Who says it's fictional chemistry and biology?
According to this page by the food experts at http://www.realfoodrealkitchens.com (emphasis mine):
there was one scene where she puts a few drops of water in to a tray and bread instantly rises. Is this possible? Well from what I could find yes and no. With the heavy military context of the entire underlying story of the Star Wars franchise, it would only make sense that 'regular' people might eat military styled ration packs. And in military styled rations there does exist a simple bread called Hardtack that is made from just water, flour, and sometimes salt. Is this what 'Rey' was eating? The more modern version of Hardtack is something I eat all the time with cheese, it's called a 'water biscuit', think of the British crackers made by Carr's.
They even include a recipe for water crackers further down on the same page!
A recent news article has confirmed that the chemistry used to make Rey's bread was definitely real, not CGI as many believed. Quoting from the article (emphasis mine):
“Surprisingly that was done practically, although so many people have said to me, ’we thought that was a digital effect!'” [special effects supervisor Chris] Corbould said.
The idea for the bread came from director J.J. Abrams himself, and it was actually a huge task considering that you only see it on screen for three or four seconds.
“You wouldn’t believe how long it took to actually perfect that one, that little tiny gag in the film,” Corbould said. “It started off with the mechanics of getting the bread to rise and the liquid to disappear, but then there was the ongoing problem of what color should the bread be? What consistency should it be? Should it have cracks in it? Should it not have cracks in it?”
“It took about three months,” he added. “The actual mechanics of it was fairly simple, but the actual cosmetic side took a lot longer.”
But in case you’re one of those people clamoring for a home version of Rey’s food portions, don’t get too excited — the bread might have been a real practical effect, but it also tasted terrible and probably had next to no real nutritional value.
“No, you wouldn’t want to eat it!” Corbould admitted.
In-universe, here's a picture of the description found in the (canonical) TFA Visual Dictionary: