Konjac mannan

32 Konjac mannan S. Takigami, Gunma University, Japan

Abstract: Konjac mannan is a main component of tubers of konjac which is a perennial plant of Araceae. It is a heteropolysaccharide consisting of -Dglucose (G) and -D-mannose (M), with a G/M ratio of 1 to 1.6. Konjac mannan contains very small amounts of acetyl groups and the viscosity of its aqueous solution is quite high. Deacetylation occurs with alkali treatment and a chewy irreversible gel is prepared. The gel has been used as a traditional dietary food in Japan for a long time. Konjac mannan interacts synergistically with other polysaccharides and forms thermoreversible gels. In this chapter, the following subjects are explained: cultivation of the konjac tuber, the production process and purification of konjac flour, the chemical structure and molecular weight of konjac mannan, the component analysis of commercial konjac flour, the properties of mixture gel synergistically prepared by konjac mannan and other polysaccharides, the uses and applications of konjac flour, and regulatory status. Key words: konjac mannan, glucomannan, konjac tuber, cultivation, production process, purification, structure, gel, viscosity.

32.1

Introduction

Konjac (Lasioideae Amorphophallus) is a perennial plant and a member of the family of Araceae. The original home of the konjac plant is not certain, but is considered to be in Southeast Asia. There are many species of konjac plants in the Far East and Southeast Asia that belong to the Amorphophallus,1 for example, A. konjac C. Koch (Japan, China, Indonesia), A. bulbifer Bl. (Indonesia), A. oncophyllus Prain ex Hook. f. (Indonesia), A. variabilis Blume (the Philippines, Indonesia, Malaysia), etc. Only Amorphophallus konjac C. Koch grows in Japan. They contain konjac mannan in their tubers. Konjac

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Fig. 32.1

Konjac plants.

mannan is a heteropolysaccharide consisting of -D-glucose (G) and -Dmannose (M), with a G/M ratio of 1 to 1.6. The konjac tuber grows in size year by year and three- to five-year-old plants bloom with purplish-red flowers in the spring. Konjac is an allogamous plant and plant breeding is performed by crossfertilisation. Figures 32.1 and 32.2 show konjac plants and tubers. The main component of the konjac tuber is konjac mannan (KM), which varies in composition from 8±10% of a raw tuber. Starch, lipid and minerals are also present in the tuber. KM is accumulated in egg-shaped cells covered with scale-like cell walls2, 3 and the KM cells are observed within the parenchyma of the tuber. The size and number of the KM cells increase with distance from the epidermis, reaching ~650 m at the central part of the tuber. Other types of organelles in the parenchyma surround the KM cells. Starch exists in spherical organelles as small particles. Bunches of needle-like crystals are also observed in the tuber and the size of a crystal is ca. 150 m  5 m. Since a high content of calcium was detected in the crystal by energy dispersive X-ray (EDX) analysis, the needle-like crystal is considered to be calcium oxalate. The konjac

Konjac mannan 891

Fig. 32.2

Two-year-old konjac tubers.

tuber, unprocessed, has a harsh taste. This can be removed from the konjac flour by processing. Irreversible konjac mannan gel is prepared by alkali treatment of grated konjac tuber or konjac flour aqueous solution. KM has very small amount of acetyl groups and deacetylation occurs with the alkali treatment.4 It is considered that the gelation of konjac mannan is induced by deacetylation. The lowest critical concentration of konjac flour aqueous solution necessary for gel formation is about 0.5%. The konjac gel (Kon-nyaku in Japanese) is classified as a dietary fibre and it has a chewy texture. The first description of konjac gel and its preparation process are found in an old Chinese poem composed by Zuo Shi and its annotation written in the third century.5 It is thought in Japan that the production method of konjac gel was introduced from Korea with Buddhism in the sixth century as a medicine. However, it took a long time before konjac gel became a popular food and this was due to two important investigations for the production process of konjac flour. T. Nakajima (1745±1826) developed a manufacturing technique to produce konjac flour by pulverising dried chips of konjac tuber (Arako). K. Mashiko (1745±1854) improved on this technique to obtain cleaner konjac flour (Seiko). He polished Arako using a mortar worked by a water wheel and separated impurities from the konjac flour by wind sifting. Nowadays, konjac flour is produced in very modern factories controlled by computer systems. However, the principle of the production manufacturing process is the same. It is well known that konjac mannan interacts synergistically with kappa carrageenan6 and xanthan gum7, 8 and forms elastic thermoreversible gels. These synergistic gels are major products in the food industry as new healthy gel foods,

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particularly in Japan. In the United States, the US Department of Agriculture recently accepted the use of konjac flour as a binder in meat and poultry products. Konjac flour is suitable for thickening, gelling, texturing, and water binding. It may be used to provide fat-replacement properties in fat-free and low-fat meat products.

