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Thermal properties of curdlan in aqueous suspension and curdlan gel
Food Hydrocolloids Vol.5 no.5 pp.427-434, 1991
Thermal properties of curdlan in aqueous suspension and curdlan gel Akira Konno and Tokuya Harada! Kinran Junior College, Suita 565 and lKobe Women's University, Suma-ku, Kobe 654, Japan Abstract. The thermal properties of curdlan in aqueous suspension and curdlan gel were studied by differential scanning calorimetry (DSC). The DSC curve of curdlan in aqueous suspension showed two sharp endothermic peaks, at 50-64 and 140-160°C, and a broad endothermic peak between the two peaks. An exothermic peak at ~140°C was also observed. The two endothermic peaks are caused by the swelling of curdlan and the melting of gel formed. The broad peak may be caused by the formation of firm gel stabilized by hydrophobic interaction. The exothermic peak may be due to the formation of microfibrils. The swelling temperature and enthalpy of curdlan in aqueous suspension were 49.7°C and 10 Jig, respectively. The swelling temperature of curdlan in the presence of sodium chloride or urea shifted to a higher or a lower temperature range, respectively.
Introduction
Harada et al. (1-6) found that curdlan, a bacterial polysaccharide composed entirely of (1 ~ 3)-I3-D-glucosidic linkage, forms two types of gel: one is formed by neutralization of its alkaline solution or by heating aqueous suspension at about 60°C and has a much lower gel strength and syneresis than the other, which is obtained by heating an aqueous suspension to >80°C. Curdlan is now produced commercially by Takeda Chemical Ind. Ltd and is used as a food additive and industrial material. Studies on curdlan have been reviewed by Harada (7,8) and Kasai and Harada (9). Harada et al. (5) mentioned that 60°Cset curdlan gel is formed by an endothermic reaction since the inside temperature of the gel is lower than that of the outside during gel formation. Previously Konno et al. (10,11) reported that curdlan powder swells in water similar to the starch granule at the first stage of heating, and forms a network structure with micelles stabilized by hydrophobic interaction at the second stage in a higher temperature range, whereas the thermo-reversible gel prepared by heating to ~60°C has micelle structures ordered by intermolecular hydrogen bonding of swollen curdlan. In this paper we investigate heat-induced gel formation of curdlan by differential scanning calorimetry. Materials and methods
Materials Spray-dried curdlan powder was provided by Takeda Chemical Ind. Ltd. The average degree of polymerization (DPn) of the polymer determined according to Manners' method (12) was ~450. 427
A.Konno and T.Harada
Methods
Differential scanning calorimetry was carried out using Seiko SSC 5200 DSC 120 (Seiko Instruments Inc). The dispersion of curdlan (2 mg) obtained using a Waring blender was sealed in a Ag capsule of 70111. An appropriate quantity of distilled water was used as a reference material to obtain a flat baseline. Calibration for temperature and enthalpy was done using indium pressed on a clean glass plate, or benzoic acid or diphenyl in a Ag capsule as the standard. The temperature was raised from 30 to 170°C at the rate of 1°C/min. Curdlan gel prepared by keeping at 5°C for 2 days after heating at 70°C for 10 min was also used. Enthalpy was calculated from the peak area of the DSC curve. The breaking strength and strain of gels obtained by heating 2% curdlan aqueous suspension to various temperatures were measured using a creep meter RE-330S (Yamaden Co.) at the compression rate of 1 mm/s. The size of the gel sample was 15 mm in diameter and 10 mm in thickness. Results and discussion
The effect of heating temperature on the physical properties of curdlan gel prepared from aqueous suspension was examined by a creep meter as shown in Figure 1. Breaking strength and strain of 2% curdlan gels were measured at 3000 r - - - - - - - - - - - - - - - - . .
