Effects of Ultrasound Treatment on Physiochemical Properties and Antimicrobial Activities of Whey Protein-Totarol Nanoparticles

Totarol is a natural antimicrobial compound extracted from the heartwood of Podocarpus totara, a conifer native to New Zealand. The effects of whey protein totarol nanoparticles treated with ultrasound on the physiochemical properties and the growth of Staphylococcus aureus were investigated. The particle size of whey protein totarol nanoparticles was reduced by ultrasound treatment from 31.24 +/- 5.31 to 24.20 +/- 4.02 nm, and the size distribution was also narrowed by the treatment. Viscosity and modulus data indicated that the flow behaviors of whey protein totarol nanoparticles seemed to be Newtonian and exerted a typical viscoelastic fluid at protein content of 15% (w/v). Rheological properties were more insensitive to ultrasonic time. Time-killing assays, agar diffusion tests, the cell membrane damage analysis, and microstructure were exploited to study the antibacterial properties of whey protein totarol nanoparticles. The MIC of whey protein totarol nanoparticles after ultrasound treatment decreased from 4 to 2 mu g/mL compared with that without ultrasound treatment. Whey protein totarol nanoparticles treated with ultrasound resulted in a significant (P < 0.05) decrease in time killing after 24 h. The agar diffusion results showed that the inhibition zones of whey protein totarol nanoparticles were 12 and 36 mm for untreated and treated with ultrasound, respectively. The cell membrane damages and the microstructure changes also proved that whey protein totarol nanoparticles treated with ultrasound had strong antibacterial activities against S. aureus and that the antibacterial effectiveness enhanced with the increasing of ultrasonic time. These findings suggested that whey protein totarol nanoparticles treated with ultrasound were more effective against S. aureus than untreated nanoparticles.