Glucomannan is a general term which applies to hemicelluloses consisting of glucose and mannose which are found as constituents in many plants (especially Liliaceae, Araceae and softwood species). Depending on their origin glucomannans differ by their polymer structure (branched or linear), by their molecular weight, by their glucose/mannose ratio and by the sequence of the glucose and mannose units. In the following we describe a glucomannan which occurs in the corms of East Asian Amorphophallus species where it forms the principal reserve carbohydrate. For this glucomannan the term "Konjac Mannan" or "Konjac Glucomannan" ("KGM") should be used as the common trade name "Glucomannan" is unspecific. Konjac Mannan is mostly produced from the species Amorphophallus konjak (which is synnonym with A. rivieri). Konjac Mannan consists of glucose and mannose in the ratio
of 5 : 8. SHIMAHARA (1975(I)) proposes the following sequence of individual hexoses which should represent the basic unit in the polymer chain:

-G-G-M-M-G-M-M-M-M-M-G-G-M-

(M for beta-D-Mannose, G for beta-D-Glucose, the bond between the sugar units is 1,4.)

The sequence was determined by hydrolysis of Konjak Mannan by acids, by cellulase and by beta-mannanases occurring in germinating Amorphophallus tubers and a subsequent evaluation of the resulting mono- and oligosaccharide patterns (SHIMAHARA, 1975(II); KATO, 1969; KATO, 1970; SATOH, 1970). Konjac Mannan is slightly branched (every 50 to 60 sugar units) via a C3 bond on hexoses of the main polymer chain (SHIMAHARA, 1975(II); KATO, 1973) and it contains approximately one

acetyl group per 19 sugar residues (MAEKAJI, 1974). The molecular weight of Konjak Mannan depends to a certain degree on the species or even variety of Amorphophallus it is derived from and also on the method of extraction. SUGIYAMA (1972) found values of 0.67 to 1.9 million Dalton (weight average MW) depending on the Amorphophallus variety. Processing diminishes the molecular weight in most cases. Even the drying of Amorphophallus tubers in the sun has a depressing effect on the molecular weight either due to bacterial action or due to mannanases present in the Amorphophallus tuber (SUGIYAMA, 1972). Konjak Mannan has been used since a long time under the name "Konyaku" as a gelling agent in the Japanese cuisine. It is extracted from fresh or dried Amorphophallus corms either by mechanical means or by a wet (ethanol) extraction process (SUGIYAMA, 1972, US PATENT (I)). Aqueous solutions of insufficiently purified Konjak Mannan still containing significant quantities of tuber matrix material show a low viscosity. This can possibly be explained by beta-mannanases present in the adhering matrix material. SHIMAHARA (1975(II)) isolated two beta-mannanases in germinating Amorphophallus corms. The mode of action of both was found to be a random or endo mechanism, i.e. the enzymes cleave the Konjac Mannan polymer at random distance from the end of the chain (albeit preferentially on certain types of bond) which leads to a rapid degradation of the molecular weight and hence of the viscosity of the Konjac Mannan solution. Konjac Mannan solution gels if heater after alkali addition. This gelling is explained by MAEKAJI (1978) by the hydrolysis of the acetyl groups of the Konjac Mannan. (The OH-groups no longer protected by acetyl groups form intramolecular hydrogen bonds thus "cross linking" the Konjac Mannan molecules). A further interesting characteristic of Konjac Mannan lies in its synergism with other hydrocolloids: Konjac Mannan and Xanthan form mixed gels at a total concentration of as low as 0.1%. Thermoreversible gelation is observed in mixtures of Konjac Mannan and Agarose or Carrageenan solutions (DEA, 1981).
The ingestion of Konjac Mannan has been found to be cholesterol lowering in many investigations (VORSTER, 1985; TSUJI, 1968; US PATENT (II); US PATENT (III); VENTER, 1978; KIRIYAMA, 1970). The hypocholesterolaemic activity is common to several hemicelluloses and was found to be connected to water solubility and high viscosity of aqueous solutions (due to high molecular weight). It is assumed that the effect is based on an interference of the transport of cholesterol in the jejunum and of bile acids (cholesterol intermediates) in the ileum (DAVIS, 1975). KIRIYAMA (1970) found that Konjac Mannan gel formed by the reaction of Konjac Mannan solution with lime does not lower the serum cholesterol level. Hemicellulases do not occur in the mucosal lining of the human alimentary tract (nor have they been found in any vertebrate) wherefore Konjac Mannan cannot be digested by humans. However, a certain fraction of ingested Konjac Mannan is nevertheless degraded by colon inhabiting bacteria (e.g. Aerobacter mannanolyticus) (INOUE, 1957; INNAMI, 1961). MARINI (1975) found that only 71% of 3 g of ingested Konjac Mannan could be recovered in the feces. It was also reported that Konjac Mannan ingestion has a weight reducing effect (BIANCARDI, 1989; JAPAN KOKAI). It is assumed that such an effect is caused by an increase of the gastric emptying time (EUR. PATENT). A reduction of the blood glucose level and the serum insulin concentration by the administration of Konjac Mannan in a glucose tolerance test was found by OKU (1983) and DOI (1979, 1982). However, improved glucose tolerance could not be confirmed by MORGAN (1990); they found however a postprandial lowering of the insulin level. The recommended daily dosage of Konjac Mannan varies from 3.0 g to 7.2 g per adult person and day (DOI, 1979; BIANCARDI, 1989; VENTER, 1978). Konjac Mannan was found to be non-toxic by OKETANI (1984) although a hypertrophic effect on cecum and colon of rats was observed when excessive amounts of Konjac Mannan (20% in the diet) were administered (KONISHI, 1984(I); 1984(II)). DOI (1983) studied the resorption of Vitamin B12 and Vitamin E in the presence of Konjac Mannan. They found no interference with the resorption of Vitamin B12 but a reduced resorption of Vitamin E. They conclude that the resorption of fat soluble vitamins is affected because of the removal of bile acids by Konjac Mannan.
 

