Foam-mat freeze-drying approach for preserving stability and antimicrobial activity of clove essential oil nanoemulsion

Abstract

Foam-mat freeze-drying technique was used to maintain stability and antimicrobial activity of clove essential oil nanoemulsion (CN). The CN was prepared from 1% (v/v) clove essential oil, 3% (v/v) Tween® 80, and deionized water as a continuous phase, using an ultrasonication. The optimal condition for fabrication of foam-mat freeze-dried clove essential oil nanoemulsion (FFD-CN) was achieved using response surface methodology (RSM) with Box-Behnken design (BBD). The optimal condition was found at 3% (v/v) of foaming agent (Methocel™), 60 °C drying temperature, and 72 h drying time. The obtained FFD-CN had the droplet size of 26.14 nm, polydispersity index (PDI) of 0.193, water activity (aw) of 0.273, solubility of 88.71%, and viscosity of 12.85 cP. The FFD-CN exhibited good stability and maintained antimicrobial activity, as analyzed using minimum inhibitory concentration (MIC), against gram-negative Escherichia coli, Pseudomonas aeruginosa and gram-positive Staphylococcus aureus during 3 months storage at 25 °C. When observed using a scanning electron microscope and energy dispersive x-ray spectrometer (SEM-EDS), bacterial cell morphological destruction caused by both FFD-CN and CN was evident. Gas chromatography-mass spectrometry (GC-MS) results suggested that FFD-CN can preserve eugenol, a major volatile bioactive compound in clove oil, in its structure better than liquid-formed nanoemulsion. Transformation of liquid CN into FFD-CN can effectively preserve stability and antimicrobial activity throughout the storage.