In situ nanomechanical properties of natural oil bodies studied using atomic force microscopy.

Natural oil bodies (OBs) from plant organs represent an important category of functional ingredients and materials in a variety of industrial sectors. Their applications are closely related to the membrane mechanical properties on a single droplet level, which remain difficult to determine. In this research, the mechanical properties of the membranes of OBs from soybean, sesame, and peanut were investigated in-situ by atomic force microscopy (AFM). Different regions of the force-deformation curves obtained during compression were analyzed to extract the stiffness Kb or Young's modulus of the OB membranes using Hooke's law, Reissner theory, and the elastic membrane theory. At higher strains (ε = 0.15-0.20), the elastic membrane theory breaks down. We propose an extension of the theory that includes a contribution to the force from interfacial tension based on the Gibbs energy, allowing effective determination of Young's modulus and interfacial tension of the OB membranes in the water environment simultaneously. The mechanical properties of the OBs of different sizes and species, as well as a comparison with other phospholipid membrane materials, are discussed and related to their membrane compositions and structures. It was found that the natural OBs are soft droplets but do not rupture and can fully recover following compressive strains as large as 0.3. The OBs with higher protein/oil ratio, have smaller size and stronger mechanical properties, and thus are more stable. The low interfacial tension due to the existence of phospholipid-protein membrane also contributes to the stability of the OBs. This is the first report measuring the mechanical properties of OB membranes in-situ directly.

Trivalent iron induced gelation in Artemisia sphaerocephala Krasch. polysaccharide.

Artemisia sphaerocephala Krasch. polysaccharide (ASKP) has attracted growing attention in the field of food and medical engineering due to its biological activity and colloidal property. In this study, the binding between ASKP and ferric ions was found and the binding mechanism was explored. The results showed that ASKP could form a hydrogel with three-dimensional network structure in the presence of ferric ions. Ferric ions could specifically bind with the carboxyl and hydroxyl groups of the high molecular weight fraction of 60P with the binding stoichiometry of [M3+]/[repeating unit] = 2.5. The possible mechanism of the formation of ASKP-Fe3+ complex was proposed as two binding modes of monodentate and bridging binding. ASKP-Fe3+ complex exhibited higher thermal stability than ASKP revealed with DSC thermograms. The study indicated that ASKP would be a novel gelation biopolymer and the ASKP-Fe3+ complex hydrogel could be exploited as a new iron fortifier.

Structural Characterization and Chain Conformation of Water-Soluble ß-Glucan from Wild Cordyceps sinensis.

Water-soluble ß-d-glucan was obtained from wild Cordyceps sinensis by alkali solution and ethanol precipitation. The structure characteristics were determined using high-performance anion-exchange chromatography coupled with pulsed amperometric detection (HPAEC-PAD), methylation combined with gas chromatography-mass spectrometry, and one-/two-dimensional nuclear magnetic resonance spectroscopy. Results showed that ß-d-glucan had a structure of every seven (1→3)-ß-d-Glcp backbone residues with two (1→6)-ß-d-Glcp branches. Additionally, conformation properties in different solvents were investigated by static light scattering, dynamic light scattering, and HPSEC with multiple detectors. It was found that ß-d-glucan in 0.5 M NaOH had a narrow unimodal distribution of hydrodynamic radius displaying a spherical coil conformation, whereas it formed severe aggregation in dimethyl sulfoxide. In 0.1 M NaNO3, ß-d-glucan mainly existed as a rod-like conformation corresponding to a helical structure together with small aggregates (10%). This work added more information to the understanding of C. sinensis polysaccharides.

Modulation of calcium-induced gelation of pectin by oligoguluronate as compared to alginate.

Previous studies have demonstrated that oligoguluronate (guluronate block extracted from alginate, GB) was an efficient modulator of the gelation and gelling properties of macromolecular alginate in the presence of calcium. Here we report totally different modulatory effects of the oligomer when used to modify the gelation of low methoxyl pectin (LMP). GB was found to promote the gelation of LMP in the range of R ([Ca]/[guluronate + galacturonate]) < 0.25 and could make non-gelling systems gellable. This is significantly different from the case of alginate where no gelation could be induced at all. In the range of 0.25 < R < 0.60, the addition of GB was found to inhibit the gelation of LMP, whereas it had a negligible effect on the gelation of alginate as long as a fixed R was considered. In the range of R > 0.60, GB was found to promote the gelation of LMP again, which is similar to the case of alginate. The results were in consistence with microstructural observations by AFM. The different modulatory effects of GB were thought to arise from the different gelation mechanisms of LMP and alginate, that is, a progressive dotting growth of LMP dimers vs. a critical zippering growth of alginate dimers during Ca-induced crosslinking. The mechanism of GB modulating the gelation of LMP was proposed and compared to that for alginate.

