Learn English – Nourishment vs food

The word outdated isn't typically used in the context of food freshness.

The adjective outdated suggests something is no longer widely in use or is unfashionable. Wearing poodle skirts is generally considered outdated. Super Nintendo and Windows 95 are outdated. You wouldn't typically say your food is outdated (unless its presentation or ingredient usage has gone out of style).

"Expiration date" is a term used to indicate when either the shop has to stop selling the item ('sell by' date) or the date by which it must be consumed ('use by' date). If the freshness and overall quality of the product is more of a concern than health issues (for example, crackers go stale but are largely safe to consume after their expiration date), one might use the term 'best before' or 'best used by' date.

But, generally speaking, if food is said to have "expired", that generally means it should not be consumed. It is only historically that items (like milk) were labeled somewhat ambiguously with terms like "expiration date" — which, in the case of milk, was the date it needed to be sold by — but laws varied by product. Product labeling of expiration dates is typically a bit more specific now.

Fast food is served at a fast food restaurant, such as Mcdonalds, &c; while junk food a specific type of food such as chips, pop, &c.

Obviously, junk food could be served as fast food.

Essentially, the adjective: "fast" modifies the noun "food" in describing how long it takes the consumer to get the food ready to be eaten, that is the food will be ready very soon.

The adjective: "junk" describes the quality and nutritional value of the food. Here "junk" is considered as that which is not usable, i.e. garbage.

Therefore to answer your question, different aspects of the food are being described.

1. preparation time
2. quality