Production of nisin-loaded solid lipid nanoparticles for sustained antimicrobial activity

Nisin is a natural antimicrobial agent that is used as a preservative in heat processed and low pH foods. However, its bioactivity is lost by interaction with food components. Slow release nisin-loaded solid lipid nanoparticles (SLN) were produced by high pressure homogenization to provide protection from the food environment and prolong the biological activity. The optimized conditions for the preparation of Imwitor 900 based SLNs was a pressure of 1500 bars in a high pressure homogenizer for three cycles, with 5% (w/v) poloxamer 188 and 0.125% (w/v) sodium deoxycholate as the surfactant and co-surfactant, respectively. Unloaded SLN produced under this condition had the smallest nanometric particle size (119 ± 15.1 nm) with a narrow polydispersity (0.38 ± 0.03). Nisin-loaded SLNs, prepared from 0.5 to 3.0% (w/w) nisin, were larger than the unloaded SLN, with a size range of 159 ± 6.4-167 ± 8.6 nm, had a zeta-potential of −28.3 ± 0.15 to −29.2 ± 0.12 mV and nisin entrapment efficiency of 69.2 ± 0.04-73.6 ± 0.04%, the optimal being at 2% (w/w) nisin. During 28 day of aqueous suspension at 30 °C, the size of the SLNs increased to 214 ± 10.8-245 ± 15.7 nm and zeta-potential decreased to −21.6 ± 0.43 to −25.9 ± 0.34 mV. Scanning electron microscopy (SEM) demonstrated that nanoparticles had platelet shape. In vitro release studies revealed that nisin was released from the SLNs throughout the 25 day period but the release rate decreased as the pH of buffer increased from 2.0 to 7.4 and as the salt concentration increased, up to 0.5 M sodium chloride, whereupon high nisin was released within the first day. The antibacterial activity of nisin-loaded SLNs against Listeria monocytogenes DMST 2871 and Lactobacillus plantarum TISTR 850 was evident for up to 20 and 15 days, respectively, compared to only one and three days, respectively, for free nisin.