gellan gum was heated at 3* Cfor 0* min For preparation To whom correspondence should be addressed of NaCl-containing samples, NaCl solution dissolved in moritaka@swuacjp minimum volume of hot ultrapure water was added to the

2 346 H MORITAKA et al hot sample solution Agar, gellan gum and k-carrageenan min Figure, shows those of *2 agar, gellan gum and were prepared at *2 concentration and NaCl concentra- k-carrageenan solutions each with *- NaCl and Fig - tions were *+, *- and */ shows those of sample solutions with */ NaCl The Rheological measurements Thermal scanning rheolog- storage modulus of each polysaccharide solution increased ical measurements were performed within the linear with lowered temperature and slowed cooling rate However viscoelastic region using a dynamic stress rheometer from Physica Co (MCR-**, Germany) with a parallel-plate ge- the storage moduli of gellan gum solutions with *+ */ NaCl increased with lowering the temperature to a ometry of,/ mm diameter with latticed grooves to avoid certain level and then decreased with further lowering of gel slippage The hot sample solution was poured into the the temperature when measured at a **1 C/min cooling cup The storage modulus (G ) and loss modulus (G ) were rate Figure shows storage moduli of various sample measured at +* Hz, at *+ deformation and at,-mm gaps gels at / C The storage modulus of gellan gum gel The solution was then cooled from 0/ C to / C at a cooling rate of **1, */, +*, -* and 0* C/min containing NaCl showed the highest storage modulus At any cooing rate, the storage moduli of gellan gum and DSC measurements Di# erential scanning calorimetry k-carrageenan gels increased with increasing NaCl con- (DSC) measurements were carried out using a Setaram centration The e# ect of adding NaCl on the storage micro DSC-III calorimeter (Caluire, France) Approximate- moduli of gellan gum and k-carrageenan gels became ly 2** mg of the sample solution was sealed herme- stronger as the cooling rate was slowed, and the e# ect of tically into the DSC pan, and then the pan was accurately cooling rate became stronger with increasing NaCl concentration weighed A reference pan was filled with an equal amount However, NaCl did not have a clear e# ect on of deionized water The temperature was then cooled the storage modulus of agar gel measured at a cooling from 0/ C to / C at a cooling rate of **1, *- and +, C/min rate above */ C/min cooling rate Figure / shows the temperature at which the storage modulus begins to increase Results (the gelation temperature) The slower the cooling Mechanical properties Figure + shows the storage rate, the higher the gelation temperature In gellan gum moduli of *2 agar, gellan gum and k-carrageenan solu- and k-carrageenan solutions, the gelation temperature intions measured at cooling rate of **1*/+*-*,,, and 0* C/ creased with increasing NaCl concentration The details will be described later Thermal property Figure 0 shows the cooling DSC Table + Metal contents of the agar, gellan gum and curves of *2 agar, gellan gum and k-carrageenan solutions without NaCl at a cooling rate of **1, *- and +, C/ k-carrageenan samples min Figure 1 shows the cooling DSC curves of those with *- NaCl The exothermic peak in the cooling DSC curve became broader as the cooling rate slowed The cooling DSC curves of agar without NaCl at cooling rates of **1 C/min and *- C/min showed two transition peaks The DSC curves of agar and gellan gum solutions without NaCl at a cooling rate of +, C/min had a sharper peak Fig + Temperature dependence of storage modulus for various polysaccharide solutions (a) *2 agar solution; (b) *2 gellan gum solution; (c) *2 k-carrageenan solution;, **1 C/min;, */ C/min;, +* C/ min;, -* C/min;, 0* C/min

3 ects of Cooling Rate and Sodium Chloride on Polysaccharide Gelation 347 Fig, Temperature dependence of storage modulus for various polysaccharide solutions with *- NaCl (a) *2 agar solution; (b) *2 gellan gum solution; (c) *2 k-carrageenan solution;, **1 C/min;, */ C/min;, +* C/ min;, -* C/min;, 0* C/min Fig - Temperature dependence of storage modulus for various polysaccharide solutions with */ NaCl (a) *2 agar solution; (b) *2 gellan gum solution; (c) *2 k-carrageenan solution;, **1 C/min;, */ C/min;, +* C/ min;, -* C/min;, 0* C/min Fig ect of NaCl concentration on the storage modulus of various polysaccharide gels at / C (a) *2 agar gel; (b) *2 gellan gum gel; (c) *2 k-carrageenan gel;, **1 C/min;, */ C/min;, +* C/min;, -* C/ min;, 0* C/min

