Effect of pH and ionic strength on competitive protein adsorption to air/water interfaces in aqueous foams made with mixed milk proteins

Quantitative analysis of competitive milk protein adsorption to air/water interfaces in aqueous foam was performed by capillary electrophoresis (CE). Foams were made by whipping protein solutions, in which skim milk powder (SMP) and whey protein isolate (WPI) were mixed at 0.5% protein in different proportions at different pH values and NaCl concentrations. Preferential adsorption of beta-casein into foam phases occurred under most solution conditions, if partial dissociation of the casein micelles had occurred. Preferential adsorption of beta-casein was not observed with added Ca2+, due to the re-association of casein micelles. Enrichment of caseins into the foam phase was more apparent than that of whey proteins. The foamability of SMP demonstrated a continuous improvement due to the gradually increasing dissociation of casein micelles when the concentration of NaCl increased from 0 to 0.8 M. The foamability of WPI increased when NaCl concentration rose from 0 to 0.1 M, and decreased with further increase in NaCl concentration. NaCl at low concentration (I < or = 0.4) did not show a significant effect on the competitive adsorption among milk proteins, indicating that electrostatic interactions do not play a key role in competitive adsorption. NaCl at higher concentration, e.g., 0.6 M, caused less whey protein to be adsorbed to the air/water interfaces. The whippability of WPI was highest at pH 4.5 and lowest at pH 3, and that of SMP was the opposite. The proportions of beta-lactoglobulin and alpha-lactalbumin in the foam phase were lower at acidic pH and higher at basic pH, compared with that at natural pH of WPI.     

Physico-chemical characteristics of oil-in-water emulsions based on whey protein-phospholipid mixtures

Emulsions prepared with whey proteins, phospholipids and 10% of vegetable oil were used for a model typifying dressings, coffee whitener and balanced diets. For the present study, two whey proteins (partial heat-denatured whey protein concentrate (WPC) and undenatured whey protein isolate (WPI)) in combination with different phospholipids (hydrolysed and unmodified deoiled lecithin) were chosen to investigate the interactions between proteins, phospholipids and salt (sodium chloride) in such emulsion systems. Oil-in-water (o/w) emulsions (10 wt.% sunflower oil) containing various concentrations of commercial whey proteins (1-2%), phospholipids (0.39-0.78%) and salt (0.5-1.5%) were prepared using a laboratory high pressure homogeniser under various preparation conditions. Each emulsion was characterised by droplet size, creaming rate, flow behaviour and protein load. The dynamic surface activity of the whey proteins and lecithins at the oil-water interface was determined using the drop volume method. The properties of emulsions were significantly influenced by the content of whey protein. Higher protein levels improved the emulsion behaviour (smaller oil droplets and increased stability) independent of the protein or lecithin samples used. An increase of the protein content resulted in a lower tendency for oil droplet aggregation of emulsions with WPC to occur and emulsions tending towards a Newtonian flow behaviour. The emulsification temperature was especially important using the partial heat-denatured WPC in combination with the deoiled lecithin. A higher emulsification temperature (60 degrees C) promoted oil droplet aggregation, as well as an increased emulsion consistency. Emulsions with the WPC were significantly influenced by the NaCl content, as well as the protein-salt ratio. Increasing the NaCl content led to an increase of the droplet size, higher oil droplet aggregation, as well as to a higher creaming rate of the emulsions. An increase of the lecithin content from 0.39 to 0.78% in the emulsion system resulted in a small reduction of the single droplet size. This effect was more pronounced when using the hydrolysed lecithins.     

Temperature and pH-Sensitive Polymers with Hydrophobic Spacers for the Controlled Delivery of Drugs

Summary: Acid methacrylates containing hydrophobic aliphatic and aromatic spacers were used to prepare pH-sensitive ampholytic hydrogels and bidimensional temperature- (T) and pH-sensitive hydrogels. Their swelling behaviour was studied by changing the pH and temperature of buffer solutions. Salicylamide, salicylic acid and green fluorescent protein (GFP) as model drugs were loaded into the gels and their release kinetics studied under simulated gastric and intestinal conditions. T- and pH-sensitive hydrogels containing aliphatic spacers show sustained release of analgesics depending on pH (e.g. 7.4); while longer aliphatic spacers resulted in drug release depending on pH and temperature (T < transition T). GFP was released from temperature- and pH-sensitive ampholytic hydrogels after different lag times depending on hydrogel composition.     

