Effect of temperature and pH on the properties of skim milk gels made from a tamarillo (Cyphomandra betacea) coagulant and rennet.

Reconstituted skim milk was gelled with a crude protease extract from tamarillo [Cyphomandra betacea or Solanum betacea (syn.)] fruit and compared with gels prepared with calf rennet. The effects of temperature and pH on the gelation of skim milk were investigated by small deformation oscillatory rheology. The tamarillo extract-induced gels had a faster rate of increase in the elastic modulus (G') at the early stage of gelation than rennet-induced milk gels. This was probably due to the broader proteolytic activity of tamarillo protease extracts as shown by sodium dodecyl sulfate-PAGE analysis. Confocal microscopy also showed that the milk gels resulting from the addition of tamarillo extracts had larger voids than rennet-induced milk gels. The proteolytic activity of tamarillo extracts was found to be optimal at pH 11. For both rennet and tamarillo extracts, the aggregation time was similar between pH 6.7 and 6.5, but the aggregation time of rennet-induced milk gels was lower than that of milk gels obtained by the addition of tamarillo extracts at pH lower than 6.5. An increase in temperature was found to have a significant effect on aggregation time, particularly at 20°C, where rennet did not coagulate milk in 3 h but the tamarillo extracts coagulated milk within 2 h. The results of this study suggest that extracts from tamarillo fruit could be used for milk gelation, particularly under lower temperature or high pH conditions.

Changes in the viscoelastic behaviour of the rumenal digesta mat of sheep fed on pasture or chaffed lucerne hay.

We examined the changes in the physical properties of the digesta mat over a period of 24 h after cessation of feeding, in sheep that had been maintained on pasture or fed chaffed lucerne hay. The dry matter content of the digesta mat declined at similar rates in both dietary groups, although it was generally higher in sheep fed lucerne. Median particle size declined in the digesta mats of both dietary groups at similar rates in samples taken after 8 h, but median particle size was significantly greater in sheep fed chopped lucerne hay than in those fed grass. Thus, particles were not reduced to a common size suggesting that factors in addition to particle size governed the rate of breakdown of the rumen mat. The relationship between the elastic and loss moduli was of a consistent pattern in all samples taken from the rumen mat indicating that it behaved as a weak gel. The elastic and loss moduli of the digesta mats of sheep that had been fed pasture or chopped lucerne hay converged to similar values after 12 h and declined broadly at similar rates after this. The relationship between these two moduli and the dry matter content of the rumen mat were of similar curvilinear form for sheep on both diets. These findings suggest that the rate of breakdown of the rumen mat is more likely to be governed by its composite behaviour than by the size of the constituent particles.

Direct measurements of interfacial interactions between pectin and kappa-casein and implications for the stabilisation of calcium-free casein micelle mimics.

Using Surface Plasmon Resonance (SPR) it has been shown that the fine structure of the anionic polysaccharide pectin strongly influences its interfacial interaction with a kappa-casein layer coated onto a gold surface (via a dextran linker) in the pH range 3.5-6.8, with the highest SPR signal being observed for pectin with the lowest charge density tested (a degree of methylesterification (DM) around 90%). Furthermore, the Brownian motions of kappa-casein coated polystyrene beads (used to provide calcium-free 'model casein micelles') were studied in pectin solutions using Diffusing Wave Spectroscopy (DWS) and microscopy, and were compared with measurements made on naked beads. At every pH value studied (with the exception of 3.5), bridging of the protein-covered probe particles was observed for pectins of both DM 28 and DM 78. However, no aggregated complexes were found in these model casein micelle systems when pectin of an unusually high DM was used (90%). It was hypothesised that having a limited number of binding regions of spatially limited extent maximises the number of chains binding to the protein layer (as found with the SPR measurement), encourages the formation of loops and trains, and additionally limits the potential for destabilisation via bridging.

Bio-inspired network optimization in soft materials--insights from the plant cell wall.

The dynamic-mechanical responses of ionotropic gels made from the biopolymer pectin have recently been investigated by microrheological experiments and found to exhibit behaviour indicative of semi-flexible polymer networks. In this work we investigate the gelling behaviour of pectin systems in which an enzyme (pectinmethylesterase, PME) is used to liberate ion-binding sites on initially inert polymers, while in the presence of ions. This is in contrast to the previous work, where it was the release of ions (rather than ion-binding groups) that was controlled and the polymers had pre-existing cross-linkable moieties. In stark contrast to the semi-flexible network paradigm of biological gels and the previous work on pectin, the gels studied herein exhibit the properties of chemically cross-linked networks of flexible polymers.

Microrheological studies reveal semiflexible networks in gels of a ubiquitous cell wall polysaccharide.