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428
Thermal properties of curdlan
30°C. The breaking strength of curdlan gels was constant between 60 and 75°C, whereas it increased linearly at >75°C. The breaking strain of curdlan gels was also constant within the temperature range 60-75°C, increasing slightly between 75 and 85°C and then remaining constant at > 85°C. Data shown in Figure 1 indicate that gels formed by heating to > 75°C are firmer than those obtained by heating to < 75°C. These results correspond to those of gel strength evaluated by curd-meter (13) . Some researchers (14,15) have reported that the DSC curve of starch in aqueous suspension gave only one peak at ~70°C. However, starch in aqueous suspension is unable to form a firm gel by heating. The DSC curve of curdIan in aqueous suspension gave two sharp endothermic peaks , at 50-64 and 140160°C, a broad endothermic peak shown by the dotted line between the two sharp peaks and an exothermic peak at ~ 140°C (Figure 2). Previously Konno et at. (10,11) measured the change of viscosity and the transmittance of curdlan suspension during heating and observed swelling of curdlan by photomicroscopy. These results supported the observation that the suspended curdlan swelled remarkably in the temperature range of 50-60°C. Therefore, we concluded that the endothermic peak at -56°C could be related to the swelling of curdlan. Since the results also suggested that swelled curdlan could form a network structure with micelles stabilized by hydrophobic interaction at >60°C
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429
A.Konno and T.Harada
(10,11), and Kasai (9), Saito (16), Okuyama (17) and Harada (18) and their colleagues indicated that the helix structure was transformed from single strand to triple strands at > 110°C, the broad endothermic peak may indicate structural transition such as the formation of firm gel stabilized by hydrophobic interaction. The exothermic peak at ~ 140°C may correspond to thermal structural transition such as the partial crystallization of curdlan helices, i.e. the construction of microfibril, which is resistant to zymolyase [(1 ~ 3)-f3-Dglucanase] and 32% sulfuric acid treatment at 32°C (4,6). The endothermic peak at 154°C may be due to the melting of the firm gel since Kuge et at. (19) found that curdlan gel melts within the temperature range 140-160°C. 55
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Thermal properties of curdlan
The swelling temperature of the suspended curdlan was recorded as the onset temperature of the endothermic peak in a lower temperature range, as shown in Figure 2. The swelling temperature of curdlan was 49.7°C, independent of the concentration, as shown in Figure 3. Figure 4 shows the effect of curdlan concentration on the swelling enthalpy (Ii.H). Swelling enthalpy was also independent of the concentration and was determined to be 10 Jig of curdlan, corresponding to the gelatinization enthalpy of 10-12 Jig for wheat starch granules (14). The DSC curve of curdlan gel (4%) prepared by heating to 70°C indicated two endothermic peaks, at 44-65°C and at 140-160°C, a broad endothermic peak (indicated by a dotted line) between the two peaks and an exothermic peak at -143°C are shown in Figure 5. This is very similar to the results obtained by DSC for curdlan suspension, as shown in Figure 2. Konno et al. (10,11) reported that the gel which was set by heating the aqueous suspended curdlan to 65°C prior to cooling to <40°C melted at 65°C. Therefore the first peak is related to the thermal transition from gel to sol, whereas the second broad peak relates to the sol-gel transition stage. The exothermic peak at -140°C may be due to the formation of microfibrils. The melting enthalpy of curdlan gel was independent of the gel concentration within 2-5%. The melting enthalpy of 4% gel was 5 Jig as the basis of measuring curdlan quantity, which was half of the value for the swelling enthalpy in aqueous suspension. This means that the gel
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431
A.Konno and T.Harada
I
I
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Fig. 6. Effect of sodium chloride on the swelling temperature of curdlan in aqueous suspension (4%).
