Dynamic viscoelastic study on the gelation of konjac glucomannan with different molecular weights

Food Hydrocolloids 13 (1999) 227–233

Dynamic viscoelastic study on the gelation of konjac glucomannan with different molecular weights M. Yoshimura a,*, K. Nishinari b a

School of Humanity for Environment Policy and Technology, Himeji Institute of Technology, 1-1-12 Sinzaikehoncho, Himeji, Hyogo 670-0092, Japan; b Department of Food and Nutrition, Faculty of Human Life Science, Osaka City University, Sumiyoshi, Osaka 558-0022, Japan Accepted 23 December 1998

Abstract Gelation kinetics of konjac glucomannan in the presence of an alkaline coagulant was studied by dynamic viscoelastic measurements. The gelation time became shorter and the rate constant of gelation increased with increasing molecular weight or concentration of konjac glucomannan or heating temperature. The plateau value of the storage shear modulus increased with increasing molecular weight or concentration of konjac glucomannan. The plateau value as a function of heating temperature or pH showed a maximum at a certain temperature or pH. 䉷 1999 Elsevier Science Ltd. All rights reserved. Keywords: Konjac glucomannan; Alkaline coagulant; Gelation; Dynamic viscoelasticity; Molecular weight

1. Introduction Konjac glucomannan (KGM) is a neutral polysaccharide derived from the tuber of Amorphophallus konjac C. Koch. It is composed of b-1,4 linked D-mannose and D-glucose; the ratio of mannose and glucose is 1.6:1 (Kato & Matsuda, 1969). There are some branching points at the C-3 position of the mannoses (Maeda, Shimahara, & Sugiyama, 1980). An acetyl group is attached to one per 19 sugar residues (Maekaji, 1978a,b). It forms a thermally stable gel on heating in the presence of an alkaline coagulant. It is used as konnyaku in traditional Japanese cooking (Nishinari, Williams, & Phillips, 1992). Maekaji (1974) suggested that deacetylation occurs on alkaline treatment, and causes the association of molecular chains, leading to the formation of junction zones, and a three dimensional network. Maekaji (1978a,b) used an Amylograph to study the rheological changes accompanying KGM gelation. He showed that the KGM gelation by alkaline treatment occurred after a certain induction period (gelation time). Considering the induction reaction as a chemical reaction, he estimated the activation energy from the Arrhenius plot as 11.6 kcal/mol. This value was almost constant irrespective of the gelling conditions, and agreed fairly well with the activation energy 11.8 kcal/mol for deacetylation. From * Corresponding author. Fax: ⫹ 81-792-93-5710. E-mail address: [email protected] (M. Yoshimura)

these results he concluded that the induction reaction corresponded to deacetylation. However, as refined KGM has a few or no acetyl groups, the mechanism of the gelation still remains a problem to be studied. Kohyama and Nishinari (1990) reported solution properties of KGM by measuring a specific volume with different temperatures, concentration and pH. The specific volume was almost constant between pH 3 and 11 and then increased steeply at about pH 11 with increasing pH. They also reported that the gelation of KGM occurs from pH 11.3 to 12.6. It was suggested that the change in molecular structure is necessary for gelation of KGM. It was introduced in the USA and Europe as a food additive. When KGM is dissolved in water, it gives highly viscous solutions. Jacon, Rao, Cooley, and Walter (1993) reported that the intrinsic viscosity of KGM was 1320 ml/g which is one of the highest of the polysaccharides used in food industries. According to Case, Knopp, Hamann, and Schwartz (1992), gelation rate is increased in the presence of sodium sulfate and decreased in the presence of sodium nitrate. They reported that KGM gels have similar properties to those of covalently bonded gels, however, there is no evidence for covalent crosslinking. They examined fracture properties of 2% KGM gel by torsion testing at several temperatures, and found that breaking strain decreased while breaking stress increased with increasing temperature (Case & Hamann, 1994). The physico-chemical properties have not been fully elucidated mainly because of the difficulty in obtaining

