When talking about quantity use "de" because you're talking about the objects in a general way and not referring to specific objects.  Tomatoes in general and not "the" specific tomatoes.  Examples: Beaucoup de gens; tant de choses; tas de questions.  Etc.  Of course, you could say, "Un kilo des tomates que je préfère" because now you're talking about the specific kind of tomatoes you prefer.  Hah, I had to correct my misspelling of tomatoes in my native language!

Merci beaucoup Sean. Much appreciated, and yes my native spelling has been shot since I've been learning another language too