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

Abstract

The interactions that lead to structure transitions in oil-in-water emulsions were investigated using highresolution ultrasonic spectroscopy. High methoxyl pectin (HMP) was added to the emulsions at various concentrations and the dynamics of aggregation induced by changes in pH were observed. Two independent ultrasonic parameters, velocity and attenuation, were measured as a function of time or pH. At pH 6.8, both velocity and attenuation of sound changed as a function of HMP concentration. During acidification, caused by the addition of glucono-δ-lactone, there were small changes in the overall ultrasonic velocity, but it was possible to relate these changes to the structural changes in the emulsion. The values of ultrasonic attenuation decreased at high pH with increasing amount of HMP, indicating changes in the flocculation state of the oil droplets caused by depletion forces. During acidification at pH 5.4, emulsions containing HMP showed a steep increase in the ultrasonic attenuation, and this pH corresponds to the pH of association of HMP with the casein-covered oil droplets. The adsorption of HMP onto the interface causes a rearrangement of the oil droplets, and the emulsions containing sufficient amounts of HMP no longer gel at acid pH. This is well described by the ultrasonic attenuation changes in the various emulsions. This research demonstrated for the first time that ultrasonic spectroscopy can be employed for in situ monitoring and analysis of acid-induced destabilization of food emulsions.

Changes in Colloidal Properties of Oil in Water Emulsions Stabilized with Sodium Caseinate Observed by Acoustic and Electroacoustic Spectroscopy

“…In the present study, all pectin was added to the emulsions immediately prior to measurements in one step rather than a stepwise addition. Therefore, in the present case, addition of high concentrations of pectin could have already reached a plateau value for attenuation 15 and the increase in medium viscosity would lower the propensity of reorganization of the phase separated clusters.…”

“…15 As the pectin concentration was stepwise increased, low but constant attenuation was observed. In the present study, all pectin was added to the emulsions immediately prior to measurements in one step rather than a stepwise addition.…”

“…In the case of sodium caseinate emulsions containing 0.1% HMP, however, a slight reduction in attenuation was observed after about 4 h from mixing. It is known that HMP induces depletion flocculation of emulsions 15,23 . The interfacial layer at this pH is negatively charged, and charge repulsion between the casein and the HMP is expected.…”

“…Using ultrasonic resonance, Liu et al (2007) showed that addition of HMP to emulsions was characterized by a rapid reduction in attenuation at low concentrations. 15 As the pectin concentration was stepwise increased, low but constant attenuation was observed.…”

“…Attenuation measurements have been employed to study the interactions, flocculation and aggregation of emulsion systems. 14,15 For example as depletion flocculation of the oil droplets takes place, a significant extent of reduction is observed in ultrasonic attenuation. 2 Electroacoustic spectroscopy couples ultrasonic and electric fields.…”