32.2

Manufacture

32.2.1 Cultivation Only Amorphophallus konjac C. Koch grows in Japan and selective breeding of konjac plants has been carried out. Recently, five species of the A. konjac have been cultivated, namely, Zairai, Shina, Haruna-kuro, Akagi-ohdama and Miyogi-yutaka. The latter three species are improved breeds and Haruna-kuro and Akagi-ohdama account for more than 90% of the total tuber output. The cultivation process of the konjac tuber in Japan is as follows. Seed tubers (Kigo) and/or one-year-old tubers are planted in the spring. The tubers push out new shoots and are consumed completely. The konjac plants grow during the summer and have new tubers. In the late autumn, the plants die and new tubers are dug from the ground. The new tuber has seed tubers at the top of its suckers. The two-year-old tubers are used to produce konjac flour. One-year-old tubers and the seed tubers are kept in a storehouse with heating during the winter to avoid freezing. This cycle is repeated in the following spring. In China, there are six kinds of konjac plants containing konjac mannan and two species can be cultivated, namely, A. rivieri Duieu and A. aldus Lie et Chen. The selective breeding of konjac plants is also actively carried out. 32.2.2 Production process of konjac flour The two-year-old konjac tubers are brought to a storehouse in containers from farmhouses. The tubers are transported to a washing apparatus using conveyer belts and are washed with water, brushing away mud and epidermis and then distributed to each line. The washed konjac tubers are sliced into thin chips, and the chips are dried in a hot-air drier equipped with a heavy oil burner. This is because konjac flour contains a small amount of sulphur dioxide as an impurity. Sulphur dioxide bleaches konjac chips and for this reason the colour of lowerquality konjac flour is extremely white. The dried konjac chips are called Arako in Japanese. The dried chips are pulverised and konjac mannan (KM) particles (i.e., konjac flour) are obtained. Since the KM particles are very tough, they are polished after being produced to remove impurities surrounding the KM cells. Then konjac flour is separated by wind shifting. The polished konjac flour is called Seiko. Micro-fine powder obtained as a by-product is collected using a dust collector. The by-product is called Tobiko in Japanese, which literally means flying powder. The main components of Tobiko are starch and fine KM powder. Protein (ca. 24%) and ash (ca. 10%) are also included in Tobiko.

Konjac mannan 893 The viscosity of konjac flour is dependent on the raw tubers and is controlled by the mixing of flours produced from different types of tubers. Then the konjac flour thus prepared is packed into the bags and is kept in a cool storehouse to avoid a change in quality. 32.2.3 Purification of konjac flour Commercial konjac flour (Seiko) is a light-coloured powder with fish-like smell and a slightly harsh taste. The current practice of several companies is to wash konjac flour with ethanol aqueous solution to remove the micro-fine powders remaining on the surface and the impurities trapped inside the konjac particles. The konjac flour is whitened by washing. Figures 32.3 and 32.4 show the SEM images of commercial konjac flour, with Fig. 32.4 being the one which has been highly purified. The surface of konjac flour shows scale-like patterns and seems to have been worn smooth (Fig. 32.3). After purification, the scale-like patterns are more clearly observed. Table 32.1 shows the composition of the various components in konjac flour before and after purification. Since the protein content was determined by nitrogen analysis, the value represents not only protein but also all nitrogencontaining substances. The carbohydrate content increased with washing but the concentration of the other components decreased by washing. The carbohydrate value parallels that of KM. The fish-like smell decreases remarkably by washing. It has been reported that alkali treated konjac gel contains trimethylamine and that the fish-like smell of the flour is caused by the amine.9, 10 Konjac flour with and without

Fig. 32.3 Scanning electron micrograph of konjac flour (Seiko).

