Y. Hemar

Recent advances in the characterisation of heat-induced aggregates and intermediates of whey proteins

Abstract This article reviews the latest developments in understanding the mechanisms of heat-induced aggregation of whey proteins, in particular β-lactoglobulin. The influences of the reaction conditions (pH, concentration, temperature, ionic strength) on protein aggregation are briefly described. The aggregation process can be interpreted using reaction schemes consisting of several, complex intermediate states. The aggregates and intermediates are held together via disulphide bridges and non-covalent interactions, mainly hydrophobically driven associations.

Viscosity, microstructure and phase behavior of aqueous mixtures of commercial milk protein products and xanthan gum

Abstract The behavior of commercial milk protein/xanthan mixtures was studied at neutral pH. Four milk protein ingredients; skim milk powder, milk protein concentrate, sodium caseinate and whey protein isolate were considered. For the xanthan concentrations used, up to 1wt%, the viscosity of the mixtures was dominated by the viscosity of xanthan. Mixtures of xanthan with skim milk powder or milk protein concentrate showed phase separation, as seen by confocal micrographs, and phase diagrams have been established for these two systems. No visible phase separation was observed in the case of mixtures of sodium caseinate or whey protein isolate systems. However, mixtures of sodium caseinate and xanthan, under certain conditions, showed formation of ‘thread-like’ xanthan-rich regions by confocal microscopy. We believe that the phase separation occurring in milk protein concentrate/xanthan or skim milk powder/xanthan mixtures was a result of depletion flocculation of casein micelles by the xanthan macromolecules, but thermodynamic incompatibility was likely to occur in sodium caseinate/xanthan mixtures.

Influence of xanthan gum on the formation and stability of sodium caseinate oil-in-water emulsions

Abstract Studies have been made of the changes in droplet sizes, surface coverage and creaming stability of emulsions formed with 30% (w/w) soya oil, and aqueous solution containing 1 or 3% (w/w) sodium caseinate and varying concentrations of xanthan gum. Addition of xanthan prior to homogenization had no significant effect on average emulsion droplet size and surface protein concentration in all emulsions studied. However, addition of low levels of xanthan (≤0.2xa0wt%) caused flocculation of droplets that resulted in a large decrease in creaming stability and visual phase separation. At higher xanthan concentrations, the creaming stability improved, apparently due to the formation of network of flocculated droplets. It was found that emulsions formed with 3% sodium caseinate in the absence of xanthan showed extensive flocculation that resulted in very low creaming stability. The presence of xanthan in these emulsions increased the creaming stability, although the emulsion droplets were still flocculated. It appears that creaming stability of emulsions made with mixtures of sodium caseinate and xanthan was more closely related to the structure and rheology of the emulsion itself rather than to the rheology of the aqueous phase.

Process-induced changes in whey proteins during the manufacture of whey protein concentrates

Abstract Samples of cheese whey or acid casein whey obtained from commercial plants during the different stages of whey protein concentrate (WPC) manufacture were analyzed for protein composition and the extent of protein aggregation. Protein composition was determined by size exclusion chromatography (SEC) and electrophoresis, while the degree of protein aggregation was determined through the changes in molecular weight distributions monitored by SEC combined with multi-angle laser light scattering. The results show that there were some differences between the compositions of protein between the two types of wheys with the acid casein whey also containing lower proportions of “soluble” aggregates than the cheese whey. Ultrafiltration/diafiltration processes caused a removal of the non-protein, low molecular weight components, but had no significant effect on protein composition and aggregation. Evaporation and subsequent spray drying of the ultrafiltered retentates caused no significant changes in any of the whey protein components. Differences in functionality of WPC products are probably more related to modifications of proteins and other components caused by the processes that are used in cheese or casein manufacture rather than to the WPC manufacturing process.

