Rheology of whey protein–dextran conjugate films at the air/water interface

Impact of the Maillard reaction on the antioxidant capacity of bovine lactoferrin

Studies raise the notion that the Maillard reaction (MR) may be harnessed to modify the antioxidant capacity of alimentary proteins. However, little is known about the impact of MR on bioactive proteins. Glucose and fructose were used as model moieties reacting with lactoferrin (LF). UV absorbance and SDS-PAGE analyses were used to monitor MR progression during 36 h of mild thermal processing (60 °C, 79% RH). FTIR and CD did not reveal changes in LF structure; However, dynamic light scattering showed MR increased mean particle sizes and sample turbidity at 3<pH<10. DPPH and FRAP antioxidant assays showed marked increases in antioxidant capacity of Maillard conjugates as a function of reaction time (12 and 36 h), protein:monosaccharide mole ratio (1:1 or 1:3) and moiety type, compared to unprocessed LF. Overall, a link between conjugates' antioxidant capacity and processing parameters is described to enable future attempts to enhance LF functionality in foods containing carbohydrates.

Interfacial rheology: an overview of measuring techniques and its role in dispersions and electrospinning

Interfacial rheological properties have yet to be thoroughly explored. Only recently, methods have been introduced that provide sufficient sensitivity to reliably determine viscoelastic interfacial properties. In general, interfacial rheology describes the relationship between the deformation of an interface and the stresses exerted on it. Due to the variety in deformations of the interfacial layer (shear and expansions or compressions), the field of interfacial rheology is divided into the subcategories of shear and dilatational rheology. While shear rheology is primarily linked to the long-term stability of dispersions, dilatational rheology provides information regarding short-term stability. Interfacial rheological characteristics become relevant in systems with large interfacial areas, such as emulsions and foams, and in processes that lead to a large increase in the interfacial area, such as electrospinning of nanofibers.

Effect of the ionic strength of pulsed electric field treatment medium on the physicochemical and structural characteristics of lactoferrin

Pulsed electric field (PEF) treatment (35 kV cm(-1) for 19.2 μs using bipolar 2 μs pulses) was conducted on bovine lactoferrin (LF; 0.4 mg mL(-1)) prepared in simulated milk ultrafiltrate (SMUF), at concentrations between 0.2× and 2× normal strength, with electrical conductivities ranging from 0.17 to 1.04 S m(-1). The physicochemical and structural characteristics (LF content by a spectrophotometric and an ELISA method, surface hydrophobicity, electrophoretic mobility, far-UV circular dichroism spectra, and tryptophan fluorescence) of LF dissolved in SMUF of all strengths tested were not changed after PEF treatment. The PEF treatment of LF in 0.2 strength SMUF did not cause the release of LF-bound ferric ion into the aqueous phase, with a concentration of LF-bound iron being the same as that of the untreated LF control (174 μg L(-1)). However, in treatment media with higher ionic strengths, ferric ion was released from the LF molecule into the aqueous phase. The concentration of LF-bound iron decreased from 174 μg L(-1) for the LF treated in 0.2 strength SMUF to 80 μg L(-1) for that treated in double-strength SMUF. The results suggest that the PEF-induced iron depletion of LF does not appear to cause an appreciable conformational change in LF molecules. PEF treatment could be developed as a novel physical way to produce iron-depleted LF, as an alternative to the existing chemical method.

Physicochemical and emulsifying properties of whey protein isolate (WPI)-dextran conjugates produced in aqueous solution

The physicochemical and emulsifying properties of protein and polysaccharide conjugates prepared under mild conditions were investigated. The covalently linked conjugates of whey protein isolate (WPI) and dextran (DX, 440 kDa) were produced by incubating aqueous solutions containing 10% WPI and 30% DX at pH 6.5 and 60 degrees C for 48 h. After purification by anion-exchange chromatography and affinity chromatography, the conjugate had a weight-average molecular weight (M(w)) of 531 kDa and a radius of gyration (R(g)) of 30 nm as determined by size exclusion chromatography-multiangle laser light scattering (SEC-MALLS); the molar binding ratio of WPI to DX was calculated to be approximately 1:1. The purified conjugate had significantly improved heat stability when subjected to 80 degrees C for 30 min and remained soluble over a range of pH from 3.2 to 7.5 and ionic strengths from 0.05 to 0.2 M in contrast to native WPI. The emulsifying ability and emulsion stability made with WPI-DX conjugate were also improved compared to WPI and gum arabic (an emulsifier containing naturally derived glycoproteins).