Preparation and stability of nano-scaled gel beads of λ-carrageenan bound with ferric ions.

Iron-deficiency anemia (IDA) is a major global public health problem, and the iron fortifiers in diet are clearly needed in the prevention and improvement of IDA for humans. A novel nano-scaled gel beads of λ-carrageenan (λ-car) specifically binding with ferric ions was developed to be a promising iron fortifier with no adverse organoleptic changes on food. Turbidity measurement, thermogravimetric analysis and Fourier transform infrared spectroscopy confirmed the successful chelating. The gel beads of λ-car-Fe3+ complex showed good dispersibility and solvent stability. The in vitro cell viability of HepG2 cells treated with λ-car-Fe3+ was over 75% at 5 mg/mL of ferric ions, indicating a significant cytotoxicity reduction of ferric ions. The stability of λ-car-Fe3+ complex powder was obviously increased against browning during 60 d storage with zein coating, which was attributed to the prevention of moisture permeation. Zein coated gel beads also performed a slow release of ferric ions in simulated gastrointestinal juices, resulting from the compact and hydrophobic zein surface delaying the dissociation of λ-car-Fe3+ in acidic environment. This λ-car-Fe3+ complex would have a great potential as a safe iron fortifier and facilitate iron supplementary with the advantage to relieve the side effects of iron ions.

Stability and Oil Migration of Oil-in-Water Emulsions Emulsified by Phase-Separating Biopolymer Mixtures.

Oil-in-water (O/W) emulsions with varying concentration of oil phase, medium-chain triglyceride (MCT), were prepared using phase-separating gum arabic (GA)/sugar beet pectin (SBP) mixture as an emulsifier. Stability of the emulsions including emulsion phase separation, droplet size change, and oil migration were investigated by means of visual observation, droplet size analysis, oil partition analysis, backscattering of light, and interfacial tension measurement. It was found that in the emulsions prepared with 4.0% GA/1.0% SBP, when the concentration of MCT was greater than 2.0%, emulsion phase separation was not observed and the emulsions were stable with droplet size unchanged during storage. This result proves the emulsification ability of phase-separating biopolymer mixtures and their potential usage as emulsifiers to prepare O/W emulsion. However, when the concentration of MCT was equal or less than 2.0%, emulsion phase separation occurred after preparation resulting in an upper SBP-rich phase and a lower GA-rich phase. The droplet size increased in the upper phase whereas decreased slightly in the lower phase with time, compared to the freshly prepared emulsions. During storage, the oil droplets exhibited a complex migration process: first moving to the SBP-rich phase, then to the GA-rich phase and finally gathering at the interface between the two phases. The mechanisms of the emulsion stability and oil migration in the phase-separated emulsions were discussed.

Protein/Polysaccharide Electrostatic Complexes and Their Applications in Stabilizing Oil-in-Water Emulsions.

Consumers are becoming increasingly fastidious in demanding food products with improved quality and functionality. This largely relies on rational design of food structures. As the two key food ingredients, protein and polysaccharides play important roles in food structuring. The combination of protein and polysaccharide provides rich opportunities for food structure and function designs through molecular interaction and assembly. This paper provides a brief review on the formation and characterization of protein/polysaccharide electrostatic complexes and their applications in stabilizing oil-in-water emulsions, particularly those containing polyunsaturated fatty acids.

Synthesis and antioxidant properties of gum arabic-stabilized selenium nanoparticles.

Selenium nanoparticles (SeNPs) were prepared by using gum arabic (GA) as the stabilizer in a facile synthetic approach. The size, morphology, stability and antioxidant activity in vitro of the gum arabic-selenium nanocomposites (GA-SeNPs) were characterized by transmission electron microscopy (TEM), dynamic light scattering (DLS), Fourier-transform infrared spectroscopy (FTIR), atomic force microscopy (AFM) and ultraviolet-visible spectrophotometry (UV-vis). SeNPs (particle size of ∼34.9 nm) can be stabilized in gum arabic aqueous solutions for approximately 30 days. FTIR results show that SeNPs were combined to the hydroxyl groups of GA. In the present work, the alkali-hydrolyzed GA (AHGA) was also prepared and its efficiency in stabilizing SeNPs was compared with GA. It was concluded that the branched structure of GA was a significant factor for the functionality. The hydroxyl radical scavenging ability and DPPH scavenging ability of GA-SeNPs were higher than those of AHGA-SeNPs and could reach 85.3±2.6%, 85.3±1.9% at a concentration of 4 mg/ml, respectively.

