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. Utilizing the method or device, liquid product is sprayed into a cavity of a reactor using a nozzle that produces a flat spray to provide means for efficient heating and treatment of the liquid. The reactor comprises at least two spaced-apart walls that define a space therebetween into which a flat stream of liquid is sprayed. The interior surfaces of the walls may be heated to the same or different temperatures using a variety of techniques. 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.
