If you went back in time and ate food there, would it not agree with your stomach?

This occurred to me while conjuring up an idea for a time travel story. I have the characters eating food in the past and then realized, “Wait, if they were used to processed foods all the time in the future, would eating all this raw food suddenly make them gassy or disagreeable?”

Keep in mind I have little idea what “processed food” means. I always assumed it means laced with chemicals/generally sterilized.

Also, a lot more bacteria would be around back then during food preparation, right? Or maybe bacteria that’s not around today we never knew about? By “back then” I mean earlier than any common food preservation in human history, so, as a random example, ancient Greece.

On the contrary too, what if you were used to eating raw foods all the time and suddenly had processed foods?