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Fig. 32.4 Table 32.1 purification

Scanning electron micrograph of purified konjac flour.

Analytical results of components in konjac flour before and after Contents (g/100g of sample)

Konjac flour Purified konjac flour

Water

Protein

Lipid

Carbohydrate

Fibre

Ash

7.2 7.5

2.2 0.8

2.3 0.9

82.6 88.6

0.5 0.5

5.2 1.7

purification showed mass spectra attributable to nitrogen-containing substances, but they were not identical to trimethylamine.2 This demonstrates that konjac flour does not contain trimethylamine as an impurity. Trimethylamine should be separated from other nitrogen-containing substances by the alkali treatment. The purification of konjac flour is very effective in preventing the putrefaction of konjac gel prepared by alkali treatment and the syneresis of the mixed gels prepared by konjac mannan and other gums.

32.3

Structure

The main component of konjac flour is a glucomannan called konjac mannan (KM), whose main chain consists of D-glucose and D-mannose linked by -D-1,4 bonds. The ratio of glucose (G) to mannose (M) is reported to be 1 to 1.611±13 or 2 to 3.14, 15 Although the repeating structural unit of the main chain is still uncertain, typical proposals for the unit by research scientists are as follows:

Konjac mannan 895 1. 2. 3.

G-G-M-M-M-M-G-M or G-G-M-G-M-M-M-M11 M-M-M-G-G13 G-G-M-M-G-M-M-M-M-M-G-G-M.13, 16

It is also reported that KM has side chains and the branching position is considered to be the C3 position of mannose residues11, 17 or C3 positions on both glucose and mannose13 in the main chain. The degree of branching is estimated at approximately three for every 32 sugar units14 or at one for 80 sugar residues.11 The length of the branched chain was also evaluated as 11 to 16 hexose residues17 or as several hexose units.11 KM contains acetyl groups in the main chain. Figure 32.5 shows a Fourier transformation infra-red (FT-IR) spectrum of purified KM. An absorption due to stretching vibration of C = O group in acetyl group is observed at 1730 cmÿ1. The acetyl group content was estimated at one for 19 sugar residues.4 Figure 32.6 shows the chemical structure of KM proposed by Okimasu.1, 18 The crystalline form of KM was studied by the X-ray diffraction method.19 KM shows a different X-ray diffraction powder pattern from both crystalline polymorphs of other glucomannans (mannan I and mannan II) which have been studied. The fibre pattern of the annealed KM indicated that it exists in an extended two-fold helical structure. Since konjac flour forms very viscous solutions, measurement of the weight average molecular weight (Mw) and the mean square radius of gyration (1/2) of KM was carried out using partially methylated KM samples.20 The average values of Mw and 1/2 were determined to be 10105 and 110nm. It was also reported that both Mw and 1/2 were found to be dependent on species of konjac plant, cultivation districts and preparation method. The authors21 measured molecular weight (Mw), molecular dispersity and root mean square (RMS) of KM (Akagi ohdama species obtained in Gunma prefecture, Japan) using the Dawn multi-angle laser light scattering method, associated with a gel permeation

Fig. 32.5 FT-IR spectrum of konjac mannan analysed by the attenuated total reflection (ATR) method.

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Fig. 32.6

Chemical structure of konjac mannan.

chromatographic (GPC) fractionation. The Mw, molecular dispersity and RMS were 13.2105, 2.1 and 130nm, respectively.

