Back in the day I worked at at Burger King for several years, and then a mom-and-pop burger shop. I ended up closing and often worked drive thru. I had to take the shake machine apart every single night and clean and soak the parts in sanitizer and then put it back together. We threw out the shake machine mix once a week and started over, otherwise it was put into a big bucket and kept in the freezer. I am more inclined to think it was in the mix and the mix kept getting used.