Β-glucan Gels Research Articles

Freeze-thaw Properties of β-glucan Gels with Different Concentrations

The β-glucan gel was prepared in freeze-thawing method. The formation of freeze-thawed gel was studied by low-field nuclear magnetic resonance (NMR). Three relaxing components with different transverse relaxation time (T21, T22 and T23) were founded in the β-glucan system. With the increase of the number of freezing and thawing, the T22 of barley β-glucan increased rapidly after 5th freezing and thawing and A22 increased significantly after 3th freezing and thawing. The relaxation time T21, T22 and T23 rose first and then did not change. A21 and A22 first increased and then changed little while A23 first declined and then changed little. By performing the principal component analysis on the relaxation distribution curve, β-glucan gel of different concentrations. The rheological study showed that β-glucan gel with higher mass fraction has a larger elastic viscous modulus. This indicated that it formed more more physical ross-linked structure in gel. The results of scanning electron microscopy showed that β-glucan solut ion with higher mass fraction formed structure compact gel while β-glucan solution with lower mass fraction formed structure loose gel.

Freeze-thaw Properties of β-glucan Gels

The network structure of β-glucan polymers and the presence of water have significant effects on the properties of β-glucan gels produced by freeze-thaw cycles. The properties of the gel are influenced by its fine structure, molecular size, mass fraction, number of freeze-thaw cycles, and temperature of the β-glucan. To characterize β-glucan freeze-thaw gels, we used low-field nuclear magnetic resonance (LF-NMR) to measure water proton transverse relaxation in aqueous β-glucan solutions during storage. The results indicated that microphase separation occurred during cryogelation, and three water components were identified in the cryostructure. The spin-spin relaxation time was analyzed on the basis of chemical exchange and diffusion exchange theory. The location of each water component was identified in the porous microstructure of the cryogel. The pore size measured from scanning electron microscopy (SEM) images agreed with the pore size estimated from relaxation time. The formation of cryogel was confirmed by rheological. The results suggested that LF-NMR can monitor the polysaccharide cryogelation process by the evolution of spin-spin relaxation characteristics.

Microstructure and mechanical properties of arabinoxylan and (1,3;1,4)-β-glucan gels produced by cryo-gelation

The interactions between heteroxylans and mixed linkage glucans determine the architecture and mechanical properties of cereal endosperm cell walls. In this work hydrogels made of cross-linked arabinoxylan with addition of β-glucan were synthesised by cryogelation as a biomimetic tool to investigate endosperm walls. Molecular and microstructural properties were characterised by nuclear magnetic resonance (13C NMR), scanning electron microscopy (SEM) and immunolabelling/confocal laser scanning microscopy (CLSM). The response to mechanical stress was studied by compression-relaxation experiments. The hydrogels consisted of a scaffold characterised by dense walls interconnected by macropores with both hemicelluloses co-localised and homogeneously distributed. The gels showed a high degree of elasticity reflected in their ability to resist compression without developing cracks and recover 60–80% of their original height. Our results highlight the compatibility of these hemicelluloses to coexist in confined environments such as cell walls and their potential role in determining mechanical properties in the absence of cellulose.

Textural and Rheological Properties of Oat Beta-Glucan Gels with Varying Molecular Weight Composition

The impact of oat β-glucan concentration and molecular weight (MW) on gel properties was investigated. Mixed MW gels/viscous solutions at 3, 4, and 5% β-glucan with high molecular weight (HMW):low molecular weight (LMW) ratios of 0:100, 25:75, 50:50, 75:25, and 100:0 were evaluated. The 100:0 and 50:50 gels had the lowest tan δ values. The 50:50 gels had the highest storage moduli (G'), whereas 100:0 solutions did not gel. Peak melting temperature (TP) was highest for 0:100 gels and decreased with the addition of HMW β-glucan. Hardness, at 40% compression, increased with concentration, and 25:75 and 50:50 gels were hardest at each concentration. Ordered microstructure, apparent in 0:100 gels, diminished with HMW β-glucan addition. Glucose addition resulted in lower tan δ values and firmer, harder gels compared to gels without glucose. Thus, the textural properties and melting profiles of β-glucan gels can be manipulated by adjusting the ratios of molecular weight fractions or addition of sugar for different applications.

