Microencapsulation of oils using whey protein/gum arabic coacervates

Microencapsulating sunflower oil, lemon and orange oil flavour was investigated using complex coacervation of whey protein/gum arabic (WP/GA). At pH 3.0-4.5, WP and GA formed electrostatic complexes that could be successfully used for microencapsulation purposes. The formation of a smooth biopolymer shell around the oil droplets was achieved at a specific pH (close to 4.0) and the payload of oil (i.e. amount of oil in the capsule) was higher than 80%. Small droplets were easier to encapsulate within a coacervate matrix than large ones, which were present in a typical shell/core structure. The stability of the emulsion made of oil droplets covered with coacervates was strongly pH-dependent. At pH 4.0, the creaming rate of the emulsion was much higher than at other pH values. This phenomenon was investigated by carrying out zeta potential measurements on the mixtures. It seemed that, at this specific pH, the zeta potential was close to zero, highlighting the presence of neutral coacervate at the oil/water interface. The influence of pH on the capsule formation was in accordance with previous results on coacervation of whey proteins and gum arabic, i.e. WP/GA coacervates were formed in the same pH window with and without oil and the pH where the encapsulation seemed to be optimum corresponded to the pH at which the coacervate was the most viscous. Finally, to illustrate the applicability of these new coacervates, the release of flavoured capsules incorporated within Gouda cheese showed that large capsules gave stronger release and the covalently cross-linked capsules showed the lowest release, probably because of a tough dense biopolymer wall which was difficult to break by chewing.

Stability of Casein Micelles Cross-Linked by Transglutaminase

In this study, caseins micelles were internally cross-linked using the enzyme transglutaminase (TGase). The integrity of the micelles was examined on solubilization of micellar calcium phosphate (MCP) or on disruption of hydrophobic interactions and breakage of hydrogen bonds. The level of monomeric caseins, determined electrophoretically, decreased with increasing time of incubation with TGase at 30 degrees C; after incubation for 24 h, no monomeric beta- or kappa-caseins were detected, whereas only a small level of monomeric alphaS1-casein remained, suggesting near complete intramicellar cross-linking. The ability of casein micelles to maintain structural integrity on disruption of hydrophobic interactions (using urea, sodium dodecyl sulfate, or heating in the presence of ethanol), solubilization of MCP (using the calcium-chelating agent trisodium citrate) or high-pressure treatment was estimated by measurement of the L*-value of milk; i.e., the amount of back-scattered light. The amount of light scattered by casein micelles in noncross-linked milk was reduced by >95% on complete disruption of hydrophobic interactions or complete solubilization of MCP; treatment of milk with TGase increased the stability of casein micelles against disruption by all methods studied and stability increased progressively with incubation time. After 24 h of cross-linking, reductions in the extent of light scattering were still apparent in the presence of high levels of dissociating agents, possibly through citrate-induced removal of MCP nanoclusters from the micelles, or urea- or sodium dodecyl sulfate-induced increases in solvent refractive index, which reduce the extent of light-scattering.

Structural and mechanical study of a self-assembling protein nanotube

We report a structural characterization of self-assembling nanostructures. Using atomic force microscopy (AFM), we discovered that partially hydrolyzed alpha-lactalbumin organizes in a 10-start helix forming tubes with diameters of only 21 nm. We probed the mechanical strength of these nanotubes by locally indenting them with an AFM tip. To extract the material properties of the nanotubes, we modeled the experiment using finite element methods. Our study shows that artificial helical protein self-assembly can yield very stable, strong structures that can function either as a model system for artificial self-assembly or as a nanostructure with potential for practical applications.

Growth and structure of α-lactalbumin nanotubes

Partial hydrolysis of α-lactalbumin by a protease fromBacillus licheniformisunder appropriate conditions leads to the formation of nanotubular structures. The growth of α-lactalbumin nanotubes and their dimensions were analysed using transmission electron microscopy, static light scattering and small-angle X-ray scattering. The scattering data were fitted using a model describing the growth of the tubes and using the form factor of a hollow tube. The cylinder diameter was calculated to be 19.9 (2) nm and the cavity diameter 8.7 (7) nm. The elongation rate of the nanotubes was about 10 nm min−1under the experimental conditions.