4 348 H MORITAKA et al Fig / ect of NaCl concentration on the gelation temperature of various polysaccharide solutions (a) *2 agar solution; (b) *2 gellan gum solution; (c) *2 k-carrageenan solution;, **1 C/min;, */ C/min;, +* C/ min;, -* C/min;, 0* C/min Fig 0 DSC cooling curve for various polysaccharide solutions at **1 C/min, *- C/min, and +, C/min (a) *2 agar solution; (b) *2 gellan gum solution; (c) *2 k-carrageenan solution Fig 1 DSC cooling curve for various polysaccharide solutions with *- NaCl at **1 C/min, *- C/min, and +, C/min (a) *2 agar solution; (b) *2 gellan gum solution; (c) *2 k-carrageenan solution than those at a slower cooling rate, indicating a more or in Table, The peak temperatures of gellan gum and less uniform transition from sol to gel upon gelling Table k-carrageenan solutions were higher in the presence than, shows the peak temperature (the mid-point of transition in the absence of *- NaCl The slower the cooling rate, temperature) and enthalpy calculated from the area the higher was the peak temperature Similar results surrounded by the cooling DSC curve and the base line were obtained for the gelation temperature calculated Since the base line of DSC curve obtained at a cooling rate from the storage modulus In any polysaccharide solu- +, C/min could not be observed clearly, enthalpies were shown only for cooling rates of *- C/min and **1 C/min tion, the enthalpy was increased by adding NaCl, but was not influenced by the cooling rate

5 ects of Cooling Rate and Sodium Chloride on Polysaccharide Gelation 349 Table, Enthalpy and peak temperature of agar, gellan gum and k-carrageenan Discussion on the agarose gel structure They reported that the gel The storage modulus of each polysaccharide solution rapidly cooled had a much tighter and homogeneous increased with slowed cooling rate (Fig + ) The fact that structure, and the agarose gel slowly cooled had a much the storage modulus increased greatly at a slow cooling higher order and heterogeneous structure Labropoulos rate suggests that the structures of junction zone in the et al (,**,) reported the random association of agar fibers gel of polysaccharide and molecular structure between may be enhanced by a more rapid cooling rate Mohamjunction zones became sti# er at a slow cooling rate Since med et al ( +332) stated that the increase of the equilibrium the formation of the junction zone and the aggregation of storage moduli values of agar sol at a slower cooling rate molecular in between junction zones are essentially the is probably related to the formation of longer sti# er same process, the process of assembling the grown mole- suprafibers and the better packing e$ ciency of the cules is the important point for gelation of polysaccha- double helices within the suprafibers rides It is well known that polymer chains of polysaccha- The slower the cooling rate, the higher the gelation rides transit from a random coil to helix coil in response temperature (Fig /) A possible interpretation of the to the change of temperature, ph and ion concentration at ascent in gelling temperature is that the slower rate of the first stage of the gelation process Triggered by this cooling allows formation of longer and well-grown helichange, helical chains aggregate to form a gel at the ces The longer and well-grown helical fibers aggregate second stage If the speed of the transition from random easier than the shorter and immature helical fibers at a coil to helix coil is su$ ciently high, the slope of the higher temperature The gelation temperatures of gellan storage modulus plotted against temperature may be gum and k-carrageenan solutions with NaCl shifted changed by the cooling rate However, the storage modulus- higher than those without NaCl (Fig /, Table,) Since temperature curves shown in Figs + - shifted in parallel with slowing the cooling rate, suggesting that the random coil changed to a helix coil slowly in the first stage The gellan gum and k-carrageenan primarily comprise carboxyl group and sulfate group respectively, those are negatively charged in an aqueous solution The Na ion may shield equilibrium storage modulus at / C varied with the cool- the electric repulsion of molecules, and promote the coiling rates (Fig ), suggesting that the process of network to-helix conformational transition and then participate in formation was qualitatively influenced by the cooling the formation of contacts between the ordered segments rate In the process of aggregation of helical fiber, the of the macromolecules These e# ects may become stronger longer and well-grown helical fibers aggregate easier than with increasing NaCl concentration the shorter and immature helical fibers Therefore, under a semi-equilibratory state (ie, at an extremely slow cool- The enthalpy change calculated from the area surrounded by DSC curve and the base line was not influenced by the ing rate) the fibers may aggregate more heterogeneously than under a non-equilibratory state (ie, at an extremely cooling rate in Table, This means that even if the degree and distribution of fiber aggregation were changed, the rapid cooling rate) It is assumed that the spatial distribution enthalpy change for the formation of cross linking resition of the fibers is also heterogeneous and the number of mains almost the same The enthalpy change may be branched molecular fibers is smaller at a slow cooling principally dominated by the random coil to helical tranrate Since the dynamic response is supported by the In the DSC curve, the temperature zone that in- network of well-grown fibers, the storage modulus may creased the storage modulus of gellan gum and k-carra- increase at a slow cooling rate In contrast, at a rapid geenan deviated from the temperature zone that started cooling rate, randomly aggregated fibers grow independently resulting in a uniform fiber aggregation and formaan exothermic reaction The exothermic reaction of gellan gum solution at **1 C/min cooling rate started about -* C tion of a network with many branching points and thus In contrast, storage modulus started to increase at about the storage modulus may be lowered * C and reached a mechanical equilibrium state at -* C Sugiyama et al (,**) reported that the storage modulus of gelatin gel increased at a slow cooling rate Kusukawa et al ( +333 ) examined the e# ect of cooling rate The time required for the transition from random coil to helix coil in the first stage in +* C/ **1 C/min +* min This indicates that the change of enthalpy is very small in