0268-005X/99/$ - see front matter 䉷 1999 Elsevier Science Ltd. All rights reserved. PII: S0268-005 X( 99)00 003-X

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easily soluble and well-fractionated KGM samples. Recently the interaction of KGM with other hydrocolloids was studied (Cairns, Miles, & Morris, 1988; Cairns, Atkins, Miles, & Morris, 1991; Nishinari et al., 1992; Nishinari, Miyoshi, Takaya, & Williams, 1996; Ross-Murphy, Shatwell, Sutherland, & Dea, 1996; Kohyama, Iida, & Nishinari, 1993; Kohyama, Sano, & Nishinari, 1996; Kohyama & Nishinari, 1997; Miyoshi, Takaya, Williams, & Nishinari, 1996; Yoshimura, Takaya, & Nishinari, 1996; Yoshimura, Takaya, & Nishinari, 1998). In the present study, we investigated the rheological properties during KGM gelation on addition of an alkaline coagulant as a function of molecular weight and concentration of KGM by observing dynamic viscoelasticity. The effects of heating temperature and pH on the gelation process were also investigated. 2. Experimental 2.1. Materials and methods Four fractions of KGM with different molecular weights were gifts from Shimizu Chemical Co. (Hiroshima, Japan). These were prepared by an enzymatic degradation. The native KGM was treated with an enzyme (SP-249, Novo Nordisk A/S, Copenhagen, Denmark) for different reaction times at an ambient temperature, and LM4, LM3, LM2 and LM1 fractions with low molecular weights were obtained. The details of KGM samples were described in previous reports (Kohyama et al., 1993, 1996). Na2CO3 (Wako Pure Chemical Industries, Ltd., Osaka, Japan) of reagent grade was used as an alkaline coagulant. The molecular weight of KGM was determined by Kohyama et al. (1996) in the previous report. Cadoxen was used as a solvent for measurements of molecular weight and intrinsic viscosity. The molecular weight of LM1, LM2, LM3, and LM4 were 2.56 × 10 5, 4.38 × 10 5, 4.44 × 10 5, and 5.96 × 10 5, respectively. The intrinsic viscosity of LM1, LM2, and LM4 were 1.98, 2.63, and 3.50 dl/g, respectively. Details of molecular weight determination were described in previous reports (Kohyama et al., 1993, 1996). 2.2. Preparation of KGM dispersion Powders of KGM fractions (LM1, 2, 3, 4) were dispersed in distilled water at room temperature for 1 h and were heated to 80⬚C and then maintained at 80⬚C for 1 h and cooled to room temperature. The KGM concentrations used were 1–3 w/w%. 2.3. Dynamic viscoelasticity Dynamic viscoelastic measurements were carried out using a Fluids Spectrometer RFS II (Rheometrics Co.

Fig. 1. Time dependence of G 0 ,G 00 and tan d for 2% dispersions of konjac glucomannan A: LM1, S: LM2, K: LM3, × :LM4. Measurement temperature: 60⬚C Symbols represent the experimental values and the solid lines represent calculated curves.

Ltd., NJ, USA) with a parallel plate geometry (25 mm in diameter, 15 mm gap) and a strain of 5% (limit of linear viscoelastic strain was about 8%). The KGM dispersion of 1 ml was poured directly onto the plate of the instrument, which was kept at each measurement temperature. The gel formation curves were studied at constant temperatures from 40 to 80⬚C. Twenty microliters of Na2CO3 solution, an alkaline coagulant, was added at time t ˆ 0 to the KGM dispersion and mixed, and then the storage shear modulus G 0 and the loss shear modulus G 00 were measured as a function of time at a constant frequency 1 rad/s.

M. Yoshimura, K. Nishinari / Food Hydrocolloids 13 (1999) 227–233 Table 1 Parameters of the first order kinetic model for the gelation of 2% KGM with different molecular weights (G 0 sat, the plateau value of G 0 after a long time; k, the rate constant of gelation of KGM; t0, the gelation time; r, correlation coefficient) Sample

G 0 sat (Pa)

k (min )

t0 (min)

r

LM1 LM2 LM3 LM4

41 453 562 2300

0.121 0.248 0.228 0.214

11 7 6 5

0.978 0.976 0.973 0.942

-1

3. Results and discussion 3.1. Effects of KGM molecular weight Fig. 1 shows the time evolution of the storage shear modulus G 0 , the loss shear modulus G 00 for 2% dispersions of KGM with different molecular weights in the presence of 20 ml 1 M Na2CO3 solution at 60⬚C. G 0 and G 00 increased with time and attained the plateau value. The mechanical loss tangent tan d ˆ G 00 /G 0 decreased with time. The plateau value of tan d for LM1 was larger than 0.1, and 2% LM1 dispersion formed a weak gel, while the values for 2% LM 2, 3 and 4 dispersions were smaller than 0.1, and these 2% KGM dispersions formed elastic gels. The plateau values of G 0 and G 00 at time t ˆ 60 min increased with increasing molecular weight. Some gelation processes at a constant temperature were treated by an equation of first order kinetics or other modified equations (Nishinari, 1997). These curves could be well approximated by the following equation: G(t) ˆ G 0 sat(1 ⫺ e ⫺k(t⫺t0), where G 0 sat is the plateau value of G 0 after a long time, k is the rate constant of gelation of KGM, and t0 is the gelation time. The gelation time t0 is taken as the time at which G 0 begins to deviate from the

229

baseline. The baseline is defined as 10 0 Pa of G 0 as it is the lower limit value to detect G 0 and the reproducibility of G 0 is good above this baseline. G 0 sat and k were determined from the best fitting of the experimental and calculated values of G 0 . The symbols represent the experimental values. The solid lines represent calculated curves. The gelation time t0 became shorter, the rate constant k and the plateau value G 0 sat increased with increasing molecular weight (Table 1). As molecular weights of LM2 and LM3 were not so different, the rate constant k and the plateau value G 0 sat were not so different. Generally, rheological properties of gels depend on the molecular structure of gelling agent. However, the dependence of the elastic modulus of gels on molecular weight has not been studied in depth. Relationships between the elastic modulus of gels and molecular weight were studied for gelatin (Ward & Saunders, 1955), agarose (Watase & Nishinari, 1983), kappa-carrageenan (Rochas, Rinaudo, & Landry, 1990), casein micelles (Niki, Kohyama, Sano, & Nishinari, 1994), and methyl cellulose (Nishinari, Hofmann, Moritaka, Kohyama, & Nishinari, 1997). For a few polymer gels, the elastic modulus was found to increase with increasing molecular weight and becomes independent above a certain molecular weight (Mitchell, 1980). The probability that junction zones formed increases, and the flexible molecular chains connecting junction zones will become longer with increasing molecular weight. Therefore, the number of elastically active chains increases and hence the elastic modulus of gels increases with increasing molecular weight. If this picture is valid for konjac gels, both the breaking stress and breaking strain also increase with increasing molecular weight. As a result of the limited quantity of the specimens with different molecular weights, it was not easy to perform large deformation rheology, however, this should be clarified in the near future.

Fig. 2. Time dependence of G 0 for dispersions of LM1 with various concentrations S: 1%, A: 1.5%, K: 2%, × :3%. Measurement temperature: 65⬚C Symbols represent the experimental values and the solid lines represent calculated curves.

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Table 2 Parameters of the first order kinetic model for the gelation of LM1 with various concentrations (abbreviations as in Table 1) Sample (%)

