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Effect of Change in Cooling Rate on the Gelation of Sodium―type Gellan GumHatsue Moritaka*§　Atsuko Sagawa*　Takahiro Funami**　Kenji Kubota***

　The effects of change in cooling rate on the gelation of 0 . �％ sodium―type gellan gum solutions with and with-out 0 . �％ sodium chloride （NaCl） were investigated based on the storage moduli of the solutions. The cooling rate was changed at the temperature where the storage modulus began to increase. The storage modulus of 0 . �％ so-dium―type gellan gum solution with 0 . �％ NaCl was larger in the measurement for cooling rate change from slow to fast than in that for change from fast to slow.　After the cooling rate change, in cases where the rate was equal to that before the change, the slower the rate, the larger the storage modulus. When the cooling rate was changed from slow to fast, the lower the temperature at which the rate was changed, the larger the storage modulus became. These results show that the cooling rate before the increase of storage modulus has a remarkably large influence on the formation of three―dimensional network structure.

Keyword： sodium―type gellan gum, gelation, cooling rate, storage modulus, change of cooling rate

* Graduate School of Human Science, Showa Women＇s University ** San―Ei Gen F. F. I., Company *** Graduate School of Engineering　§ inquiry　 Graduate School of Human Science, Showa Women＇s

University �―� Taishido, Setagaya―ku, Tokyo, ��4―���� , Japan TEL 0�（�4��）4�2�　FAX 0�（�4��）����

Introduction

　Gellan gum is used in the food industry to manufac-ture/prepare a wide variety of products. Gellan gum is a microbial polysaccharide produced by aerobic fermenta-tion in Sphingomonas elodea. It is a linear anionic het-eropolysaccharide composed of polymeric tetrasaccharide

（β―D―glucose, β―D―glucronic acid, β―D―glucose and α―L―rhamnose） with one carboxyl side group per unit. Polysaccharides like gellan gum are known to show a conformational transition from a random coil to a double helix coil, and gel formed by the polysaccharide is com-posed of partial bundles of double helical polymer chains. The gelation behavior of gellan gum is dependent on many factors, including gellan gum concentration, the thermal history and the presence of additives. The gela-tion of gellan gum is particularly accelerated by the addi-tion of salt :　gels with excellent heat resistance can be formed in a short time through such addition. To date, many studies on gellan gum （Miyoshi et al., ���4 a, ���4 b, ����, Takahashi et al., ����, Moritaka et al., ����, ���2, 2002, 200�, Ogawa et al., 200� a, 200� b, 2006） have been published.　Previously, we reported about the relationship between storage modulus and cooling rate in a gelation of 0 . �％ agar, 0 . �％ potassium―type gellan gum and 0 . �％ κ―car-

rageenan solutions with and without sodium chloride （NaCl） （Moritaka and Shimada, 2006, Moritaka et al., 200�）. The storage moduli at � . 0℃ of 0 . �％ agar, 0 . �％ potassium―type gellan gum and 0 . �％ κ―carrageenan so-lutions without NaCl showed high values when the cool-ing rate was low. However, the enthalpy of the cooling DSC curve was not influenced by the cooling rate in the DSC measurement. By decreasing the cooling rate, the gelation temperature of each of the polysaccharide solu-tions was elevated and the effect of NaCl addition on the storage modulus of potassium―type gellan gum and κ―carrageenan was intensified （Moritaka et al., 200�）.　The gellan gum used in the food industry is usually a potassium―type gellan gum. Potassium―type gellan gum has a greater ability to gel than sodium―type gellan gum, as the K＋ ion promotes the gelation of gellan gum more than the Na＋ ion. However, to examine the effect of the cooling rate and the addition of NaCl on gelation, sodium―type gellan gum is considered to be more suitable than the potassium―type gellan gum.　Many studies on the influence of thermal history on the gelation （Sugiyama et al., 2004, Labropoulos et al., 2002, Aymard et al., 200�, Kusukawa et al., ����, Mohammed, et

al., ����, Michon et al., ����） have been published. To our knowledge at least, there have been no reports describ-ing effects of changes in the cooling rate in the cooling process for gellan gum. However, it is known that the three dimensional structure and mechanical properties of gel are remarkably influenced by the thermal history of the subatance. Therefore, the research on the thermal history of gels is important for the food industry and cooking. The effects of change of the cooling rate in the

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日本調理科学会誌 Vol. 44，No. �，�～�4（20��）〔Article〕

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J. Cookery Sci. Jpn.　Vol. 44　No. �（20��）

cooling process for sodium―type gellan gum with and without sodium chloride were investigated by the au-thors.

