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MANUFACTURE AND PROPERTIES OF GLUCOMANNANS AND GLUCOMANNOOLIGOSACCHARIDES DERIVED FROM KONJAC AND OTHER SOURCES Belen Gómez, Beatriz Miguez, Remedios Yañez, and Jose L. Alonso J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.6b05409 • Publication Date (Web): 14 Feb 2017 Downloaded from http://pubs.acs.org on February 19, 2017
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MANUFACTURE
AND
PROPERTIES
OF
GLUCOMANNANS
AND
2
GLUCOMANNOOLIGOSACCHARIDES DERIVED FROM KONJAC AND OTHER SOURCES
3
Belén Gómez, Beatriz Míguez, Remedios Yáñez, José L. Alonso*
4
Chemical Engineering Department, Polytechnic Building, University of Vigo (Campus Ourense). 32004
5
Ourense, Spain.
6
CITI, Avda. Galicia, nº 2, Tecnopole, San Cibrao das Viñas, 32900, Ourense, Spain.
7
CINBIO, University Campus, 36.310, Vigo, Pontevedra, Spain.
8 9
*Corresponding author: telephone 988387233, e-mail address
[email protected] 10 11
Abstract
12
Glucomannans (GM) are polymers which can be found in natural resources, such as
13
tubers, bulbs, roots, and in both hard- and softwoods. In fact, mannan-based
14
polysaccharides represent the largest hemicellulose fraction in softwoods. In addition
15
to their structural functions and their role as energy reserve, they have been assessed
16
for their healthy applications, including their role as new source of prebiotics. This
17
article summarizes the scientific literature regarding the manufacture and the
18
functional properties of GM and their hydrolysis products with a special focus on their
19
prebiotic activity.
20 21
Keywords: glucomannans, glucomannooligosaccharides, konjac, emerging prebiotics,
22
gut microbiota, health benefits.
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1. Introduction
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The market for functional foods is continuously increasing due to the growing demand
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for safe products with capacity for improving the human health through their regular
26
consumption. In this context, the intake of prebiotics is considered as a good strategy
27
for achieving this objective. These compounds have been recently defined as “non-
28
digestible compounds that through its metabolism by microorganisms in the gut,
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modulate the composition and/or the activity of the gut microbiota, thus conferring
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physiological benefit effects on the host health”.1
31
The gut microbiota constitutes a complex association of bacteria (comprising more
32
than 1000 species and around 1014 microorganisms) that gradually increase along the
33
jejunum and the ileum up to reach the maximum concentration in the colon.2 This gut
34
microbiota reflects an interrelationship between the different bacterial groups, that
35
might work together for the benefit of the host performing three essential primary
36
functions: (i) metabolic, (ii) trophic and (iii) defensive.3 In fact, there is a large list of
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pathologies that are linked to the alteration of this intestinal microbiota, including
38
hepatic encephalopathy, diarrhoea, diabetes, obesity or colon cancer (see Table 1).4,5
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The most widely accepted prebiotics are lactulose, inulin, fructooligosaccharides,
40
galactooligosaccharides
41
glucomannans
42
glucomannoligosaccharides or GMOS) could also play a role as prebiotics but, before,
43
they must fulfil, as for the rest of candidates, several requirements:6 i) they cannot be
44
hydrolysed or absorbed in the upper gastrointestinal tract, ii) they have to encourage
45
the development of beneficial bacteria such as bifidobacteria and lactobacilli, and iii)
(GM)
and and
the
human
their
milk
derived
oligosaccharides. oligomers
However,
(denoted
as
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they must induce beneficial physiological effects on the host health (that have to be
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clearly demonstrated through well conducted human trials).
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The best-known raw material for GM and GMOS production is konjac (Amorphophallus
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konjac), which has been employed for centuries with health purposes, although there
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are other potential natural resources that contain these kind of components, such as
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eastern white pine, higanbana, orchid or redwood.7 It is necessary to remark that the
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konjac glucomannan (KGM) has been approved as a GRAS (generally recognized as
53
safe) food additive in USA and Canada and it is also accepted by the European Union as
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emulsifier and thickener with the E-number E425.7
55
Research studies on GM are limited and especially focused on its production from
56
konjac and on both its structural characteristics and physical properties.8–12 However,
57
other properties such as the impact of this polysaccharide and its derived oligomers on
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human intestinal microbiota have not been yet deeply investigated.
59
In order to contribute to the development of new mannan-derived prebiotics, this
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article summarizes the scientific literature regarding specially the manufacture and the
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biological properties of GM and their derived oligosaccharides.