G 0 sat (Pa)

k (min )

t0 (min)

r

1.0 1.5 2.0 3.0

19 27 185 229

0.019 0.133 0.149 0.296

47 10 7 4

0.798 0.976 0.995 0.970

-1

3.2. Effects of KGM concentration Fig. 2 shows the time evolution of G 0 and G 00 for dispersions of LM1 with various concentrations at 65⬚C. The gelation time t0 became shorter, the rate constant k and the plateau value G 0 sat increased with increasing concentration (Table 2). Many researchers reported that G 0 of biopolymer gels (e.g. gelatin, agar, casein micelle) showed a square power law in concentration dependence (Nishinari, 1997). The concentration dependence of the elastic modulus of biopolymer gels was discussed by Clark and Ross-Murphy (1985) using a cascade treatment and by Oakenfull (1984) using a modified theory of rubber elasticity. Fig. 3 shows a double logarithmic plot of G 0 sat versus the concentration c. The exponent m in the relation G 0 sat ⬃ c m was found to be 2.55 for KGM in the concentration range studied. As both theories of Clark and Ross-Murphy (1985) and of Oakenfull (1984) predict a larger exponent at a lower concentration range and a smaller exponent at a higher concentration range, this should be further studied for a wide concentration range. 3.3. Effects of heating temperature Fig. 4 shows the time dependence of G 0 and G 00 for the 1% LM2 dispersion at different heating temperatures. Gelation time t0 became shorter, rate constant k increased with increasing heating temperature. The plateau value G 0 sat increased with increasing heating temperature up to 75⬚C and then decreased at 80⬚C (Table 3).

Fig. 4. Time dependence of G 0 ,G 00 for dispersions of 1% LM2 at different heating temperatures A: 50⬚C, K: 60⬚C, × : 70⬚C, *: 75⬚C, W: 80⬚C. Symbols represent the experimental values and the solid lines represent calculated curves.

Fig. 5 shows G 0 and G 00 for 2% LM2 dispersions as a function of time at different heating temperatures from 45 to 80⬚C. The gelation time t0 became shorter, rate constant k increased with increasing heating temperature. The plateau value G 0 sat increased with increasing heating temperature up to 50⬚C and then decreased. G 0 increased quickly and decreased at 75 and 80⬚C (Table 4). Fig. 6 shows G 0 and G 00 for 3% LM2 dispersions as a function of time at different heating temperatures from 40 to 80⬚C. The gelation time t0 became shorter and the rate constant k became larger with increasing heating temperature. The plateau values G 0 sat increased with increasing heating temperature up to 50⬚C and then decreased (Table 5). G 0 increased quickly and decreased at 70, 75, and 80⬚C. Generally, the gelation proceeds faster at higher temperatures for heat-setting systems such as methyl cellulose (Nishinari et al., 1997; Haque & Morris, 1992; Vigouret, Rinaudo, & Table 3 Parameters of the first order kinetic model for the gelation of 1% LM2 at different heating temperatures (abbreviations as in Table 1)

Fig. 3. Double logarithmic plot of storage shear modulus of LM1 and the concentration.

Temperature (⬚C)

G 0 sat (Pa)

k (min )

t0 (min)

r

50 60 70 75 80

35 311 420 910 115

0.082 0.120 0.098 0.158 0.691

38 12 6 1 1

0.981 0.970 0.969 0.975 0.992

-1

M. Yoshimura, K. Nishinari / Food Hydrocolloids 13 (1999) 227–233

Fig. 5. Time dependence of G 0 ,G 00 for dispersion of 2% LM2 at different heating temperatures S: 45⬚C, A: 50⬚C, K: 60⬚C, × : 70⬚C, *: 75⬚C, W: 80⬚C. Symbols represent the experimental values and the solid lines represent calculated curves.

231

Fig. 6. Time dependence of G 0 ,G 00 for dispersions of 3% LM2 at different heating temperatures S: 40⬚C, A: 50⬚C, K: 60⬚C, × : 70⬚C, *: 75⬚C, W: 80⬚C. Symbols represent the experimental values and the solid lines represent calculated curves.