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432
Thermal properties of curdlan
structure stabilized by intra- and intermolecular interactions, such as hydrogen bonds, may be weaker than that of the suspended curdlan. The exothermic peak at -143°C is very large in comparison with the DSC curve obtained using curdlan in aqueous suspension (Figure 2). However, the endothermic peak at -153°C, which was independent of the gel concentration, became much smaller. The reason for the difference is unclear. Figure 6 shows the effect of sodium chloride on the swelling temperature of suspended curdlan (4%). The swelling temperature increased with increasing concentration of sodium chloride, but the iliI values were unchanged in the presence of sodium chloride. Konno et al, (10,11) reported that the temperature producing the change of specific viscosity and transmittance of curdlan suspension during the swelling stage shifted to a higher temperature range in the presence of sodium chloride. It is therefore considered that this increase is caused by the depressive action of salt on osmotic swelling. Next we examined the effect of urea (1 mol/drrr') on the thermal behavior of suspended curdlan (4%). The DSC curve obtained is shown in Figure 7. All peaks of suspended curdlan shifted more or less to a lower temperature range in the presence of urea, but the swelling enthalpy was virtually constant (-10 Jig) up to 4 mol/drrr'. The DSC curve above -100°C differs.from those in Figures 2 and 5. The endothermic peak at ~ 154°C is particularly small, but the exothermic peak near 170°C is very large. These facts are not easily explained. With 8 mol/ dnr' of urea, the JiH value was 2.5 Jig. Figure 8 shows the effect of urea on the swelling temperature of aqueous suspended curdlan (4%). The swelling temperature shifted to a lower temperature range with the increase in urea concentration. Harada (7) has already shown that the addition of 8 mol/dm" urea reduced the initial temperature of gel formation of curdlan to 33°C, and
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433
A.Konno and T.Harada
that a firm gel could not be formed at the presence of 8 mol/drrr' urea. Therefore these phenomena may be due to the breaking of intra- and intermolecular hydrogen bonds of curdlan with urea. Acknowledgements
The authors thank Misses Ikuko Asai, Misuzu Tsukagawa, Ayako Hamamura and Asami Wakamatsu for their skillful assistance in differential scanning calorimetry. References 1. Harada,T., Masada,T., Fujimori,K. and Maeda,I. (1966) Agric. Bioi. Chern., 30,196-198. 2. Harada,T., Misaki,A. and Saito,H. (1968) Arch. Biochern. Biophys., 124,292-298. 3. Kanzawa,Y., Harada,T., Koreeda,A. and Harada,A. (1987) Agric. Bioi. Chern., 51,1839-1843. 4. Kanzawa,Y., Harada,T., Koreeda,A., Harada,A. and Okuyama,K. (1989) Carbohydr. Polyrn., 10, 299-313. 5. Harada,T., Sato,S. and Harada,A. (1987) Bull. Kobe Wornen's Univ., 20, 143-164. 6. Takahashi,F., Harada,T., Koreeda,A. and Harada,A. (1986) Carbohydr. Polyrn., 6, 407-421. 7. Harada,T. (1977) In Sandford,P.A. and Laskin,A. (eds), Exocellular Microbial Polysaccharides. ACS Syrnp. Ser. 45. American Chemistry Society, pp. 265-283. 8. Harada,T. (1979) In Blanshard,J.M.V. and Mitchell, (eds), Polysaccharides in Food. Butterworths, London, pp. 283-300. 9. Kasai,N. and Harada,T. (1980) In French,A.D. and Gardner,K.H. (eds), Fiber Diffraction Methods. ACS Symp, Ser. 141. American Chemistry Society, pp. 363-384. 10. Konno,A., Kimura,H., Nakagawa,T. and Harada,T. (1978) Nippon Nogei Kagaku Kaishi, 52, 247-250 (in Japanese). 11. Konno,A., Azechi,Y. and Kimura,H. (1979) Agric. Bioi. Chern., 43,101-104. 12. Manners,D.J., Masson,A.J. and Sturgeon.J, (1971) Carbohydr. Res., 17, 109-114. 13. Maeda,I., Saito,H., Masada,M., Misaki,A. and Harada,T. (1967) Agric. Bioi. Chern., 31, 11841188. 14. Stevens,D.J. and Elton,G.A. (1971) Starke, 23, 8-11. 15. Wootton,M. and Bamunuarachchi,A. (1979) Starke, 31, 201-204. 16. Saito,H., Yoshioka,Y., Yokoi,M. and Yamada,Y. (1990) Biopolyrners, 19, 1689-1698. 17. Okuyama.K., Otsubo,A., Fukuzawa,Y., Ozawa,M., Harada,T. and Kasai,N. (1991) Carbohydro Chern., 10, 645-656. 18. Harada,T., Kanzawa,Y., Kanenaga,K., Koreeda,A. and Harada,A. (1991) Food Structure, 10, 1-18. 19. Kuge,T., Suetsugu,N. and Nishiyama,K. (1971) Agric. Bioi. Chem., 41,1315-1316.