32.4

Technical data

The quality of commercial konjac flour is appraised by the size of KM particles, viscosity, whiteness, moisture and admixing of impurities such as pieces of scorched epidermis and denatured KM particles during the hot-air drying. Some kinds of bacteria are observed in konjac flour but these are not colon bacilli.22 They cause putrefaction of konjac gel and degradation of molecular weight of konjac mannan. The most important criterion of the quality of konjac flour is its high viscosity in aqueous solution, which in turn depends on the molecular weight of the polysaccharide. Table 32.2 shows typical technical data of two types of commercial konjac flours and purified flour of them. The data is kindly given by Ogino Shoten Co. Ltd. in Gunma Prefecture, Japan. The Chinese konjac flour is a bonded one and was pulverised by Ogino Shoten Co. Ltd. Konjac mannan is a water-soluble polymer but it needs a special technique to dissolve it in water completely. To dissolve at room temperature, konjac flour must be added to water with stirring until the powder is completely dissolved. It is important to stir the solution continuously so that the powder does not lump. Hot water is not effective to dissolve konjac flour. The relationship between viscosity of purified commercial konjac flour and stirring time is shown in Fig. 32.7. The konjac flour, Rheolex RS, was characterised by a very fine mesh size (80 mesh sieve) and the measurements were carried out at 25 ëC using a viscometer. The data was kindly provided by the Shimizu Chemical Co. Ltd. in Hiroshima Prefecture, Japan. The viscosity of KM aqueous solution increases with stirring time and reaches a constant value after two hours. The viscosity of KM aqueous solution increases gradually with increasing concentration until 1% and then increases remarkably. As seen in Fig. 32.7, the viscosity of a 2% aqueous solution is more than 12 times higher than that of 1% solution. The viscosity of KM aqueous solution is not affected by salt

Konjac mannan 897 Table 32.2

Analytical results of components in commercial konjac flours Japanese konjac tuber

Viscosity (mPas)+ Whiteness Water* Protein* Lipid* Carbohydrate* Fibre* Ash* Sulphur dioxide Arsenic* Lead* Trimethyl amine** Number of germ Coliform bacteria

Ordinary flour

Purified flour

15.0±15.2 66±68 6.5 2.1 1.3 84.6 0.5 5.0 0.65 g/kg Not detected Not detected 490 ppm Less than 300/g negative

17.0±18.0 73 6.6 1.1 0.3 89.2 0.6 2.2 0.17 g/kg Not detected Not detected 85 ppm Less than 300/g negative

Chinese konjac tuber (bonded) Ordinary flour

Purified flour

13.5±13.6 18.0 69.9 68.9 8.4 5.3 3.0 1.5 0.9 0.3 82.2 90.0 0.6 0.8 4.9 2.1 2.1 g/kg 0.64 g/kg Not detected Not detected Not detected Not detected 760 ppm 170 ppm 420/g Less than 300/g negative negative

+ 1% konjac aqueous solution at 35 ëC after 4 h stirring at 90 rpm. * g/100 g of konjac flour. ** nitrogen-containing substances.

concentration, but is affected by pH of the solution. The effect of pH on viscosity change for 1% and 2% KM solutions is listed in Table 32.3. The viscosity of KM solution decreases with decreasing pH value. At a high pH, KM solution changes to gel.

Fig. 32.7 Relationships between viscosity of purified commercial konjac flour, Rheolex RS, and stirring time: () 1%, (·) 2%.

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Table 32.3

Effect of pH on viscosity change for 1% and 2% KM solutions Viscosity (cps)

KM* concentration Water (%) (no pH adjust.) 1 2

31,600 341,000

pH 4

pH 3

pH 2.5

31,800 340,000

29,900 301,000

18,600 251,000

* ± Rheolex RS.

Table 32.4 Viscosity of mixtures of KM and other gums with various composition. Total concentration of the mixtures is 1% KM* Other gums concentration concentration (%) (%) 0.0 0.2 0.4 0.6 0.8 1.0

1.0 0.8 0.6 0.4 0.2 0.0

Viscosity (cps) Xan

LBG

Gel

Pec

Car

Aga

8,250 225 0 0 300 0 8,800 650 125 75 12,750 60 12,000 2,700 1,525 600 17,500 725 13,250 7,500 5,860 3,750 51,000 3,740 161,000 15,750 14,700 11,640 113,600 12,500 29,500 29,500 29,500 29,500 29,500 29,500

GG

CMC

4,250 75 6,800 225 10,000 1,065 14,750 4,075 20,750 12,200 29,500 29,500

* ± Rheolex RS; Xan ± xanthan gum; LBG ± Locust bean gum; Gel ± Gelatin; Pec ± Pectin; Car ± carrageenan; Aga ± Agar; GG ± Guar gum; CMC ± Carboxymethyl cellulose.

