The young coconut liquid endosperm used as suspending medium in this study had a UV-C absorbance of 0.90 and UV-C transmittance of 12.74%. From the UV-C absorbance of diluted liquid endosperms, an extinction coefficient (ε) at 254 nm equal to 0.09 cm−1 was calculated (Fig. 1a). This value is close to the previously reported value equal to 0.13 cm−1 for young coconut liquid endosperm (Gabriel, 2015). The ε value represents the degree of light attenuation of a specific liquid or chemical at a given wavelength (Ingle & Crouch, 1988), and is due to UV-C-absorbing chemical species in the menstruum that could reduce the inactivation efficacy against the suspended organisms. Suspending the cells of the test organisms in the young coconut liquid endosperm significantly increased the UV-C absorbance by 0.08–0.18 units, resulting in a corresponding decrease in UV-C transmittance by 2.36–4.35% (Table 1). Despite the similarity in the number of suspended cells, the optical properties of the inoculated liquid endosperm significantly varied, with UV-C absorbance values ranging from 0.98 to 1.08 (8.39–10.38% transmittance). This may be due to differences in the amounts of UV-C-absorbing cellular components in the test microorganisms.