star wars - What are the rations Rey eats?

In The Force Awakens, we see Rey pouring some flour-like polystarch rations into water so that the flour amazingly transforms into something that looks like bread. What are these rations? Is there any documentation of the fictional chemistry and biology that makes the starch rations behave as they do?

Answer

There's a pretty reasonable description of how they work in the film's two novelisations (Official / Junior);

Green slab-stuff sizzled in a makeshift cook pan. Opening the packet of beige powder, she dumped it into a tin half full of water. A brief stir activated the mixture, which promptly expanded and solidified into a loaf of something like bread. She slid the cooked meat off its pan and onto a plate, then slipped the loaf out of its container. Taking a seat, she dug into both as if she had not eaten in weeks. It seemed that these days all too many meals were like that.

and

Rey made dinner in her humble home. She opened both ration packets and dumped the green protein square into a pan on a burner. While that was cooking, she mixed the brownish flour with water in a container. The chemical reaction worked its magic and a doughy loaf rose.

“Meat and bread” was what Plutt called this packet combination, though Rey doubted it tasted anything like the real thing. She ate from a plate that she licked clean of any leftover food.

Which matches the description in the film's script

She cooks for one. Does everything for one. She opens the POWDER, moves to the makeshift WOK, where the GREEN MEAT SIZZLES. Pours the powder into milky WATER in a tin. Stirs it. It GROWS INTO A LOAF as she puts the meat on an old plate. Grabs the loaf.

So it's an...erm...chemical reaction. And then you get bread. Ta-da!

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As to how the effect was accomplished practically, it may interest you that it wasn't actually CGI.

Chris [Corbould - SFX Supervisor]: We had a little bowl of liquid. At the bottom of that bowl you had a bread molded inflatable bladder. And as we inflated that, we sucked all the liquid out. So that as that was coming up, the liquid was disappearing by using a vacuum dump. A very simple procedure, but quite effective.