A micro- and macro-scale approach to probe the dynamics of sol–gel transition in cereal β-glucan solutions varying in molecular characteristics

Cereal β-glucans are known to display functional properties with health benefits, such as reduction of plasma cholesterol and of postprandial serum glucose levels in humans and animals; such effects have been lately attributed to viscosity and gelation potential of these water soluble fibres. The local dynamics of β-glucan solutions differing in molecular size and the molar ratio of trimers to tetramers (DP3/DP4) chain segments, which undergo gelation upon ageing, was investigated. Confocal microscopy, particle tracking microrheology and conventional bulk shear rheological measurements have been employed to study the microstructure and mechanical properties of the polysaccharide networks on various length scales. The structural features of β-glucans, such as molecular weight and ratio of DP3/DP4, were found to be important determinants of their gelling properties and microstructure. For the β-glucan gels cured at 25 °C both the microrheology and the bulk rheology revealed that with decreasing molecular weight and increasing DP3/DP4 molar ratio the gelation time decreased, while the gelation rate increased along with the storage modulus. In all samples studied, particulate clusters of β-glucan assemblies were generated with size ∼1–15 μm and the clusters were eventually interconnected to expand over the available space to form an elastic network. The pore size and the structural entities were found to increase in size at lower values of the DP3/DP4 molar ratio and with preparations of high molecular weight. The size and the compactness of the structural entities seem to play an important role in the network reinforcement. The embedded tracer particles were found to experience relatively homogeneous microenvironments at DP3/DP4 ∼2.1, reflecting a rather slow rate of chain aggregates structuring. The behaviour of the polysaccharide network dynamics at a microscopic level does not always seem to match the overall bulk macroscopic response.

Attenuation of glycemic responses by oat β-glucan solutions and viscoelastic gels is dependent on molecular weight distribution

Oat β-glucan attenuates postprandial glycemic responses when solubilized to form viscous solutions. High molecular weight (MW) β-glucan is associated with high solution viscosity, which is in turn associated with lower glycemic responses. However, low MW β-glucan is also able to form viscoelastic gels. The effect of low (145,000 g mol(-1)) and high (580,000 g mol(-1)) MW β-glucan presented as liquid drinks and gels on glycemic responses was determined. Healthy subjects (n = 15) consumed 50 g glucose drinks with no β-glucan; 4 g low MW; or 4 g high MW β-glucan; and gels containing 4 g low MW; 2 g low plus 2 g high MW; or 3 g high plus 1 g low MW β-glucan. Overall, β-glucan solutions elicited lower glycemic responses than gels. For gels, peak blood glucose rise (PBGR) decreased with increasing dose of high MW β-glucan (r(2) = 0.976, P > 0.05), and PBGR for the gel with 3 g high-MW was lower than for the control (P < 0.05). However, β-glucan gels retained glucose better than solutions under in vitro analysis. Observed effects were found to be related to the rheological properties of the foods. β-Glucan solutions and not gels effectively attenuated in vivo glycemic responses.

Barley beta-glucan aerogels via supercritical CO2 drying

Low density, highly porous aerogels have a variety of applications. Through supercritical CO2 (SCCO2) drying, pore liquid can be removed with little to none of the damage associated with conventional drying techniques for wet gels. Gels formed using a barley β-glucan concentrate at 5, 6 and 7% (w/v) levels were dried using SCCO2. These aerogels were compared to those formed using freeze drying and air drying, as well as SCCO2+ethanol, and analyzed in terms of density, shrinkage, color and structural properties. SCCO2 aerogels had lower densities than air-dried gels and a more consistent structure than freeze-dried gels. β-Glucan concentration had no significant effect on density or surface area, with values of 0.20g/cm3 and 166m2/g, respectively, obtained for 5% β-glucan gels. As β-glucan aerogels are biodegradable, biocompatible, renewable and edible, they have great potential for application as delivery vehicles for nutraceuticals or pharmaceuticals.

