Event Triggered When Food Changes State?

nope not at the moment, there isn't also an event for when you are placing or picking them up from the grill

Event added with the new update ItemTransformEvent

As @Minotorious mentioned, the new ItemTransformEvent can be used for this purpose It's triggered when raw meat turns into cooked meat, or if ores turn into ingots, for example. We still have to add more events to give more control over furnaces.