Normally, the smell is a result of poor cleaning after use. A simple hot water rinse directly after you consume your shake should do the trick. However, if you want to go one (or two) steps further, you can pour hot water and soap in it, shake it up like a soapy smoothie and rinse through. If it’s too late to go through with that step and you already have a very stubborn, very caked-on protein residue, you can fill it with hot water and drop dish soap or a denture tablet in it, leave it for 10–15 minutes and rinse it out.