Phase separation induced molecular fractionation of gum arabic--sugar beet pectin systems.

This paper investigates the phase separation and phase separation-induced fractionation of gum arabic (GA)/sugar beet pectin (SBP) mixed solutions. A phase diagram, including cloud and binodal curves, was established by visual observation and phase composition analysis. The deviation of the binodal curve from the cloud curve was a result of phase separation-induced fractionation of polydisperse GA and SBP molecules. Fractionation of GA increased the content of arabinogalactan-protein complex (AGP) from ca. 13% to 27%. The fractionated GA (FGA) showed improved emulsifying functionality, whereas the fractionated SBP (FSBP) had a reduced emulsifying functionality. The changes in emulsifying efficiency can be explained by interfacial adsorption behaviors at the oil-water interface as indicated by interfacial tension measurements.

Characterisation of commercial LM-pectin in aqueous solution.

Fourteen commercial LM-pectin samples were investigated in this study. The degree of esterification (DE) varied from 9.6% to 42.0% and the degree of amidation (DA) from 0% to 25%. Chemical and structural characteristics were examined using atomic absorption (AA), dilute solution capillary viscometry, GPC-MALLS and dynamic light scattering. The intrinsic calcium was in the range of 47-1388 ppm, the intrinsic viscosity varied from 2.9 to 4.9 (dL/g) and the weight average molecular weight (M(w)) from 113 to 290 (kDa). Most of the samples had Huggins constants of ~0.5. However, for samples having acidic pH, Huggins constant values greater than 1, which can be as an indication of aggregation, were obtained. The high Huggins constant value could be reduced to ~0.5 by the addition of 3M urea, indicating that the aggregation was stabilised by hydrogen bonding. Shear flow viscosity revealed three types of rheological behaviour. Type A showed pronounced shear thinning behaviour, which was reduced by the addition of hydrogen bonding breaking agent urea. Type B with intrinsic Ca of 1mM and pH ~4 showed two shear thinning regions, with significantly enhanced shear viscosity upon addition of calcium. Type C showed the least aggregation due to its pH ~4 and low intrinsic Ca, but could be converted to type B upon addition of Ca. The effect of Ca on the rheological behaviour of types B and C was further confirmed by CaCl(2) and Ca-chelating agent (EDTA). Temperature affected the molecular conformation of all types and most significantly type A by eliminating the hydrogen bonding.

Improved sugar beet pectin-stabilized emulsions through complexation with sodium caseinate.

The study investigates the complexes formed between sodium caseinate (SC) and sugar beet pectin (SBP) and to harness them to stabilize SBP emulsions. We find that both hydrophobic and electrostatic interactions are involved in the complexation. In SC/SBP mixed solution, soluble SC/SBP complexes first form on acidification and then aggregate into insoluble complexes, which disassociate into soluble polymers upon further decreasing pH. The critical pH's for the formation of soluble and insoluble complexes and disappearance of insoluble complexes are designated as pH(c), pH(φ), and pH(d), respectively. These critical pH values define four regions in the phase diagram of complexation, and SC/SBP emulsions were prepared in these regions. The results show that the stability of SBP-stabilized emulsion is greatly improved at low SC/SBP ratios and acidic pH's. This enhancement can be attributed to an increase in the amount of adsorbed SBP as a result of cooperative adsorption to sodium caseinate. Using a low ratio of SC/SBP ensured that all caseinate molecules are completely covered by adsorbed SBP chains, which eliminates possible instability induced by thermal aggregation of caseinate molecules resulting from stress acceleration at elevated temperatures. A mechanistic model for the behavior is proposed.

Competitive adsorption between sugar beet pectin (SBP) and hydroxypropyl methylcellulose (HPMC) at the oil/water interface.

The emulsification performance, stability and competitive adsorption of two natural food emulsifiers, sugar beet pectin (SBP) and hydroxypropyl methylcellulose (HPMC) have been investigated. Both can reduce the surface tension and emulsify oil in water. However, due to their different structure and conformation they operate via different mechanisms. Using 15% middle chain triglycerides (MCTs) oil, the amounts of SBP and HPMC adsorbed in emulsions made with these individually and in mixtures were determined. The interfacial concentration (Γ) for SBP stabilized emulsion was ∼1.25mg/m(2) and for HPMC 3.5mg/m(2). The higher adsorption of HPMC was due to multilayer adsorption, whereas SBP adsorbed as a monolayer. Competitive adsorption between SBP and HPMC was also investigated. When the HPMC concentration approached that of adsorbed SBP, the effect of HPMC became dominant and at 1.5wt.% controlled the behavior of the mixed emulsions, which were then almost independent of SBP. The minor role of SBP was mainly to decrease the proportion of large droplets in the emulsion. A model to describe the competitive adsorption between SBP and HPMC is proposed.

