"The man eats two eggs and drinks apple juice."
Translation:Bărbatul mănâncă două ouă și bea suc de mere.
I've been thinking of your question throughout the day, but I couldn't find any discussion on it. Bottom line is that it's always "suc de mere" and not "suc de măr" and most fruits and vegetables appear in the plural form when describing something using "de", be it juice, tea, jam, pie etc:
mere, pere, căpșuni, cireșe, banane, prune, piersici, caise, nectarine, portocale, mandarine, struguri, trandafiri, roșii, mure, gutui, castraveți, cartofi, dovlecei, morcovi etc.
There are exceptions:
lămâie, zmeură, pepene, dovleac, cactus, ceapă, varză , ananas etc.
One of the most striking pairs is: portocală (orange) - lămâie (lemon), because you wouldn't expect there to be any difference.
I think that you shouldn't view them as singular nouns, but rather uncountable forms. Indeed they can be preceded by the plural indefinite article "niște", which can show an unspecified quantity of a collective noun:
vrei niște lămâie? - would you like some lemon?
vrei niște portocală? - would you like some orange?
The second sentence sounds less natural, possibly even wrong. I'm not a native English speaker, so I may be off, but it also seems that way in English.
My best guess is that we memorize these idiomatically and there's probably no solid rule which can predict what words appear in the singular or in the plural. One interesting tidbit is that I'm like 99% percent sure that I've never heard anyone use the plural of "zmeură" (raspberry).
Lastly, there are some fruits and vegetables which are either invariable or have no plural form (especially borrowed words for exotic fruits):
ardei, mango, avocado, kiwi, papaya etc.