Use of FTIR and mass spectrometry for characterization of glycated caseins

This study investigates the potential of Fourier transform infrared spectroscopy (FTIR) to monitor glycation-induced changes in protein structure. Aqueous solutions of sodium caseinate and glucose (1:2 w/w, pH 6·7) were heated at 90°C for 0, 10, 20, 40 and 60 min. Evidence for caseinate glycation was obtained by mass spectrometry techniques (electrospray (ESI) and matrix-assisted laser desorption ionisation (MALDI)). FTIR was able to discriminate between glycated and non-glycated sodium caseinate, when the data were analysed by multivariate statistical methods; principal component analysis (PCA) and soft independent modelling of class analogy (SIMCA). The techniques used were complementary and provided different levels of information about the glycated samples.

Interfacial shear rheology of protein-surfactant layers

The shear rheology of adsorbed or spread layers at air/liquid and liquid/liquid phase boundaries is relevant in a wide range of technical applications such as mass transfer, monolayers, foaming, emulsification, oil recovery, or high speed coating. Interfacial shear rheological properties can provide important information about interactions and molecular structure in the interfacial layer. A variety of measuring techniques have been proposed in the literature to measure interfacial shear rheological properties and have been applied to pure protein or mixed protein adsorption layers at air/water or oil/water interfaces. Such systems play for example an important role as stabilizers in foams and emulsions. The aim of this contribution is to give a literature overview of interfacial shear rheological studies of pure protein and protein/surfactant mixtures at liquid interfaces measured with different techniques. Techniques which utilize the damping of waves, spectroscopic or AFM techniques and all micro-rheological techniques will not discuss here.

Implications of interfacial characteristics of food foaming agents in foam formulations

The manufacture of food dispersions (emulsions and foams) with specific quality attributes depends on the selection of the most appropriate raw materials and processing conditions. These dispersions being thermodynamically unstable require the use of emulsifiers (proteins, lipids, phospholipids, surfactants etc.). Emulsifiers typically coexist in the interfacial layer with specific functions in the processing and properties of the final product. The optimum use of emulsifiers depends on our knowledge of their interfacial physico-chemical characteristics - such as surface activity, amount adsorbed, structure, thickness, topography, ability to desorb (stability), lateral mobility, interactions between adsorbed molecules, ability to change conformation, interfacial rheological properties, etc. -, the kinetics of film formation and other associated physico-chemical properties at fluid interfaces. These monolayers constitute well defined systems for the analysis of food colloids at the micro- and nano-scale level, with several advantages for fundamental studies. In the present review we are concerned with the analysis of physico-chemical properties of emulsifier films at fluid interfaces in relation to foaming. Information about the above properties would be very helpful in the prediction of optimised formulations for food foams. We concluded that at surface pressures lower than that of monolayer saturation the foaming capacity is low, or even zero. A close relationship was observed between foaming capacity and the rate of diffusion of the foaming agent to the air-water interface. However, the foam stability correlates with the properties of the film at long-term adsorption.

Molecular modeling directed by an interfacial test apparatus for the evaluation of protein and polymer ingredient function in situ

A simplified apparatus is described that measures the damping of a suspended measuring device. The movement of the device (bob) is damped by the properties of the air-water surface adsorbed material. Its value lies in describing the surface chemomechanical properties of ingredients and excipients used in food, nutraceutical, cosmetic (cosmeceutical), and natural drug-food product formulations that traverse the food sciences. Two surfactants, two food and drug-grade polymers, and five naturally occurring food and serum proteins were tested and used to estimate and model interfacial viscoelasticity. Equilibration times of >15 min were found to give sufficiently stable interfaces for routine assessment. The viscoelasticity of the air-water interface was estimated with reference to model solutions. These model solutions and associated self-assembled interfacial nanostructured adsorbed layers were fabricated using a preliminary screening process with the aid of a specialized foaming apparatus ( C(300) values), surface tension measurements (23-73 mN/m), and referential surface shear and dilation experiments. The viscoelasticity measured as a percentage of surface damping ( D) of a pendulum was found to range from 1.0 to 22.4% across the samples tested, and this represented interfacial viscosities in the range of 0-4630 microNs/m. The technique can distinguish between interfacial compositions and positions itself as an easily accessible valuable addition to tensiometric and analytical biochemistry-based techniques.

