easy Jackie, many fruit juices will begin to ferment in a few hours.. apple juice first becomes fizzy as fermentation starts, and the fermantation actually acts as a preservative, converting sugars into more stables alcohols..

no reason not to think that lemonade, with extra sugar (and in times past, a common sugar was barley sugar, which ferments even faster than cane sugar) wouldn't have become slightly fizzy in a few hours... till people got to thinking fizzy is the way lemonade should be. Like orangeina, (which you can get here in US) UK lemonade is not as fizzy as soda, but not flat either.

the US has had temperance societies right from the very beginning, and people would have learned, or practiced ways not to encourage fermentation in lemonade here.