Ever Since I can remember we have been told to boil our Enviro Cloths for 10 minutes if they develop an odour.
Just recently, word has come out that Norwex no longer suggests this method of sanitizing your cloths!! It has been stated that boiling for long periods of time can break down the microfiber and may shorten the effective life span of the cloth.
The new train of thought is to soak your "sour " cloths in a solution of Vinegar, Baking Soda and Water and then drain them, pour boiling water over them, rinse well, wring out, and hang to dry or dry in your dryer.
If you use just water and rinse your cloths thoroughly, wring them out and dry them you will have no need to treat with either solution above. A couple of minutes now and no hassle later. :)