Encapsulation by Solid Lipid Nanoparticles
SLNs are aqueous colloidal dispersions in nanosized solid lipid shells. High encapsulation efficiency, no need for organic solvents, easiness of the scale-up and sterilization, flexible control of release, possibility of delayed release, and the effective protection of the core material against environmental factors are the unique advantages of the SLNs in comparison to nanoemulsions and nanoliposomes (Saupe & Rades, 2006). On the other hand, release of the core material in SLNs takes longer time than other techniques due to longer degradation period of the solid matrix (Muller, Dingler, Schneppe, & Gohla, 2000).
The production of SLNs is achieved by either hot homogenization method or cold homogenization method in food processing. In the hot homogenization, the lipid is melted approximately 10°C above the melting point. Then the core material and the surfactant which is heated to same temperature is added into the melted lipid. The mixture is homogenized using a high-pressure homogenization at the controlled temperature. SLNs are produced by cooling of the emulsion which leads to recrystallization (Fathi et al., 2012). However, this method is not suitable for the heat-sensitive compounds. Therefore, cold homogenization must be applied for this type of products. In cold homogenization method, first core materials are added into the melted lipid. After cooling and solidification of the lipid, it is grounded by a mill. The obtained nanoparticles are dispersed in a cold surfactant and homogenized using a high-pressure homogenizer. However, special care must be taken to prevent temperature rising during homogenization (Fathi et al., 2012).
Only a study on production of SLNs containing fish oils was found in the literature. Salminen, Gdmmel, Leuenberger, and Weiss (2016) produced SLNs from fish oil using tristearin as carrier and quillaja extract alone or in combination with lecithin (high- or low-melting point) as surfactant. Physical and chemical stability was tested during storage at dark for 51 days. The particle size and polydispersity index of the SLNs were found to be stable except quillaja-low-melting lecithin. On the other hand, in terms of chemical stability, SLNs produced with quillaja-hot-melting lecithin combination was found to be more protective than those of quillaja-low-melting lecithin and quillaja alone.