Electrospinning of composite nanofibers based on chitosan,poly(ethylene oxide), and chitin nanofibrils

Composite chitosan nanofibers containing 20 wt % chitin nanofibrils and 10 wt % PEO are obtained via the electrospinning method. Additions of 0.5–20.0 wt % chitin nanofibrils into chitosan solutions with concentrations of 3–7 wt % in acetic acid (70 vol %) insignificantly increase the electrical conductivity, surface-tension coefficient, and viscosity of these mixed solutions. Decreases in the viscosities of chitosan solutions containing chitin nanofibrils with increases in shear rate provide evidence for the structuring of solutions and the orientation of chitosan macromolecules and chitin nanofibrils in the shear flow. The effects of shear stress and a high-voltage electric field on chitosan solutions containing chitin nanofibrils and PEO result in a decrease in the imperfection of composite nanofibers. The introduction of chitin nanofibrils allows the content of PEO in the composite nanofibers to be reduced.     

Effects of poly(ethylene oxide) and pH on the electrospinning of whey protein isolate

Poly(ethylene oxide) (PEO) is known for facilitating the electrospinning of biopolymer solutions, which are otherwise not electrospinnable. The objective of this study was to improve the understanding of the positive effects of PEO on the electrospinning of whey protein isolate (WPI) solutions under different pH conditions. Alterations in protein secondary structure and polymer solution properties (viscosity, conductivity, and dynamic surface tension), as induced by pH changes, significantly affected the electrospinning behavior of WPI/PEO (10% w/w: 0.4% w/w PEO) solutions. Acidic solutions resulted in smooth fibers (707 ± 105 nm) while neutral solutions produced spheres (2.0 ± 1.0 μm) linked with ultrafine fibers (138 ± 32 nm). In comparison, alkaline solutions produced fibers (191 ± 36 nm) that were embedded with spindle‐like beads (1.0 ± 0.5 μm). ¹³C NMR and FTIR spectroscopies showed that the increase in random coil and α‐helix secondary structures in WPI were the main contributors to the formation of bead‐less electrospun fibers. The electrospinning‐enabling properties of PEO on aqueous WPI solutions were attributed to physical chain entanglement between the two polymers, rather than specific polymer–polymer interactions. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Preparation, characterization, swelling and in vitro drug release behaviour of poly[N-acryloylglycine-chitosan] interpolymeric pH and thermally-responsive hydrogels

Novel biodegradable pH- and thermal-responsive interpenetrating polymer network (IPN) hydrogels were prepared for controlled drug delivery studies. The IPN hydrogels were obtained in mild aqueous acid media by irradiation of solutions of N-acryloylglycine (NAGly) mixed with chitosan, in the presence of glutaraldehyde as a crosslinking agent and using 2,2-dimethoxy-2-phenyl acetophenone as photoinitiator. These hydrogels were subjected to equilibrium swelling studies at different temperatures (25 °C, 37 °C and 45 °C) in buffer solutions of pH 2.1 and 7.4 (similar to that of gastric and intestinal fluids respectively). 5-Fluorouracil (5-FU) was entrapped in the hydrogels, and drug release studies carried out at 37 °C in buffer solutions at pH 2.1 and 7.4.     

Emulsifying properties of whey protein–dextran conjugates at low pH and different salt concentrations

The emulsifying behaviour of glyco-protein complexes of the non-ionic polysaccharide dextran (500 kDa) with whey protein isolate (WPI) have been investigated in systems containing (20 vol.% oil phase) medium-chain triglyceride oil, silicone oil, orange oil, and n-tetradecane under acidic and high electrolyte concentrations. Covalent coupling of protein to polysaccharide is achieved by dry heat treatment of protein+polysaccharide mixtures. Emulsions were made with WPI and whey protein isolate-dextran (WPI-DX) conjugate, and stability was followed by determining changes in average droplet size and extent of serum separation with time, with gum arabic (GA) chosen as reference emulsifier. The results show that the WPI-DX conjugate gives much better stability than the whey protein alone or GA under similar conditions. The improved emulsifying properties of WPI on complexing with dextran is probably due to the enhanced steric stabilization provided by the bulky hydrophilic polysaccharide moiety.     

RADIATION INDUCED ACRYLAMIDE/CITRIC ACID HYDROGELS AND THEIR SWELLING BEHAVIORS

Citric acid (CAc) moieties containing acrylamide (AAm) hydrogels were prepared by gamma irradiation of their aqueous solutions. A possible polymerization and crosslinking mechanism for acrylamide/citric acid (AAm/CAc) hydrogels is proposed. The effects of irradiation dose and citric acid content on swelling behavior were investigated. Swelling took place in water at 25°C and was followed gravimetrically. Incorporation of a relatively low amount of citric acid to acrylamide hydrogel increased its swelling up to 950% from 700%. The diffusion of water into AAm/CAc hydrogels was found to be a non-Fickian type. Diffusion coefficients of AAm/CAc hydrogels found as 5 × 10^?7? 10 × 10^?7 cm² sec^?1. It has also been found that the number average molar mass between crosslinks is increased with the CAc content and decreased with irradiation dose.     