Interfacial compositions, microstuctures and properties of oil-in-water emulsions formed with mixtures of milk proteins and κ-carrageenan: 1. Sodium caseinate

Abstract Studies have been carried out on the changes in droplet size, surface coverage and composition, and stability of emulsions formed with 30% (w/w) soya oil, and aqueous solution containing 0.5 and 3.0% sodium caseinate and varying concentration of κ-carrageenan. In all emulsions, except those made using 3.0% sodium caseinate, addition of κ-carrageenan up to 0.4% (w/w), before homogenisation, slightly increased the average droplet size, but had no significant effect on protein surface coverage and composition. Addition of low levels of κ-carrageenan (

Small and large deformation rheology and microstructure of κ-carrageenan gels containing commercial milk protein products

Abstract The rheological behaviour of commercial milk protein/ κ -carrageenan mixtures in aqueous solutions was studied at neutral pH. Four milk protein ingredients; skim milk powder, milk protein concentrate, sodium caseinate, and whey protein isolate were considered. As seen by confocal laser microscopy, mixtures of κ -carrageenan with skim milk powder, milk protein concentrate, and sodium caseinate showed phase separation, but no phase separation was observed in mixtures containing whey protein isolate. For κ -carrageenan concentrations up to 0.5xa0wt%, the viscosity of the mixtures at low shear rates increased markedly in the case of skim milk powder and milk protein concentrate addition, but did not change by the addition of sodium caseinate or whey protein isolate. For κ -carrageenan concentrations from 1 to 2.5xa0wt%, small and large deformation rheological measurements, performed on the milk protein/ κ -carrageenan gels, showed that skim milk powder, milk protein concentrate or sodium caseinate markedly improved the strength of the resulting gels, but whey protein isolate had no effect on the gel stength.

High definition mapping of circular and longitudinal motility in the terminal ileum of the brushtail possum Trichosurus vulpecula with watery and viscous perfusates

Longitudinal and radial movements during spontaneous contractions of isolated segments of terminal ileum of the brushtail possum, a species of arboreal folivore, were studied using high definition spatiotemporal maps. Segments obtained from specimens were continuously perfused with solutions of various apparent viscosities at 3xa0cm and 5xa0cm hydrostatic pressure. A series of sustained tetrodotoxin-sensitive peristaltic events occurred during perfusion. The leading edge of each peristaltic event progressed by a succession of rhythmic surges of circular contraction with concerted concurrent phasic longitudinal contractions. Three types of peristaltic event were observed, with differing durations of occlusion and patterns of cyclic, in phase, circular and longitudinal contractions. Each peristaltic event was preceded by a change of shade on the D map that indicated circumferential dilatation. Differences in the slopes of these phasic shade changes from those occurring during peristalsis indicate that this distension is passive and likely results from aboral displacement of fluid. Tetradotoxin insensitive longitudinal contraction waves of frequency 9.2xa0min−1 occurred during and in the absence of peristalsis, originating at a variety of sites, and propagating either in an orad or aborad direction but predominantly in the latter. Perfusion with 1% guar gum, at 5xa0cm hydrostatic pressure caused the lumen to become distended and the generation of peristaltic events to cease pending reduction of the hydrostatic head to 3xa0cm but longitudinal contractile activity was preserved. Neither the frequencies nor the rates of progression of circular and longitudinal contractile events, nor the temporal coordination between these events, varied with the apparent viscosity of the perfusate or altered in a manner that could facilitate mixing.

Interfacial compositions, microstructure and stability of oil-in-water emulsions formed with mixtures of milk proteins and κ-carrageenan: 2. Whey protein isolate (WPI)

Abstract The changes in droplet size, surface coverage and composition, and stability of emulsions formed with 30% (w/w) soya oil and aqueous solutions containing 0.5 or 3.0% whey protein isolate (WPI) and various concentrations of κ-carrageenan were examined. At neutral pH, addition of κ-carrageenan up to 0.4% (w/w), before homogenization, slightly increased the average droplet size, but had no significant effect on protein surface coverage and composition. Addition of low levels of κ-carrageenan (approximately 0.025% in the aqueous phase) caused extensive flocculation of oil droplets through depletion interactions, which resulted in very low creaming stability. At higher κ-carrageenan concentrations, the creaming stability increased, apparently due to the formation of a network of flocculated droplets. All emulsions formed without κ-carrageenan showed Newtonian behaviour, but upon addition of κ-carrageenan, the emulsions became pseudoplastic. When the pH of the emulsions was lowered to pH≤6, the apparent viscosity of the emulsions, particularly at low shear rates increased, leading to a marked increase in stability to creaming. This effect was attributed to the bridging of the oil droplets through electrostatic interactions between WPI and κ-carrageenan at the interface of the oil droplets, forming a droplet network.