62 63
2. Basic structure and composition of mannans
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Four types of mannan polysaccharides can be distinguished: pure mannan,
65
glucomannan, galactomannan and galactoglucomannan. These carbohydrates are
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widespread in nature and are considered as one of the major components of
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hemicellulose in the cell wall plants. They can be linear or branched polymers
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consisting of mannose, galactose and glucose, as well as acetyl substituents. The
69
degree of acetylation and also the mannose to glucose ratio are parameters that 3 ACS Paragon Plus Environment
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depend on the raw material and affect the functionality of the polysaccharide. The
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typical structure of GM is shown in Figure 1.
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It is widely accepted that the presence of acetyl groups in the GM backbone, as well as
73
glucuronic acid and phosphoric acid groups, confer solubility in aqueous solution.
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Thereby, the deacetylation of the chain by treatments with alkalis is responsible of
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their insolubility and the formation of thermally stable gels.13
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Katsuraya et al. (2003)9 reported that the main chain of KGM is made up by (1,4)-
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linked D-mannose and D-glucose sugars, with side chains at C-3 position of both D-
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glucosyl and D-mannosyl residues. Specifically, they indicated that the degree of
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branching is about 8%. On the other hand, Chen et al. (2013)14 indicated that the
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branch chains, made up of about 3 or 4 monomers, are linked to either the C-3 or the
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C-6 hydroxyl groups of the main chain. On the other hand, the molecular weight of
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KGM can vary in the range 200-2000 kDa, depending on the cultivars, origin,
83
production methods or storage.15 Tester and Al-Ghazzewi (2016)7 reported that the DP
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of glucomannans is > 200 (except for eastern white pine and redwood). Meanwhile,
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galactoglucomannan is one of the major components in softwood hemicelluloses (it
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accounts for 10-25% of the wood weight), although it can also be found in lower
87
amounts in hardwoods.16–18 Galactoglucomannan backbone consists of a random
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sequence of glucose and mannose molecules joined by links β-(1-4), and galactose
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branches joined through β-(1-6) links to the mannose units from the backbone. On the
90
other hand, galactomannan is made up by a main chain of mannose units linked by β-
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(1-4) links and galactose side chains linked by β- (1-6).19
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Mannans are structural elements in plant cell walls, exhibit a storage function as non-
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starch carbohydrate reserve and provide resistance to mechanical damage. In addition, 4 ACS Paragon Plus Environment
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some heteromannans are widely used as stabilizing, thickening and gelling agents in
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the food industry.20 GM is characterized by its exceptional gelling properties and high
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viscosity. In fact its aqueous solution is considered the most viscous among the natural
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colloidal solutions.15 Moreover, it can be used as a source of dietary fibre and a
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complement for weight loss. However, the highly depolymerized GM do not seem to
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be effective for weight reduction because it does not swell or form gels like native GM
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when hydrated.21
101 102
3. GM industrial production
103
The industrial processes to obtain GM usually employ Amorphophallus konjac as the
104
main source and consist of a sequence of stages which include washing, slicing into
105
chips and drying steps. Then, according to Chua et al. (2010),22 the dried chips are
106
pulverized, and the common konjac flour (food grade) is obtained after removing
107
impurities such as starch, protein, cellulose and low molecular weight sugars from the
108
crude pulverized flour, either by wind shifting or by alcohol precipitation.23,24 The latter
109
involves several ethanol washing steps to remove low molecular weight sugars (such
110
as D-glucose and D-fructose), a process followed by an aqueous extraction at room
111
temperature.23
112
It is important to remark that the extraction method can strongly affect the yield, the
113
molecular weight and the structure of the resulting mannans, as it was observed by
114
Lundqvist et al. (2003)25 by comparing several methods for galactoglucomannan
115
production from spruce (Picea abies).
116 117
4. GMOS manufacture and characterization 5 ACS Paragon Plus Environment
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Glucomannan polysaccharides have many uses as native polymers, requiring different
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purity degrees depending on the application. For example, a flour with the highest
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glucomannan content is used in nutraceutical applications.26 However, this polymer
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can also be hydrolysed to give low molecular-weight saccharides by a variety of
122
strategies.27
123 124
4.1. GMOS Production
125
A variety of techniques (including enzymatic, chemical or physical treatments) have
126
been assayed for manufacturing GMOS mixtures from mannans (see Table 2), being
127
the enzymatic route the most employed due to: a) its better selectivity towards the
128
type of polymer and links and ii) the use of mild conditions of temperature and
129
pressure, reducing the equipment and operation costs.
130
The two major plant mannan-degrading enzymes are mannan endo-1,4-β-
131
mannosidase or 1,4-β-D-mannan mannanohydrolase (commonly known as β-
132
mannanase) and β-D-mannoside mannohydrolase or β-mannosidase.20
133
He et al. (2001)38 used β-mannanase from Bacillus licheniformis for hydrolyzing konjac-
134
derived powder and they correlated the initial substrate, enzyme concentrations and
135
the reaction time with the degree of hydrolysis, obtaining a good agreement between
136
predicted and experimental data. The β-mannanase was also employed by Chen et al.