Structural and topographical characteristics of adsorbed WPI and monoglyceride mixed monolayers at the air-water interface

In this work we have analyzed the structural and topographical characteristics of mixed monolayers formed by an adsorbed whey protein isolate (WPI) and a spread monoglyceride monolayer (monopalmitin or monoolein) on the previously adsorbed protein film. Measurements of the surface pressure (pi)-area (A) isotherm were obtained at 20 degrees C and at pH 7 for protein-adsorbed films from water in a Wilhelmy-type film balance. Since the surface concentration (1/A) is actually unknown for the adsorbed monolayer, the values were derived by assuming that the A values for adsorbed and spread monolayers were equal at the collapse point of the mixed film. The pi-A isotherm deduced for adsorbed WPI monolayer in this work is practically the same as that obtained directly by spreading. For WPI-monoglyceride mixed films, the pi-A isotherms for adsorbed and spread monolayers at pi higher than the equilibrium surface pressure of WPI are practically coincident, a phenomenon which may be attributed to the protein displacement by the monoglyceride from the interface. At lower surface pressures, WPI and monoglyceride coexist at the interface and the adsorbed and spread pi-A isotherms (i.e., the monolayer structure of the mixed films) are different. Monopalmitin has a higher capacity than monoolein for the displacement of protein from the air-water interface. However, some degree of interactions exists between proteins and monoglycerides and these interactions are higher for adsorbed than for spread films. The topography of the monolayer corroborates these conclusions.     

Effect of Gel Structure on the In Vitro Gastrointestinal Digestion Behaviour of Whey Protein Emulsion Gels and the Bioaccessibility of Capsaicinoids

This study investigated the effect of gel structure on the digestion of heat-set whey protein emulsion gels containing capsaicinoids (CAP), including the bioaccessibility of CAP. Upon heat treatment at 90 °C, whey protein emulsion gels containing CAP (10 wt% whey protein isolate, 20 wt% soybean oil, 0.02 wt% CAP) with different structures and gel mechanical strengths were formed by varying ionic strength. The hard gel (i.e., oil droplet size d_4,3 ~ 0.5 μm, 200 mM NaCl), with compact particulate gel structure, led to slower disintegration of the gel particles and slower hydrolysis of the whey proteins during gastric digestion compared with the soft gel (i.e., d_4,3 ~ 0.5 μm, 10 mM NaCl). The oil droplets started to coalesce after 60 min of gastric digestion in the soft gel, whereas minor oil droplet coalescence was observed for the hard gel at the end of the gastric digestion. In general, during intestinal digestion, the gastric digesta from the hard gel was disintegrated more slowly than that from the soft gel. A power-law fit between the bioaccessibility of CAP (Y) and the extent of lipid digestion (X) was established: Y = 49.2 × (X − 305.3)^0.104, with R² = 0.84. A greater extent of lipid digestion would lead to greater release of CAP from the food matrix; also, more lipolytic products would be produced and would participate in micelle formation, which would help to solubilize the released CAP and therefore result in their higher bioaccessibility.     

Structure and physiochemical characteristics of whey protein isolate conjugated with xylose through Maillard reaction at different degrees

The purpose of this study is to prepare Maillard reaction products (MRPs) with both good emulsification and antioxidant activity, and the relationship between the Maillard reaction degree and structural changes of whey protein isolate (WPI) conjugated with xylose through wet-heating was explored. Browning intensity didn’t change while the free amino groups reduced significantly at the initial stage of Maillard reaction (MR). Amino acid analysis indicated that the lysine and arginine reduced significantly. The antioxidant property of the MRPs was significantly improved. The best emulsifying properties could be obtained in the middle degree of MR. The sodium dodecyl sulphate-polyacrylamide gel electrophoresis analysis illustrated that WPI and xylose formed high molecular weight conjugates. The circular dichroism spectra suggested that α-helix and random coil were increased while the β-sheet and β-turns were decreased after the MR. All of the MRPs exhibited a marked red shift in the maximum fluorescence intensity, indicating that the hydrophilicity of MRPs was enhanced.     

