Abstract: This study evaluated the stability of bilayer emulsions as a function of secondary layer composition and pH. Primary emulsions were formulated with 5% soybean oil, 1% protein from nonfat dry milk (NDM) powder as emulsifier and ι-carrageenan (ι-carr), low-methoxyl pectin (LMp), high-methoxyl pectin (HMp), or gelatin as secondary layers. ζ-Potential values increased for each emulsion as the pH decreased, with ι-carr emulsions being consistently more negatively charged than primary emulsions and significantly more stable. ζ-Potential values were not always correlated to emulsion stability. Gelatin secondary emulsions at pH 3 and HMp secondary emulsions at pH 7 were unstable due to the presence of depletion flocculation. In addition, LMp secondary emulsions stability at pH 7 might be due to calcium bridging, which increased the emulsion's viscosity. Overall, the stability of NDM emulsions was improved when ι-carr and LMp were used as secondary layers at pH 7 and 5, and when ι-carr and HMp were used as secondary layers at pH 3. Increased stability of these systems can be attributed to a second homogenization step used to formulate the secondary emulsions and to the presence of Ca+2 in the NDM. Results from this research show that the stability of bilayer emulsions is driven by the presence of depletion flocculation, droplet charge, droplet size and distribution and viscosity.
Practical Application: The use of everyday ingredients (nonfat dry milk powder, gelatin, pectin, and carrageenan), which are understood and accepted by the average consumer, creates label-friendly products that are the wave of the future. Stable emulsions can be formed using these ingredients at various pH. Understanding the stability and how the pH impacts the physicochemical characteristics and stability of these emulsions will enable manufactures to use ordinary ingredients to create healthier products (for example, low-fat dressings, sauces, dips, and beverages).