Received on May 2, 1991; accepted on August 12, 1991
434
Thermal properties of curdlan in aqueous suspension and curdlan gel Akira Konno and Tokuya Harada! Kinran Junior College, Suita 565 and lKobe Women's University, Suma-ku, Kobe 654, Japan Abstract. The thermal properties of curdlan in aqueous suspension and curdlan gel were studied by differential scanning calorimetry (DSC). The DSC curve of curdlan in aqueous suspension showed two sharp endothermic peaks, at 50-64 and 140-160°C, and a broad endothermic peak between the two peaks. An exothermic peak at ~140°C was also observed. The two endothermic peaks are caused by the swelling of curdlan and the melting of gel formed. The broad peak may be caused by the formation of firm gel stabilized by hydrophobic interaction. The exothermic peak may be due to the formation of microfibrils. The swelling temperature and enthalpy of curdlan in aqueous suspension were 49.7°C and 10 Jig, respectively. The swelling temperature of curdlan in the presence of sodium chloride or urea shifted to a higher or a lower temperature range, respectively.
Introduction
Harada et al. (1-6) found that curdlan, a bacterial polysaccharide composed entirely of (1 ~ 3)-I3-D-glucosidic linkage, forms two types of gel: one is formed by neutralization of its alkaline solution or by heating aqueous suspension at about 60°C and has a much lower gel strength and syneresis than the other, which is obtained by heating an aqueous suspension to >80°C. Curdlan is now produced commercially by Takeda Chemical Ind. Ltd and is used as a food additive and industrial material. Studies on curdlan have been reviewed by Harada (7,8) and Kasai and Harada (9). Harada et al. (5) mentioned that 60°Cset curdlan gel is formed by an endothermic reaction since the inside temperature of the gel is lower than that of the outside during gel formation. Previously Konno et al. (10,11) reported that curdlan powder swells in water similar to the starch granule at the first stage of heating, and forms a network structure with micelles stabilized by hydrophobic interaction at the second stage in a higher temperature range, whereas the thermo-reversible gel prepared by heating to ~60°C has micelle structures ordered by intermolecular hydrogen bonding of swollen curdlan. In this paper we investigate heat-induced gel formation of curdlan by differential scanning calorimetry. Materials and methods
Materials Spray-dried curdlan powder was provided by Takeda Chemical Ind. Ltd. The average degree of polymerization (DPn) of the polymer determined according to Manners' method (12) was ~450. 427
A.Konno and T.Harada
Methods
Differential scanning calorimetry was carried out using Seiko SSC 5200 DSC 120 (Seiko Instruments Inc). The dispersion of curdlan (2 mg) obtained using a Waring blender was sealed in a Ag capsule of 70111. An appropriate quantity of distilled water was used as a reference material to obtain a flat baseline. Calibration for temperature and enthalpy was done using indium pressed on a clean glass plate, or benzoic acid or diphenyl in a Ag capsule as the standard. The temperature was raised from 30 to 170°C at the rate of 1°C/min. Curdlan gel prepared by keeping at 5°C for 2 days after heating at 70°C for 10 min was also used. Enthalpy was calculated from the peak area of the DSC curve. The breaking strength and strain of gels obtained by heating 2% curdlan aqueous suspension to various temperatures were measured using a creep meter RE-330S (Yamaden Co.) at the compression rate of 1 mm/s. The size of the gel sample was 15 mm in diameter and 10 mm in thickness. Results and discussion
The effect of heating temperature on the physical properties of curdlan gel prepared from aqueous suspension was examined by a creep meter as shown in Figure 1. Breaking strength and strain of 2% curdlan gels were measured at 3000 r - - - - - - - - - - - - - - - - . .
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100
2000
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Temperature
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Fig. 1. Effect of heating temperature on the physical properties of curdlan gel (2%). 0, Breaking strength;., breaking strain.
428
Thermal properties of curdlan
30°C. The breaking strength of curdlan gels was constant between 60 and 75°C, whereas it increased linearly at >75°C. The breaking strain of curdlan gels was also constant within the temperature range 60-75°C, increasing slightly between 75 and 85°C and then remaining constant at > 85°C. Data shown in Figure 1 indicate that gels formed by heating to > 75°C are firmer than those obtained by heating to < 75°C. These results correspond to those of gel strength evaluated by curd-meter (13) . Some researchers (14,15) have reported that the DSC curve of starch in aqueous suspension gave only one peak at ~70°C. However, starch in aqueous suspension is unable to form a firm gel by heating. The DSC curve of curdIan in aqueous suspension gave two sharp endothermic peaks , at 50-64 and 140160°C, a broad endothermic peak shown by the dotted line between the two sharp peaks and an exothermic peak at ~ 140°C (Figure 2). Previously Konno et at. (10,11) measured the change of viscosity and the transmittance of curdlan suspension during heating and observed swelling of curdlan by photomicroscopy. These results supported the observation that the suspended curdlan swelled remarkably in the temperature range of 50-60°C. Therefore, we concluded that the endothermic peak at -56°C could be related to the swelling of curdlan. Since the results also suggested that swelled curdlan could form a network structure with micelles stabilized by hydrophobic interaction at >60°C
r-:
~
<,:
56.3 C
0 ..J
123 .0 C
U.
