In response to a question, I would like to take a moment and explain the relationship of this last batch of food idioms to English speech and discourse.
Idioms are, by their very nature, informal, for the simple reason that they cannot be read literally. You will never find idioms used in throne speeches by the Queen of England, or in government regulations.
You will find idioms everywhere else.
There are degrees of formality. Idioms may not be used to the same degree in high level business meetings as they are in workplace gossip about the local post office, but they will be used, because for English natives, they make comprehension easier, not harder. Often something can be expressed with an idiom far quicker and better than with a long sentence.
For the last three idioms we covered, the big cheese, a bad egg, and a bad apple, these expressions might seem slightly dated to the Facebook generation, but they are nonetheless very, very common in mainstream American English, the sort heard and spoken around the world. They are used in classic works of American literature. They are used without regard for the fact a non-native speaker might not understand them; all graduates of secondary education in America are expected to know such basic expressions. They are not slang; they are used across a wide range of speech, from the highly informal to the quite formal. They are just not used in perfectly formal contexts, such as those I mentioned earlier.
The reason I have not been assigning formality ratings or commonness of use ratings to my Idioms posts, with some sort of five-star system, is for two reasons.
One, I have made a point of posting only common English idioms, those commonly encountered precisely because they are used in a wide variety of circumstances. This may not have been obvious. Unfortunately, since that is the approach I have taken, it is a waste of everyone's time for me to explicitly say an idiom is common. Right now, they all are, because food idioms are so common in general.
Two, there is no systematic data on how common a particular idiom is. Furthermore, any such data would only be good within a particular region of the English-speaking world. An idiom commonly used in Alabama may be as rare as hen's teeth on Rhode Island. Even if I knew, frequency would vary by region and dialect to the point of being useless in a rating system.
So, I'm just doing the best I can with this. Thank you for your understanding, and do not be afraid of asking more questions. I appreciate them and am happy to reply.