Jun-Ru Qi

Adsorption and dilatational rheology of heat-treated soy protein at the oil-water interface: relationship to structural properties.

We evaluated the influence of heat treatment on interfacial properties (adsorption at the oil-water interface and dilatational rheology of interfacial layers) of soy protein isolate. The related structural properties of protein affecting these interfacial behaviors, including protein unfolding and aggregation, surface hydrophobicity, and the state of sulfhydryl group, were also investigated. The structural and interfacial properties of soy protein depended strongly on heating temperature (90 and 120 °C). Heat treatment at 90 °C induced an increase in surface hydrophobicity due to partial unfolding of protein, accompanied by the formation of aggregates linked by disulfide bond, and lower surface pressure at long-term adsorption and similar dynamic interfacial rheology were observed as compared to native protein. Contrastingly, heat treatment at 120 °C led to a higher surface activity of the protein and rapid development of intermolecular interactions in the adsorbed layer, as evidenced by a faster increase of surface pressure and dilatational modulus. The interfacial behaviors of this heated protein may be mainly associated with more flexible conformation and high free sulfhydryl group, even if some exposed hydrophobic groups are involved in the formation of aggregates. These results would be useful to better understand the structure dependence of protein interfacial behaviors and to expand utilization of heat-treated protein in the formulation and production of emulsions.

Formation of soy protein isolate-dextran conjugates by moderate Maillard reaction in macromolecular crowding conditions.

BACKGROUND Several methods have been reported for the conjugation of proteins with polysaccharides. Protein-polysaccharide conjugates can be formed by traditional dry heating, but this process is not attractive from an industrial viewpoint, and no commercial conjugates have been manufactured in this way. In the present study, in order to develop a more practical reaction method, macromolecular crowding was used to attach polysaccharides to proteins. RESULTS Soy protein isolate-dextran conjugates (SDCs) were prepared via the initial stage of the Maillard reaction in macromolecular crowding conditions. The impact of various processing conditions on the formation of SDCs was investigated. The optimal conditions chosen from the experiments were a soy protein isolate/dextran ratio of 1:1 (w/w), a pH of 6.5, a reaction temperature of 60 °C and a reaction time of 30 h. Circular dichroism spectroscopy showed that the secondary and tertiary structures of the conjugates were changed significantly. Structural flexibility increased, allowing better display of their functional characteristics. The conjugates had a composition with various sizes, especially macromolecules, according to gel permeation chromatography. Thermal analysis showed that the thermal stability of the conjugates was improved. CONCLUSION The production of SDCs under macromolecular crowding conditions appears to be an effective and promising technique, representing an advance over classic protein glycosylation methods.

Soy lipophilic protein nanoparticles as a novel delivery vehicle for conjugated linoleic acid

Soy lipophilic protein nanoparticles (LPP), which present a novel delivery vehicle for conjugated linoleic acid (CLA), were fabricated by ultrasonication of the soy lipophilic protein (LP), which exhibits unique characteristics including a high loading capacity, oxidation protection and a sustained releasing profile in vitro for CLA. The CLA-loaded LPP exhibited a mean diameter of 170 ± 0.63 nm and a loading capacity of 26.3 ± 0.40% (w/w). A coating of sodium caseinate (SC) on the surface improved the colloidal stability of the CLA-loaded LPP. This encapsulation conferred protection against the oxidation of CLA, by which the head space-oxygen consumption and hydrogen peroxide value were obviously decreased in comparison with the SC-encapsulated CLA and CLA alone. The delivery system enables a sustained releasing profile of CLA in a simulated gastrointestinal tract (GIT). These findings illustrate that the LPP could act as an effective delivery device for CLA, which could provide oxidation stability and a sustained release property.

In vitro assessment of the bioaccessibility of fatty acids and tocopherol from soybean oil body emulsions stabilized with ι-carrageenan.

The present investigation aimed to expand the knowledge of the in vitro bioaccessibility of fatty acids and tocopherol from natural soybean oil body emulsions stabilized with different concentrations of ι-carrageenan. Several physicochemical parameters including proteolysis of the interfacial layer, interfacial composition, and microstructure were evaluated with regard to their impact on the bioaccessibility of fatty acids and tocopherol. Results from simulated human digestion in vitro indicated that the bioaccessibility of total fatty acids and tocopherol decreased (62.7-8.3 and 59.7-19.4%, respectively) with the increasing concentration of ι-carrageenan. During the in vitro digestion procedure, ι-carrageenan affected physicochemical properties of the emulsions, thereby controlling the release of fatty acids and tocopherol. These results suggested that soybean oil body emulsions stabilized with ι-carrageenan could provide natural emulsions in foods that were digested at a relatively slow rate, the important physiological consequence of which might be increasing satiety.

Physicochemical and structural characterisation of protein isolate, globulin and albumin from soapnut seeds (Sapindus mukorossi Gaertn.)

The amino acid (AA) composition and physicochemical and conformational properties of protein isolate (SNPI), globulin (SNG) and albumin (SNA) fractions from soapnut seeds were evaluated. The essential AA of SNG, SNA and SNPI (except sulfur-containing AA) are sufficient for the FAO/WHO suggested requirements for 2-5year old infants. SNG and SNPI showed similar electrophoresis patterns and AA compositions, the subunit of those proteins consisted of two polypeptides linked by disulfide bonds. In contrast, SNA showed a different AA compositions and SDS-PAGE pattern. Both SNG and SNPI presented a typical U-shape protein solubility (PS)-pH profile, SNA showed a completely different PS-pH profile, especially at pH 2.0-4.0. The near-UV circular dichroism (CD), differential scanning calorimetry (DSC) and tryptophan fluorescence spectra analyses indicated that the flexibility in tertiary conformations decreased in the order: SNA>SNPI>SNG, while soapnut proteins had a similar secondary conformation, with a highly ordered structure (the β-types), as evidenced by far-UV CD spectra.

