Ultraviolet photoreactors are commonly used to disinfect drinking water and wastewater. Recently these reactors have been proposed for treating liquid foods such as dairy products, which are much more opaque. The performance of a UV reactor is related to the distribution of UV dose delivered by the reactor. UV-sensitive microspheres have recently been demonstrated as a way to quantify the UV dose distribution in conventional reactors, but have never been used in liquid foods, which present new challenges.