Electrostatic effects on the yield stress of whey protein isolate foams

The mechanisms responsible for foam structure are of practical interest within the food industry. The yield stress (tau) of whey protein isolate (WPI) foams as affected by electrostatic forces was investigated by whipping 10% (w/v) protein solutions prepared over a range of pH levels and salt concentrations. Measurements of foam overrun and model WPI interfaces, i.e. adsorption kinetics as determined via dynamic surface tension and dilatational rheological characterization, aided data interpretation. Interfacial measurements were also made with the primary whey proteins, beta-lactoglobulin (beta-lg) and alpha-lactalbumin (alpha-la). Yield stress of WPI foams was dependent on pH, salt type and salt concentration. In the absence of salt, tau was highest at pH 5.0 and lowest at pH 3.0. The addition of NaCl and CaCl2 up to 400 mM significantly increased tau at pH 7.0 but not at pH 3.0. Furthermore, at pH 7.0, equivalent molar concentrations of CaCl2 as compared to NaCl increased tau to greater extents. Salts had minimal effects on tau at pH 5.0. Comparisons with interfacial rheological data suggested the protein's capacity to contribute towards tau was related to the protein's potential at forming strong, elastic interfaces throughout the structure. The dynamic surface tension data for beta-lg and alpha-la were similar to WPI, while the interfacial rheological data displayed several noticeable differences.     

Surface shear rheology of WPI-monoglyceride mixed films spread at the air-water interface

Surface shear viscosity of food emulsifiers may contribute appreciably to the long-term stability of food dispersions (emulsions and foams). In this work we have analyzed the structural, topographical, and shear characteristics of a whey protein isolate (WPI) and monoglyceride (monopalmitin and monoolein) mixed films spread on the air-water interface at pH 7 and at 20 degrees C. The surface shear viscosity (etas) depend on the surface pressure and on the composition of the mixed film. The surface shear viscosity varies greatly with the surface pressure. In general, the greater the surface pressure, the greater are the values of etas. The values of etas for the mixed WPI-monoolein monolayer were more than one order of magnitude lower than those for a WPI-monopalmitin mixed film, especially at the higher surface pressures. At higher surface pressures, collapsed WPI residues may be displaced from the interface by monoglyceride molecules with important repercussions on the shear characteristics of the mixed films. A shear-induced change in the topography and a segregation between domains of the film forming components were also observed. The displacement of the WPI by the monoglycerides is facilitates under shear conditions, especially for WPI-monoolein mixed films.     

Competitive adsorption between egg yolk lipoproteins and whey proteins on oil-in-water interfaces

Adsorption characteristics of mixtures of egg yolk lipoproteins and whey protein isolate (WPI) were studied in emulsions (20% oil, v/v 0.5% protein, w/v pH 7.0) made with pure triolein or n-tetradecane. Emulsions stabilized by granule lipoproteins (GLP) or low-density lipoproteins (LDL) had smaller particle sizes than emulsions stabilized by WPI. In protein mixtures containing egg yolk lipoproteins and WPI, there was a decrease in particle size with increased concentration of the yolk lipoproteins. The reduction in particle size of emulsions was greater when WPI was mixed with LDL than with GLP, for both n-tetradecane and triolein. Emulsions made with triolein had smaller particle sizes than those made with n-tetradecane, irrespective of the type or ratio of lipoproteins used. Therefore, the protein concentration per unit area of the interface was greater for emulsions containing n-tetradecane than for triolein. In displacement experiments, emulsions made with only WPI were mixed with 0.1 and 0.5% GLP or LDL for a given period of time and the relative concentrations of β-lactoglobulin and -lactalbumin determined. Displacement of β-lactoglobulin by LDL increased with time and was greater in emulsions made with n-tetradecane than with triolein. However, displacement of β-lactoglobulin by GLP was greater in emulsions made with triolein than with n-tetradecane. -lactalbumin was completely displaced from the interface within 1 min of addition of either 0.5% GLP or LDL, whereas addition of 0.1% GLP or LDL resulted only in a partial displacement. The results suggest that egg yolk lipoproteins are more surface active than WPI and that LDL penetrates the n-tetradecane–water interface more than GLP, while GLP penetrates the triolein–water interface more than LDL.     