Materials and Methods

1．　Sample preparation　Sodium―type gellan gum was kindly supplied by San―Ei Gen F.F.I. Co., Ltd., Osaka, Japan. The molecular weight of sodium―type gellan gum was Mn＝� .��×�0�, Mw＝2 .�0×�0� and Mz＝� .��×�0� . Molecular weight was measured by the HPLC method. After the sample was added to �00 g mobile phase and dissolved by stir-ring at 2 , 000 rpm for �0 min. at �0℃, the measurement was performed under the following conditions :　TSG gel, G 6000 PW × L, GMPW × L （exclusion limit :　�×�0�～�×�0�）, T :　40℃, mobile phase :　0 . 0� M NaNO�＋0 .0�％ NaN�, flow rate :　0 . � ml　/　min, injected volume :　�00μl, dn/dc :　0 . ��� （TOSHO Ltd., Tokyo）. Table � shows the inorganic ion contents of sodium―type gellan gum. The sodium chloride （NaCl） used was extra―fine―grade re-agent （Wako Pure Chemical Industries, Osaka, Japan） and the water was ultra pure water. Sodium―type gellan gum was dispersed into the ultra pure water by stirring, left to swell at 2� . 0℃ for �2 h and then heated at �00℃ for 4 h.　Sodium chloride is often used in the manufacturing and cooking of food. NaCl shields the electric repulsion of car-boxyl side groups in gellan gum fibers, while promoting the tight binding of helical fibers and strengthening junc-tion zones （Moritaka et al., ���2）. Consequently, it is very important to examine the influence of the NaCl addition. For preparation of samples containing NaCl, NaCl solu-tion dissolved in a minimum volume of hot ultra pure wa-ter was added to the hot sample solutions.　Concerning the ＂sodium―type gellan gum alone＂ sam-ples, the concentrations of sodium―type gellan gum were 0 . �％, � . 0％ and � . 2％ for the measurement at the fixed cooling rate, and 0 . �％ for the measurement for cooling rate change. For the ＂sodium―type gellan gum with NaCl＂ sample, NaCl concentration was 0 . �％ and sodium―type gellan gum concentration was 0 . �％.2．　Rheological measurements　Thermal scanning rheological measurements were per-formed within the linear viscoelastic regime using a dy-