Table 4 Parameters of the first order kinetic model for the gelation of 2% LM2 at different heating temperatures (abbreviations as in Table 1) Temperature (⬚C)

G 0 sat (Pa)

k (min )

t0 (min)

r

45 50 60 70 75 80

2370 4418 453 171 1522 614

0.090 0.110 0.248 0.532 0.641 1.260

27 8 7 5 2 1

0.969 0.992 0.976 0.908 0.953 0.964

⫺1

Table 5 Parameters of the first order kinetic model for the gelation of 3% LM2 at different heating temperatures (abbreviations as in Table 1) Temperature (⬚C)

G 0 sat (Pa)

k (min )

t0 (min)

r

40 50 60 70 75 80

9337 11803 1652 3443 3800 3788

0.038 0.097 0.226 0.441 0.744 1.076

21 10 3 2 1 1

0.998 0.992 0.983 0.954 0.990 0.968

-1

Table 6 Parameters of the first order kinetic model for the gelation of 1% LM1 with alkaline coagulant of different concentrations (abbreviations as in Table 1) Sample

G 0 sat (Pa)

k (min )

t0 (min)

r

0.5 M 1.0 M 1.5 M 2.0 M

251 520 634 98

0.059 0.167 0.171 0.206

13 4 4 2

0.964 0.985 0.976 0.994

-1

Fig. 7. Time dependence of G 0 ,G 00 for dispersions of 1% LM1 with alkaline coagulant of different concentrations S: 0.5 M, A: 1.0 M, K: 1.5 M, × : 2.0 M. Measurement temperature 60⬚C. Symbols represent the experimental values and the solid lines represent calculated curves.

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Desbrieres, 1996), xyloglucan (Reid, Edwards, & Dea, 1988; Shirakawa, Yamatoya, & Nishinari, 1998), curdlan (Hirashima, Takaya, & Nishinari, 1997), glycinin, and bconglycinin (Kohyama & Nishinari, 1993; Nagano, Akasaka, & Nishinari, 1994; Nagano, Mori, & Nishinari, 1994). The reason why the plateau value G 0 sat decreased with increasing temperature beyond a certain temperature may be attributed to one of the following possibilities: (1) Dispersions of KGM form gels so rapidly at higher temperatures that disordered network are formed, and weak gels result. (2) The slip between the plate and sample solutions may occur during measurement because of the exuded water from KGM gel. This should be explored in the future. 3.4. Effects of alkaline concentration Fig. 7 shows G 0 and G 00 for the 1% LM1 dispersion as a function of time at 60⬚C in the presence of an alkaline coagulant with different concentrations. The pH value of the 1% LM1 dispersion was 5.84 at t ˆ 0. When 20 ml Na2CO3 (0.5, 1.0, 1.5, and 2.0 M) was added to the KGM dispersions, the values of pH increased gradually and became 10.56, 10.75, 10.92, and 10.98, respectively after 30 min. The gelation curves for KGM dispersions with 1.0, 1.5, and 2.0 M Na2CO3 increased with time and attained plateau values faster than for KGM dispersion with 0.5 M Na2CO3. The gelation time t0 became shorter with increasing alkaline concentration. The plateau value G 0 sat seemed to become maximum at 1.5 M Na2CO3 (Table 6). The reason why G 0 sat seemed to become maximum in the presence of 1.5 M Na2CO3 may also be attributed to one of the two possibilities mentioned before. In addition to this, the molecular degradation at high pH should also be taken into account. The phenomenon that gels formed rapidly show a smaller elastic modulus than gels formed slowly was observed for many gels such as rennet-induced casein gels (Niki & Sasaki, 1987; Zoon, van Vliet, & Walstra, 1989), gellan gum gels (Nakamura et al., 1993), and amylose gels (Amari & Nakamura, 1977). If this phenomenon is induced by the slippage in the shear mode viscoelastic measurements, this should be checked by adopting indentation or other methods which are free from slippage. If this phenomenon is caused by the formation of the disordered structure in a rapid gelling process, the structural observation should be useful. References Amari, T., & Nakamura, M. (1977). Stress relaxation of aqueous gels of amylose, amylopectin, and their blends. Nihon Reorogi Gakkaishi, 6, 28–31 (in Japanese with summary and figure captions in English). Cairns, P., Atkins, E. D. T., Miles, M. J., & Morris, V. J. (1991). Molecular transforms of kappa carrageenan and furcellaran from mixed gel systems. Int. J. Biol. Macromol., 13, 65–68. Cairns, P., Miles, M. J., & Morris, V. J. (1988). X-ray diffraction studies on