High-definition spatiotemporal mapping of contractile activity in the isolated proximal colon of the rabbit

Four types of contractile activity were identified and characterised in the isolated triple haustrated proximal colon of the rabbit using high-definition spatiotemporal mapping techniques. Mass peristalses were hexamethonium-sensitive deep circular contractions with associated taenial longitudinal contractile activity that occurred irregularly and propagated rapidly aborad, preceded by a zone of local lumen distension. They were sufficiently sustained for each event to occupy the length of the isolated colonic segment and the contraction persisted longer orally than aborally, the difference being more pronounced when lumen contents were viscous. Haustra were bounded by deep even-spaced ring contractions that progressed slowly aborad (haustral progression). Haustral formation and progression were hexamethonium-sensitive and coordinated across intertaenial domains. Ripples were hexamethonium-resistant phasic circular contractions that propagated predominantly orad at varying rates. In the presence of haustra, they were uncoordinated across intertaenial domains but were more coordinated when haustra were absent. Fast phasic contractions were relatively shallow hexamethonium-resistant contractions that propagated rapidly in a predominantly aborad direction. Fast phasic circular contractions were accompanied by taenial longitudinal muscle contractions which increased in amplitude prior to a mass peristaltic event and following the administration of hexamethonium. On the basis of the concurrence and interaction of these contractile activities, we hypothesise that dual pacemakers are present with fast phasic contractions being modulated by the interstitial cells of Cajal in the Auerbach’s plexus (ICC-MY) while ripples are due to the submucosal ICC (ICC-SM). Further, that ICC-SM mediate the enteric motor neurons that generate haustral progression, while the intramuscular ICC (ICC-IM) mediate mass peristalsis. The orad movement of watery fluid was possibly due to ripples in the absence of haustra.

Periodic fluid extrusion and models of digesta mixing in the intestine of a herbivore, the common brushtail possum (Trichosurus vulpecula)

The digesta in four gut compartments (proximal and distal halves of small intestine, caecum, and proximal colon) of a wild hindgut fermenting herbivore, the common brushtail possum (Trichosurus vulpecula), were investigated by rheometry and permeametry. Digesta from all compartments were highly viscous and exhibited shear-thinning. Apparent viscosity was positively related to dry matter content, and increased from proximal small intestine to colon. Dynamic rheological measurements showed that in small intestinal digesta the elastic modulus was greater than the viscous modulus and their ratios were characteristic of weak gels, indicating that digesta could sustain compression. The apparent viscosity of distal small intestinal digesta was markedly lower when measured by capillary viscometry than by rotatory viscometry, indicating that plug flow was likely to be facilitated by lubrication from a peripheral layer of less viscous fluid; i.e., there was an augmented plug flow. Permeametry showed that fluid was extruded from all digesta on compression at physiological pressures, that there was significant permeability of proximal and distal small intestinal digesta, but that digesta became progressively compacted during permeation, with a concomitant reduction in permeability as dry matter content increased. It is proposed that conditions within the small intestine differ from those of an ideal plug flow reactor as radial mixing and turbulence cannot occur. Instead, we suggest that segmentation and peristalsis aid radial mixing of the fluid phase by compressing the solid phase, with extrusion of fluid through the digesta plug. This extrusion may be followed by resorption of fluid back into the plug when the elasticity of the solid phase of digesta is Hookean, thus aiding the mixing of secreted enzymes with insoluble substrates within the plug.