namic stress rheometer from Physica Co., Ltd., （MCR �00, Germany） with a parallel―plate geometry of 2� . 0―mm diameter with latticed groove to avoid gel slippage. Each of the hot sample solutions was poured into cups. The sample surface was covered with silicon oil to pre-vent evaporation. The storage modulus （G′） and loss modulus （G″） were measured at � . 0 Hz, at 0 . �％ defor-mation and at 2―mm gaps.　The solution was then cooled from 6� .0℃ to � . 0℃ at a fixed cooling rate of 0 . �℃/min, � . 0℃/min, � . 0℃/min or 6 . 0℃/min. All samples （0 .�％, � . 0％, � . 2％ sodium―type gellan gum and 0 . �％ sodium―type gellan gum with 0 . �％ NaCl） were measured by this method. For the same kind of sample, different portions were used in measurements at 0 . �℃/min, � . 0℃/min, � . 0℃/min and 6 . 0℃/min cool-ing rates.　When the cooling rate was changed from slow to fast in the cooling process from 6� .0℃ to � . 0℃, it started at 0 . �℃/min cooling rate and then the rate was changed to � . 0℃/min （0 .� ― � . 0℃/min cooling rate） at �2 . 0℃ for 0 . �％ sodium―type gellan gum alone and at �� . 0℃ for 0 . �％ sodium―type gellan gum with 0 . �％ NaCl. Similarly, the measurement of other portions of the two samples was started at 0 . �℃/min cooling rate and then the rate was changed to � . 0℃/min cooling rate （0 .� ― � . 0℃/min cooling rate） in the cooling process from 6� .0℃ to � . 0℃. The measurement of still other portions of the same sam-ples was started at 0 . �℃/min cooling rate and then the rate was changed to 6 . 0℃/min （0 .� ― 6 . 0℃/min cooling rate）. Thus, for the same two kinds of sample, different portions were used in measurements for cooling rate changes between 0 . � ― � . 0℃/min, 0 . � ― � . 0℃/min and 0 . � ― 6 . 0℃/min.　In contrast, when the cooling rate was changed from fast to slow in the cooling process from 6� .0℃ to � . 0℃, cooling rate was started at 6 . 0℃/min and then changed to 0 . �℃/min （6 .0 ― 0 . �℃/min cooling rate） at �� . 0℃ for 0 . �％ sodium―type gellan gum alone and at �� . 0℃ for 0 . �％ sodium―type gellan gum with 0 . �％ NaCl. Similarly, the measurement of other portions of the two samples was started at 6 . 0℃/min cooling rate and then the rate was changed to � . 0℃/min （6 .0 ― � . 0℃/min cooling rate） in the cooling process from 6� .0℃ to � . 0℃. And the measurement of still other portions of the same sam-ples was started at 6 . 0℃/min and then changed to � . 0℃/min （6 .0 ― � . 0℃/min cooling rate）. Thus, different portions of the same two kinds of sample were used in the measurements for cooling rate changes.

Table 1．　Metal contents of gellan gum （ppm）

Ca K Mg Na

220� N.D. N.D. �����

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Effect of Change in Cooling Rate on the Gelation of Sodium―type Gellan Gum

Results and Discussion

1．　Cooling at a fixed rate　Figure � shows the storage moduli of 0 . �％ and � . 2％ ＂sodium―type gellan gum alone＂ solutions, as a represen-tatives, measured at cooling rates of 0 . �℃/min, � . 0℃/min, � . 0℃/min and 6 . 0℃/min. The temperature at which the storage modulus begins to increase （the gela-tion temperature） shifted to higher temperatures with in-creasing concentration of sodium―type gellan gum and as the cooling rate was slowed down. The storage modulus increased as the temperature was lowered and as the cooling rate slowed. In the present report, the tempera-ture at which the storage modulus begins to increase （G′＞G″） is defined to be the gelation temperature. What is to be noted is that this gelation （G′＞G″） temperature depends on the frequency in the measurement. However, this temperature is frequently used as a gelation temper-ature. Therefore, the temperature at which the storage modulus begins to increase was used as a gelation tem-perature in the present report.　Figure 2 shows changes in the storage moduli and the loss moduli of 0 . �％ sodium―type gellan gum solutions with 0 . �％ NaCl at various cooling rates. At all cooling rates, the storage modulus increased and the gelation temperature was shifted to a higher temperature through the addition of NaCl. The change in gelation temperature according to the cooling rate of 0 . �％ sodium―type gellan

gum solution with 0 . �％ NaCl was smaller than that of 0 . �％ ＂sodium―type gellan gum alone＂ solution. However, the change in the storage modulus according to the cool-ing rate of 0 . �％ sodium―type gellan gum solution with 0 . �％ NaCl was larger than those of all ＂sodium―type gel-lan gum alone＂ solutions. These results showed a tenden-cy similar to those of κ―carrageenan and potassium―type gellan gum presented in our previous report （Moritaka et

al., 200�）.　It often takes a remarkably long time for the storage modulus of gel to reach a condition of equilibrium. For in-stance, the elasticity modulus of gelatin solution keeps in-creasing for a long time （Djabourov et al., ����）. Howev-er, in the present investigation, the storage modulus at the measured temperature of � . 0℃ was shown as the fi-nal storage modulus.　The value of storage modulus reflects the structural stiffness of segments between junction zones and that of the junction zones themselves （Nishinari et al., ���0, ����）. The junction zones in the gel and the segments between junction zones become stiff at a slow cooling rate.　The gelation of agarose is known to show remarkable thermal hysteresis. This is reported to be due to the for-mation of a stable cohesion body of the helix at far higher temperature than the usual helix formation temperature. Therefore, with the slow cooling, agarose molecules form a long helix chain and cohere easily ;　hence the stability