konjac mannan-kappa carrageenan mixed gels. Carbohydr. Polym., 8, 99–104. Case, S. E., & Hamann, D. D. (1994). Fracture properties of konjac mannan gel: effect of gel temperature. Food Hydrocoll., 8, 147–154. Case, S. E., Knopp, J. A., Hamann, D. D., & Schwartz, S. J. (1992). Characterization of gelation of konjac mannan using lyotropic salts and rheological measurements. In G. O. Phillips & P. A. Williams & D. J. Wedlock (Eds.), (pp. 489). Gums and stabilisers for the food industry, 6. Oxford: Oxford University Press. Clark, A. H., & Ross-Murphy, S. B. (1985). The concentration dependence of biopolymer gel modulus. Brit. Polym.J., 17, 164–168. Haque, A., & Morris, E. R. (1992). Thermogelation of methylcellulose. Carbohydr. Polym., 22, 161. Hirashima, M., Takaya, T., & Nishinari, K. (1997). DSC and rheological studies on aqueous dispersions of curdlan. Thermochim. Acta., 306, 109–114. Jacon, S. A., Rao, M. A., Cooley, H. J., & Walter, R. H. (1993). The isolation and characterization of a water extract of konjac flour gum. Carbohydr. Polym., 20, 35–41. Kato, K., & Matsuda, K. (1969). Studies on the chemical structure of konjac mannan. Agric. Biol. Chem., 33, 1446–1453. Kohyama, K., Iida, H., & Nishinari, K. (1993). A mixed system composed of different molecular weights konjac glucomannan and kappa carrageenan: large deformation and dynamic viscoelastic study. Food Hydrocoll., 7, 213–226. Kohyama, K., & Nishinari, K. (1990). Dependence of the Specific Volume of Konjac Glucomannan on pH. In G. O. Phillips & D. J. Wedlock & P. A. Williams (Eds.), (pp. 459). Gums and Stabilisers for the Food Industry, 5. Oxford: IRL Press. Kohyama, K., & Nishinari, K. (1993). Rheological studies on the gelation process of soybean 7S and 11S properties in the presence of glucono-dlactone. J. Agric. Food Chem., 41, 8–14. Kohyama, K., & Nishinari, K. (1997). New application of konjac glucomannan as a texture modifier. JARQ, 31, 301–306. Kohyama, K., Sano, Y., & Nishinari, K. (1996). A mixed system composed of different molecular weights konjac glucomannan and kappa carrageenan2. Molecular weight dependence of viscoelasticity and thermal properties. Food Hydrocoll., 10, 229–238. Maeda, M., Shimahara, H., & Sugiyama, N. (1980). Detailed examination of the branched structure of konjac glucomannan. Agric. Biol. Chem., 44, 245–252. Maekaji, K. (1974). The mechanism of gelation of konjacmannan. Agric. Biol. Chem., 38, 315–321. Maekaji, K. (1978). A method for measurement and kinetic analysis of the gelation process of konjac mannan (kinetic study on the gelation of konjac mannan). Nippon Nogeikagakukaishi , 52, 251–257 (in Japanese with summary and figure captions in English). Maekaji, K. (1978). Determination of acidic component of konjac mannan. Agric.Biol.Chem., 42, 177–178. Mitchell, J. R. (1980). Rheology of gels. J. Text Stud., 11, 315–337. Miyoshi, E., Takaya, T., Williams, P. A., & Nishinari, K. (1996). Effects of monovalent and divalent cations on the interaction between gellan gum and konjac glucomannan. J. Agric. Food Chem., 44, 2486–2495. Nagano, T., Akasaka, T., & Nishinari, K. (1994). Dynamic viscoelastic properties of glycinin and b-conglycinin gels from soybeans. Biopolymers, 34, 1303–1309. Nagano, T., Mori, H., & Nishinari, K. (1994). Effect of heating and cooling on the gelation kinetics of 7S globulin from soybeans. J. Agric. Food Chem., 42, 1415–1419. Nakamura, K., Harada, K., & Tanaka, Y. (1993). Viscoelastic properties of aqueous gellan solutions: the effects of concentartion on gellan. Food Hydrocoll., 7, 435–447. Niki, R., Kohyama, K., Sano, Y., & Nishinari, K. (1994). Rheological study on the rennet-induced gelation of casein micelles with different sizes. Polymer Gels and Networks, 2, 105–109. Niki, R., & Sasaki, T. (1987). Studies on gelling process of renneted casein

