It's amazing how much learning another language teaches you about your native one as well... My first thought was that fruit is simply fruit... until I started reading comments and the complaints about plural fruit. Then the more I read and thought about it, English has several food items that work like this: fish, meat, fruit, cereal. Basically any word that refers to both an item that you eat multiple pieces of and is a category of food.
Great catch, VickiWonder! A partial explanation for this in English is the concept of "countable nouns" (1 chicken, 2 cows, 3 piggies) vs "mass nouns" (some chicken, some beef, some pork).
Plurals (typically or always, depending on how you think about them and how strongly you feel about it) only work on countable nouns.
"Das Obst" is a singular term (look at the article) it has no plural even though it used like one in everyday speech.
It is a mass noun like butter, blood, wine or water, for example. A part of water is still water but a part of a table isn't a table (as a whole). The same goes for the term "Obst". A piece of "Obst" is still "Obst".
Most mass nouns don't have a plural form but in case they have those don't refer to the mass noun itself but to the brand. Take " der Wein" (wine), for example. A part of the wine you have in your glass is still wine, right? Right. But "Wein" has a plural called "die Weine". That is because in the case of "Wein" the plural doesn't refer to the general term of wine but to different brands. So if you say "die Weine" you mean different brands like white wine, red wine, Riesling etc.
Mass nouns also aren't countable. You can't say one "Obst" or fifty "Obst". If you want to make it countable you need a word to help with that: "Ein Stück Obst" (one piece of fruit) or "fünfzig Stück Obst" (fifty pieces of fruit).
"Das Gemüse" is similar to "Obst", it has a plural (die Gemüse) though, which I never heard used by anyone btw. People use "Gemüse" the same way they use "Obst" in everyday speech. I'm unsure if it is actually a mass noun. I even looked it up but couldn't find anything specific. I think it is if you want an opinon.
CreativityBrain asked a legit question. You are forgetting that we DO make a distinction in English and use the word "fruits" for different types of fruit, so it is not always a collective noun. Your example of milk is not parallel, as it is all the same kind of thing. However, if you had cow milk, soy milk, and almond milk, there could be a valid argument that "milks" could be used.
Yes, KatTancock. You're absolutely right.
In India I've rarely heard "fruit" used as a plural - it's almost always "fruits". And it's not necessarily while talking about different species / varieties.
"I've bought fruits from the supermarket" - could mean multiple quantities of the same fruit, or different species / varieties.