Fig. 1．　 Storage modulus of 0 . �％ and � . 2％ sodium―type gellan gum at cooling rates of 0 . �℃/min, � . 0℃/min, � . 0℃/min and 6 . 0℃/min.

　（a）　0 .�％ sodium―type gellan gum, （b）　� .2％ sodium―type gellan gum■：0.�℃/min cooling rate,　◆：�.0℃/min cooling rate,▲：�.0℃/min cooling rate,　●：6.0℃/min cooling rate　

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J. Cookery Sci. Jpn.　Vol. 44　No. �（20��）

under heat increases （Mohammed et al., ����）.　With a slow cooling rate, the helical fibers comprising the gel grow thick and long. The longer and well―grown helical fibers aggregate more easily than do the shorter and immature helical fibers （Moritaka et al. 200�）. As a result, the spatial distribution of the fibers becomes het-erogeneous and the number of branched molecular fibers is smaller at a slow cooling rate （Kusukawa et al. ����）. Mohammed et al. （����） reported that the slowly cooling gel is more opaque than fast cooling gel at � . 0℃ and the level of cohesion of the slowly cooling gel was more pro-nounced than that of fast cooling gel. In the present in-vestigation, the gel formed at fast cooling rate was trans-parent, while the gel formed at slow cooling rate was opaque. These phenomena support the results of Kusuka-wa et al. （����）.　Aymard et al. （200�） reported that the gelation pro-cess of 2 ％ agarose solution could be examined by mea-suring temporal dependence of the storage modulus and the loss modulus at a constant temperature. Their results showed that the semi―equilibrium storage moduli mea-sured at �℃ and �0℃ were larger than those measured at higher temperature. This report contradicts the results of our investigation and those of Mohamed et al. （����）. This discrepancy may be due different cooling conditions in the experiments. The three dimensional network struc-ture of the gel is strongly influenced by the thermal his-tory. Therefore, it appears necessary to clarify the rela-tionship between the structure of the gel and its thermal

history by experimenting under the various cooling con-ditions, as use of gel in food industries and cooking can be extended if gels of different structures can be formed from the solution of gel material of the same concentra-tion.　Since gellan gum with glucuronic acid has a carboxyl group as a side chain, gellan gum molecules take on a negative charge in an aqueous solution. Na＋ ions shield the electric repulsion of the carboxyl side groups in gel-lan gum fibers, promote the coil―to―helix conformational transition and then participate in the formation of con-tacts between the ordered segments of the macromole-cules （Moritaka et al. ���2）. The formation of network structure of sodium―type gellan gum with NaCl was sig-nificantly affected by the cooling rate. Since the core of each of the aggregated molecules grows independently at a fast cooling rate, the effect of the NaCl addition at a fast cooling rate is smaller than that at a slow cooling rate （Fig. 2（a））. These results showed a comparable ten-dency with those of κ―carrageenan and potassium―type gellan gum in our previous report （Moritaka et al., 200�）.2．　Cooling at various rates　The preliminary results of the present investigation in-dicated that the storage modulus becomes larger at a slow cooling rate than at a fast cooling rate. Furthermore, we examined in which of the temperature zones that are higher and lower than the gelation temperature, the cool-ing rate has a stronger effect on the storage modulus.

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Fig. 2．　 Storage modulus and loss modulus of 0 . �％ sodium―type gellan gum with 0 . �％ NaCl at cooling rates of 0 . �℃/min, � . 0℃/min, � . 0℃/min and 6 . 0℃/min.