Composition and structure of whey protein/gum arabic coacervates

Complex coacervation in whey protein/gum arabic (WP/GA) mixtures was studied as a function of three main key parameters: pH, initial protein to polysaccharide mixing ratio (Pr:Ps)(ini), and ionic strength. Previous studies had already revealed under which conditions a coacervate phase was obtained. This study is aimed at understanding how these parameters influence the phase separation kinetics, the coacervate composition, and the internal coacervate structure. At a defined (Pr:Ps)(ini), an optimum pH of complex coacervation was found (pH(opt)), at which the strength of electrostatic interaction was maximum. For (Pr:Ps)(ini) = 2:1, the phase separation occurred the fastest and the final coacervate volume was the largest at pH(opt) = 4.0. The composition of the coacervate phase was determined after 48 h of phase separation and revealed that, at pH(opt), the coacervate phase was the most concentrated. Varying the (Pr:Ps)(ini) shifted the pH(opt) to higher values when (Pr:Ps)(ini) was increased and to lower values when (Pr:Ps)(ini) was decreased. This phenomenon was due to the level of charge compensation of the WP/GA complexes. Finally, the structure of the coacervate phase was studied with small-angle X-ray scattering (SAXS). SAXS data confirmed that at pH(opt) the coacervate phase was dense and structured. Model calculations revealed that the structure factor of WP induced a peak at Q = 0.7 nm(-1), illustrating that the coacervate phase was more structured, inducing the stronger correlation length of WP molecules. When the pH was changed to more acidic values, the correlation peak faded away, due to a more open structure of the coacervate. A shoulder in the scattering pattern of the coacervates was visible at small Q. This peak was attributed to the presence of residual charges on the GA. The peak intensity was reduced when the strength of interaction was increased, highlighting a greater charge compensation of the polyelectrolyte. Finally, increasing the ionic strength led to a less concentrated, a more heterogeneous, and a less structured coacervate phase, induced by the screening of the electrostatic interactions.

Impaired rennetability of heated milk; study of enzymatic hydrolysis and gelation kinetics

Casein micelles in milk are stable colloidal particles with a stabilizing hairy brush of kappa-casein. During cheese production rennet cleaves kappa-casein into casein macropeptide and para-kappa-casein, thereby destabilizing the casein micelle and resulting in aggregation and gel formation of the micelles. Heat treatment of milk causes impaired clotting properties, which makes heated milk unsuitable for cheese production. In this paper we compared five different techniques, often described in the literature, for their suitability to quantify the enzymatic hydrolysis of kappa-casein. It was found that the technique is crucial for the yield of casein macropeptide and this yield then affects the calculated enzymatic inhibition caused by heat treatment, ranging from 5 to 30%. The technique, which we found to be the most reliable, demonstrates that heat-induced calcium phosphate precipitation does not affect the enzymatic cleavage, while whey protein denaturation causes a very slight reduction of enzyme activity. By using diffusing wave spectroscopy, a very sensitive technique to monitor gelation processes, we demonstrated that heat-induced calcium phosphate precipitation does not affect the clotting. Whey protein denaturation does not affect the start of flocculation but has a clear effect on the clotting process. This work adds to a better understanding of the processes causing the impaired clotting properties of heated milk.

Gelation mechanism of milk as influenced by temperature and pH; studied by the use of transglutaminase cross-linked casein micelles

Casein micelles in milk are colloidal particles consisting of four different caseins and calcium phosphate, each of which can be exchanged with the serum phase. The distribution of caseins and calcium between the serum and micellar phase is pH and temperature dependent. Furthermore, upon acidification casein micelles lose their colloidal stability and start to aggregate and gel. In this paper, we studied two methods of acid-induced gelation, i.e., 1) acidification of milk at temperatures of 20 to 50 degrees C and 2) decreasing the pH at 20 degrees C to just above the gelation pH and subsequently inducing gelation by increasing the temperature. These two routes are called T-pH and pH-T, respectively. The gelation kinetics and the properties of the final gels obtained are affected by the gelation route applied. The pH-T milks gel at higher pH and lower temperature and the gels formed are stronger and show less susceptibility to syneresis. By using intramicellar cross-linked casein micelles, in which release of serum caseins is prevented, we demonstrated that unheated milk serum caseins play a key role in gelation kinetics and characteristics of the final gels formed. This mechanism is presented in a model and is relevant for optimizing and controlling industrial processes in the dairy industry, such as pasteurization of acidified milk products.

