I used to believe that, in general, I should use a singular countable noun with an article.  However, reading one book talking about articles, I started to get myself confused whether or not an article should be really omitted (= should not be used) in the cases as follows:

1. This one smells like old dog. (not "an old dog"?)
2. It tastes like apple./It tastes something like apple. (not "an apple"?)
3. You may put pineapple on your pizza.
4. Add carrot and onion after heating the pan.

Well, according to the author of this book, he says that "an article is not necessary if the noun specified is not recognized as having its original form."

As for the above 3 and 4, I kind of understand why "a/an" cannot be used--because we do not mean that we put/add a whole pineapple, carrot, or onion.  However, even so, I still have doubts if it is okay to leave the nouns (pineapple/carrot/onion) without any articles or making them plural instead.

Professional teachers/native speakers, please help me clear this problem.

Ant_222 wrote:

To me, in all these examples singular nouns denote a substance rather than a thing. That's why indefinite articles are omitted. P.S.: Anyway, I'd use "an old dog" for #1... because... it doesn't make much sense to speak of the substance (matter) of an old dog.