Using particle tracking to probe the local dynamics of barley β-glucan solutions upon gelation

The sol–gel transition of aqueous barley β-glucan solutions which undergo gelation with ageing has been studied by conventional bulk rheology, phase contrast microscopy and particle tracking microrheology. Characterisation of the primary structure of the β-glucan isolate was carried out by enzymic methods and HPLC. The Brownian diffusion of fluorescent microspheres (0.75μm diameter, carboxylate-coated particles) was used to probe the spatial mechanical properties of the gelling systems at the scale of microns; the potential use of passive particle tracking to study biopolymer gelling systems that present spatial heterogeneities is thus explored. For the β-glucan gels cured at 25°C both microrheology and bulk rheology revealed that with increasing the polysaccharide concentration the gelation time decreased, while the gelation rate and gel strength of the barley β-glucan gels increased. The particle tracking method had higher sensitivity and could map molecular ordering and structural heterogeneities in the evolving polysaccharide network at a micro-level. That is, different size pores were generated upon ageing with regions of depleted or less amount of β-glucan molecules. Furthermore, this method could detect changes in the fine structure of the system before such events can be registered by bulk rheological measurements; i.e. microheterogeneity and aggregation of β-glucan chains were revealed by particle tracking at earlier temporal stages of the experiment.

Effects of oxidative treatment on the physicochemical, rheological and functional properties of oat β-glucan

The objective of this work was promote oxidation of β-glucan from oat bran with hydrogen peroxide at different concentration levels (0.3%, 0.6% and 0.9% H2O2) and reaction times (30 and 60min), and evaluate the physicochemical and functional properties of isedoxidised β-glucan with in-vitro tests. An increase in carbonyl and carboxyl groups and alterations in swelling power were verified in the oxidised β-glucan. The cholic acid binding capacity increased in the oxidised β-glucan; however, the fat binding capacity was not affected. After chemical digestion, the available glucose of the oxidised β-glucan was increased. Oxidation with hydrogen peroxide decreased the viscosity, hardness, adhesiveness and gumminess of the β-glucan gels. More studies are necessary to determine the effect of the oxidative treatment of β-glucan on its technological properties in food products, and biological properties should be examined with in-vivo studies.

Using particle tracking to probe the local dynamics of barley β-glucan solutions

The sol-gel transition of barley isolated β-glucan solutions which undergo gelation with ageing has been studied by conventional bulk rheology, phase contrast microscopy and particle tracking microrheology. Also, characterization of primary structure of the β-glucan isolate was carried out by HPLC. The Brownian diffusion of fluorescent microspheres (0.75μm diameter, carboxylate-coated particles) was used to probe spatial mechanical properties of the gels at the scale of microns. The potential use of passive particle tracking as a new method of studying food systems that present spatial heterogeneities is explored. For the β-glucan gels cured at 25oC both the microrheology and the bulk rheology revealed that with increasing concentration of the polysaccharide the gelation time decreased, while the gelation rate and gel strength of the barley β-glucan gels increased. Moreover, the melting point increased with increasing concentration of the β-glucans indicating a better organization of the ordered domains in the network structure. The particle tracking method had higher sensitivity and could map molecular ordering and structural heterogeneities at a micro level. Furthermore, this method could detect changes in the structuring of the system before such events can be registered by the bulk rheological measurements. For these reasons, the timescales of the microscopic dynamic behavior do not always seem to match the timescales of the overall macroscopic behavior.