Complexation of bovine serum albumin and sugar beet pectin: stabilising oil-in-water emulsions.

In a previous study (Langmuir 28 (2012) 10164-10176.), we investigated the complexation of bovine serum albumin (BSA) with sugar beet pectin (SBP). A pH-composition phase diagram was established and structural transitions in relation to the phase diagram during complexation were identified. The present study examines the implications of these interactions on the emulsifying performance of BSA/SBP mixtures. Middle-chain triglycerides (MCTs) in water emulsions were prepared using conditions corresponding to different regions of the phase diagram. At high pHs and in the stable region of mixed individual soluble polymers where complexation is absent, there is no improved emulsifying performance, compared with the individual protein and polysaccharide. For these mixtures, the emulsion characteristics are controlled by the major component in the solutions, as determined by the competitive adsorption of the two components at the oil-water interface. At low pHs and low BSA/SBP ratios, and so mainly within the stable region of intramolecular soluble complexes, BSA/SBP mixtures greatly improve the stability of emulsions. Here, stabilisation is controlled by the cooperative adsorption of the two components at the oil-water interface. Through electrostatic complexation BSA promotes the adsorption of SBP on to interfaces to form a thick steric layer around emulsion droplets and thus providing better stability. At low pHs and high BSA/SBP ratios, that is, mainly within the unstable region of intermolecular insoluble complexes, emulsions prepared are extremely unstable due to bridging flocculation between emulsion droplets.

Complexation of bovine serum albumin and sugar beet pectin: structural transitions and phase diagram.

The complexation between bovine serum albumin (BSA) and sugar beet pectin (SBP) was studied in situ by coupling glucono-δ-lactone (GDL) induced acidification with dynamic light scattering and turbidity measurements. Individual measurements at specific pHs and mixing ratios were also carried out using zeta potentiometry, gel permeation chromatography-multiangle laser light scattering (GPC-MALLS), and isothermal titration calorimetry (ITC). These investigations together enabled the establishment of a phase diagram of BSA/SBP and the identification of the molecular events during protein/polysaccharide complexation in relation to the phase diagram, which showed five regions: (I) a stable region of mixed individual soluble polymers, (II) a stable region of intramolecular soluble complexes, (III) a quasi-stable region of intermolecular soluble complexes, (IV) an unstable region of intermolecular insoluble complexes, and (V) a second stable region of mixed individual soluble polymers, on lowering pH. We found for the first time that the complexation could take place well above the critical pH(c), the value that most previous studies had regarded as the onset occurrence of complexation. A model of structural transitions between the regions was proposed. The borderline between region II and region III represents the BSA/SBP stoichiometry for intramolecular soluble complex at a specific pH, while that between region III and region IV identifies the composition of the intermolecular insoluble complex. Also studied was the effect of NaCl and CaCl(2) on the phase diagram and structural transitions.

Molecular weight, tertiary structure, water binding and colon behaviour of ispaghula husk fibre.

Molecular variables, using aqueous and alkaline extracts, of the polysaccharide from ispaghula husk (IH) were examined using gel-permeation chromatography linked to multi-angle laser light scattering. Progressive extraction can yield a component with a molecular weight (MW)value up to about 7 x 106 Da, and gels, which accompany the extraction, have MW ranging from 10-20 x 106 Da. To mimic the polysaccharide degradation, particularly in the colon, the solid IH was degraded progressively using ionising radiation. A chain break occurs every 7.5 kGy in NaOH and every 15 kGy in water. The solid-state matrix is opened by the radiation to yield increased visco-elasticity of the aqueous extracts at critical radiation doses, before further degradation occurs after about 12 kGy. Differential scanning calorimetry is used to study the mechanism of interaction of water with IH. The first water to be taken up is non-freezing water and represents about twelve water molecules/disaccharide unit of the polysaccharide. As the water content is increased, the water becomes bound to the polysaccharide and freezes and melts at a temperature different from free water. This water is thermodynamically distinguishable from free water. It forms amorphous ice on cooling which crystallises exothermically and subsequently melts endothermically. Saturation occurs at a water content of 2-3 g water/g polymer, showing that about 60% of the water in the system is 'bound'. The most surprising conclusion is that despite the fact that the IH swells in water to form a solid and stiff gel, the greater part of that water in the gel is still free and behaves like liquid water.