Transitions in Structure in Oil-in-Water Emulsions As Studied by Diffusing Wave Spectroscopy

Abstract

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.

Real-Time Determination of Structural Changes of Sodium Caseinate-Stabilized Emulsions Containing Pectin Using High Resolution Ultrasonic Spectroscopy

“…This suggested that adding more HMP does not change the structure of the flocs much, but simply crowded the separated phases. A similar observation was reported for depletion flocculation of emulsion droplets with added xanthan (0-0.2%) 8,9,24 and HMP. Statistical analysis of the data of ultrasonic attenuation as a function of concentration of HMP indicated that there was an effect of pectin type on the values of attenuation, although the overall trend is the same.…”

“…5,7 During acidification with glucono-δ-lactone (GDL), the pH is gradually lowered, and at a pH <5.0, the steric stabilizing layer of casein micelles collapses, there is a decrease in charge repulsion, and the caseins aggregate (Liu et al, submitted). 8,9 It has been demonstrated that the interactions between HMP and casein micelles are pH-dependent and occur at pH of about 5. The HMP adsorbed on the casein micelles prevents sedimentation via electrostatic and steric stabilization.…”

“…In emulsions stabilized with sodium caseinate, when the pH is lowered towards the isoelectric point of the proteins, the oil droplets approach and aggregate, and a sol to gel transition occurs. 8,10 HMP adsorbs onto the sodium caseinatestabilized oil droplets at pH just above the isoelectric point of the caseins, inhibiting the aggregation of the oil droplets (Liu et al, submitted). 10,11 In low pH systems, pectin can interact with the adsorbed protein to form interfacial complexes that improve the emulsion stability.…”

“…Recently, diffusing wave spectroscopy has been successfully used to study these emulsion systems without the need for dilution (Liu et al, submitted). 8,9,11 Ultrasound spectroscopy also shows great potential for the study of in situ changes in emulsion systems. This noninvasive technique has been successfully employed to study droplet-droplet interactions in situ to distinguish differences between bridging flocculation and depletion flocculation in whey protein-stabilized emulsions.…”

Food Functionality and the Physics of Bionanotechnology: Some Examples and Challenges

“…In the case of sodium caseinate emulsions, it was found that diffusing wave spectroscopy is a very useful technique to measure the different structural transitions that take place within the system. It was found that two transitions took place in a system with an excess of stabilizer [43] .…”