Synergistic Interfacial Properties of Casein and Small Molecule Surfactants for Fabrication of Essential Oil Nanoemulsions

Nanoemulsions of essential oils are important for delivery of flavors and antimicrobial preservatives in food systems. The overall goal of this work was to study the formation and properties of essential oil nanoemulsions fabricated with sodium caseinate (NaCas) or its hydrolysates and small molecule surfactants (SMSs). The first group of lemon oil nanoemulsion was prepared with NaCas and Tween 20 using a phase inversion temperature (PIT) method. The combination of NaCas and Tween 20 reduced the turbidity and droplet dimension of emulsions than using them individually. Heating at 90 °C for >1 h resulted in transparent nanoemulsions for samples with 1.5% lemon oil. Negative and positive effects on nanoemulsion formation were observed with 0.2-0.4 and 0.6-0.8mM NaCl, respectively. The nanoemulsion prepared with 2% NaCas, 0.4-1.2% Tween 20 and 1.5% lemon oil had a volume-area mean diameter of around 100 nm that was optically stable during 15-day storage at room temperature, while creaming occurred after longer time storage.

Protein hydrolysates have a higher diffusion rate and flexibility than proteins. In order to improve the emulsification efficiency and emulsion stability, the second group of thymol nanoemulsions fabricated with combinations of casein hydrolysates (CH) and sucrose stearate (SS) was studied. For NaCas hydrolyzed by pancreatin for different durations, the product hydrolyzed for 10 min resulted in the most transparent and stable emulsions due to limited reduction of casein molecular weight. Thermal treatment further improved the emulsion stability because of the improved solubility of SS and the strengthened interactions between CH and SS during heating. Addition of 0.25-1% SS improved the emulsification capacity of 2% CH and the emulsion clarity, while excess (2%) SS significantly increased turbidity and particle dimension of emulsions. With the increased thymol concentration (0-3%), stability of emulsions improved. Emulsions prepared with 0.25% SS and 1.5-3% thymol, or 1% SS and 3% thymol, had stable droplet dimensions during ambient storage for over two months. The combination of SS and CH also enabled stable emulsions at pH 5. Therefore, combinations of SMSs with NaCas and its hydrolysate can be used as novel approaches to prepare essential oil nanoemulsions for various applications.