（a）　Storage modulus, （b）　Loss modulus■：0.�℃/min cooling rate,　◆：�.0℃/min cooling rate,▲：�.0℃/min cooling rate,　●：6.0℃/min cooling rate

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Effect of Change in Cooling Rate on the Gelation of Sodium―type Gellan Gum

1）　Gellan gum alone　Figure �（a） shows the storage modulus of 0 . �％ ＂sodi-um―type gellan gum alone＂ solution obtained by the mea-surement for cooling rate change from 6 .0℃/min to 0 . �℃/min （6 .0 ― 0 . �℃/min cooling rate） at �� . 0℃. Fig-ure �（b） represents the storage modulus of 0 . �％ ＂sodi-um―type gellan gum alone＂ solution obtained by the mea-surement for cooling rate change from 0 .�℃/min to 6 . 0℃/min （0 .� ― 6 . 0℃/min cooling rate） at �2 . 0℃. In Fig. �（a） and （b）, the storage moduli obtained by chang-ing the cooling rate in the cooling process are shown be-tween that at a fixed cooling rate of 6 . 0℃/min and that at a fixed cooling rate of 0 . �℃/min. Figure 4 shows the storage modulus obtained by the measurement of 0 . � ― 6 . 0℃/min cooling rate at �2 . 0℃, at 2� . 0℃ （� .0℃ lower than �2 .0℃） and at �� . 0℃ （� .0℃ higher than �2 .0℃）. However the storage moduli measured by changing the cooling rate at 2� . 0℃, at �2 . 0℃ and at �� . 0℃ were al-most the same as that at � . 0℃. Thus, the cooling rate did not remarkably affect the formation of the network struc-ture. This may be due to the inter―molecule repulsion and small molecular number in low concentrations of 0 . �％ sodium―type gellan gum.2）　Gellan gum with NaCl　Figure �（a） shows the storage moduli of 0 . �％ sodi-um―type gellan gum solution with 0 . �％ NaCl at �� . 0℃ obtained by the measurements for cooling rate change from 6 .0℃/min to 0 . �℃/min （6 .0 ― 0 . �℃/min cooling

rate）, � . 0℃/min （6 .0 ― � . 0℃/min cooling rat） and � . 0℃/min （6 .0 ― � . 0℃/min cooling rate） respectively. Figure �（b） shows the storage moduli of 0 . �％ sodium―type gellan gum solution with 0 . �％ NaCl at �� . 0℃ ob-tained by the measurements for cooling rate changes from 0 .�℃/min to � . 0℃/min （0 .� ― � . 0℃/min cooling

Fig. 3．　Storage modulus of 0 . �％ sodium―type gellan gum at various cooling rates（a）　Storage modulus changed from 6 .0℃/min cooling rate to 0 . �℃/min cooling rate at �� . 0℃,（b）　Storage modulus changed from 0 .�℃/min cooling rate to 6 . 0℃/min cooling rate at �2 . 0℃

◆：0.�℃/min cooling rate, ▲：6.0℃/min cooling rate,（a）　●：Cooling rate changed from 6 .0℃/min to 0 . �℃/min, （b）　●：Cooling rate changed from 0 .�℃/min to 6 . 0℃/min

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Fig. 4．　 Storage modulus of 0 . �％ sodium―type gellan gum at various cooling rates

△：0.�℃/min cooling rate, ○：6.0℃/min cooling rate,●：Cooling rate changed from 0 .�℃/min to 6 . 0℃/min at 2� . 0℃,□：Cooling rate changed from 0 .�℃/min to 6 . 0℃/min at �2 . 0℃,▲：Cooling rate changed from 0 .�℃/min to 6 . 0℃/min at �� . 0℃

