Why are so many foods referred to as singular collective nouns? I can understand it for things like "rice." Some words are problematic in translation: salume, salame, salami, salamis. Some words seem to be commonly used in the singular, but didn't used to be: I see "shrimps" a lot on older cookbooks, but rarely see or hear the word in the plural anymore at all. But the one that strikes me the most is "eggplant." I almost never see or hear the word "eggplants." And it's not like they're so commonly small that one would think of them as a collective noun.
I guess the word "pea" is a tragic counterexample from centuries past, when the plant was known as a "pease" plant. Boys and girls, can you say "back-formation"? :)
Perhaps some of my, ahem, more literate Gentle Readers would spare a comment or two, not that I have any particularly illiterate ones. :)