Table 32.5 Gel strength of mixtures of KM and other gums with various compositions. Total concentration of the mixtures is 1% KM* Other gums concentration concentration (%) (%) 0.0 0.2 0.4 0.6 0.8 1.0

1.0 0.8 0.6 0.4 0.2 0.0

Gel strength (g) Xan

LBG

Gel

Pec

Car

Aga

GG

CMC

± 7.8 161.7 84.3 34.7 ±

± ± ± ± ± ±

± ± ± ± ± ±

± ± ± ± ± ±

24.1 118.7 185.3 129.0 ± ±

21.4 25.7 20.3 11.7 4.0 ±

± ± ± ± ± ±

± ± ± ± ± Ð

* ± Rheolex RS; Xan ± xanthan gum; LBG ± Locust bean gum; Gel ± Gelatin; Pec ± Pectin; Car ± carrageenan; Aga ± Agar; GG ± Guar gum; CMC ± Carboxymethyl cellulose.

Konjac mannan interacts synergistically with other polysaccharides and forms thermoreversible gels. The viscosity of the mixtures and the gel strength are listed in Tables 32.4 and 32.5, respectively. The synergism is observed for the combination of KM and xanthan gum, KM and carrageenan, and KM and agar. Table 32.6 shows the effect of sugar concentration on the gel strength for 1% mixed gel with various ratios of KM to -carrageenan. The addition of sugar

Konjac mannan 899 Table 32.6 Effect of sugar concentration on gel strength of mixed gel of KM and carrageenan with various compositions. Total concentration of the mixtures is 1% Gel strength (g) KM*/Car ratio 8:2 7:3 6:4 5:5 4:6 3:7 2:8

Sugar concentration (%) 0

5

10

15

± ± 121.5 331.0 299.7 216.8 137.5

± 141.6 205.4 275.3 285.2 213.8 100.1

± 134.9 195.8 299.1 297.4 187.8 100.3

± 145.3 220.7 260.9 222.4 101.1 109.7

* ± Rheolex RS; Car ± -carrageenan. Table 32.7 Effect of salt concentration on gel strength of mixed gel of KM and carrageenan with various compositions. Total concentration of the mixtures is 1% Gel strength (g) *

KM /Car ratio 8:2 7:3 6:4 5:5 4:6 3:7 2:8

Salt concentration (%) 0

1

3

5

± ± 121.5 331.0 299.7 216.8 137.5

41.0 72.0 120.7 247.7 342.2 529.0 265.4

± ± ± ± 27.9 87.5 126.5

± ± ± ± ± ± ±

* ± Rheolex RS; Car ± -carrageenan.

enhances the gel strength slightly for the gel with higher composition of KM but reduces the strength for the gel with lower composition of KM. Table 32.7 shows the influence of the addition of salt on the gel formation for a 1% of mixture of KM and -carrageenan. The synergistic gel formation is inhibited by addition of salt.

32.5

Uses and applications

Konjac flour has been used as an important food ingredient for more than a thousand years. With the addition of a mild alkali such as calcium hydroxide, konjac flour aqueous solution (ca. 3% of concentration) changes to a strong, elastic and irreversible gel. The alkali treated konjac gel is quite a popular traditional Japanese food and is called Kon-nyaku in Japanese. Recently,

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Table 32.8

Applications and functional uses of konjac mannan

Application

Function

Confectionery Jelly Yoghurt Pudding Pasta Beverage Meat Edible film

Viscosity, texture improver, moisture enhancer Gel strength, texture improver Fruit suspension, viscosity, gelation Thickening, mouthfeel Water-holding capacity Fibre content, mouthfeel Bulking, fat replacer, moisture enhancer Water soluble, water insoluble

synergistic gels prepared by mixing of other hydrocolloids are major products in the food industry as new types of healthy jellies. Clinical studies indicate that konjac mannan solution has the ability to reduce serum cholesterol and serum triglyceride. Konjac mannan also has an influence on glucose tolerance and glucose absorption. However, the alkali treated gel food does not have such effects. Konjac flour is suitable for thickening, gelling, texturing, and water binding. It may be used to provide fat replacement properties in fat-free and low-fat meat products. Applications and functional uses of konjac mannan are listed in Table 32.8.