M. Yoshimura, K. Nishinari / Food Hydrocolloids 13 (1999) 227–233 micelle using a newly developed gel firmness tester: influence of casein micelle concentration and temperature. Jpn. J. Zootech. Sci., 58, 1032– 1039 (in Japanese with summary and figure captions in English). Nishinari, K. (1997). Rheological and DSC study of sol–gel transition in aqueous dispersions of industrially important polymers and colloids. Colloid Polym. Sci., 275, 1093–1107. Nishinari, K., Hofmann, K. E., Moritaka, H., Kohyama, K., & Nishinari, N. (1997). Gel–sol transition of methylcellulose. Macromol. Chem. Phys., 198, 1217–1226. Nishinari, K., Miyoshi, E., Takaya, T., & Williams, P. A. (1996). Rheological and DSC studies on the interaction between gellan gum and konjac glucomannan. Carbohydr. Polym., 30, 193–207. Nishinari, K., Williams, P. A., & Phillips, G. O. (1992). Review of the physico-chemical characteristics and properties of konjacglucomannan. Food Hydrocoll., 6, 199–222. Oakenfull, D. (1984). A method for using measurements of shear modulus to estimate the size and thermodynamic stability of junction zones in noncovalently cross-linked gels. J. Food Sci., 49, 1103, 1104 & 1110. Reid, J. S. G., Edwards, M., & Dea, I. C. M. (1998). Enzymatic modification of natural seed gums. In G. O. Phillips & D. J. Wedlock & P. A. Williams (Eds.), Gums and stabilisers for the food industry 4, (pp. 391). Oxford: IRL Press. Rochas, C., Rinaudo, M., & Landry, S. (1990). Role of molecular weight on

233

the mechanical properties of kappa carrageenan gels. Carbohydr. Polym., 12, 255–266. Ross-Murphy, S. B., Shatwell, K. P., Sutherland, I. W., & Dea, I. C. M. (1996). Influence of acyl substituents on the interaction of xanthans with plant polysaccharides. Food Hydrocoll., 10, 117–122. Shirakawa, M., Yamatoya, M., & Nishinari, K. (1998). Tailoring of xyloglucan properties using an enzyme. Food Hydrocoll., 12, 25–28. Vigouret, M., Rinaudo, M., & Desbrieres, J. J. (1996). Thermogelation of methylcellulose in aqueous solutions. J. Chim. Phys., 93, 858–869. Ward, A. G., & Saunders, P. R. (1955). Mechanical properties of degraded gelatins. Nature, 176, 26–29. Watase, M., & Nishinari, K. (1983). Rheological properties of agarose gels with different molecular weights. Rheol. Acta, 22, 580–587. Yoshimura, M., Takaya, T., & Nishinari, K. (1996). Effects of konjacglucomannan on the gelatinization and retrogradation of corn starch as determined by rheology and differential scanning calorimetry. J. Agric. Food Chem., 44, 2970–2976. Yoshimura, M., Takaya, T., & Nishinari, K. (1998). Rheological studies on mixtures of corn starch and konjac-glucomannan. Carbohydr. Polym., 35, 71–79. Zoon, P., van Vliet, T., & Walstra, P. (1989). Rheological properties of rennet-induced skim milk gels.4. The effects of pH and NaCl. Nelh Milk Dairy J., 43, 17–34.