That not true. '... don't eat apple' is perfectly sound, as apple can be both count and non count. When you cut apple in pieces, it's not AN apple anymore (it could be exactly one apple, or a half or two or even a bowl of apple), but the substance is still called apple. This applies to virtually all foods that are otherwise count nouns: as soon as they become food they can be either count our non count. The typical joke is: I like dog. As opposed to: I like dogs. If you're still doubting, replace dog with chicken. [edited for spelling/grammar]
I have a question about this - I read in a textbook that if the subject is feminine plural, and the sentence is negative and हैं is dropped, then the imperfect participle (here, खाती) is nasalized. Is this just a formal grammatical rule that's generally not used in everyday speech? Is it a regional thing? Thanks!