Fabrication and Characterization of Stable Soy β-Conglycinin–Dextran Core–Shell Nanogels Prepared via a Self-Assembly Approach at the Isoelectric Point

The preparation of soy β-conglycinin-dextran nanogels (∼90 nm) went through two stages, which are safe, facile, and green. First, amphiphilic graft copolymers were formed by dextran covalently attaching to β-conglycinin via Maillard dry-heating reaction. Second, the synthesized conjugates were heated above the denaturation temperature at the isoelectric point (pH4.8) so as to assemble nanogels. The effects of pH, concentration, heating temperature, and time on the fabrication of nanogels were examined. The morphology study displayed that the nanogels exhibited spherical shape with core-shell structures, which was reconfirmed by zeta-potential investigation. Both circular dichroism spectra and surface hydrophobicity analyses indicated that the conformations of β-conglycinin in the core of nanogels were changed, and the latter experiment further revealed that the hydrophobic groups of β-conglycinin were exposed to the surface of protein. The nanogels were stable against various conditions and might be useful to deliver hydrophobic bioactive compounds.

Subcritical Water Induced Complexation of Soy Protein and Rutin: Improved Interfacial Properties and Emulsion Stability.

Rutin is a common dietary flavonoid with important antioxidant and pharmacological activities. However, its application in the food industry is limited mainly because of its poor water solubility. The subcritical water (SW) treatment provides an efficient technique to solubilize and achieve the enrichment of rutin in soy protein isolate (SPI) by inducing their complexation. The physicochemical, interfacial, and emulsifying properties of the complex were investigated and compared to the mixtures. SW treatment had much enhanced rutin-combined capacity of SPI than that of conventional method, ascribing to the well-contacted for higher water solubility of rutin with stronger collision-induced hydrophobic interactions. Compared to the mixtures of rutin with proteins, the complex exhibited an excellent surface activity and improved the physical and oxidative stability of its stabilized emulsions. This improving effect could be attributed to the targeted accumulation of rutin at the oil-water interface accompanied by the adsorption of SPI resulting in the thicker interfacial layer, as evidenced by higher interfacial protein and rutin concentrations. This study provides a novel strategy for the design and enrichment of nanovehicle providing water-insoluble hydrophobic polyphenols for interfacial delivery in food emulsified systems.

The influence of heat treatment on acid-tolerant emulsions prepared from acid soluble soy protein and soy soluble polysaccharide complexes

This research presents a green procedure to prepare oil in water (O/W) emulsion from acid soluble soy protein (ASSP) and soy soluble polysaccharide (SSPS), a long-term stable nanoscale system for delivering the lipophilic components. The emulsion technique involved the preparation complexion using ASSP and SSPS by electrostatic and hydrophobic interactions as well as high pressure homogenization. The average diameter of the droplet of emulsions (fresh and heated) is 263±2nm. Such emulsions resulted in heating stable dispersions containing corn oil at the concentration of 20.0%, even at the pH around the isoelectric points of ASSP. After 90days storage at 4°C, the mean diameter of emulsions after heating at 80°C for 60min is 314±1nm compared with 341±3nm of emulsions unheated. The heat-stability of dispersions were affected by emulsion conditions, so the present research demonstrates the emulsion stability against heat treatment, ionic strength and pH change.

Rheological Properties of Soybean β-Conglycinin in Aqueous Dispersions: Effects of Concentration, Ionic Strength and Thermal Treatment

Steady shear and dynamic oscillatory measurements were used to investigate the effect of concentration, ionic strength and thermal treatment on rheological properties of soybean β-conglycinin in aqueous dispersions. SDS-PAGE and Differential Scanning Calorimetry (DSC) showed that β-conglycinin exhibited partial denaturation and formation of aggregates during isolation. Under steady shear flow, strong shear-thinning behavior was observed with increasing shear rate from 0.001 to 1200 s−1. A dispersion of β-conglycinin (≥5% w/v, without applying thermal treatment) exhibited gel-like dynamic mechanical spectra at 20°C. This suggested that β-conglycinin in aqueous dispersions showed rheological properties of typical weak gel-like (entanglement) or semi diluted polymeric solution. Weak gel network of β-conglycinin was susceptible to ionic strength, suggesting that electrostatic forces play an important role in the formation of weak gel network. These properties of β-conglycinin have practical significance for the food processors in the formulation of new products.

Fractionation and characterization of soy β-conglycinin-dextran conjugates via macromolecular crowding environment and dry heating.

Conjugates of β-conglycinin and dextran were prepared by heating in solution under macromolecular crowding environment and dry-heating methods, and then fractionated by solubility at pH 4.8 and pH 6.5 and characterized. The results showed that the degree of glycation of the conjugates extracted from pH 4.8 were higher than the conjugates extracted from pH 6.5. Corresponding to the higher degree of glycation, it was supposed that the β-conglycinin of groups 4.8 of macromolecular crowding environment was completely surrounded by the dextran molecular while that of groups 6.5 were encircled partially with a lower degree of glycation. Compared to β-conglycinin, groups 4.8 demonstrated a decreasing surface hydrophobicity and sulfhydryl group content while groups 6.5 increased. The secondary structure of β-conglycinin soluble at pH 4.8 after conjugating under macromolecular crowding environment tended to stretch out and the highly ordered structure turn to random structures. The differences between the extraction of pH 4.8 and pH 6.5 conjugated by dry-heating methods were not as remarkable as the difference between the extraction conjugated by macromolecular crowding environment.