A fractal analysis approach to viscoelasticity of physically cross-linked barley β-glucan gel networks

The structure and gelation kinetics of mixed linkage barley β-glucans of varying Mw have been investigated. The fractal concept has been applied to describe the structure development of barley β-glucan gels using a scaling model and dynamic rheometry data. The model supports that the gel structure consists of fractal clusters that upon aggregation lead to a three-dimensional network. The analysis showed that with increasing Mw a denser (more packed) network is formed as indicated by the corresponding fractal dimension ( d f) values. The microelastic parameter of the model, α, showed that all gel structures were in the transition regime implying structural reordering upon ageing. The description of the microstructure as a fractal network seems to be able to explain syneresis and other observations from large deformation testing of such systems. The molecular treatment of the gelation kinetics suggests that the gelling behavior is governed by the probability of collision of chain fragments with consecutive cellotriosyl units. This is greater for small chains due to their higher diffusion rates, for chains having lower amounts of cellulose like fragments and finally for those showing smaller degree of intrachain interactions. As a result, the faster gelling systems exhibit lower fractal dimensionality (more disordered systems) something that is in accordance with current kinetic theories.

Evaluation of structure in the formation of gels by structurally diverse (1→3)(1→4)-β- d-glucans from four cereal and one lichen species

The (1→3)(1→4)-β- d-glucans from four cereal sources (oats, wheat, barley and rye) and one lichen source (Icelandic moss) were used to test two proposed structurally based hypotheses about the gelling mechanism of these polymers. Structures were evaluated using high performance anion exchange chromatography of the oligosaccharide fragments released by a (1→3)(1→4)-β- d-glucan-4-glucanohydrolase. This determined the relative amounts of cellodextrin units, of different degrees of polymerisation, which are joined by β-(1→3) linkages in the intact polysaccharide chain. Oat β-glucan had the lowest β-(1→3)-linked cellotriosyl unit content and lichenan had the highest. Strong correlations were found between the fraction of β-(1→3)-linked cellotriosyl units in the β-glucans and the elasticity of 6% gels in water, as measured by dynamic rheometry. Differential scanning calorimetry showed that the β-(1→3)-linked cellotriosyl unit content was also correlated with the onset and peak temperatures when 6% β-glucan gels were melted. No correlation was found between the longer (DP 6–9) β-(1→3)-linked cellodextrin oligosaccharide content and either the gel elasticity or melting characteristics. These findings are consistent with a model in which runs of consecutive β-(1→3)-linked cellotriosyl units form the junction zones in the gel network, but not with a model in which longer β-(1→3)-linked cellodextrins associate, as in cellulose fibres, to produce the gel network. Microscopic images of the β-glucan gels from the five species revealed that the microstructure was not homogeneous in any of the samples, which may be related to the variability in the enthalpy of melting of gels. There was a coarsening of gel structure as the β-(1→3)-linked cellotriosyl unit content increased.

A comparative study on structure–function relations of mixed-linkage (1→3), (1→4) linear β- d-glucans

