Silicone base emulsions containing either glycerine or propylene
glycol were formulated based on the solubility and stability of
the extract. The ingredients were varied to produce a physically
stable emulsion that was preliminarily evaluated by the centrifugation
assay (Table 4). Three stable bases were included for the
preference test. Base formula 2 gained the highest preference in
terms of spreadability, texture, color, smoothness, adsorption,
softness, moisturizing and odor during and after use (Churienthong
et al., 2010). Therefore, this base was selected for the development
of rice husk cosmetics. The rice husk extract from 120 min of reflux
was incorporated in the preservative-free base (50 mg/ml). The
extract was compatible with the base and remained stable following
accelerated stability tests, as shown in Table 5. Although the
pH of the formulation was slightly shifted, it was in the acceptable
range for cosmetic formulations; this pH shift may have been
because no preservatives were added to the formulation. Furthermore,
the sun protection efficacy of the developed formulation
was better than the silicone base. Although UVB protection was
minimal due to the decreased SPF, UVA protection efficacy was
better (Table 6). The increased UVA/UVB ratio is in agreement with
the boot star rating and was confirmed by the wider range of
critical wavelengths of the rice husk cosmetics. The protection efficacy
was reduced in terms of the UVA/UVB ratio following the
accelerated stability test. However, the range of critical wavelengths
and boot star rating displayed high protection. In addition,
the chemical stability of the TPC was determined based on its
contribution to antioxidant activity and is related with the fatty
acid content, as described above. Freshly prepared rice husk