Upon storage, the particle instability may be induced possibly due to the rupture of surfactant film which led to the incomplete coverage of the lipid surface. Therefore, crystal growth between particles could take place yielding the change in rheological behaviors. By means of rheological investigation, similar observations were reported in literatures. Lippacher et al. (2004) found the influence of storage time and temperature on storage modulus of semi-solid cetyl palmitate nanoparticles.