Microrheological measurements have been carried out on ionotropic gels made from an important cell wall polysaccharide, using diffusing wave spectroscopy and multiple particle tracking. These gels were formed by the interaction of calcium ions with negatively charged groups on the polymer backbone, which is a copolymer of charged and uncharged sugars, galacturonic acid, and its methylesterified analog, respectively. The results suggest that semiflexible networks are formed in these systems, with a low frequency, frequency independent storage modulus (G'>G''), and a high frequency scaling of both G' and G'' with omega3/4. The differences observed between gels obtained using polysaccharide samples with different amounts and patterns of the charged ion-binding groups could comfortably be accommodated within this theoretical framework, assuming that the elementary semiflexible elements of the network are filaments consisting of two polymer chains bridged with calcium. In particular, a sample that was engineered to possess a blockwise intramolecular distribution of calcium chelating moieties clearly exhibited the high frequency scaling of both moduli with omega3/4 across some three orders of magnitude, and the concentration dependences of the elastic modulus, at both high and low frequency, were found to follow power laws with predicted exponents. Furthermore, quantitative agreement of the moduli with theory was found for realistic estimates of the molecular parameters, suggesting that the physics of semiflexible networks is not only exploited by protein components of the cytoskeleton but also by polysaccharides in plant cell walls.

Characterization of flow and mixing regimes within the ileum of the brushtail possum using residence time distribution analysis with simultaneous spatio-temporal mapping.

We studied the flow and mixing regimes in isolated segments of the terminal ileum of brushtail possums during spontaneous circumferential and longitudinal contractions under conditions that allowed backflow and compared them with those of inactive segments. Residence time distributions (RTDs) were determined by perfusion with two probes of different rheological properties to which an inert dye marker was added. Ileal segment volume and oscillatory flow during the period of RTD determination were derived from spatiotemporal maps. High viscosity guar gum solution generated RTDs characteristic of laminar flow in inactive ileal segments which confirmed that no slip was occurring at the mucosal layer. In active segments, motility and consequent oscillatory flow imparted significant additional axial dispersion to the flow patterns of both probes. Mixing occurred episodically during periods when intestinal volume was reduced and onflow was augmented by peristalsis, which may prevent the establishment of steady state conditions. Marker concentration rose more steeply when active ileal segments were being perfused with a probe of similar viscosity to normal digesta than with low viscosity Earle's/Hepes solution, each being subject to similar levels of oscillatory flow. This indicated that a coarser mixing regime prevailed and that absorption of nutrients from viscous digesta would rely to a greater degree on molecular diffusion.

DWS microrheology of a linear polysaccharide.

Diffusing wave spectroscopy has been used to measure the rheological behavior of pullulan (M(w) = 1 x 10(5)) aqueous solutions up to concentration of 40 g/dL. It was found that these solutions were mainly viscous, with the loss modulus G'' higher than the elastic modulus G'. The plot of the specific viscosity eta(sp) as a function of pullulan concentration showed two critical concentrations c = 4 g/dL and c = 15 g/dL. For c < c, eta( sp) approximately c(1.25+/-0.05); for c < c < c, eta( sp) approximately c(2+/-0.05); and for c > c, eta( sp) approximately c(4.5+/-0.5). These results are in very good agreement with those reported in the literature.

Monitoring of flocculation and creaming of sodium-caseinate-stabilized emulsions using diffusing-wave spectroscopy.

Diffusing-wave spectroscopy (DWS) has been used to study the stability of sodium-caseinate-stabilized emulsions. The emulsions underwent creaming as a result of depletion flocculation when excess sodium caseinate was added. The creaming process was monitored over a 3-h period and each autocorrelation function was collected for 2 min to ensure adequate signal-to-noise ratio. The temporal variation of average particle size times the coefficient of viscosity of the continuous phase was derived from the backscattering measurements, and the variation of the scattering mean free path length with time was found from the backscattering and transmission measurements. It was confirmed that the creaming process was delayed at high oil concentrations, presumably due to the formation of oil droplet networks.

Dynamic light scattering investigation of sodium caseinate and xanthan mixtures.

Aqueous solutions of sodium caseinate and xanthan at pH 7 and containing 0.1 M NaCl, and their mixtures were investigated using dynamic light scattering. Sodium caseinate solutions showed a bimodal distribution of relaxation rates; with the aggregate peak distribution predominating. Xanthan solutions showed a single distribution at low concentrations (< or =0.06 wt.%) and a bimodal distribution at higher concentrations. The sodium caseinate-xanthan mixture modes were independent of the total biopolymer concentration, and behaved as a superposition of sodium caseinate solution alone and xanthan solution alone. This indicates that there is no interaction between xanthan and sodium caseinate in the range of concentrations considered in this study.

Electrostatic Interactions in Adsorbed Protein Layers Probed by a Sedimentation Technique.

A simple centrifugation technique has been used to determine the thicknesses of layers of alphaS1-casein and beta-casein adsorbed onto monodisperse polystyrene latex particles. The influence of electrostatic interactions within the layers on their thickness and stabilizing ability is investigated by varying the ionic strength of the suspension or by including calcium ions, known to bind specifically to the caseins. Copyright 1998 Academic Press.