Serenade is an excellent prophylactic measure, but it will only retard growth. A solution of 0.25% rosemary oil, or neem oil should do the trick. The nice thing about this is that it is also a pretty good miticide agent. Try Home Depot. About $6 per quart spray bottle. Milk contains lactose which can provide an excellent disaccharide for bacteria metabolism, so while it does work, it is not a great choice.