> Bring to a boil then leave on low for a couple of hours.

Alternatively, since you mention this above, one can use a Blendtec or Vitamix grade blender to friction-heat the liquid. Apparently this is more energy efficient than microwaving for instance: https://news.ycombinator.com/item?id=11399036 . Obviously this won't work for soups where one desires very coarse pieces of vegetables; nonetheless I often find the above useful.