6 350 H MORITAKA et al this process and the storage modulus increases mainly by conformational change of molecular chain In gellan gum and k-carrageenan containing much potassium and calcium, the molecule may not completely disperse even at a high temperature, and there is a possibility that some aggregated cores existed before the first stage This e# ect may be intensified when measured at a slow cooling rate If the aggregated core does not spread in the aqueous solution, a weak network structure or entanglement with the core may be formed In this case, the enthalpy change for the conformational change in the first stage is very small Consequently, the total enthalpy change is considered to correspond mainly to the energy for the fiber aggregation in the second stage However, this point remains to be considered after purification of the sample The storage modulus of gellan gum with */ NaCl measured at a cooling rate of **1 C/min showed the maximum value at /* C and then slightly decreased with decreasing temperature This behavior is considered to be caused by the looseness of the network formed by potassium and calcium However, the storage modulus of k-carrageenan did not show such behavior This may be because k-carrageenan had a sulfate group of strong acidity at the dissociation group, and contains a smaller amount of potassium and calcium compared with gellan gum The e# ect of added NaCl on the storage modulus of gellan gum and k-carrageenan became stronger with slowing the cooling rate Moritaka et al ( +33, ) reported that NaCl shields the electric repulsion of the carboxyl side groups in gellan gum fibers and promotes the tight binding of helical fibers and strengthens the junction zones At a slow cooling rate, the molecules aggregate and grow from the initial stage of gelation, and the formation of network structure is greatly a# ected by intermolecular repulsion Consequently, the addition of NaCl at a slow cooling rate influences remarkably the formation of network structure At a rapid cooling rate, on the other hand, the addition of NaCl did not significantly a# ect the formation of the network structure, because each aggregated molecule core grows independently References Hermansson, AM ( +323) Rheological and microstructural evidence for transient states during gelation of k-carrageenan in the presence of potassium Carbohydr Polm, +*, Kusukawa, N, Ostrovsky, MV and Garner, MM ( +333 ) ect of gelation conditions on the gel structure and resolving power of agarose-based DNA sequencing gels Electrophoresis,,*, +// +0+ Labropoulos, KC, Niesz, DE, Danforth, SC and Kevrekidis, PG (,**,) Dynamic rheology of agar gels, theory and experiments Part II : gelation behavior of agar sols and fitting of a theoretical rheological model Carbohydrate Polymers, /*, *1 +/ Lai, VMF, Wong, PA-L and Lii, C-Y (,***) ects of cation properties on sol-gel transition and gel properties of k- carrageenan J Food Sci, 0/, +--, +--1 Lundin, L and Hermansson, A-M ( +332) Multivariate analysis of the influences of locust bean gum, as-casein, k-casein on viscoelastic properties of Na-k-carrageenan gels Food Hydrocolloids, +,, +1/ +21 Miyoshi, E, Takaya, T and Nishinari, K ( +33 a) Gel-sol transition in gellan gum solutions Rheological studies on the e# ects of salts Food Hydrocolloids, +2, /*/ /,1 Miyoshi, E, Takaya, T and Nishinari, K ( +33 b) Gel-sol transition in gellan gum solutions II DSC studies on the e# ects of salts Food Hydrocolloids, +2, /,3 /, Miyoshi, E and Nishinari, K ( +333) Rheological and thermal properties near the sol-gel transition of gellan gum aqueous solutions Progr Colloid Ploymer Science, ++, 02 2, Mohammed, ZH, Hember, MWN, Richardson, RK and Morris, ER ( +332) Kinetic and equilibrium processes in the formation and melting of agarose gels Carbohydrate Polymers, -0, +/,0 Moritaka, H, Fukuba, H, Nakahama, N and Nishinari, K ( +33+ ) ect of monovalent and divalent cations on the rheological properties of 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