Gelling Mechanisms of Glucomannan Polysaccharides and Their

Chapter 23

Gelling Mechanisms of Glucomannan Polysaccharides and Their Interactions with Proteins Downloaded by UNIV OF GUELPH LIBRARY on June 9, 2013 | http://pubs.acs.org Publication Date: December 10, 2002 | doi: 10.1021/bk-2003-0834.ch023

1

I. C. Baianu and Ε. M. Ozu 1

2

2

College of ACES, FSHN Department and AFC-NMR Facility, University of Illinois at Urbana, 101 Bevier Hall, 908 South Goodwin Avenue, Urbana, IL 61801

Gelling properties of glucomannan polysaccharides (GP) are important in good science and technology. The functional properties of GP gels and related polysaccharides were recently reviewed in relation to gelling mechanisms that are likely to operate both in gels made from purified GP or konnyaku flour (1). We are now presenting a more detailed account of the most likely gelling mechanism in glucomannans mixed with water and in the presence of added wheat gluten proteins than it has been previously reported. Our results suggest a model of glucomannan gels in which divalent cations, such as Ca , act as a crosslinker, both in the presence or absence of wheat gluten proteins. ++

Konnyaku flour obtained from plant cultivars of Amorphophallus konjac K. Koch is an important food material because of its remarkable gelling properties at very low solids concentrations in water (less than -0.5%). It has been employed in certain regions of Asia, such as Japan, to make a variety of foods that were claimed to have significant health benefits. Glucomannan polysaccharides (GP) are the main active component of Konnyaku flour, and can

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© 2003 American Chemical Society

In NMR Spectroscopy of Polymers in Solution and in the Solid State; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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be readily extracted and purified from konnyaku flour. Gelling properties of glucomannan polysaccharides are therefore important in food science and technology. The functional properties of GP gels and related polysaccharides were recently reviewed in relation to gelling mechanisms that are likely to operate both in gels made from purified GP or konnyaku flour (I). New experimental results are here presented together with a detailed model of the gelling mechanisms of glucomannans in water with, or without, added wheat gluten proteins.

Materials and Methods Konnyaku flour was obtained from tubers of plant cultivars of Amorphophallus konjac K. Koch that were kindly provided by Ajinomoto Co. Glucomannan polysaccharides were extracted from konnyaku flour and further purified from precipitates in ethanolic mixtures. The range of glucomannan oligosaccharides that are obtained by partial hydrolysis of GP is illustrated in Table I.

N M R Spectroscopy and Relaxation Pulsed Nuclear Magnetic Resonance (NMR) instrumentation and methodology that we employed to study GP and/or wheat gluten protein hydration in gels are as described in our previous report (J). An approach and analysis that combines low-field N M R relaxation data with high-field N M R spectroscopic data obtained on the same samples was recently reported (2) that is also applicable to structural and hydration studies of GP gels with/without added wheat gluten proteins in water. High-resolution N M R spectra of solutions were obtained with a Varian Unity 400 spectrometer operating at 400 M H z H resonance frequency.

IR Spectroscopy Transmission, mid-IR cw-spectra of GP were obtained by employing K B r pellets with GP powder homogenously embedded in the pellets. Reflectance N I R spectra of GP were obtained from purified powders (without KBr) on a PerkinElmer Spectrum O N E N T S spectrometer equipped with an integrating sphere and an extended-range InGaAs detector.



Glucopyranose

c

^annopyranose

Mannose fraction

a

A Β C D Ε F G H I

Oligosaccharide

A Β C D Ε F G H I

Oligosaccharide

2 2 2 3 4 5 5 7

2

DP

0.83 0.67 0.53 0.46 0.36 0.29 0.20 0.16 0.07

Man°

K

β-0->4)

β-(1->·4) β-(1->4)

β-0->4)

β-(1->4)

β-0->4)

β-(1->4) β-(1->4) β-(1->4)

Type and Configuration of the Bond

G, M (1:1) G M G, M (1:1) G, M (1:2) G, M (1:3) G, M (2:3) G, M (1:4) G, M (3:4) Structure

