Manufacture and Properties of Glucomannans and

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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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Journal of Agricultural and Food Chemistry

1

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

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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

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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

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functional properties of GM and their hydrolysis products with a special focus on their

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prebiotic activity.

20 21

Keywords: glucomannans, glucomannooligosaccharides, konjac, emerging prebiotics,

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gut microbiota, health benefits.

ACS Paragon Plus Environment

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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

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consumption. In this context, the intake of prebiotics is considered as a good strategy

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for achieving this objective. These compounds have been recently defined as “non-

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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

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The gut microbiota constitutes a complex association of bacteria (comprising more

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than 1000 species and around 1014 microorganisms) that gradually increase along the

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jejunum and the ileum up to reach the maximum concentration in the colon.2 This gut

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microbiota reflects an interrelationship between the different bacterial groups, that

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might work together for the benefit of the host performing three essential primary

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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

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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,

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galactooligosaccharides

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glucomannans

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glucomannoligosaccharides or GMOS) could also play a role as prebiotics but, before,

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they must fulfil, as for the rest of candidates, several requirements:6 i) they cannot be

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hydrolysed or absorbed in the upper gastrointestinal tract, ii) they have to encourage

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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

2 ACS Paragon Plus Environment

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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

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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

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Research studies on GM are limited and especially focused on its production from

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konjac and on both its structural characteristics and physical properties.8–12 However,

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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.

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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.

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2. Basic structure and composition of mannans

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Four types of mannan polysaccharides can be distinguished: pure mannan,

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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

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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

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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,

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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

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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

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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

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The industrial processes to obtain GM usually employ Amorphophallus konjac as the

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main source and consist of a sequence of stages which include washing, slicing into

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chips and drying steps. Then, according to Chua et al. (2010),22 the dried chips are

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pulverized, and the common konjac flour (food grade) is obtained after removing

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impurities such as starch, protein, cellulose and low molecular weight sugars from the

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crude pulverized flour, either by wind shifting or by alcohol precipitation.23,24 The latter

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involves several ethanol washing steps to remove low molecular weight sugars (such

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as D-glucose and D-fructose), a process followed by an aqueous extraction at room

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temperature.23

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It is important to remark that the extraction method can strongly affect the yield, the

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molecular weight and the structure of the resulting mannans, as it was observed by

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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

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strategies.27

123 124

4.1. GMOS Production

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A variety of techniques (including enzymatic, chemical or physical treatments) have

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been assayed for manufacturing GMOS mixtures from mannans (see Table 2), being

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the enzymatic route the most employed due to: a) its better selectivity towards the

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type of polymer and links and ii) the use of mild conditions of temperature and

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pressure, reducing the equipment and operation costs.

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The two major plant mannan-degrading enzymes are mannan endo-1,4-β-

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mannosidase or 1,4-β-D-mannan mannanohydrolase (commonly known as β-

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mannanase) and β-D-mannoside mannohydrolase or β-mannosidase.20

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He et al. (2001)38 used β-mannanase from Bacillus licheniformis for hydrolyzing konjac-

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derived powder and they correlated the initial substrate, enzyme concentrations and

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the reaction time with the degree of hydrolysis, obtaining a good agreement between

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predicted and experimental data. The β-mannanase was also employed by Chen et al.

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(2013)14 to produce GMOS from konjac tubers achieving high yields. Moreover, dimers

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and trimers were obtained from KGM by enzymatic hydrolysis using cellulase and β-

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mannanase.8 Meanwhile, Mikkelson et al. (2013)39 described the production of GMOS

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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.

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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,

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hydrolysing more than 90% polysaccharides in the konjac flour solution into

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oligosaccharides and some monosaccharides; Albrecht et al. (2011),41 who investigated

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the action of enzymes from faecal bacteria on KGM and compared the

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oligosaccharides produced to the oligosaccharides obtained from KGM by the action of

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fungal endo-β-(1,4)-glucanase or endo-β-(1,4)-mannanase; and Liu et al. (2015)37 that

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investigated the factors affecting the enzymatic hydrolysis of KGM using β-mannanase

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and reported the optimum conditions (t=2 h, T=50°C, pH=6 and enzyme charge=150

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U/g).

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On the other hand, acids can also be used for GM degradation. Chen et al. (2005)42

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obtained partially hydrolysed KGM by a treatment with 0.2 N HCL after boiling for 20

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min, and Rivas et al. (2012)34 subjected samples of Pinus pinaster wood to

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hydrothermal processing (a reaction catalysed by hydronium ions coming from water

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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

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mannans with a minimal generation of sugars and sugar degradation compounds.

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Other methods that have been assayed for obtaining oligosaccharides by

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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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165 166

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.

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When the final objective is to use these products as food ingredients for human

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nutrition, the obtained liquors obtained must be refined.

172

In spite of the limited studies regarding GMOS purification, it can be stated that

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membrane technology is the most employed and suitable technique for the

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purification of these oligosaccharide mixtures. Besides being a simple technology, the

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use of membranes offers numerous advantages such as: high-energy efficiency, no

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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

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and its scale up is relatively easy. 43,44

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Rivas et al. (2012)34 subjected liquors obtained from Pinus pinaster wood by

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hydrothermal processing to a two-step discontinuous diafiltration using regenerated

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cellulose membranes of 5 and 1 kDa cutoff (Millipore) with a filtration area of 41.8 ×

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10-4 m2 and an operating pressure of 4 bars for refining and fractionating their

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hemicellulose-derived

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glucomannan or galactoglucomannan breakdown, substituted with one or more acetyl

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groups) obtaining two streams with different molecular mass distribution (GMOS 1, >5

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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

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molecular weight cut-off (3x105 Da) to a minimum one (1x103 Da), obtaining 6

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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,

Manufacture and Properties of Glucomannans and

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