title_body: Increase cold brew coffee concentration
upvoted_answer: You should reduce your coffee concentrate by placing it in a vacuum to lower the boiling point of the water down to room temperature. Lowering the pressure of water decreases its boiling point. This way you will boil off the water quickly without adding heat and risking any changes in flavor. 
  
  Otherwise you can put your coffee concentrate in an oven on a low heat setting poured into a glass or ceramic baking pan to maximize the surface area. Use the convection setting if you have it. The added heat shouldn't cause too much of a flavor change since the coffee beans have been removed at this point and won't continue to brew/extract harsh flavors.