In Back to the Future II, we see Grandma Lorraine rehydrate a pizza - very well too I might add. The tool used for this is a hydrator. Looking at the linked article for a hydrator on Futurepedia, though, there is only reference to it being used for a pizza. It wouldn't really make much sense if it was only usable on pizzas though - do we know of any other foods in Back to the Future sources (canon and non-canon accepted) that can also be dehydrated and rehydrated?
It certainly seems very likely. When she puts the pizza into the hydrator, she says "Hydrate level 4 please":
This suggests that the device is capable of rehydrating a wide range of products. I have not been able to find any references to exactly what these other food products might be.
According to the BTTF 2 novelisation, you can also hydrate...erm...smaller pizzas.
Grandma Lorraine stuck one of those expandable pizzas in the Hydrator - a real four-incher! That was one thing you could say about Marty McFly Senior’s parents: They weren’t cheap.
'so I thought,’ she continued cheerily as she distributed the plates, ‘it would be nice if we threw a little party for him.’
The Hydrator beeped. His mother bustled happily back to the machine, pulling the now fifteen-inch pizza free. And she was nice enough not to mention that it had taken the Hydrator a full twelve-second cycle to finish their dinner - Marty wished he could afford one of those new six-second models.
Even if it can only be used for pizza, this would also explain why the hydrator has variable settings since not all sizes of pizza would need the same level of hydration.
Additionally, the concept art for the film describes it as a "PIZZA HYDRATOR". This is hardly conclusive, but it is at least strongly suggestive that that is what it's main purpose is.
And the officially licensed BTTF card game also refers to it as a "Pizza Hydrator"