137
(2013)14 to produce GMOS from konjac tubers achieving high yields. Moreover, dimers
138
and trimers were obtained from KGM by enzymatic hydrolysis using cellulase and β-
139
mannanase.8 Meanwhile, Mikkelson et al. (2013)39 described the production of GMOS
140
with DP 2-6 by enzymatic hydrolysis of KGM using two T. reesei endoglucanases and
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one mannanase and stated that these oligosaccharides (with defined structure) are
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susceptible to be used as substrates in bioactivity assays.
143
Other current enzymatic studies on GM were reported by Zhang et al. (2009),40 who
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incubated 10 mg/ml of konjac flour with 10 U/mg of MAN5 enzyme at 50°C for 24 h,
145
hydrolysing more than 90% polysaccharides in the konjac flour solution into
146
oligosaccharides and some monosaccharides; Albrecht et al. (2011),41 who investigated
147
the action of enzymes from faecal bacteria on KGM and compared the
148
oligosaccharides produced to the oligosaccharides obtained from KGM by the action of
149
fungal endo-β-(1,4)-glucanase or endo-β-(1,4)-mannanase; and Liu et al. (2015)37 that
150
investigated the factors affecting the enzymatic hydrolysis of KGM using β-mannanase
151
and reported the optimum conditions (t=2 h, T=50°C, pH=6 and enzyme charge=150
152
U/g).
153
On the other hand, acids can also be used for GM degradation. Chen et al. (2005)42
154
obtained partially hydrolysed KGM by a treatment with 0.2 N HCL after boiling for 20
155
min, and Rivas et al. (2012)34 subjected samples of Pinus pinaster wood to
156
hydrothermal processing (a reaction catalysed by hydronium ions coming from water
157
ionization and from in situ generated acids) under optimum conditions (175°C, 26 min)
158
and obtained liquors with high contents of oligomers and polymers derived from
159
mannans with a minimal generation of sugars and sugar degradation compounds.
160
Other methods that have been assayed for obtaining oligosaccharides by
161
depolymerisation of GM are oxidative degradation, physical treatments and ϒ-
162
irradiation. In particular, the combination of ϒ-irradiation and β-mannanase was an
163
efficient method to obtain konjac oligosaccharides with molecular mass lower than
164
2200 Da.35 7 ACS Paragon Plus Environment
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4.2. Purification and fractionation of GMOS
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During the oligomer production (mainly by chemical or physical methods), a variety of
168
unwanted compounds, such as monosaccharides, extractives, and/ or sugar
169
decomposition products, among others, can be generated in the reaction medium.
170
When the final objective is to use these products as food ingredients for human
171
nutrition, the obtained liquors obtained must be refined.
172
In spite of the limited studies regarding GMOS purification, it can be stated that
173
membrane technology is the most employed and suitable technique for the
174
purification of these oligosaccharide mixtures. Besides being a simple technology, the
175
use of membranes offers numerous advantages such as: high-energy efficiency, no
176
harmful organic solvents are employed, it allows modify the operating conditions (such
177
as pressure, temperature, agitation or feed rate), it does not require too much space
178
and its scale up is relatively easy. 43,44
179
Rivas et al. (2012)34 subjected liquors obtained from Pinus pinaster wood by
180
hydrothermal processing to a two-step discontinuous diafiltration using regenerated
181
cellulose membranes of 5 and 1 kDa cutoff (Millipore) with a filtration area of 41.8 ×
182
10-4 m2 and an operating pressure of 4 bars for refining and fractionating their
183
hemicellulose-derived
184
glucomannan or galactoglucomannan breakdown, substituted with one or more acetyl
185
groups) obtaining two streams with different molecular mass distribution (GMOS 1, >5
186
kDa and GMOS 2, 1-5 kDa) and high purity (97.5 and 100%), which compared well to
187
the commercial prebiotics. Likewise, Jian et al. (2013)35 used ultrafiltration to
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fractionate konjac oligosaccharides, operating successively with membrane separating
oligosaccharides
(mainly
made
up
of
hexoses
from
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devices (Shanghai Shiyuan biological engineering equipment Co., Ltd., China), with the
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Centramate Tseries cassette membrane (Pall Corporation, USA), from a maximum
191
molecular weight cut-off (3x105 Da) to a minimum one (1x103 Da), obtaining 6
192
fractions with different molecular mass range (Group 1, >300 kDa; Group 2, 100-300
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kDa; Group 3, 50-100 kDa; Group 4, 5-50 kDa; Group 5, 1-5 kDa; and Group 6,