Surface hydrophobicity of physicochemically and enzymatically treated milk proteins in relation to techno-functional properties

Surface hydrophobicity (SH) of milk proteins treated physicochemically (by heating and Maillard reaction) or modified enzymatically (by transglutaminase, lactoperoxidase, laccase, and glucose oxidase) was assessed in relation to their techno-functional properties. Heat-treatment increased SH of whey protein isolate and decreased SH of sodium caseinate and bovine serum albumin. Maillard reaction of milk proteins caused time-depended decreases of SH. Only for total milk protein reacting with glucose and lactose elevated SH-values were detected. Protein modification with transglutaminase, laccase, and lactoperoxidase strongly increased the SH of whey protein isolate and total milk protein. Incubation with glucose oxidase elevated SH values of sodium caseinate, whey protein isolate, and total milk protein. When correlating SH with techno-functional properties, a positive correlation was observed between SH and foam formation, and a negative correlation was observed between SH and foam stability as well as emulsion stability. No clear correlation was detected between SH and emulsifying activity, surface tension, viscosity, and heat stability of enzymatically and physicochemically treated milk proteins.

The emulsion flocculation stability of protein–carbohydrate diblock copolymers

The effect of the steric layer thickness on the flocculation stability of beta-lactoglobulin-carbohydrate diblock copolymers was assessed. The diblock copolymers were created by conjugating beta-lactoglobulin to maltose or a series of different M(n) maltodextrins using the Maillard reaction. The thickness and spatial arrangement of the interfacial layers were assessed via latex adsorption and selective enzymatic digestion studies. An increase in the molecular weight of the maltodextrin (900, 1900 and 3800 Da) increased the interfacial thickness (1.1, 2.5 and 7.3 nm, respectively). No detectable change to interfacial thickness was observed upon the attachment of maltose. The increase in the interfacial layer thickness scaled with the hydrodynamic size of the carbohydrate. The beta-lactoglobulin-maltodextrin conjugates were found to have a diblock architecture, with the protein anchored at the surface and the carbohydrate protruding into the aqueous continuous phase. The stability of oil-in-water emulsions formed using the conjugates was assessed by exposing them to salt (150 mM NaCl or 0-20 mM CaCl(2)), heat alone or heat in the presence of 150 mM NaCl. Conjugation of a 900 Da maltodextrin provided sufficient steric stabilization to prevent flocculation in high salt environments. The effect of the (number) density of the steric layer was also assessed by controlling the average number of maltodextrins attached per beta-lactoglobulin molecule. The steric layer density at which emulsions became unstable was a function of carbohydrate M(n). Emulsions made from the 900 Da maltodextrin conjugate became unstable below a steric layer density of one tail per 7.5 nm(2), whilst emulsions made from the 1900 Da maltodextrin were unstable below a steric layer density of one tail per 9.5 nm(2). This trend was expected and can be explained by the stronger van der Waals attraction that arises from the closer interdroplet separations that are permissible with the shorter maltodextrins. The excellent flocculation stability of Maillard conjugate emulsions is thought to arise from the combined effects of weak electrostatic repulsion from the screened protein surface charge and steric repulsion from the attached carbohydrate layer. This means that attachment of a relatively thin steric layer is enough to stabilize the emulsions against flocculation. These findings have important implications for the development of commercial processes to manufacture protein-carbohydrate Maillard conjugate emulsifiers. Furthermore the work provides a greater empirical understanding of the relationship between interfacial architecture and colloidal stability, and may provide the means for greater theoretical understanding of biopolymer stabilization of interfaces.