When the holding samples were filled with milk, the extent of UV penetration decreased significantly at both levels of impact energy, to 0.75 ± 0.38 and 1.15 ± mJ pulse1, respectively, resulting in measured penetrations of only 0.25% and 0.27%. This finding agrees with the results reported by other authors who found that the depth of UV light penetration into a liquid medium depends on the incident light irradiance, the transmittance and the optical and physico-chemical characteristic of the substance.