A method and device to treat liquid to reduce the amount of microorganisms in the liquid to a preselected level and/or to mitigate the growth of microorganisms are disclosed. The reactor and method described herein may be used to treat a variety of liquids and may be particularly well suited for the treatment of food substances, such as dairy, non-frozen concentrate juices, and the like. The process for reducing the number of pathogens in a liquid utilises a reactor having at least (i) two spaced-apart walls, where each spaced-apart wall has an interior surface, (ii) an interior cavity between the interior surfaces of the at least two spaced-apart walls, (iii) an entrance to the interior cavity, and (iv) a nozzle centred at the entrance to the interior cavity. The method has the steps of: (a) heating the liquid before the liquid enters the nozzle； (b) spraying the liquid by the nozzle into the interior cavity； (c) subjecting the liquid to a pressure drop as it is being sprayed； and (d) increasing the temperature of the liquid when the liquid is in the interior cavity.
