Look if it doesn’t work for you don’t do it. Not everything has to be debated ad nauseam.

20 hours ago, dan williams said:

Killing the bacteria that cause the smell.

If you only want to kill the bacteria that causes the smell , you could try putting the helmet in a bag , then put the helmet in a freezer for a few hours .

I haven't tried this myself , but I have a friend who has flameproof overalls & helmet for car racing - he tries to avoid washing as he reckons this reduces the flame retardant properties.

when his kit gets a bit smelly { but not dirty } he puts it all in a sealed bag and leaves overnight in freezer

Makes sense. Freezing will kill most bacteria. Ice crystals destroy the cell walls. Not sure of the effect of repeated heat cool cycles. Usually wreaks havoc on adhesive joints like the liner to shell in most helmets. 

Easiest and cheapest stuff by miles is laundry sanitizer. From the supermarket. 
 

I use a large builders tub, fill with cold water and the sanitizer. Dunk/rinse all my kit through it. Rinse. Leave to dry. Everything smells like new again after. 
 

I do the same for my enduro helmet and body armour. 

I remove the liner and wash it with hair shampoo, If you intend to machine wash it then you should put it in a pillow case and tie it up, still gets washed but helps protect from getting damaged.

Amazon Have, sweat wicking Helmet liner cooling skull caps    i have the, KTM one  my helmet smells great, i wash after each ride