32.6

Regulatory status

In Japan, konjac flour is accepted as a food ingredient and a food additive for thickening and as a stabiliser according to the provisions of the Food Sanitation Act. For regulatory purposes, a distinction must be drawn between konjac flour and konjac mannan, the separated polysaccharide. The Food Chemical Codex lists the current uses of konjac flour in the United States as gelling agent, thickener, film former, emulsifier, and stabiliser. Konjac flour is also used as a binder in meat and poultry products. Konjac mannan has been recognised as GRAS (generally recognised as safe) by the Food and Drug Administration (FDA) since 1994 and the US Department of Agriculture (USDA) accepted the use of konjac flour as a binder in meat and poultry products in 1996. In Sweden, it was recognised that konjac mannan has the ability to reduce serum cholesterol and indication of the effect was officially accepted enabling claims to be made for its use as a functional food. Konjac flour imported into Europe for diet food and pet food is rarely of consistent quality and does not meet EU standards. However, konjac mannan received a provisional European classification number as a food additive (E425) in 1998. Konjac mannan can thus be imported into Europe because it has achieved an E number.

Konjac mannan 901

32.7 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

References

(ed.), Science of Konjac, Keisuisha, Hiroshima (1984). and G. O. PHILLIPS Gums and Stabilisers for the Food Industry, 8, 391 (1996), (eds G. O. Phillips, P. A. Williams and D. J. Wedlock), IRL Press, Oxford, UK. S. TAKIGAMI, T. TAKIGUCHI and G. O. PHILLIPS Food Hydrocolloids, 11, 479 (1997). K. MAEKAJI Agric. Biol. Chem., 38, 315 (1974). S. OKIMASU (ed.) (1984) Science of Konjac, Keisuisha, Hiroshima, Japan. P. A. WILLIAMS, S. M. CLEGG, M. J. LANGDON, K. NISHINARI and G. O. PHILLIPS Gums and Stabilisers for the Food Industry, 6, 209 (1992), (Eds. G. O. Phillips, P. A. Williams and D. J. Wedlock), IRL Press, Oxford, UK. G. J. BROWNSEY, P. CAIRNS, M. J. MILES and V. J. MORRIS Carbohydr. Research, 176, 329 (1988). P. A. WILLIAMS, D. H. DAY, K. NISHINARI and G. O. PHILLIPS Food Hydrocolloids, 4, 489 (1991). T. KASAI and Y. KOBATA Proceeding of Hokkaido University, 5, 145 (1965). N. KIMURA, K. MOTOKI, T. TAKIGUCHI and Y. SATOU Annual Report of Gunmaken Industrial Research Laboratory (1994), p. 147, Gunma, Japan. K. KATO and K. MATSUDA Agric. Biol. Chem., 33, 1446 (1969). H. SHIMAHARA, H. SUZUKI, N. SUGIYAMA and K. NISHIDA Agric. Biol. Chem., 39, 301 (1975). M. MAEDA, H. SHIMAHARA and N. SUGIYAMA Agric. Biol. Chem., 44, 245 (1980). F. SMITH and C. SRIVASTA J. Am. Chem. Soc., 81, 1715 (1959). T. SATO, A. MORIYA, J. MIZUKUCHI and S. SUZUKI Nippon Kagaku Zasshi, 91, 1071 (1970). R. TAKAHASHI, I. KUSUKABE, S. KUSANO, Y. SAKURAI, K. MURAKAMI, A. MAEKAWA and T. SUZUKI Agric. Biol. Chem., 48, 2943 (1984). T. NAKAJIMA and K. MAEKAWA Matsuyama Shinonome Gakuen Kenkyuronshu, 2, 55 (1966); 3, 117 (1967). S. OKIMASU and N. KISHIDA Hiroshima Joshi Daigaku, Kaseigakubu Kiyo, 13, 1 (1982). K. OGAWA, T. YUI and T. MIZUNO Agric. Biol. Chem., 55, 2105 (1991). N. KISHIDA, S. OKIMASU and T. KAMATA Agric. Biol. Chem., 42, 1645 (1978). Unpublished data. T. TAKIGUCHI, T. NARITA, K. SEKIGUCHI, I. YOSHINO and I. KAWANO Annual Report of Gunmaken Industrial Research Laboratory (1990), p. 168, Gunma, Japan. S. OKIMASU

S. TAKIGAMI