I
J: U ~ 0:: W
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0 0
z
w
! 40
72 . 5
105
TEMP C
(Heating)
137 .5
170
Fig. 2. The DSC curve of curdlan in aqueous suspension (4%). Bar represents 60 flW.
429
A.Konno and T.Harada
(10,11), and Kasai (9), Saito (16), Okuyama (17) and Harada (18) and their colleagues indicated that the helix structure was transformed from single strand to triple strands at > 110°C, the broad endothermic peak may indicate structural transition such as the formation of firm gel stabilized by hydrophobic interaction. The exothermic peak at ~ 140°C may correspond to thermal structural transition such as the partial crystallization of curdlan helices, i.e. the construction of microfibril, which is resistant to zymolyase [(1 ~ 3)-f3-Dglucanase] and 32% sulfuric acid treatment at 32°C (4,6). The endothermic peak at 154°C may be due to the melting of the firm gel since Kuge et at. (19) found that curdlan gel melts within the temperature range 140-160°C. 55
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~
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50
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430
Thermal properties of curdlan
The swelling temperature of the suspended curdlan was recorded as the onset temperature of the endothermic peak in a lower temperature range, as shown in Figure 2. The swelling temperature of curdlan was 49.7°C, independent of the concentration, as shown in Figure 3. Figure 4 shows the effect of curdlan concentration on the swelling enthalpy (Ii.H). Swelling enthalpy was also independent of the concentration and was determined to be 10 Jig of curdlan, corresponding to the gelatinization enthalpy of 10-12 Jig for wheat starch granules (14). The DSC curve of curdlan gel (4%) prepared by heating to 70°C indicated two endothermic peaks, at 44-65°C and at 140-160°C, a broad endothermic peak (indicated by a dotted line) between the two peaks and an exothermic peak at -143°C are shown in Figure 5. This is very similar to the results obtained by DSC for curdlan suspension, as shown in Figure 2. Konno et al. (10,11) reported that the gel which was set by heating the aqueous suspended curdlan to 65°C prior to cooling to <40°C melted at 65°C. Therefore the first peak is related to the thermal transition from gel to sol, whereas the second broad peak relates to the sol-gel transition stage. The exothermic peak at -140°C may be due to the formation of microfibrils. The melting enthalpy of curdlan gel was independent of the gel concentration within 2-5%. The melting enthalpy of 4% gel was 5 Jig as the basis of measuring curdlan quantity, which was half of the value for the swelling enthalpy in aqueous suspension. This means that the gel
~
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a::
w
:r:
ro a
z
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40
72.5
105
TEMP C
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137.5
1 TO
Fig. 5. The DSC curve of curdlan gel (4%) prepared by heating at 70°C. Bar represents 60 ....W.
431
A.Konno and T.Harada
I
I
O. 5
J. 0
Na C! (Illole/L)
Fig. 6. Effect of sodium chloride on the swelling temperature of curdlan in aqueous suspension (4%).
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Fig. 7. Effect of urea (1 rnol/drrr' ) on the DSC curve of curdlan in aqueous suspension (4%) . Bar represents 70 J.L W.