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J. Cookery Sci. Jpn.　Vol. 44　No. �（20��）

rate）, � . 0℃/min （0 .� ― � . 0℃/min cooling rate） and 6 . 0℃/min （0 .� ― 6 . 0℃/min cooling rate） respectively. The storage moduli obtained by the measurements for cooling rate changes were observed to be the differential value between that at a fixed fast cooling rate and that at a fixed slow cooling rate.　After the cooling rate change, the slower the cooling rate, the larger the storage modulus.　In Fig. �（a）, the storage modulus of 6 . 0 ― 0 . �℃/min cooling rate was larger than those of 6 . 0 ― � . 0℃/min cooling rate and 6 . 0 ― � . 0℃/min cooling rate. However, no remarkable difference was observed in the storage modulus with 6 . 0 ― � . 0℃/min cooling rate and that with 6 . 0 ― � . 0℃/min cooling rate. The storage modulus at � . 0℃ obtained with � . 0℃/min fixed cooling rate and that obtained with � . 0℃/min fixed cooling rate were remark-ably different, as shown in Fig. 2.　When the storage modulus of 6 . 0 ― � . 0℃/min cooling rate in Fig. �（a） was compared with that of 0 . � ― � . 0℃/min cooling rate in （b） or the storage modulus of 6 . 0 ― � . 0℃/min cooling rate in Fig. �（a） and that of 0 . � ― � . 0℃/min cooling rate in （b）, the storage modulus at � . 0℃ was different though the cooling rate after change

in cooling rate was the same. In all measurements, when the cooling rate after change in cooling rate was the same, the storage modulus was large in the measurement started from the slow cooling rate. These results suggest that the cooling rate before change in cooling rate influ-ences the formation of three―dimensional network struc-ture more strongly than the cooling rate after change in cooling rate.　When the transition from random coil to helix coil is in-sufficient at a fast cooling rate before change in cooling rate, the helical fibers are immature and short. It is hard-er for the short and immature fibers to aggregate than the longer and well―grown helical fibers. Consequently, it may be difficult to affect the network formation by the cooling rate after slowing down the cooling rate. Con-versely, a slower rate of cooling allows formation of lon-ger and well―grown helical fibers. The cooling rate, after having been quickened, is considered to strongly influ-ence the network formation, because the longer and well―grown helical fibers aggregate easily.　Figure 6 shows the storage moduli measured for cool-ing rate change from 0 .�℃/min to 6 . 0℃/min at 4� . 0℃

（�0 .0℃ lower than �� .0℃）, at 46 . 0℃ （� .0℃ lower than

Fig. 5．　 Storage modulus of 0 . �％ sodium―type gellan gum with 0 . �％ NaCl at various cooling rates

（a）　 Storage modulus changed from 6 .0℃/min cooling rate to 0 . �℃/min, � . 0℃/min and � . 0℃/min cooling rates at �� . 0℃,

（b）　 Storage modulus changed from 0 .�℃/min cooling rate to � . 0℃/min, � . 0℃/min and 6 . 0℃/min cooling rates at �� . 0℃,

（a）　 ◆：6.0℃/min cooling rate, ■：Cooling rate changed from 6 .0℃/min to 0 . �℃/min, ▲：Cooling rate changed from 6 .0℃/min to � . 0℃/min, ●：Cooling rate changed from 6 .0℃/min to � . 0℃/min

（b）　 ◆：0.�℃/min cooling rate, ▲：Cooling rate changed from 0 .�℃/min to � . 0℃/min, ●：Cooling rate changed from 0 .�℃/min to � . 0℃/min, ■：Cooling rate changed from 0 .�℃/min to 6 . 0℃/min

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Effect of Change in Cooling Rate on the Gelation of Sodium―type Gellan Gum

�� .0℃）, at �� . 0℃ and at �6 . 0℃ （� .0℃ higher than �� .0℃）. For comparison, the storage modulus of 0 . �℃/min, 6 . 0℃/min and 0 . � ― 6 . 0℃/min cooling rates, re-spectively, are shown in Fig. 6. The storage modulus measured by the fixed cooling rate of 0 . �℃/min was the largest and that measured by the fixed cooling rate of 6 . 0℃/min was the smallest. In the measurement for 0 . � ― 6 . 0℃/min cooling rate, the storage modulus obtained for the change of cooling rate at 4� . 0℃ was the largest, followed, in order of decreasing size, by that at 46 . 0℃, �� . 0℃, and �6 .0℃.　Mohammed et al. （����） reported that agarose formed a cohesion body of the helix at far higher temperature than the temperature of formation of each helix. There-fore, agarose molecules form a long helix chain and be-come able to cohere easily. The results of the present in-vestigation correspond to those of Mohammed et al.