Adsorption of Uranyl Ions into Poly(Acrylamide‐co‐Acrylic Acid) Hydrogels Prepared by Gamma Irradiation

Acrylamide (AAm)/Acrylic Acid (AAc) copolymers have been prepared by gamma irradiation of binary mixtures at three different compositions where the acrylamide/acrylic acid mole ratios varied around 15, 20, and 30%. Threshold dose for 100% conversion of monomers into hydrogels was found to be 8.0 kGy. Poly(Acrylamide‐co‐Acrylic Acid) (poly(AAm‐co‐AAc)) hydrogels have been considered for the removal of uranyl ions from aqueous solutions. Swelling behavior of these hydrogels was determined in distilled water at different pH values and in aqueous solutions of uranyl ions. The results of swelling tests at pH 8.0 indicated that poly(AAm‐co‐AAc) hydrogel, containing 15% acrylamide showed maximum % swelling. Diffusion of water and aqueous solutions of uranyl ion into hydrogels was found to be non‐Fickian in character and their diffusion coefficients were calculated. The effect of pH, composition of hydrogel, and concentration of uranyl ions on the adsorption process were studied at room temperature. It was found that one gram of dry poly(AAm‐co‐AAc) hydrogel adsorbed 70–320 mg and 70–400 mg uranyl ions from aqueous solutions of uranyl nitrate and uranyl acetate in the initial concentration range of 50–1500 mg UO₂ ²⁺L^?, depending on the amount of AAc in the hydrogels, respectively. Adsorption isotherms were constructed for poly(AAm‐co‐AAc)–uranyl ion system indicating an S type of adsorption in the Giles classification system. It is concluded that crosslinked poly(AAm‐co‐AAc) hydrogels can be successfully used for the removal of uranyl ions from their aqueous solutions.     

Formation of concentrated biopolymer particles composed of oppositely charged WPI and pectin for food applications

The application of protein–polysaccharide complexes as potential structure modifier, fat replacer, or emulsifying agents in food dispersions has gained increasing interest amongst scientists and manufacturers. Based on associative complexation, low biopolymer concentrations are typically used to generate particulated complexes. The current study, however, presents results that focused on the formation of concentrated biopolymer dispersions. A simple heat treatment was applied to tailor the overall water content of the biopolymer dispersion. For that purpose, whey protein isolate (WPI) and citrus pectin (DE 71%) solutions were mixed at different pH and biopolymer ratios to induce complex coacervation and subsequently heat-treated (??=?90–95°C). Phase separation behavior, microstructural, rheological, and electrical properties of the complexes were investigated by surface charge, turbidity, particle size, rheometry, and light microscopy measurements. Results revealed that complexation was induced under acidic conditions, whereas high WPI:citrus pectin ratios led to positive surface charges, promoting the formation of large and dense particles. In addition, concentrated complex dispersions with water contents ≥80% could be manufactured and easily re-dispersed, whereas complexes maintained their particulate structures. Results are of importance for future studies where we intend to incorporate concentrated biopolymer particles as structuring agents in complex food matrices.     

Synthesis and Properties of IPN Hydrogels Based on Konjac Glucomannan and Poly（acrylic acid）

As a random copolymer of β-(1,4) linked D-mannose and D-glucose, konjac gluco- mannan (KGM) is a naturally occurring water-soluble polysaccharide, and has been paid attention in the field of drug controlled release carriers potentially used in colon1,2. …     

Stereocomplex Mediated Gelation of PEG-(PLA)2 and PEG-(PLA)8 Block Copolymers

Stereocomplex mediated hydrogels have been prepared by mixing solutions of polymers of opposite chirality of either PEG-(PLA)₂ triblock copolymers or PEG-(PLA)₈ star block copolymers. The critical gel concentrations of the mixed enantiomer solutions were considerably lower compared to polymer solutions containing only the single enantiomer. Moreover, gel-sol transition temperatures were increased and gel regions were expanded due to stereocomplexation. Rheology measurements showed that stereocomplexed hydrogels based on PEG-(PLA)₈ have higher storage moduli compared to those based on PEG-(PLA)₂. Stereocomplexed hydrogels prepared from 13 wt% PEG-(PLA)₂ solutions in PBS showed a storage modulus of 0.9 kPa at 37 °C, while at similar conditions stereocomplexed hydrogels of PEG-(PLA)₈ showed a storage modulus of 1.9 kPa at 10 wt%.     

One-step hydrolysis for the preparation of carboxylated cellulose nanofibrils with highly stable dispersibility from pomelo peel

Cellulose - One-step acid hydrolysis was successfully employed to prepare carboxylated cellulose nanofibrils (CCNFs) from pomelo peel waste with the mixture of hydrochloric acid and citric acid...     

Development of quality evaluation method for cellulose fibrils

Cellulose - Highly fibrillated cellulose, including cellulose micro or nanofibrils (CMFs or CNFs), has been extensively investigated. However, its morphological properties defined as the degree of...