Hello everybody,
It's a very interesting discussion, let me say my oppinon...
1. des fraises, du pain - it is not a question of singular or plural. The bread is simply cosidered uncountable (not only in french), because one "normally" eats pieces or "parts" of a loaf and not whole loaves ... In Slovak and in Czech, it is almost common to say "dám si chleba" ("napiju se vody",...) which is exactly the same as du pain (genitivus partitivus). For the same reason, "Qanto costano le ouva"? but "Quanto costa il pane ?"
2. In Hungarian: "Mennyibe kerül a tojás"? and "Mennyibe kerül a kenyér?". An other example, that has nothing to do with the ingredients: "Ennek a lánynak hosszú haja van" but in Italian "Questa ragazza ha capelli lunghi" or in Czech "To děvče má dlouhé vlasy". In Hungarian we have singular in all these cases.
3. I think, the reason is the following: originally, in the earliest phases of the evolution of the languages (both Uralic and Indoeuropean) there was no singular nor plural, a noun itself was number-indifferent, only in context could it took the sense of singular or plural (I am not sure, but as far as I know, in the Turkish it is so even today). This feature, though only partially, but seems to survive in the Hungarian language.
4. An other situation: három ember, tre uomini, three men... This is, of course, not the case of the original question about the ingredients, but perhaps the same logic: ember is formally in singular ("indifferent") but the word három gives it the sense of plural.
5. With other words, in contrast to IE languages, we could say that the Hungarian uses the plural explicitly when it is semantically needed or important, othewise the singular is preferred.