The effects of fine structure and molecular size on the rheological properties of six mixed-linkage (1→3), (1→4)-β- d-glucans (β-glucans) in the solution and gel state were studied. Molecular size characterization was carried out with high-performance size exclusion chromatography combined with a refractive index detector. Samples were divided into two groups according to the values of apparent molecular weight ( M w) of the peak fraction of the main eluting peak calculated as ∼200×10 3 for an oat, a barley, and a wheat β-glucan and ∼100×10 3 for an oat and a barley β-glucan, and a lichenan sample. All polysaccharides analyzed by 2D NMR spectroscopy and high-performance anion-exchange chromatography of the cellulosic oligomers released by the action of lichenase showed the typical fine structure of mixed-linkage linear (1→3), (1→4)-β- d-glucan. Following lichenase digestion of β-glucans, the molar ratios of tri- to tetrasaccharides (DP3/DP4) were found to follow the order of lichenan (24.5)>wheat (3.7)>barley (2.8–3.0)>oat (2.1). Differences in critical concentration (c ∗ ∗), viscosity, viscoelastic and shear thinning properties among samples were dependent mainly on differences in molecular size of the polymeric chains as well as on the β-glucan fine structure. All β-glucan isolates were able to form gels, as probed by dynamic rheometry; with decreasing molecular size and increasing DP3/DP4 ratio, the gelation time decreased and the gelation rate (I E =[ d log G′/ dt] max ) increased. Differential scanning calorimetry (DSC) showed that cereal β-glucan gels exhibit rather broad endothermic gel→sol transitions at 55–80 °C, while lichenan gels give a sharper transition, implying a more cooperative process. The DSC kinetic data showed similar responses to that from dynamic rheometry; the rate of development of the endotherm increased with increasing DP3/DP4 ratio of the polysaccharide. Furthermore, the storage modulus ( G′) and the apparent melting enthalpy values (plateau Δ H) increased with decreasing molecular size and with increasing DP3/DP4 ratio. The melting temperature of the gel network, as determined by DSC and dynamic rheometry, was found to increase with the molecular size and the DP3/DP4 ratio of β-glucans; the T m for lichenan was ∼89 °C and for cereal β-glucans varied in the narrow range of ∼65–72 °C. Large deformation mechanical tests (compression mode) up to failure revealed an increase in strength and a decrease in brittleness of mixed-linkage β-glucan gels with increasing DP3/DP4 ratio and molecular size of the polysaccharide.

Molecular size effects on rheological properties of oat β-glucans in solution and gels

The effects of molecular size on rheological properties of highly pure oat β-glucans in solution and gel state were studied. The fine structure of the β-glucan preparations was assessed by 13C-NMR spectroscopy and high-performance anion-exchange chromatography (HPAEC) of the cellulosic oligomers released by the action of lichenase. The tri- and tetra-saccharides accounted for 90.8–96.4% of the total oligomers analyzed and the calculated molar ratios of trimers/tetramers varied between 1.95 and 2.30. Molecular size characterization was carried out with high performance size exclusion chromatography (HPSEC) combined with a refractive index detector; for samples with apparent molecular weight ( M w) of the peak fraction of the main eluting peak ranging between 35 and 250×10 3, the values of limiting viscosity ([ η]), critical concentration (c ∗∗) and coil overlap parameter (c ∗∗[η]) were within 0.67–3.83 dl/g, 0.70–2.10 g/dl and 1.41–2.67, respectively. The shear thinning and viscoelastic behavior of fresh β-glucan solutions were typical of a random coil type polysaccharide, and dependent on the molecular size and concentration of the polymer. On the other hand, time-dependent rheological behavior was revealed by thixotropic loop experiments for the lowest molecular size sample, implying the formation of intermolecular networks. All β-glucan samples, except the preparation with the highest M w, were able to form gels, as revealed by dynamic rheometry; with increasing M w the gelation time increased and the gelation rate (I E =[ d( log G′)/ dt] max ) decreased. The gelation rate also increased with increasing concentration and gel curing temperatures reaching a maximum at ∼25–35 °C; for higher temperatures the I E values decreased. The dependence of storage modulus on β-glucan concentration ( C) followed power law relationships; G′ varied as C 7.2–7.5. A two-step melting behavior was observed for gels cured under conditions favoring a quick gelation. Differential scanning calorimetry (DSC) showed that gels exhibit rather broad endothermic gel→sol transitions at 55–75 °C. The DSC kinetic data showed similar responses to those from dynamic rheometry; the rate of endotherm development increased with decreasing molecular size of the polysaccharide. Also, the apparent melting enthalpy values (plateau Δ H) increased with decreasing molecular size. The melting temperature of the gel network, as determined by DSC and dynamic rheometry, was found to increase with the molecular size of β-glucans. Large deformation mechanical tests (compression mode) revealed an increase in strength and a decrease in brittleness of oat β-glucan gels with increasing M w. The true stress ( σ TR) and strain ( ε TR) at failure of β-glucan gels increased with increasing polysaccharide concentration reaching a maximum at a certain concentration; at higher concentrations the σ TR, and ε TR values decreased.