The food I use (PERO) has ingredients in it which prevent nasty smells and they are natural plant extracts. I presume other foods have these in.

I have just changed Hedley from pets at home to Pero (adding gradually) and his bowels have been a bit loose, but I discussed this with the vet and he said it was just whilst he gets used to the new food. He gave me some tablets to crush up to firm his stools.

http://www.pero-petfood.co.uk/images...DATA_SHEET.pdf