Joseph asked: “Maybe you can clarify what is the difference between a fruit and a vegetable REALLY?”
When I was a kid, it was endless fun to blow my friends’ minds, and my own, with the information that, even though we put it on our burgers and in pizza sauce, a tomato is not a vegetable—it’s a fruit!!
Well, I didn’t have a TV.
Neither, as it happens, did the United States Supreme Court of 1893, which unanimously ruled that, for purposes of taxation, a tomato is a vegetable. That ruling still stands, even while the European Union in 2001 decreed that (when it comes to jam, at least) “tomatoes, the edible parts of rhubarb stalks, carrots, sweet potatoes, cucumbers, pumpkins, melons and watermelons are considered to be fruit.” And the state of Arkansas decided to have it both ways when it declared in 1987 that its local pink tomato “shall hereafter be the official State Fruit and the official State Vegetable.”
Clearly the answer you get depends on who you ask: a lawmaker, a proud Arkansan, a juvenile pedant.
If you ask a botanist, a tomato is unambiguously a fruit, which The New Oxford Book of Food Plants defines as “the structure, usually containing seeds, which develops from the flower ovary,” That definition encompasses apples and oranges, tomatoes and cucumbers, pinto beans and wheat kernels, and bolls of cotton.
That botanist will also tell you that “vegetable” is not a botanical term at all, just a culinary one. The field of botany categorizes the parts of plants according to the function they serve for the plant, but any part of a plant’s anatomy, from root to leaf, can play the savory role of “vegetable” in the kitchen. So a number of foods that are fruits in botany, like the tomato—which contains seeds and develops from an ovary—turn out to be vegetables in the culinary world. (Culinary vegetables needn’t even come from a plant: take mushrooms. And don’t get me started on the Vegetable Lamb.)
The gulf between the botanical and the culinary starts to narrow if we ask the plant itself what it has in mind. Botanical fruits are typically bright-colored, juicy, and sweet, because the more attractive the fruit is to animals, the more widely the plant’s seeds (contained in the fruit) get dispersed when those animals eat and excrete them. Bright, juicy, and sweet are pretty good qualifications for a culinary fruit too. (The tomato seems to have a confused identity especially because it’s also full of the savory compound glutamate, which puts it in a vegetal realm despite its alluring red fruitiness.)
Culinary vegetables are more typically parts that it doesn’t benefit the plant to have eaten. So instead of overt attractive features, they often produce bitter, sharp flavor compounds, like the sulfury isothiocyanates in brussels sprouts (which are axillary buds to a botanist); and/or require cooking, like potatoes (a stem tuber).
Ultimately, since “vegetable” is a culinary concept, the best way to make this distinction is to ask a cook. The somewhat tautological, and hardly quibble-proof, rule of thumb is: If you’d put it in a fruit salad, it’s a fruit.