Relevant passage:
Their beverage is made of some root, and it will not keep for more than two or three days. It is rather sharp in taste, not at all heady, good for the stomach, and laxative to those who are not used to it; it is a very pleasant drink to those who are. Instead of bread they use white stuff like preserved coriander, which I have tasted; the flavour is sweetish and rather insipid."
The They:
These people inhabit the seashore, and are shut in on the landward side by a range of high mountains, which leave a strip about a hundred leagues in depth between them and the sea.
Also mentioned (in case this helps): They had long, wooden buildings ("barns"), and cotton beds hanging from the ceiling. Each barn was a village. They did not practice agriculture, but instead hunted.
Perhaps the root is manioc. The clue of the hundred leagues between them and the sea led me to believe it was somewhere like Panama or Costa Rica - somewhere where the mountains would hem people to the coast. Dr. Payson Sheets' work in Cerén, El Salvador, has shown that manioc actually is a much larger staple crop than one would think, and for some millennia besides.
Now, one of my favorite facts about humanity is that if we can grow it, someone ends up fermenting it into booze. Manioc liquor exists - known as cauim, or chicha de yuca. The cotton beds hanging from the ceiling is also pretty diagnostic for peoples living in this "Intermediate Zone" between the Mesoamerica archaeologists have intensely studied and South America.
I honestly don't know too much about cauim, but I'll see what I can find about it. I'm tentatively thinking this is your culprit.