c

M" G M G M M M M M

Terminal Residue

G->M G-»G M->M M->G G->M->M G-»M->M->M M-»G->G->M->M M->G->M-)>M->M G-^M-^G-^G-^M-^M-^M

Composition and Ratio of the Monomers

Table I. Glucomannan Oligosaccharides Formed by Partial Hydrolysis


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Results and Discussion Broadband, proton decoupled, H R - N M R of GP oligosaccharides were assigned by comparison with spectra of individual oligosaccharides and disaccharides. Such chemical shift assignments are summarized in Table II. Pulsed N M R measurements of transverse water relaxation (T2) at 10 M H z H resonance frequency were carried out for GP gels and GP-wheat gluten mixtures in water. Whereas the transverse relaxation curve of water in GP gels was found to be a single exponential for the concentration range from 0 to 6% GP solids by weight, that of wheat gluten protein mixtures with GP in varying proportion was found to be non-exponential. The transverse relaxation curve of water protons in fully hydrated GP-wheat gluten mixtures was best fitted by a model with three different relaxation components that was previously introduced for suspensions of homogenized muscle in water (2). Although GP hydration in gels exhibits a constant slope in the range from 0.1 to 5% GP gels in water, G P micro-rheological properties change markedly above 1% GP in water. However, in the presence of hydrated wheat gluten proteins, the GP gels and wheat gluten protein hydration properties change dramatically above 1% GP concentration. Such changes are also observed in the apparent viscosities of GP-wheat gluten mixtures in water. The addition of E D T A to the GP-wheat gluten protein mixture in water (followed by extensive dialysis at 4°C) reduces only by approximately 15% the hydration of the protein-polysaccharide mixture. These results strongly suggest a model of hydrated glucomannan gels in which divalent cations, such as Ca++, act as a cross-linker (Figure 1), both in the presence and the absence of wheat gluten proteins. (The divalent Ca-ions are represented as open circles in Figure 1). Although fewer peaks are resolved for GP in the NIR spectra in comparison with N M R , such NIR spectra (Figure 2) are strongly indicative of a complex, macromolecular structure in GP powders similar to that of starches from other plants, without any evidence of smaller oligosaccharides being present in the purified GP employed in our studies. On the other hand, X-ray diffraction patterns of dried GP films in which GP fibers were oriented in the plane of the film seem to indicate a structure with Η-bonds between adjacent GP fibers that is significantly different from that of fully hydrated GP gels. Further NIR experiments with GP gels in D2O are in progress and have the potential to identify specific divalent cation cross-linking groups in GP gels.

Conclusions Gelling mechanisms of glucomannan polysaccharides (GP) extracted from konnyaku flour obtained from plant cultivars of Amorphophallus konjac K. Koch



1 3

^annopyranose

"Glucopyranose

71.0-71.1 70.4-70.5 68.0-68.1 62.4-62.5 61.7-62.1

76.4-76.9 76.15-76.3 75.4-75.5 74.3-74.7 72.9-73.1 72.5-72.6 71.4-72.0

C Chemical Shifts 103.8-104.2 101.45-101.6 95.1-95.3 75.95-80.15 78.1-78.3 77.4-78.0

13

a

b

Assignments C 1 Glcp β C 1 Manp α, β C 1 Manp C 4 Glcp β C 5 Manp β C 4,5 Manp β C 4 Manp α C 3 Glcp β C 5 Glcp β C 5 Glcp β C 2 Glcp β C 3 Manp β C 5 Manp α C 2 Manp β and Manp α C 3 Manp α C 4 Glcp β C 4 Manp β C 6 Manp β C 6 Manp α, β C 6 Glcp β At the reducing end and Manpβ in the middle of the chain At the nonreducing end and in the middle of the chain

At the nonreducing end

In the middle of the chain At the nonreducing end At the reducing end and in the middle of the chain At the reducing end At the nonreducing end and in the middle of the chain At the nonreducing end In the middle of the chain At the nonreducing end and in the middle of the chain At the reducing end At the reducing end At the reducing end

At the nonreducing end and in the middle of the chain At the reducing end

Table II. C Chemical Shift Assignments of Glucomannan Oligosaccharides Obtained by Hydrolysis


303


A. Entangled polymer chains in a gel that is not cross-linked

B. Cross-linked polymers in a gel

Figure 1. Schematic diagram of gel networks: A . Gels without cross-links; B. Cross-linked Gels , such as G P gels, with the open circles representing cross-linkers, such as C a ^ ions in GP gels.



Figure 2. Near Infrared (NIR) spectrum of glucomannan polysaccharide


305


were investigated by Nuclear Magnetic Resonance (NMR) techniques. Detailed mechanisms of glucomannan gel formation and G P hydration were obtained by combining data from pulsed N M R transverse relaxation with high-resolution H and C N M R spectra of glucomannan gels in water. Our results suggest a model of glucomannan gels in which divalent cations, such as Ca++, act as a crosslinker, both in the presence, or absence, of wheat gluten proteins. Such a mechanism may also have technological significance for developing novel, functional foods based on both konnyaku and wheat flours that must also contain calcium ions in order to form a homogenous GP-wheat protein matrix.

References 1. Ozu, E., Baianu, I.C., and Wei, L-S. Ch.14 in Physical Chemistry of Food Processes, vol.2; Baianu, I.C., Pessen, H . , and Kumosinski, T.F. , Eds., New York, London, S. Melbourne, and Ontario: Intl. Thomson Publs. 1992. (ISBN 0422-00582-2). 2. Lee, J.R.; Baianu, I.C.; Bechtel, P. Macromol. Symp. 1999,140, 243.


Gelling Mechanisms of Glucomannan Polysaccharides and Their

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