You will need: 1 cup of vinegar, 1/2 cup of baking soda and very hot water.

Set your washing machine to very hot water. Wash the towels adding the cup of vinegar.
Once the wash cycle is complete, leave the towels in the washer, add 1/2 cup of baking soda, and run another wash with hot water.
Once complete, use the spin dryer and dry as you normally do.