432
Thermal properties of curdlan
structure stabilized by intra- and intermolecular interactions, such as hydrogen bonds, may be weaker than that of the suspended curdlan. The exothermic peak at -143°C is very large in comparison with the DSC curve obtained using curdlan in aqueous suspension (Figure 2). However, the endothermic peak at -153°C, which was independent of the gel concentration, became much smaller. The reason for the difference is unclear. Figure 6 shows the effect of sodium chloride on the swelling temperature of suspended curdlan (4%). The swelling temperature increased with increasing concentration of sodium chloride, but the iliI values were unchanged in the presence of sodium chloride. Konno et al, (10,11) reported that the temperature producing the change of specific viscosity and transmittance of curdlan suspension during the swelling stage shifted to a higher temperature range in the presence of sodium chloride. It is therefore considered that this increase is caused by the depressive action of salt on osmotic swelling. Next we examined the effect of urea (1 mol/drrr') on the thermal behavior of suspended curdlan (4%). The DSC curve obtained is shown in Figure 7. All peaks of suspended curdlan shifted more or less to a lower temperature range in the presence of urea, but the swelling enthalpy was virtually constant (-10 Jig) up to 4 mol/drrr'. The DSC curve above -100°C differs.from those in Figures 2 and 5. The endothermic peak at ~ 154°C is particularly small, but the exothermic peak near 170°C is very large. These facts are not easily explained. With 8 mol/ dnr' of urea, the JiH value was 2.5 Jig. Figure 8 shows the effect of urea on the swelling temperature of aqueous suspended curdlan (4%). The swelling temperature shifted to a lower temperature range with the increase in urea concentration. Harada (7) has already shown that the addition of 8 mol/dm" urea reduced the initial temperature of gel formation of curdlan to 33°C, and
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433
A.Konno and T.Harada
that a firm gel could not be formed at the presence of 8 mol/drrr' urea. Therefore these phenomena may be due to the breaking of intra- and intermolecular hydrogen bonds of curdlan with urea. Acknowledgements
The authors thank Misses Ikuko Asai, Misuzu Tsukagawa, Ayako Hamamura and Asami Wakamatsu for their skillful assistance in differential scanning calorimetry. References 1. Harada,T., Masada,T., Fujimori,K. and Maeda,I. (1966) Agric. Bioi. Chern., 30,196-198. 2. Harada,T., Misaki,A. and Saito,H. (1968) Arch. Biochern. Biophys., 124,292-298. 3. Kanzawa,Y., Harada,T., Koreeda,A. and Harada,A. (1987) Agric. Bioi. Chern., 51,1839-1843. 4. Kanzawa,Y., Harada,T., Koreeda,A., Harada,A. and Okuyama,K. (1989) Carbohydr. Polyrn., 10, 299-313. 5. Harada,T., Sato,S. and Harada,A. (1987) Bull. Kobe Wornen's Univ., 20, 143-164. 6. Takahashi,F., Harada,T., Koreeda,A. and Harada,A. (1986) Carbohydr. Polyrn., 6, 407-421. 7. Harada,T. (1977) In Sandford,P.A. and Laskin,A. (eds), Exocellular Microbial Polysaccharides. ACS Syrnp. Ser. 45. American Chemistry Society, pp. 265-283. 8. Harada,T. (1979) In Blanshard,J.M.V. and Mitchell, (eds), Polysaccharides in Food. Butterworths, London, pp. 283-300. 9. Kasai,N. and Harada,T. (1980) In French,A.D. and Gardner,K.H. (eds), Fiber Diffraction Methods. ACS Symp, Ser. 141. American Chemistry Society, pp. 363-384. 10. Konno,A., Kimura,H., Nakagawa,T. and Harada,T. (1978) Nippon Nogei Kagaku Kaishi, 52, 247-250 (in Japanese). 11. Konno,A., Azechi,Y. and Kimura,H. (1979) Agric. Bioi. Chern., 43,101-104. 12. Manners,D.J., Masson,A.J. and Sturgeon.J, (1971) Carbohydr. Res., 17, 109-114. 13. Maeda,I., Saito,H., Masada,M., Misaki,A. and Harada,T. (1967) Agric. Bioi. Chern., 31, 11841188. 14. Stevens,D.J. and Elton,G.A. (1971) Starke, 23, 8-11. 15. Wootton,M. and Bamunuarachchi,A. (1979) Starke, 31, 201-204. 16. Saito,H., Yoshioka,Y., Yokoi,M. and Yamada,Y. (1990) Biopolyrners, 19, 1689-1698. 17. Okuyama.K., Otsubo,A., Fukuzawa,Y., Ozawa,M., Harada,T. and Kasai,N. (1991) Carbohydro Chern., 10, 645-656. 18. Harada,T., Kanzawa,Y., Kanenaga,K., Koreeda,A. and Harada,A. (1991) Food Structure, 10, 1-18. 19. Kuge,T., Suetsugu,N. and Nishiyama,K. (1971) Agric. Bioi. Chem., 41,1315-1316.
Received on May 2, 1991; accepted on August 12, 1991
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