Fatang Jiang

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.

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.

Interactions between carboxymethyl konjac glucomannan and soy protein isolate in blended films

To elucidate biopolymer interactions between carboxymethyl konjac glucomannan (CMKGM) and soy protein isolate (SPI) in different ratios on physicochemical properties of the blended films, biodegradable CMKGM/SPI films were prepared and characterized. The results showed that CMKGM and SPI are highly compatible in blended film formation, and that Maillard reactions and hydrogen bonds interactions between CMKGM and SPI occurred. The water adsorption of the CMKGM/SPI films progressively decreased with increasing CMKGM level, the surface wettability of the blended films was improved with increasing CMKGM content; the CMKGM/SPI blend films had enhanced tensile strength (TS) and elongation at break (EAB) compared to pure CMKGM and SPI films; the oxygen permeability of blend films was decreased; the roughness was decreased with increasing CMKGM content. Moreover, the CMKGM/SPI film was biocompatible and biodegradable.

Carboxymethyl modification of konjac glucomannan affects water binding properties.

The water binding properties of konjac glucomannan (KGM) and carboxymethyl konjac glucomannan (CMKGM) are important for their application in food, pharmaceutical, and chemical engineering fields. The equilibrium moisture content of CMKGM was lower than that of KGM at the relative humidity in the range 30-95% at 25°C. The water absorption and solubility of CMKGM in water solution were lower than that of KGM at 25°C. Carboxymethyl modification of KGM reduces the water adsorption, absorption, and solubility. Both carboxymethylation and deacetylation could confer hydrophobicity for CMKGM. These data provide the basis for expanding CMKGM application.

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.

Characterization of konjac glucomannan-ethyl cellulose film formation via microscopy.

Konjac glucomannan-ethyl cellulose (KGM-EC, 7:3, w/w) blended film shows good mechanical and moisture resistance properties. To better understand the basis for the KGM-EC film formation, optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) were used to observe the formation of the film from emulsion. Optical microscopy images showed that EC oil droplets were homogeneously dispersed in KGM water phase without obviously coalescence throughout the entire drying process. SEM images showed the surface and cross-sectional structures of samples maintained continuous and homogeneous appearance from the emulsion to dried film. AFM images indicated that KGM molecules entangled EC molecules in the emulsion. Interactions between KGM and EC improved the stability of KGM-EC emulsion, and contributed to uniformed structures of film formation. Based on these output information, a schematic model was built to elucidate KGM-EC film-forming process.

The control of ice crystal growth and effect on porous structure of konjac glucomannan-based aerogels.

Konjac glucomannan (KGM)-based aerogels were prepared using a combination of sol-gel and freeze-drying methods. Preparation conditions were chosen to control ice crystal growth and aerogel structure formation. The ice crystals formed during pre-freezing were observed by low temperature polarizing microscopy, and images of aerogel pores were obtained by scanning electron microscopy. The size of ice crystals were calculated and size distribution maps were drawn, and similarly for aerogel pores. Results showed that ice crystal growth and aerogel pore sizes may be controlled by varying pre-freezing temperatures, KGM concentration and glyceryl monostearate concentration. The impact of pre-freezing temperatures on ice crystal growth was explained as combining ice crystal growth rate with nucleation rate, while the impacts of KGM and glyceryl monostearate concentration on ice crystal growth were interpreted based on their influences on sol network structure.

Whey protein isolate/gum arabic intramolecular soluble complexes improving the physical and oxidative stabilities of conjugated linoleic acid emulsions

Protein/polysaccharide electrostatic complexes have been widely used in food products to confer structure and stability. Intramolecular soluble complexes (ISCs) have superior emulsifying properties in stabilizing oil-in-water (o/w) emulsions. This paper investigates the potential application of ISCs to stabilize polyunsaturated fatty acids that were difficult to disperse and liable to oxidation. The idea was demonstrated using whey protein isolate/gum arabic (WPI/GA) ISCs and conjugated linoleic acid (CLA). Zeta potential measurements indicated a stoichiometry of r = 1.0 for the electrostatic complexation of WPI/GA. Excess of GA (r 20 mM and therefore seriously reduced the stability of ISCs-stabilized CLA emulsions. The superiority of ISCs in stabilizing polyunsaturated fatty acids is due to the cooperative adsorption of protein and polysaccharide at the emulsion interface, providing strong steric and electrostatic effects against droplet aggregation and coalescence and thus excellent physical stability. The improved oxidative stability should arise from the free radical scavenging ability of the protein at the emulsion interface, reducing lipid oxidation.

Structural characterization and properties of konjac glucomannan and zein blend films

A series of konjac glucomannan (KGM)/zein blend films were successfully prepared with zein in proportions 0-30%. The hydrophobicity of blend films were significantly stronger than pure KGM film, indicated by increased contact angle, swelling and solubility properties, and moisture absorption. Moreover, other properties including mechanical, thermal, water vapor and oxygen barrier were also found to be increased. FTIR indicated that hydrogen bond interactions and Maillard reaction occurred between KGM and zein molecules, and microstructural observations indicated that the aggregated zein was homogeneously dispersed in the KGM continuous matrix. However, these zein aggregations were larger with increased proportion of zein, leading to weakened molecular interactions with zein proportion >10%. A mixing ratio of KGM:zein=9:1 was suggested to provide best film properties. This research offers an alternative improvement for KGM-based biodegradable films.

Hydration-induced crystalline transformation of starch polymer under ambient conditions

With synchrotron small/wide-angle X-ray scattering (SAXS/WAXS), we revealed that post-harvest hydration at ambient conditions can further alter the starch crystalline structure. The hydration process induced the alignment of starch helices into crystalline lamellae, irrespective of the starch type (A- or B-). In this process, non-crystalline helices were probably packed with water molecules to form new crystal units, thereby enhancing the overall concentration of starch crystallinity. In particular, a fraction of the monoclinic crystal units of the A-type starches encapsulated water molecules during hydration, leading to the outward movement of starch helices. Such movement resulted in the transformation of monoclinic units into hexagonal units, which was associated with the B-type crystallites. Hence, the hydration under ambient conditions could enhance the B-polymorphic features for both A-type and B-type starches. The new knowledge obtained here may guide the design of biopolymer-based liquid crystal materials with controlled lattice regularity and demanded features.