Depletion-induced phase separation in colloid-polymer mixtures

Phase separation can be induced in a colloidal dispersion by adding non-adsorbing polymers. Depletion of polymer around the colloidal particles induces an effective attraction, leading to demixing at sufficient polymer concentration. This communication reviews theoretical and experimental work carried out on the polymer-mediated attraction between spherical colloids and the resulting phase separation of the polymer-colloid mixture. Theoretical studies have mainly focused on the limits where polymers are small or large as compared to the colloidal size. Recently, however, theories are being developed that cover a wider colloid-polymer size ratio range. In practical systems, size polydispersity and polyelectrolytes (instead of neutral polymers) and/or charges on the colloidal surfaces play a role in polymer-colloid mixtures. The limited amount of theoretical work performed on this is also discussed. Finally, an overview is given on experimental investigations with respect to phase behavior and results obtained with techniques enabling measurement of the depletion-induced interaction potential, the structure factor, the depletion layer thickness and the interfacial tension between the demixed phases of a colloid-polymer mixture.

Complex coacervation of whey proteins and gum arabic

Mixtures of gum arabic and whey protein (whey protein isolate, WP) form an electrostatic complex in a specific pH range. Three phase boundaries (pH(c), pHphi(1), pHphi(2)) have been determined using an original titration method, newly applied to complex coacervation. It consists of monitoring the turbidity and light scattering intensity under slow acidification in situ with glucono-delta-lactone. Furthermore, the particle size could also be measured in parallel by dynamic light scattering. When the pH is lowered, whey proteins and gum arabic first form soluble complexes. This boundary is designated as pH(c). When the interaction is stronger (at lower pH), phase separation takes place (at pHphi(1)). Finally, at pHphi(2) complexation was suppressed by the charge reduction of the gum arabic. The major constituent of the whey protein preparation used was beta-lactoglobulin (beta-lg), and it was shown that beta-lg was indeed the main complex-forming protein. Moreover, an increase of the ionic strength shifted the pH boundaries to lower pH values, which was summarized in a state diagram. The experimental pH(c) values were compared to a newly developed theory for polyelectrolyte adsorption on heterogeneous surfaces. Finally, the influence of the total biopolymer concentration (0-20% w/w) was represented in a phase diagram. For concentrations below 12%, the results are consistent with the theory on complex coacervation developed by Overbeek and Voorn. However, for concentrations above 12%, phase diagrams surprisingly revealed a "metastable" region delimited by a percolation line. Overall, a strong similarity is seen between the behavior of this system and a colloidal gas-liquid phase separation.

Association behavior of beta-casein

The association behavior of beta-casein, a protein with a distinct amphipathic character, was studied. beta-Casein exhibits markedly temperature-dependent association behavior; at low temperatures (<10-15 degrees C), monomers predominate, but as the temperature is increased, monomers associate, via hydrophobic bonding, into micelles. beta-Casein micelles have a hydrodynamic radius of approximately 12 nm, a radius of gyration of approximately 8.3 nm, and an interaction radius of approximately 15 nm. These data are fully consistent with a previous fluffy particle. The association behavior of beta-casein is also strongly affected by concentration and solvent quality. At low concentrations beta-casein exhibits a critical micelle concentration (CMC) of approximately 0.05%, w/v, at 40 degrees C. In the presence of 6 M urea the temperature dependence of beta-casein's association behavior is eliminated, leaving monomers predominantly. Temperature-dependent transformations in micelle morphology can be explained by changes in solvent quality, i.e., the temperature-protein hydrophobicity and temperature-voluminosity profiles of beta-casein. The results obtained are consistent with the shell model as developed by Kegeles, in which a distribution of micelle sizes is formed. They contrast with the traditional description of the micellization of beta-casein by a two-state model or by the closed-association model, i.e., monomers if micelles.

Electrosorption of pectin onto casein micelles

Pectin, a polysaccharide derived from plant cells of fruit, is commonly used as stabilizer in acidified milk drinks. To gain a better understanding of the way that pectin stabilizes these drinks, we studied the adsorption and layer thickness of pectin on casein micelles in skim milk dispersions. Dynamic light scattering was used to measure the layer thickness of adsorbed pectin onto casein micelles in situ during acidification. The results indicate that the adsorption of pectin onto casein micelles is multilayered and takes place at and below pH 5.0. Renneting, i.e., cleaving-off kappa-casein from the casein micelles, did not alter the adsorption pH. It did, however, show that pectin arrests the rennet-induced flocculation of casein micelles below pH 5.0. From the findings we concluded the attachment of pectin onto casein micelles is driven by electrosorption. Adsorption measurements confirmed the multilayered nature of the adsorption of pectin onto casein micelles. Both the adsorbed amount and the layer thickness increased with decreasing pH in the relevant range 3.5-5.0. The phase behavior of a casein micelles/pectin mixture was determined and could be explained in terms of thermodynamic incompatibility being relevant above pH 5.0 and adsorption, leading to either stabilization and bridging, being relevant below pH 5.0. The results confirm that electrosorption is the driving force for the adsorption of pectin onto casein micelles.

Viscoelastic properties of an exocellular polysaccharide produced by a Lactococcus lactis

The viscoelastic properties of a well-characterized exocellular polysaccharide (EPS) produced by the lactic acid bacterium Lactococcus lactis subsp. cremoris strain B40 were investigated. Dynamic rheological measurements were made as a function of frequency and EPS concentration. The bead-spring model of Rouse could reasonably describe the dynamic properties. Concentrated EPS solutions have a significant elasticity (G' > G") at high frequencies. The relatively high G' values at high concentrations and high frequencies are indicative of significant normal stress differences, and we put forward a hypothesis that suggests that these normal stresses may explain the contribution of EPSs to the ropy behavior of yogurts.

Formation of disulfide bonds in acid-induced gels of preheated whey protein isolate

Cold gelation of whey proteins is a two-step process. First, protein aggregates are prepared by a heat treatment of a solution of native proteins in the absence of salt. Second, after cooling of the solution, gelation is induced by lowering the pH at ambient temperature. To demonstrate the additional formation of disulfide bonds during this second step, gelation of whey protein aggregates with and without a thiol-blocking treatment was studied. Modification of reactive thiols on the surface of the aggregates was carried out after the heat-treatment step. To exclude specific effects of the agent itself, different thiol-blocking agents were used. Dynamic light scattering and SDS-agarose gel electrophoresis were used to show that the size of the aggregates was not changed by this modification. The kinetics of gelation as determined by the development of pH and turbidity within the first 8 h of acidification were not affected by blocking thiol groups. During gelation, formation of large, covalently linked, aggregates occurred only in the case of unblocked WPI aggregates, which demonstrates that additional disulfide bonds were formed. Results of permeability and confocal scanning laser microscope measurements did not reveal any differences in the microstructure of networks prepared from treated or untreated whey protein aggregates. However, gel hardness was decreased 10-fold in gels prepared from blocked aggregates. Mixing different amounts of blocked and unblocked aggregates allowed gel hardness to be controlled. It is proposed that the initial microstructure of the gels is primarily determined by the acid-induced noncovalent interactions. The additional covalent disulfide bonds formed during gelation are involved in stabilizing the network and increase gel strength.

Depletion interaction of casein micelles and an exocellular polysaccharide

Casein micelles become mutually attractive when an exocellular polysaccharide produced by Lactococcus lactis subsp. cremoris NIZO B40 (hereafter called EPS) is added to skim milk. The attraction can be explained as a depletion interaction between the casein micelles induced by the nonadsorbing EPS. We used three scattering techniques (small-angle neutron scattering, turbidity measurements, and dynamic light scattering) to measure the attraction. In order to connect the theory of depletion interaction with experiment, we calculated structure factors of hard spheres interacting by a depletion pair potential. Theoretical predictions and all the experiments showed that casein micelles became more attractive upon increasing the EPS concentration.

