"Do you have yeast?" "What's that?"
[looks up the history of leavened bread]
...It's on ancient Egyptian hieroglyphs and specifically mentioned in the book of Exodus.
God damnit isekai authors, stop making your vaguely Late Middle Age (500-700 years ago) fantasy worlds strangely stupider than people from the fucking Iron Age (2000-3200 years ago). At least not without thinking of some rational reason why they wouldn't know stuff that people thousands of years before their time in our world did. Do some basic fucking research for once.
(No I'm not claiming that ancient Egypt and Exodus were necessarily in the iron age, they're more late bronze age; I'm saying that by the time of the iron age, the practice of leavening bread would have been already well-established and widespread)