Stays (more) fresh (for longer)
Solution 1:
I would say "stays fresher for longer". The unspoken but inferred rest of this sentence is ...than if kept out of the fridge. Fresher is being used comparatively.
I would say "stays fresher for longer". The unspoken but inferred rest of this sentence is ...than if kept out of the fridge. Fresher is being used comparatively.