Manufacture and Properties of Glucomannans and

Feb 14, 2017 - CINBIO, University Campus, 36310 Vigo, Pontevedra, Spain. ABSTRACT: Glucomannans (GM) are polymers that can be found in natural ...
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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]

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Abstract

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Glucomannans (GM) are polymers which can be found in natural resources, such as

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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

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to their structural functions and their role as energy reserve, they have been assessed

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for their healthy applications, including their role as new source of prebiotics. This

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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.

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Keywords: glucomannans, glucomannooligosaccharides, konjac, emerging prebiotics,

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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

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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

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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

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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

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production from spruce (Picea abies).

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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)

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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-

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irradiation. In particular, the combination of ϒ-irradiation and β-mannanase was an

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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

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unwanted compounds, such as monosaccharides, extractives, and/ or sugar

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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.

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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

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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

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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,