The effect of electrostatic interactions on the critical percolation concentration (c(p)) of fibrillar beta-lactoglobulin gels at pH 2 was investigated using rheological measurements, transmission electron microscopy (TEM), and performing conversion experiments. A decreasing c(p) with increasing ionic strength was found. The fraction of nonaggregated beta-lactoglobulin was independent of ionic strength in the regime of 0.01-0.08 M. TEM experiments showed long fibrils (2-7 microm) for ionic strengths between 0.01 and 0.08 M. Since both the conversion of monomers and the contour length of the fibrils were independent of ionic strength (0.01-0.08M), the linear increase of c(p) with the Debye length can be attributed purely to an increase of electrostatic repulsion between the fibrils. This increase is explained in terms of an adjusted random contact model which takes into account the charge and semiflexibility of the fibrils.

Transitions in structure of sodium caseinate stabilized emulsions were studied using conventional rheometry as well as diffusing wave spectroscopy (DWS). Structural differences were induced by different amounts of stabilizer, and transitions in structure were induced by acidification. Special attention was given to the sol-gel transition. In this study the criterion of the sol-gel transition being frequency independent was verified for emulsions using DWS. It was shown that this sol-gel transition did not correspond to the so-called ergodic-nonergodic transition. Differences, as a function of the pH, were found for emulsions containing different amounts of stabilizer. The emulsion droplets in an emulsion without extra stabilizer formed a continuous network upon acidification, while the droplets in emulsions with an excess of stabilizer formed a network of oil droplets at neutral pH. Upon acidification of the latter one, the initial network of oil droplets fell apart, and eventually a network of sodium caseinate, in which the oil droplets were embedded, was formed. This caused the appearance of two sol-gel transitions. The breaking of the initial network as well as the network formation of sodium caseinate in time was observed by DWS.

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In this study, diffusing wave spectroscopy (DWS) is used to investigate the effect of shear on a food-related aggregating emulsion. The principle of the method is validated using a nonaggregating, nearly monodisperse latex suspension. In general, with increasing shear rate the diffusive motion of the scatterers becomes negligible compared to the convective motion. This causes a decrease in the decay time of the autocorrelation curves and a change in the form of the autocorrelation curves from nearly exponential to Gaussian. This is reflected in the exponent of the mean square displacement that changes from 1 to 2. The effect of shear on the acidification of a sodium caseinate-stabilized emulsion was studied by DWS and by rheometry. The emulsion droplets in the food-related emulsion were uniformly dispersed at neutral pH. Upon acidification down to a pH of 5.2 +/- 0.05, the emulsion showed Newtonian behavior with constant viscosity over the whole pH range. At pH 5.17 +/- 0.05, independent of the applied shear rate during acidification, the viscosity suddenly increased. From this point on, the emulsion showed shear-thinning behavior. The photon-transport mean free path ( l*) was not influenced by the applied shear rate and did not change down to pH 5.2 +/- 0.05. Close to this pH, l* increased, and the decay of the autocorrelation curves shifted to longer correlation times when shear rates smaller than 1 s (-1) were applied. At lower pH (5.05 +/- 0.05), l* started to fluctuate, and the autocorrelation curves no longer decayed to zero, indicating that at these shear rates the system behaved nonergodicly. Assuming that the convective motion and the Brownian motion are independent of each other, the mean square displacement as a result of Brownian motion was determined. From this, the sol-gel point and the radius of the aggregates at this point as a function of the shear rate was determined. The results indicated that the radius of the aggregates at the sol-gel transition decreased with increasing shear rate and suggested that shear will result in a more open structure of the network formed by the aggregates.