Further information on the viscosity of glucomannan solutions is available here.

References:

BIANCARDI, G. et al., 1989: Glucomannan in the treatment of overweight patients with osteoarthritis. Current Therapeutic Research, vol. 46, no. 5, November 1989, 908-912

DAVIS, S.E. et al., 1975: Physiological Effects of Food Carbohydrates, pp. 296-311, No. 15, Am.Chem.Soc. Symposium Series DEA, I.C.M., 1981: A.C.S. Symposium Series, 150, 439-454

DOI, K. et al., 1979: Treatment of Diabetes with Glucomannan. Lancet, May 5, 1979, 987

DOI, K. et al. 1982: Excerpta Med. 549, 306

DOI, K. et al., 1983: Tohoku J. Exp. Med., 141(Suppl.), 677-81

EUR. PAT. APPL., 1987: EP 222,967, 27 May 1987

INOUE, K. et al., 1957: Hakko Kogaku Zasshi 35, 354-5

INNAMI, S., 1961: Agr. Biol. Chem., 25, 155-63

JAPAN KOKAI TOKKYO KOHO, JP 01,224,320

KATO, K. et al., 1969: Studies of the Chemical Structure of Konjac Mannan: I. Isolation and Characterization of Oligosaccharides from the Partial Hydroyzate of the Mannan. Agr. Biol. Chem. Vol. 33(10), 1446-1453 (1969)

KATO, K. et al., 1970: Studies of the Chemical Structure of Konjac Mannan: II. Isolation ana Characterization of Oligosaccharides from the Enzymatic Hydrolyzate of the Mannan. Agr. Biol. Chem. Vol. 34(4), 532-539 (1970)

KATO, K. et al., 1973: Isolation of Oligosaccharides Corresponding to the Branching-point of Konjac Mannan. Agr. Biol. Chem. Vol. 37(9), 2045-2051 (1973)

KIRIYAMA, Sh., et al.: 1970: Changes in Hypocholesterolemic Activity in Rats by various Konnyaku Powder Treatments. Agr. Biol. Chem. Vol. 34, No. 4, p. 641-643, 1970

KONISHI, F. et al., 1984(I): J. Nutr. Sci. Vitaminol., 30(4), 373-9 KONISHI, F. et al., 1984(I): Jpn. J. Exp. Med., 1984, 54(3), 139-42 MAEKAJI, K., 1974: Agr. Biol. Chem., 38, 315-321

MAEKAJI, K., 1978: Nippon Nogeikagakukaishi, 52, 251, 485, 513 MARINI, D., 1988: Fitoterapia, 59(5), 407-13), 301-312, 1975

MORGAN, L.M. et al., 1990: Br. J. Nutr. 64(1), 103-10

OKETANI, Y. et al., 1984: Oyo Yakuri, 27(1), 127-31

OKU, T. et al, 1983: Nippon Eiyo, Shokuryo Gakkaishimi, 36(4), 301-3 SATOH, T. et al. 1970: Nippon Kagaku Zasshi, 91(11), 1071-5 SHIMAHARA, H., et al. 1975(I): Partial Purification of ß-Mannanases from the Konjac tubers and Their Substrate Specificity in Relation to the Structure of Konjac Glucomannan. Agri. Biol. Chem. Vol. 39(2),301-312, 1975

SHIMAHARA, H., et al. 1975(II): Isolation and Characterization of Oligosaccharides from an Enzymic Hydrolysate of Konjac Glucomannan. Agri. Biol. Chem. Vol. 39(2),293-299, 1975

SUGIYAMA, N., et al., 1972: Molecular Weights of Konjac Mannans of Various Sources. Agri. Biol. Chem. Vol. 36, No. 8, p. 1381-1387, 1972 TSUJI K., et al., 1968: Eiyogaku Zasshi, 26(3), 113-22

US PATENT 1973: Nr. 3,767,424, Oct. 23, 1973

US PATENT, 1974, No. 3,856,945, Dec. 24, 1974

US PATENT, 1975, No. 3,928,322, Dec. 23, 1975

VENTER, CH. S. et al.: 1990: Comparison between Physiological Effects of Konjac-Glucomannan and Propionate in Babbons Fed "Western" Diets. Journal of Nutrition, vol. 120, No. 9/1990

VORSTER, H.H. et al., 1985: J. of Plant Foods, 6(4), 263-74

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