Everything I know about this topic I learned from the Tudor Farm season of the Historical Farm series, but...

My understanding is that ale doesn't have hops, and consequently has a short shelf life and is meant to be drunk fresh. Beer, on the other hand, does have hops, which acts as a preservative, and thus has a very long shelf life. (Which is why beer was a staple beverage for the Royal Navy, how little you hear about it.)

Which brings me to my question: is the ivy meant to be a preservative, so that gill-ale is a de facto beer?

And I'm curious, what part of the ivy is ground up, did they say?