 With actual yeast, usually I kind of have to go with a gallon. But then again, yeast is relatively cheap. I can always use bread yeast, too, while I'm in the experimentation phase. It's about the cheapest yeast you can find, and it'll work. It'll carbonate things. In fact, it's really good at carbonating things. That's why it works for bread. I used to do that years ago. I remember when I was first getting into root beer making...