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:
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