If you said, "I love apple.", you would be talking about the taste or flavour, rather than a single apple. You would probably add something such as "I love apple flavoured yoghurt." If you were talking about eating whole apples, you would need the plural because it is generally speaking and apples are countable nouns.

"I love apples." - (the fruit) is correct. Put countable things in plural form, to talk about them generally. "I love Apple." - (the business) is also correct. We would know you mean the business from the context. "I love apple pie." - is correct. In this compound noun, "apple" has an adjective position, and doesn't take a plural.

"I love apples" or "I love eating an apple" is correct.

I love grapes!