Isolation and physical characterization of an exocellular polysaccharide

The physical properties of a polysaccharide produced by the lactic acid bacterium Lactococcus lactis subsp. cremoris strain NIZO B40 were investigated. Separation of the polysaccharide from most low molar mass compounds in the culture broth was performed by filtration processes. Residual proteins and peptides were removed by washing with a mixture of formic acid, ethanol, and water. Gel permeation chromatography (GPC) was used to size fractionate the polysaccharide. Fractions were analyzed by multiangle static light scattering in aqueous 0.10 M NaNO3 solutions from which a number- (Mn) and weight-averaged (Mw) molar mass of (1.47 +/- 0.06).10(3) and (1.62 +/- 0.07).10(3) kg/mol, respectively, were calculated so that Mw/Mn approximately 1.13. The number-averaged radius of gyration was found to be 86 +/- 2 nm. From dynamic light scattering an apparent z-averaged diffusion coefficient was obtained. Upon correcting for the contributions from intramolecular modes by extrapolating to zero wave vector a hydrodynamic radius of 86 +/- 4 nm was calculated. Theoretical models for random coil polymers show that this z-averaged hydrodynamic radius is consistent with the z-averaged radius of gyration, 97 +/- 3 nm, as found with GPC.

Structural and functional analysis of negatively charged milk proteins with anti-HIV activity

Several polyanionic reagents such as dextran sulfates, heparin sulfates, and negatively charged proteins have been reported to exhibit anti-HIV activity in vitro. Particularly potent inhibition has been reported for the milk protein beta-lactoglobulin (betaLG) on modification by 3-hydroxyphthalic anhydride (3HP). The introduction of multiple negatively charged carboxyl groups along the polypeptide backbone obviously leads to repulsion within the protein molecule and this is likely to affect the specific tertiary, and perhaps also secondary, structure of the protein. We used several biophysical techniques to probe the structural changes that occur on 3HP modification of betaLG. The results suggest that the protein becomes largely unstructured on chemical modification. Although a profound anti-HIV activity was measured for 3HP-betaLG, similar antiviral effects were observed with two other 3HP-modified milk proteins, alpha-lactalbumin and alpha(S2)-casein, but not with the unmodified proteins. Most potent inhibition of HIV-1 replication was obtained with 3HP-modified alpha-lactalbumin, which also demonstrated the least cytotoxicity. These combined results indicate that HIV inhibition is a general property of negatively charged polypeptides and do not support a model in which the negatively charged 3HP-betaLG protein interacts in a structure-specific manner with the CD4 cell surface receptor for HIV-1 entry.

Kappa-casein micelles: structure, interaction and gelling studied by small-angle neutron scattering

Small-angle neutron scattering (SANS) measurements on dilute and concentrated dispersions of kappa-casein micelles in a buffer at pH = 6.7 were made using the D11 diffractometer in Grenoble. Results indicate that the micelles have a dense core with a fluffy outer layer. This outer layer appears to give rise to a steeply repulsive interaction on contact. In fact, the hard-sphere model best fits the measured scattering intensities. Adding chymosin to the dispersion initiated a fractal flocculation of the micelles and consecutively a coalescence of the micelles. This unexpected second process resembled that of spinodal demixing. The dispersion phase thus separates into a water and a protein phase on a time scale of hours. The observed phenomona contribute to the understanding of the cheese-making process.

The internal structure of layered colloidal particles determined with SANS

Small-angle neutron scattering measurements at various contrasts were performed on dilute colloidal silica dispersions in order to resolve the internal structure of the particles. The particles were prepared by outgrowth of a commercially available silica core, followed by an esterification of the surface silanol groups with stearyl alcohol. Variation of the scattering contrast was achieved by using mixtures of 1H12- and 2H12-cyclohexane as dispersing medium. The scattering data were interpreted as originating from a polydisperse system of spherical three-layer particles. Model parameters were optimized sequentially, applying an increasing number of constraints. Good agreement between experimental and calculated scattering curves is obtained, even at higher K values and low contrasts. In comparison with other particles in the same size range, these silica particles represent a very suitable model system for small-angle scattering experiments at higher volume fractions.