（����）.　The storage modulus of 0 . �％ sodium―type gellan gum solution with 0 . �％ NaCl was larger in the measurement at increasing cooling rate than in that at decreasing cool-ing rate. The value of storage modulus became larger as the cooling rate became slow before and after change in cooling rate. In the measurement for cooling rate change from slow to fast, the storage modulus increased as the

temperature of cooling rate change was lowered. These results show that the cooling rate before the increase of storage modulus has a significantly large influence on the formation of three―dimensional network structure. Long, thick helical fibers are assumed to be remarkably impor-tant for improving the stiffness of junction zones and of segments between junction zones. Our results indicate that a stiff and heterogeneous gel structure with high storage modulus is formed by lowering the temperature as slowly as possible at an early stage of gelation. In con-trast, it appears possible to form a weak and homoge-neous gel structure with low storage modulus by lower-ing the temperature quickly at an early stage of gelation.

ReferenceAymard, P., Martin, D. R., Plucknett, K. Foster, T. J., Clark, A.

H. and Norton, I. T. （200�）, Influence of thermal history on the structural and mechanical properties of agarose gels, Biopolymers, 59, ���―�44

Djabourov, M., Leblond, J. and Panpon, P. （����）, Gelation of aqueous gelation solution. Ⅱ Rheology of the sol―gel transi-tion, Journal de Physique （in France）, 49, ���―�44

Kusukawa, N., Ostrovsky, M. V. and Garner, M. M. （����）, Ef-fect of gelation conditions on the gel structure and resolving power of agarose―based DNA sequencing gels, Electropho-resis, 20, �4��―�46�

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Fig. 6．　 Storage modulus of 0 . �％ sodium―type gellan gum with 0 . �％ NaCl at various cooling rates

△：0.�℃/min cooling rate, ○：6.0℃/min cooling rate,◆：Cooling rate changed from 0 .�℃/min to 6 . 0℃/min at 4� . 0℃,●：Cooling rate changed from 0 .�℃/min to 6 . 0℃/min at 46 . 0℃,□：Cooling rate changed from 0 .�℃/min to 6 . 0℃/min at �� . 0℃,▲：Cooling rate changed from 0 .�℃/min to 6 . 0℃/min at �6 . 0℃

0 20 40 60

Sto

rage

modu

lus

(Pa)

Temperature (℃)

100

102

104

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（Received May, 24, 20�0　Accepted Oct, �, 20�0）

ナトリウム型ジェランガムのゲル化に及ぼす降温速度変化の影響

森 髙 初 惠*　佐 川 敦 子*　船 見 孝 博**　窪 田 健 二***

キーワード：ナトリウム型ジェランガム，ゲル形成，降温速度，貯蔵弾性率，降温速度の変化

* 昭和女子大学 ** 三栄源 F. F. I 株式会社 *** 群馬大学　§ 連絡先　 昭和女子大学大学院生活機構研究科

〒��4―����　東京都世田谷区太子堂 �―�TEL 0�（�4��）4�2�　FAX 0�（�4��）����

和文抄録　0.�％ 塩化ナトリウム（NaCl）添加および無添加の 0.�％ ナトリウム型ジェランガムのゲル形成に及ぼす降温速度の変化の影響について，貯蔵弾性率を測定して検討した。　降温速度は貯蔵弾性率が増加し始める温度で変化させた。　0.�％ NaCl 添加 0.�％ ナトリウム型ジェランガムの貯蔵弾性率は，速い降温速度から緩慢な速度に変化させる測定よりも緩慢な降温速度から速い速度に変化させる測定において，より大きくなった。また，降温速度を変化させる前あるいは後の温度帯で降温速度を遅くすると，貯蔵弾性率の値は大きくなった。さらに，緩慢な降温速度から速い速度に変化させる測定において，降温速度を変化させ始める温度を低くすると貯蔵弾性率は大きくなった。　これらの結果は，貯蔵弾性率の増加前の温度帯での降温速度がゲルの三次元網目構造の形成に著しく大きな影響を持つことを示している。

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